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Chapter II. Production Practices as Risk Factors in Microbial Food Safety of Fresh and Fresh-Cut Produce Part III

Analysis and Evaluation of Preventive Control Measures for the Control and Reduction/Elimination of Microbial Hazards on Fresh and Fresh-Cut Produce

Table of Contents

Chapter II

Part 1   |   Part 2   |   Part 3   |   Part 4

Production Practices as Risk Factors in Microbial Food Safety of Fresh and Fresh-Cut Produce

3. Harvest and Post-harvest Operations

The focus of this section is production practices that have the potential to contaminate produce items with pathogens at harvest and post-harvest. It would be an extremely large task to identify points of potential hazards for all produce items and practices and it is beyond of the scope of this report. This section evaluates the potential points of contamination during the harvest process of selected commonly consumed fruits and vegetables. After harvesting, most produce is packed in packing houses and, occasionally, in the field. Seventy-seven percent of fruit growers packed their crop at an off-farm packing facility in 1999, while 16% used an on-farm packing facility, and 8% field packed. Forty percent of vegetable growers packed their 1999 crop in the field; 36% used an on-farm packing facility; and 23% used an off-farm packing facility. The packing operation deserves special attention. Both the quality of the water used for the cooling process as well as plant design aspects may be related to food safety. Finally, a detailed analysis of the potential for transportation of produce to be a significant contributor to the growth and survival of pathogens of concern is also presented. It is beyond the scope of this report to give guidance on good agricultural practices, in part because of the excellent documents already published. Exceptions include plant design and transportation considerations due to the unavailability of detailed guidance in such areas.

3.1. Summary of hazard control points for selected fruits and vegetables

The natural ecosystem is an uncontrolled and wild environment that includes organic debris and microorganisms, some of which may be pathogenic to man. This is the milieu in which fruits and vegetables are grown and harvested for human consumption. An analysis of common operations used to harvest and prepare fruits and vegetables for market can identify steps in the process where pathogens might be introduced, controlled, or eliminated. Despite the variability of production practices within the U. S. geographical area (for example, in certain states other potentially risky practices prevail such as hand clipping of spinach leaves at 1-2 inches above the soil that are then placed into a wooden or plastic container, loaded on a truck, topped with ice, and transported for market delivery) the ones presented here are common practices that can serve as examples.

The production practices for the fruits and vegetables listed below are described in the flow charts to follow. Hazard control points during harvest and post-harvest operations are indicated on these charts.

leafy vegetables (lettuce, celery, green onions)
spinach
culinary herbs
peas and beans
peppers
summer squash, eggplant or cucumbers
winter squash and pumpkins
bulb vegetables
root vegetables
carrots
melons
vine-ripe tomatoes
strawberries
apples
pears
citrus fruits
kiwi fruit

Relevant hazard control points include: field worker hygiene, field sanitation, equipment sanitation, container sanitation, water sanitation, truck sanitation, and temperature control. A review of the flow charts for harvest and post-harvest operations leads to the following general observations.

Field worker hygiene is an important consideration in the harvest and post-harvest processing of fruits and vegetables due to the widespread use of human hands as part of the process. In a 1999 survey of produce production practices (USDA 2001), 93% of the farms that grow fruit and 89% that grow vegetables harvest the fruit or vegetable exclusively by hand. Direct hand contact is also used to trim extraneous matter or defects, sort (for grade, color, size, defects, or maturity), tie or bind, transfer, pack or re-pack. Only about 50% of fruit and vegetable packers require their employees to wear gloves (USDA 2001).

It is common for human hands to make contact with fruits and vegetables during harvest and post-harvest operations. The extent of hand contact, however, varies with the item. Human hands touch melons, for example, at almost every step of the process. Even the minimal steps involved in apple processing include three points where human hands might be involved: harvesting, sorting, and packing. In between these two examples of produce handling are items like spinach that may be harvested, trimmed, sorted, tied, and bagged by hand. Hand contact during food-related operations is of particular importance to food safety due to the potential for an infected worker to transfer feces to hands and then to food. In theory, it should be easy to control the cleanliness of a workers' hands by requiring proper washing or wearing of gloves, but it has been extraordinarily difficult in the food-service industry to achieve this goal. A practice of potential public health concern is in-field packing of produce. A current practice by some produce processors is the coring of lettuce in the field. The additional handling step in the field has the potential of pathogen contamination if control measures are not taken. In one operation visited by this panel, control measures such as a disinfection step were in place and seemed to prevent the survival and transfer of pathogens to the core of the lettuce. Independent research addressing this possibility, however, seems necessary.

Water, either in liquid phase or in the form of ice, is an efficient vehicle for carrying microorganisms. Most, but not all, fruit and vegetables are exposed to water during harvest or post-harvest operations. Among the examples discussed in this chapter, kiwi, strawberries, winter squash pumpkins, and bulb vegetables generally are not exposed to water. Many fruits and vegetables (for example, citrus fruits and summer squash) have contact with water only for the purposes of washing. Some fruits and vegetables (for example, peas, beans, and pears) are exposed to water several times during processing. Water, which is reported to come most often from wells and municipal water supplies (USDA 2001), is used regularly in both harvest and post-harvest processes. It is used for washing (via sprays or baths), cooling (through cold water or ice), conveying produce between points (as when a flume is used), for disinfecting, or for adding fungicides or waxes. In the United States, water used for packing fruits and vegetables is treated with a disinfectant 62% and 51% of the time, respectively (USDA 2001). Control of the sanitary quality of water is technologically feasible but requires strict management of operating practices.

Fruits and vegetables routinely come into contact with harvesting equipment (knives, machetes, clippers, and scissors) and containers (bags, bins, boxes, buckets, pans, trailers and trucks). Equipment and containers in the United States are washed in about 75% of the operations and sanitized about a third of the time (USDA 2001). Equipment such as tables, conveyor belts, flumes, washing or cooling bins and additional containers is used during post-harvest operations and is washed by about 75% of the packers and sanitized by about 50% of the packers (USDA 2001). Commodities such as berries or apples that receive minimal handling may be exposed only to a container or to a few pieces of equipment. Most commodities are exposed to about 5 to 12 different containers or pieces of equipment. It is of great importance to avoid mechanical injury to the produce items during harvesting, transporting, and packing. The disturbance of the fruit or vegetable physical barrier would greatly increase the opportunities for pathogen survival and growth, if contamination occurs.

Equipment and containers may retain pathogenic microorganisms. Much of the equipment and containers used during harvest and post-harvest operations are made of materials that are difficult to clean--for example, wood. The soil typically found in a field will encrust equipment and containers. While this soil build-up may introduce pathogens, it also may deter their survival due to the presence of other microbial competitors and predators. If soil adheres to equipment used for washing and disinfecting produce, however, the organic components of the soil may affect the pH and disinfecting capacity of the water and limit the effectiveness of the disinfectant. If trucks have been previously used for animal hauling, sanitation becomes a major issue. Controlling the cleanliness and sanitation of equipment and containers is likely to be difficult due not only to their frequent exposure to dirt, but also to the materials with which they are made. Also, waxy bloom on fruit and vegetable surfaces can contaminate pickers' hands and contact surfaces.

Truck sanitation faces challenges similar to those confronted in attempting sanitation of equipment and containers. In some instances, commodities are protected from the sanitary conditions of the truck by the containers in which they are packed. In other instances, items like melons may be loaded directly into the truck. All commodities are loaded into vehicles of some kind for transportation from the field to the market. This subject is dealt with more extensively in section 3.3.

The temperature at which fruits and vegetables are stored varies according to the commodity in question. The temperature range at which fruits and vegetables are stored is narrow; temperatures that are too high or too low will impair their quality. The cool temperatures at which produce is stored inhibit the growth of some pathogenic bacteria but permit others to thrive. Cool temperatures also tend to preserve viruses and parasites. Although temperature control contributes to the safety of fruits and vegetables, especially those that are cut, its effectiveness in controlling hazard will be less significant than the hazard reduction achieved by refrigerating raw animal products.

This summary illustrates that microbiological pathogens can enter the fruit and vegetable supply at multiple stages during harvest and post-harvest processes. Sanitary habits of workers (particularly the cleanliness of their hands), potable water, clean containers, equipment and trucks are all important in reducing risk of microbiological hazards associated with fruits and vegetables.


 

Flow chart showing harvest and postharvest operations of melons.


 


Flow chart showing harvest and postharvest operations for bulb vegetables


 


Flow chart showing harvest and postharvest operations for carrots


 


Flow chart showing handling root vegetables


 


Flow chart showing harvest and postharvest operations of mature fruit: winter squash and pumpkins.


 


Flow chart showing harvest and postharvest operations of immature fruit vegetables: summer squash, eggplant or cucumbers.


 


Flow chart showing harvest and postharvest operations of leafy vegetables: lettuce, celery, and green onions.


 


Flow chart showing harvest and postharvest operations of spinach.


 


Flow chart showing harvest and postharvest operations of culinary herbs.


 


Flow chart showing harvest and postharvest operations of peas and beans.


 


Flow chart showing harvest and postharvest operations of peppers.


 


Flow chart showing harvest and postharvest operations for vine-ripe tomatoes.


 


Flow chart showing harvest and postharvest operations of strawberries.


 


Flow chart showing harvest and postharvest operations for kiwi fruit.


 


Flow chart showing harvest and postharvest operations for citrus fruits.


 


Flow chart showing harvest and postharvest operations of pears.


 


Flow chart showing harvest and postharvest operations for apples.


 


Flow chart showing harvest and postharvest operations for mature-green tomatoes.


 

3.2. Post harvest packing, cooling and storage

3.2.1. Introduction

As fresh produce moves from field to packing or cooling and shipping, the factors most likely to impact microbial food safety begin to take on many of the same dimensions, as one would anticipate in a processing plant. Field packing and in-field process preparation or light processing has been discussed in the previous section). Although incidence of pathogens was not investigated, a survey of North Carolina packing lines indicated that rot problems in bell peppers (an indication of handling and hygiene practices) were similar when packing in the field or in the packing house (Carballo and others 1994).

Incidence studies in tomato packing houses indicated that the microflora in tomatoes were most likely derived from soil (Senter and others 1985), indicating a need to improve the washing and disinfectant operations. Although microbial count monitoring is practiced on 65% and 84% of fruits and vegetables, respectively (USDA 2001), the usefulness of this measure is questionable, given the lack of good indicators (see Chapter VII) and the low frequency of pathogens on produce. Better ways to avoid public health hazards are preventive food safety programs, proper sanitation of equipment and food contact surfaces, and water disinfection which should be integrated into every facet of post harvest handling. Food safety and decay/spoilage control are concerns for produce handlers at all scales of production.

Escherichia coli O157O157:H7, Salmonella, Shigella, Listeria, Cryptosporidium, Hepatitis A virus, and Cyclospora are among the disease-causing organisms that have been associated with fresh fruits and vegetables (see Chapter IV). Several cases of food-borne illness have been traced to poor or unsanitary post-harvest practices, especially non-potable cooling water and ice. For this reason, areas of focus for preventive food safety, including common sense good housekeeping, prerequisite food safety programs, and elements of Good Manufacturing Programs, apply and will not be repeated in this section. Surface cleaning and sanitizing, rodent and vermin control (see also section 2.4.), employee hygiene and sanitary facility management, proper hand washing and optional use of gloves, utensil and tool cleaning, sanitizing, and task-segregation are among the many elements receiving adequate attention in research reports, training programs, texts, and manuals, the subject of comprehensive safety compliance and audit programs. Many of these potential sources of contamination are the subject of federal and state regulations, such as sanitary facilities under the Occupational Health and Safety Administration. In addition, some of the potential risk factors are common sense and do not need extensive research data. For example, allowing roosting birds to deposit layers of droppings in front of a cold storage room door or above a processing line is unacceptable. Similarly, no science is needed to determine the frequency with which pathogens transfer from different contact surface materials found in a packing shed, the role of biofilms developed from these droppings in resistance to cleaners. The potential risk factors are clear. Rather, some of the unique or less familiar aspects and challenges of addressing microbial contamination and cross-contamination concerns in produce packing, cooling, short-term storage, and pre-shipment handling are described below. Also, some discussion of packing facility layout and design considerations as it pertains to microbial food safety, are included. This section focuses on priority areas where additional research and innovation may be beneficial. Interestingly, all of these areas involve water as a major factor for microbial contamination. To appreciate the role of water in the packing and cooling system, it is important to briefly describe the primary elements of post-harvest handling and cooling. Detailed background may be found in many tests and technical overviews (for example, Kader 1992).

3.2.2. Water quality

3.2.2.1. The role of cooling

Temperature is the single most important tool to maintain post-harvest quality. A detailed discussion is available in Commercial Cooling of Fruits, Vegetables, and Flowers (UC 1998). Other than with field-cured or durable products, removing field heat as rapidly as possible is highly desirable. Harvesting cuts off a fruit or vegetable from its source of water, but it is still alive and will lose water, and therefore turgor, due to respiration. Field heat can accelerate the rate of respiration and therefore the rate of quality loss. Proper cooling protects quality and extends both the sensory (taste) and nutritional shelf life of produce. The capacity to cool and store produce creates greater market flexibility. Growers tend to underestimate the refrigeration capacity needed for peak cooling demand. It is often critical to reach the desired short-term storage or shipping pulp temperature rapidly to maintain the highest visual quality, flavor, texture, and nutritional content of fresh produce. Failure to cool at the optimum time may result in an elevated risk of persistent contamination or pathogen multiplication as natural plant barriers and resistance to microbial invasion is reduced. The statistical association between growth of soft-rotting bacteria and Salmonella suggests that prevention of infection or colonization by bacteria and fungi will reduce the levels of Salmonella associated with produce (Wells and Butterfield 1997). The most common cooling methods are:

Room cooling: An insulated room or mobile container equipped with refrigeration units can be used to cool produce. Room cooling is slow compared with other options. Depending on the commodity, packing unit, and stacking arrangement, the product may cool too slowly to prevent water loss, premature ripening, or decay.

Forced-air cooling: Fans are used in conjunction with a cooling room to pull cool air through packages of produce. Although the cooling rate depends on the air temperature and the rate of airflow, this method is usually 75-90% faster than room cooling. Design considerations for very small-scale to sophisticated large-scale units are available in UC 1998.

Hydrocooling: Showering produce with chilled water is an efficient way to remove heat and can serve as a means of cleaning at the same time. Hydrocooling presents a risk of pathogen internalization, as well as external contamination with pathogens. Use of a disinfectant in the water is essential. Some of the sanitizers permitted in produce are discussed in Chapter V. Hydrocooling is not appropriate for all produce. Waterproof containers or resistant waxed-corrugated cartons are required. Waxed corrugated cartons have limited recycling or secondary use outlets. Reusable, collapsible, plastic containers are gaining popularity. A list of vegetables that are suitable for hydrocooling is available in Kadar 1992 and UC 1998.

Top or liquid icing: Icing is an effective method to cool tolerant commodities and is equally adaptable to small- or large-scale operations. Ice-tolerant vegetables are listed in Kadar 1992 and UC 1998. As with water, ensuring that the ice is free of chemical, physical, and biological hazards is essential.

Vacuum cooling: Under vacuum, water within the plant evaporates and removes heat from the tissues. This system works well for leafy crops, such as lettuce, spinach, and celery, which have a high surface-to-volume ratio. Water may be sprayed on the produce prior to placing it under vacuum. As with hydrocooling, proper water disinfection is essential. The cost of the vacuum chamber system restricts its use to larger operations.

The considerations and selection of appropriate cooling methods and appropriate storage temperature and humidity conditions for a large diversity of vegetables are available in Kadar 1992 and UC 1998. Monitoring the disinfection of the chilled water is necessary to minimize the risk of pathogen contamination of produce during cooling. In large cooling operations handling both conventional and organic commodities, it is common to hydro cool (also water-spray vacuum cooling) organic produce at the beginning of daily operation, after a full cleaning and complete water exchange. This practice is intended to prevent carry-over or cross-contamination of organic produce with synthetic pesticide or other prohibited residues.

3.2.2.2. Other uses of water in packing

Other post-harvest issues involve the combined steps of unloading commodities from gondolas, trailers, harvest bins, water-assisted flume transport, fungicide applications, oil and wax applications, and other practices. These must also be evaluated for potential to contribute to elevated food safety risk. Some operators use flotation as a method of reducing damage at the point of grading and packing. Entire bins are submerged in a tank of water treated with a chemical flotation aide that allows the picked product to be gently removed and separated from the container. Lignin sulfonates are common in some fruit packing systems as a flotation aid for water-based unloading of field bins or other density separation applications. These materials can interfere with the effectiveness of water disinfection treatments (REF). As a general practice, minimizing field soil on product, bins, totes, and pallets by pre-washing will significantly reduce the disinfectant demand of the water and lower the total required volume of antimicrobial agent. These same field bins may have been placed on the orchard floor (ground) and may acquire and entrap soil with animal feces in openings, junctions, or footings. These bins may be vertically stacked for transport to the packing/cooling facility, during which time dislodged soil or feces may fall onto previously non-contaminated fruit harvested from several feet off the ground. Flotation unloading may result in distribution of this contamination from within and outside the harvested fruit-bearing surface. In this scenario, plastic harvest bins have a higher probability of transferring feces and soil to the packing shed due to the far greater surface area associated with slotting and venting and a design which typically includes multiple corrugations on all sides and footings. Growers in California and elsewhere have increased the use of bin-trailers in the field to preclude orchard floor contact and have eliminated vertical stacking of field bins containing harvested produce if soil contact is unavoidable

The risk of accumulating plant pathogens in dump tank water as well as the need to disinfect this water to minimize quality defects in produce was identified as early as 1932 (Baker and Heald 1932). Infiltration of wash-water into intact fruit has been demonstrated with several fruits and vegetables, and is thought to have contributed to an outbreak of salmonellosis associated with fresh market tomatoes (Table O8). Wash-water contaminated with microorganisms, including pathogens, can infiltrate the intercellular spaces through pores when conditions are right. Internal gas pressures and surface hydrophobicity usually prevent uptake of water. However, when produce temperature is much higher than the water temperature, the pressure difference created may be sufficient to draw water into the fruit (Bartz 1999). Addition of detergents to the water appears to enhance infiltration, likely due to reduced surface tension. Under some circumstances, wash water may enter an intact fruit through the stem scar or other opening, such as the blossom or stem end of an apple. Conditions that reduce infiltration of plant pathogens should also prevent infiltration of human pathogens.

Where pallets are used to stack finished field-packed cartons or transfer totes, soil transfer on the pallet skids or other exposed parts is not uncommon. Pre-cooling inspections are being used to screen for pallets that should be pre-cleaned prior to passage through a water-based cooling method. Pre-washing of harvested crops, which may occur in the field, also removes plant exudates released from harvest cuts or wounds that can rapidly react with oxidizers, such as hypochlorite and ozone (see Chapter V), requiring higher rates to maintain the target downstream activity.

3.2.2.3. Importance of optimal temperature

Although, rapid and adequate cooling is a primary method of post-harvest handling, many vegetables are sub-tropical in origin and susceptible to chilling injury during storage or shipping, increasing the potential for pathogen growth. Chilling injury occurs when sensitive crops are exposed to low temperatures that are above the freezing point. Damage is often induced during a short period of exposure but is not apparent for several days or until transfer to warmer conditions. Some examples of sensitive crops are basil, tomato, eggplant, green beans, okra, and yellow crookneck squash. For some vegetables different parts have distinct sensitivities. On eggplant, for example, the cap or calyx is more sensitive and turns black before the fruit itself is affected. The effects of chilling injury are cumulative in some crops. Chilling injury may not be apparent until produce is removed from low-temperature storage. Depending on the duration and severity of chilling, after several hours to a few days of warmer temperatures, chilling symptoms become evident in the following ways:

  • Pitting and localized water loss 
  • Browning or other skin blemishes 
  • Internal discoloration 
  • Increased susceptibility to decay 
  • Failure to ripen or develop color evenly 
  • Loss of flavor, especially characteristic volatiles 
  • Development of off-flavors 

Temperature management also plays a key role in limiting water loss in storage and transit. As the primary means of lowering respiration rates of fruits and vegetables, temperature has an important relationship to relative humidity and thus directly affects the rate of water loss. Relative humidity of the ambient air conditions in relation to the relative humidity of the crop (essentially 100%) directly influences the rate of water loss from produce at any point in the marketing chain. Water loss may result in wilting, shriveling, softening, browning, stem separation, or other defects.

Transport to and display at roadside stands or farmers markets often result in exposure of sensitive produce to extended periods of direct sun, warm (or even high) temperatures and low relative humidity. Water loss can be rapid under these conditions, resulting in limp, flaccid greens and loss of appealing natural sheen or gloss in fruits and vegetables. Tissue is then more prone to water uptake and infiltration, which can introduce microbes from the plant surface or from the water into protected sites or intercellular spaces. In these locations microorganisms are essentially unaffected by common mechanical or chemical inactivation. These same infiltration phenomena occur during post-shipping hydrocooling and in retail or foodservice re-hydration methods. Providing post-harvest cooling prior to and during transport and a shading structure during display can minimize rapid water loss at these market outlets. Approved fruit and vegetable waxes are effective at reducing water loss and enhancing appearance. Uniform application and coverage of waxes or oils with proper packing line brushes or rolling sponges is important. Plastic wraps or other food-grade polymer films retard water loss. Adequate oxygen exchange is necessary to prevent fermentative respiration and the development of ethanol and off-odors or flavors. Wraps or bags must have small perforations or slits to prevent these conditions, especially when temperature management is not available. Exposure of bagged or tightly wrapped produce to direct sunlight will rapidly raise the internal temperature. Water loss will result. If followed by cooling, free water condensation will develop that may result in accelerated decay.

Specialized films that create modified-atmospheres (MA) when sealed as a bag or pouch are available for many produce items that have well-characterized low oxygen and elevated carbon dioxide tolerances (see Chapter VI). Not all commodities benefit from MA.

Packaging can also be designed to minimize water loss. To minimize condensation inside the bag and reduce the risk of microbial growth, the bags may be vented, microperforated, or made of material permeable to water vapor. Barriers to water loss may also function as barriers to cooling.

In small-scale handling, reuse of corrugated containers, especially those obtained at terminal markets, wholesale distribution centers, or at retail outlets may represent a source of contamination and is strongly discouraged. Re-use of difficult to clean containers (non-plastic) is especially problematic. In addition, any trace-back effort that may be necessary would be seriously impeded by packing product from one grower in a carton from, potentially, a shipper in a different state of country.

During transportation and storage, relative humidity (more properly, vapor pressure deficit) is critical, even at low temperature. For a more complete discussion of optimal relative humidity for fruits and vegetables and the principles for prevention of water loss, see UC 1998.

3.2.2.4. The use of ethylene

Management of ethylene may be another post-harvest consideration that may potentially impact food safety during storage and transportation. Ethylene is a natural hormone produced by plants and is involved in many natural functions during development, including ripening. Ethylene treatments may be applied for degreening or accelerating ripening events in fruits harvested at mature but unripe development stages. For a detailed discussion of the role of ethylene in ripening and post-harvest management, see Kadar 1992.

In contrast to its role in ripening, ethylene from plant sources or environmental sources (for example combustion of propane in lift trucks) can damage sensitive commodities. In brief, ethylene producers should not be stored with fruits or vegetables that are sensitive to it. External ethylene will stimulate loss of quality, reduced shelf life, increased disease, and specific symptoms of ethylene injury. Such symptoms include:

  • Russet spotting of lettuce
  • Yellowing or loss of green color (in cucumbers, broccoli, kale, and spinach) 
  • Increased toughness in turnips and asparagus spears
  • Bitterness in carrots and parsnips 
  • Yellowing and abscission (dropping) of leaves in Brassicas
  • Softening, pitting, and development of off-flavor in peppers, summer squash, and watermelons 
  • Browning and discoloration in eggplant pulp and seed
  • Discoloration and off-flavor in sweet potatoes
  • Increased ripening and softening of mature green tomatoes
     

In these ways, ethylene effects on senescence may contribute to multiplication of bacterial pathogens on produce that would increase the potential for cross-contamination during downstream handling, processing, or food preparation.

In addition to providing adequate venting or fresh air exchange, ethylene adsorption or conversion systems are available to prevent damaging levels (as low as 0.1 ppm for some items) in storage and during transportation. Potassium permanganate (KMnO4) air filtration systems or absorbers are reported to eliminate human pathogens from the air in a cold room or processing plant. Other air filtration systems for ethylene removal based on glass-rods treated with a titanium dioxide catalyst and ultraviolet light inactivation are available for cold rooms. Ultraviolet light/ozone-based systems of ethylene elimination are also commercially available.

3.2.3. Design considerations for overall food safety in a processing facility

The locations and physical structures that currently support fresh fruit and vegetable packing and packaging, including minimally-processed produce, are diverse and of many different scales. In addition to the traditional packing shed or packinghouse widely used in the industry, it is well recognized that various applications of field packing, field trim and pack, in-field pre-process preparation, and specialty or small-scale direct pack operations are an integral part of the overall industry. In-the-field, post-harvest handling "facilities" range from the tailgate of a pickup truck, an in-furrow hand trailer, a shaded lean-to, a mobile packing platform, bulk harvest-aides with trailers or gondolas, and an integrated mechanized packing trailer to highly sophisticated units with grade-sort-wash-and pack capabilities. Likewise, packing shed and packinghouse design may be very simple--often, essentially a pole barn or covered shell without walls and stationary grading and packing platforms, or large, highly mechanized and computer-dependent operations. The selection of food (which includes raw produce) contact surfaces in processing facilities, like the variety in the physical structures of these facilities, is diverse. Unlike well-designed food processing facilities, some fruit and vegetable packing operations may use materials difficult to clean, such as wood, soft metal, porous polymer, and carpeting.

Across the range of post-harvest handling and packing operations, facility design and construction material selection should be planned with food safety considerations in mind. Focusing on those facilities whose operations most clearly subscribe to current Good Manufacturing Practices, this section of the chapter highlights design considerations for constructing an essentially or fully enclosed and integrated packing and shipping operation.

3.2.3.1. Overall design considerations

The overall design of the facility, from receiving area to shipping dock, is an important consideration to help eliminate opportunities for chemical, physical, and microbial contamination. Both direct contamination and cross-contamination of a product can be minimized with proper attention in a processing facility to physical design, water, air, and traffic flow, and construction material selection.

3.2.3.2. Physical design of facility

The building should be designed so that product flow is linear--that is, "in" one side and "out" the other. Incoming raw produce, grade, sort and pack lines, and outgoing packed product should, ideally, never cross-paths or co-mingle. While this is a recommended practice for all produce, it is especially important in any value-trimming or minimally processed produce operation. If the available space is not linear, the physical separation of zones achieves an equivalent one-directional traffic and product flow. In addition, separate or segregated zones for chemical storage and mixing and maintenance or fabrication shops should be planned for the facility.

Pallets and bins coming directly from the field may also be a source of contaminated soil and plant debris. Proper facility design can reduce this potential hazard. A staging area separated from primary washing and cooling, or at the point of transfer to a dry packing line, may be used to dry brush or otherwise remove field soil from cartons, bins, and pallets. At this point of receiving, when dump tanks and flumes are used, natural openings or sites of detachment as well as harvest wounds or harvest cuts increase the opportunity for contamination to produce from soil or other sources. For covered dry or wet dump areas, design structure and barriers should discourage bird and rodent activity. In keeping with a linear flow design, non-washed produce should never contact the same surfaces that will contact produce at any other step. When this is unavoidable, a thorough cleaning and sanitation procedure must precede the use of common space or contact surface. Finally, the minimally processed but unprotected product should not be stored in the same loading dock or cold room location with the raw produce or dirty containers and pallets. For example, the facility should have sufficient cold room space to keep the washed and/or graded product- being held for later shipping, packing, or packaging- separate from incoming and stored raw materials. The key element to safe facility design is ensuring that unwashed produce enter at one separated area, move in a linear or segregated flow, and exit at a terminal segregated shipping area.

3.2.3.3. Water, air, and traffic flow design

Like the overall facility design, the movement of process water, waste streams, airflow, and employees should be planned with food safety in mind. Water flow should move in a reverse direction to product flow. To conserve water and minimize wastewater discharge, many facilities re-circulate water (that is, use the same water for multiple purposes). In these cases, incoming water first contacts clean, finished product and moves opposite to product flow where it is used to wash and cool incoming raw produce before discharge.

In a similar manner, facilities may design and install an air-filtration system for central air distribution and air flow counter to product flow. Clean filtered air should move with a positive pressure from cleanest areas at packaging and packing back towards the receiving area. A positive pressure flow design helps reduce the chance of air-borne contamination along the linear facility design. Additional airflow barriers, such as air-curtains, help to isolate receiving and shipping areas that may be open to the outside environment. Operations that utilize a bulk dump for incoming materials should consider a fixed wall with pass-through conveyance to move produce from outside to inside the facility. This point of separation will reduce the potential for aerosol contamination inside the processing area during the incoming of produce.

In general, traffic flow from the outside environment and within the facility should also be carefully planned. Equipment and workers should not move between segregated areas. Cross-contamination can be avoided by preventing the movement of lift-trucks, bins, totes, tools, cleaning implements, clothing, and people from receiving or storage zones to packaging or minimal-process and pack areas. Color-coding of bins, totes, clothing, cleaning tools, and other items can help achieve this separation of traffic.

3.2.3.4. Construction materials

Proper facility construction design and selection of construction materials are major contributors to food safety programs. With regard to the area outside the facility, packed soil or even gravel over packed soil are not recommended for most receiving areas. These surfaces are very difficult to clean and may harbor pathogenic microbes. Accumulated plant debris and remains from spills or broken harvest containers may be a source of contamination by some pathogens. Ripening or decaying produce, left on soil or trapped in moisture-saturated gravel, may be an attractant for rodent or insect vectors. If the use of packed soil or gravel is unavoidable, all incoming produce should be elevated on pallets and a proper drainage system should carry water away from the holding areas.

Inside the facility, floors should be designed for easy cleaning; ideally, a smooth, non-porous floor would be used. Although not yet common, newer facilities are constructed with coving at wall junctions to prevent the entrapment of dirt and debris. Expert advice should be sought to select materials that facilitate cleaning and sanitation and to design adequate floor slope for drainage. Flooring materials should be resistant to chemical damage and cracking from equipment movement. Cracks in flooring are difficult to clean and may easily become a site for plant residue accumulation and subsequent microbial growth.

Similarly, walls should be constructed of materials that are readily cleaned and uninhabitable by pests. Sealing and screening must be used to exclude pest entry through windows and vents. Consideration should be given to permanently enclosing windows in a space being converted to applications for minimally processed fruits and vegetables.

Access doors for maintenance and services that lead directly from the external environment to processing and packaging areas should be of a plenum design. Door arrangement and spacing should preclude both doors being open at the same time. The use of access doors during operating hours should be strictly controlled. The areas outside the facility should be designed and maintained to minimize the potential for attracting or harboring rodents and other potential vectors of human pathogens. In addition, landscape design and weed control programs are part of the overall food safety planning and implementation.

Other elements can be made to greatly improve the ease of maintenance and the effectiveness of clean-up procedures. These include the location and design of drains, floor flumes, and pipelines. Expert advice should be sought to design and place these outlets, along with designing protective aides to prevent contamination from the transfer of pipe and wall condensation. Drains should be fitted with antimicrobial seals and grates capable of retarding rodent entry. The use of floor flumes should receive careful consideration due to the potential for water aerosol contamination of the room air or equipment surfaces. This is especially true for floor flumes that carry produce and water waste from one segregated area across another. Floor flume transfer from an adjacent produce cooling and packing operation into and across a packing facility for minimally processed produce should always be avoided. The design of the collection area for waste stream water should incorporate systems to prevent product or equipment contamination attraction for pests.

"Catwalks" of various designs are commonly used to access grade and sort and packing lines. Open-grate catwalks are common in packing facilities; however, an overhead design construction should be avoided over a washed produce area. Measures to prevent workers from carrying easily dislodged, potential sources of contaminants (such as soil, chemical residue, and glass or metal shards) from directly accessed overhead catwalks should be implemented. Alternately, in packing areas most sensitive to the impact of accidental worker contamination, solid-flooring catwalks with sidewalls should be installed. Materials used for harvest, in field packing, and on sorting, grading, and packing lines and associated conveyance lines are generally difficult to clean. These materials include canvas, carpeting or other fabrics, soft multi-ply rubber, wood, porous polymer sheeting, and various extruded or cut foam panels. In addition, these difficult to clean materials are often used in areas where plant debris accumulation or product damage may occur. Examples of such areas include decelerator pads, diverter pads or bumpers, pass-through spray shields, foam or rubber rollers, conveyor paddles or distributors, and packing unit accumulation. Frequent and thorough cleaning and sanitation as well as periodic replacement of damaged materials is especially essential in areas where residues and juices accumulate.

3.3. Transportation of fresh produce: factors affecting microbial food safety

Several modes of transportation are used to move harvested fruits and vegetables from production areas to packing or processing facilities, to shipping points, and to destination markets (Table II-6). Equipment used by short-haul and local distribution carriers is generally of lower quality and refrigeration capacity. Long distance transportation more often takes into consideration issues of quality and safety of fresh produce. Air carriers, railcars, marine vessels (bulk and container), highway trailers, and intermodal combinations such as trailer-on-flatcar (TOFC) and container-on-flatcar (COFC), are all used to transport fresh, perishable commodities and minimally processed foodservice and consumer convenience packaged produce. Local distribution may include refrigerated or non-refrigerated trucks (open or closed) and vans. Small-scale or limited resource growers may transport produce relatively short distances, to farmers markets, restaurants and retail outlets, or directly to subscription consumers.

Ideally, transportation continues the job of prevention of foodborne illness by Good Sanitation Practices, ensuring proper temperature and humidity management, and minimizing damage potential to the product. Highway carriers with refrigerated trailers provide the dominant portion of produce movement in North America. Much of our seasonal imported vegetables and some fruits from Mexico arrive by non-refrigerated or refrigerated highway trucks and trailers. When non-refrigerated trucks are used, cooling with ice from roadside icing stations is a common practice. Again, the quality of the water used for ice -making is a critical factor in ensuring a safe product. Recognizing the functional features and limitations for this primary method and other common modes is essential to discussing the overall development of effective systems for microbial risk reduction for fresh produce. This section covers the basic features of the primary modes of long-haul produce transportation, basic aspects of equipment cleaning and sanitation, and the issue of temperature management and measurement in relation to microbial food safety.

3.3.1. Components for temperature and atmosphere control

Refrigeration source. The majority of fresh produce transportation is within a highway trailer using a mechanical refrigeration system. Ventilation with cooler outside air is occasionally used, on a few items (that is, onions and watermelons) or on short distance hauls, to provide limited temperature reduction but it is generally inadequate for optimum temperature management. On nonrefrigerated highway trailers used for produce, small doors at the ends are open to the outside to create at least some air circulation around the load. In addition to cooling, mechanical refrigeration systems are designed to heat the storage compartment when the vehicle travels during subfreezing conditions to prevent chilling damage or freeze-injury to sensitive commodities. A properly designed and operating system can maintain a high relative humidity in the storage compartment, which is desired in most cases to prevent water loss and associated quality impacts. In reality, optimum humidity management is difficult to achieve.

In some areas or on specific produce items, top-ice (crushed ice placed on top of a bulk or pallet load) or ice placed in each individual box is occasionally used with or without mechanical refrigeration (that is, green onions, broccoli, sweet corn, some imported cantaloupes, bunched carrots and peeled 'baby' carrots). Water quality for ice production is an essential consideration, as microbial pathogens from ice have been implicated in several outbreaks on produce (see Chapter IV). Water from melted ice may also drain from one carton into another, introducing a distinct potential risk of cross-contamination during transportation and distribution. The consequences of this drainage are particularly acute in distribution where mixed loads commonly involve stacking items from various suppliers on a single pallet.

Air circulation systems. Air circulation systems are necessary to move the conditioned air through or around loads to absorb heat from the products and from external sources. In most mechanically refrigerated rail cars, highway trailers, and marine containers, the system circulates cold air through and around loads of fresh produce where respiratory heat production may be significant. Typical highway trailers have the air delivery across the top of the trailer, with an air chute attached to the ceiling distributing airflow to the rear and along the sides. The air return channels are predominantly along the sides, depending on the loading pattern, and underneath a pallet load. High-density loading in these top-air delivery systems restricts air circulation and causes product heating and accelerated ripening or senescence. Conversely, high-density loading is required for optimal performance of bottom-air delivery systems typical of refrigerated marine containers. Due to weight limit, weight distribution, or mixed load issues, empty pallet positions may occur throughout the load or at the front or rear. To ensure adequate airflow to all areas of the trailer or container, all open areas must be covered to prevent "short-cycling". Short-cycling occurs when there is an abbreviated path, created by the opening, for conditioned output air to return to the intake vent. When this occurs a portion of the load may experience greatly elevated temperatures, although the on-board temperature sensors indicate proper function and temperature management. Short-cycling can also be caused by floor loading product, which blocks the main path to the intake air duct. Produce such as onions are too often loaded while warm onto the trailer floor, causing both short-cycling and icing of the coils (from released moisture) and resulting in large temperature increases, particularly at the rear pallet positions.

Temperature control system. Mechanically refrigerated systems include one or more thermostats- automatically operated (with manual override) to provide cooling or heating- and air-circulating fan speed controls. In older systems, thermostats are generally in the return-air channel, which causes some problems. For example, warm return-air from product loaded well above the temperature control set-point will cause the controller to deliver sub-freezing air into the discharge supply stream. Product closest to the supply vent may incur freeze-injury. Newer units are available with thermostats in both discharge and return-air channels and can, with adequate provisions for air-circulation, maintain a differential (evaporator coil to air-stream) as low as 0.5 °C (1.9 °F).

Insulation. Insulation restricts heat conduction across walls, flooring, doors, and the roof of transportation vehicles. The load area is reasonably or even tightly sealed to restrict air leakage. Insulation limits the amount of ambient heat and humidity that enters the vehicle during hot weather and the amount of internal heat (mostly from the product) escaping to the outside, causing product chilling or freezing during freezing weather. Most insulation is foamed-in-place or components of extruded panels that are composed of materials that deteriorate slowly over time (about 5% of the insulating quality per year). Manufacturing improvements have brought about trailers with thinner walls, creating greater internal load space, while maintaining sufficient insulating capability for most conditions of produce transportation.

Insulation can be damaged and its thermal barrier value lessened by lift truck damage during loading and unloading operations. Water intrusion that initiates at these damage points greatly reduces the insulation quality and even facilitates temperature transfer. Highway trailers typically do not have ducted sidewalls because this feature, while improving temperature management, adds weight and reduces the available load space. In hot weather, in particular, this may be an important factor in localized product temperature gain if produce is loaded directly against the sidewalls.

Air exchange system. Ventilating the load compartment with outside air (mostly in marine containers) reduces undesirable concentrations of carbon dioxide Ventilation is used in conjunction with all modes of transport, occasionally with rail cars and trailer-on-flatcar (TOFC) vans but predominantly with specialized bulk holds in multi-deck ships and marine containers for long-haul domestic and marine export shipments. For containers, required modifications include atmosphere-injection and purge ports, special seals and membrane curtains around doors, methods for ethylene adsorption, and carbon dioxide absorbers (generally hydrated lime). For highway and air-carrier transportation, individual shipping units, such as a carton or full pallet, are specially constructed with liners or sealed polyethylene pallet covers to temporally maintain the desired injected atmospheric composition (Chapter VI).

Vibration Management. High frequency vibration within highway trailers is a much more significant source of product damage than individual sharp jolts. Improperly packed produce can be put into a spinning or rolling motion due to road-induced vibrations causing external abrasions and internal tissue damage. Air-bladders (air-ride suspensions) dampen this source of injury and are a more common feature on trailers, especially those used to haul sensitive fruits.

3.3.2. Modes of transportation

3.3.2.1. Truck and truck trailers

Highway tractor-trailers are used to haul straight loads (one commodity) or mixed loads. Currently, mixed loads account for over 75% of highway movements of fresh produce. A relatively high volume of domestic produce, especially for small volume grower/shippers, moves by LTL carriers (Less Than Load). With limited options for shipping their perishable products, users of LTL carriers frequently have to make significant compromises from optimal temperature and product mix recommendations.

Transcontinental travel time in the United States is 3 to 6 d for team drivers. Load space is intermediate in size, 70 to 100 m3 (2,000 to 3,500 cubic feet) and with a net weight load capacity of about 18,000 to 20,400 kg (40,000 to 45,000 pounds). Trucks are limited in gross weight by state regulations generally to 36,288 kg (80,000 pounds) maximum gross weight. Careful loading is required to distribute weight evenly across the axles.

Modern refrigeration units used by highway trailers have fast temperature "pull-down" capacity. An empty 53-foot long, 102-inch wide thin-walled trailer can be cooled to freezing temperatures in a few hours, even against high outside temperature conditions. The cooling capacity of modern units is important in rapid pre-cooling of the load space, but could also provide limited product temperature reduction in adequate time to maintain quality. Limitations in air-circulation largely prevent this post-load cooling from being practical or reliable. Highway trailer refrigeration operates only to maintain product temperatures established at shipping point. Frequent or prolonged door opening at the loading dock and during delivery further limit optimal temperature management.

Air Circulation. As mentioned above, air temperature is often monitored in the supply airstream about 2 to 2.5 m (6 to 8 feet) back from the discharge air duct. A preferred system, incorporated into newer refrigeration units, integrates input from both supply and return-air temperature monitors. Microprocessor temperature controllers used in modern refrigeration units incorporate thermostat control, digital thermometer, fault indication, and data recording in a self-contained controller. Satellite-communication systems can provide remote monitoring, upload, and data download capabilities in transit.

As mentioned above, the air circulation pattern in top-air delivery trailers is lengthwise, front-to-rear. Air travels from the refrigeration unit back over the top of the load, down the sides and rear of the load, back through or under the load, and up the front to the refrigeration unit. This circulation pattern is achieved only if there is adequate air return space beneath the load and a solid air return bulkhead at the front to separate the discharge and return sides of the fan, ensuring positive air circulation around the load. Without a solid (pressure) bulkhead most of the air circulates over the top of the load back to the refrigeration unit. Product temperature management at different positions within the load may be compromised. The elevated respiration of produce may be sufficient to result in localized anaerobic conditions, particularly in packaged product. The rate of temperature gain in properly pre-cooled and pallet loads of different packing materials and venting has not been adequately addressed in publicly accessible documents.

In addition to a pressure bulkhead, loads must be secured away from rear doors and away from flat sidewalls to allow air to circulate down over the load. Vertical channels (not common in highway trailers) allow some air circulation between the sidewalls and the load, and provide less contact between the walls and the load, thereby reducing the amount of heat conducted to the load.

In contrast, marine containers have vertical, bottom-to-top airflow through the load compartment, making them more effective for transporting fresh produce over an extended time period. They can provide better and more uniform product transit temperatures because they have more constant and uniform airflow, greater capacity to circulate the air through the load, and shorter air channels through the load.

Modified Atmosphere Maintenance of modified or controlled atmospheres (MA or CA) is not possible in most over-the-road truck trailers because they allow too much air infiltration. Tightness of trailers (especially doors) decreases rapidly with use and abuse.

Rear and sometimes side doors are provided for loading, unloading, and inspection. New designs, primarily for short-haul or regional distribution vans have three rear doors to reduce heat gain that can occur rapidly during multiple deliveries. As these hinged doors are constantly used, however, they are easily damaged, leading to air leaks. In addition, between 75% and 85% of loads have more than one commodity, which limits the potential usefulness of CA. These mixed commodities may have different CA prescriptions or may not benefit from CA. A common application of MA technology creates the desired, beneficial atmosphere around an individual pallet, pallet bin, and carton or unit package. Proper temperature management is essential for these shipments, as temperature abuse in a restricted oxygen atmosphere can be more detrimental to product than improper temperature alone.

Specialized marine containers are constructed to meet MA or CA requirements. Provisions are made to deliver N2 and CO2, control CO2 build-up, remove or add ethylene, remove other volatiles, and adjust oxygen depletion rates. Systems may be dockside and operators prepare individual containers for MA shipment. Alternately, self-contained CA systems for containers or on-board centralized control units are in service.

Highway Trailer and Container Sanitation Oversight of the safety of perishable produce during distribution and transportation is the responsibility of the U.S. FDA. Meat and poultry distribution safety and sanitation are the responsibility of the USDA's Food Safety & Inspection Service.

It is not uncommon for fruits and vegetables to be transported in refrigerated highway trailers or marine containers that previously contained frozen or chilled meat and fish products or other possible sources of non-food microbial contamination, such as carcasses for rendering, municipal waste, or chemical hazards.

Pallets, cartons, and bins; collapsible, returnable shipping containers; and loading lift-trucks may be a source of field soil or plant debris and residues that carries over from one load to the next. Whether these potential sources of contamination are a significant threat to produce safety remains an essentially unanswered question. Packaging may serve as an effective barrier to contamination for some products. Attention must be given to prevention and mitigation of sources of contamination or product "tainting" between transportation loads.

The first step is cleaning, generally accomplished by hand sweeping and pressure-air sweeping of the interior. Clearing floor ducts of debris will help with air circulation in the following load. Hand-held high-pressure water nozzles, hand-held pressurized steam jets and automated sidewall panel and duct cleaning equipment is typically used to clean, sanitize and deodorize the interior of vehicles or containers between loads. These systems are often designed to recycle wash water. Filtration and disinfection of these systems is essential.

Cleaning and sanitizing air chutes, refrigeration coils, and defrost pans or drains is less commonly attended to between loads or at any regular interval.

Depending on the surface and the potential for excessive corrosion, compounds used to disinfect the interior surfaces include FDA- and FSIS/USDA- approved alkaline cleaners, cationic detergents, chlorinated foams, hydrogen peroxide and peracetic acid, iodine, and bromine-based compounds (uncommon), and quaternary ammonium compounds. Concentrations generally range from 200 to 800 ppm, usually with a required follow-up clean water rinse. Ozone-based systems for aqueous clean out and gaseous cleaning of refrigeration coils are being developed.

3.3.2.2. Other modes of transportation

Air transport. Air shipment is used mainly to transport highly perishable and valuable commodities (for example, ornamentals, berries, and tropical fruits) to distant domestic and overseas markets or to supply markets with limited supplies during periods of high prices and strong demand. Products are transported in closed (mostly non-refrigerated) container units or in net-covered pallet loads, in air freighters, or in the freight compartments of passenger airplanes. Individual containers may be loaded with packages of a single commodity or with packages of many different commodities and food types.

Preload staging on the airport tarmac is required; 2 h for domestic flights and 3 h for international flights. The potential for temperature gain, or loss, of produce in airfreight containers can be extreme. Some containers and various carton designs use foam insulation lined with a reflective surface, primarily to minimize heat gain. Gel-ice pads or bricks are placed in and around the packed carton to retain product quality. Air travel time is often about 6 to 18 h, sometimes longer. Waiting time at origin, transfer, or primary or secondary destination terminals may be as long as 1 to 2 d, often at ambient temperatures. Pest exclusion inspection or fumigation and related quarantine requirements, and X-ray examination for contraband substances often delay timely transfer of highly perishable commodities to re-cooling facilities or into temporary cold storage. All of these delays may result in rapid deterioration of products. Product warming is a serious problem in air shipments.

Some airlines use cold storage rooms at origin and destination airports, but not on a regular basis for fresh horticultural products. As the movements of perishable horticultural products have increased in the past 10 years, a significant effort has been made to provide proper facilities at major terminals and freight forwarder operations. Freight forwarders with cold storage, cooling, unitizing, and shipping facilities near major airports are increasingly improving their handling of perishable products. Re-cooling and re-hydrating produce at these facilities, generally by hydrocooling, requires careful disinfection monitoring due to the potential for increased spoilage and food-safety concerns.

Break-bulk marine transport. Break-bulk marine transport is widely used to transport tree fruit, grapes, and bananas to and from ports lacking container-van loading facilities. This system is also used on older or smaller ships that have common cold storage rooms. The break-bulk designation refers to an older system in which individual packages are re-handled each time the cargo is transferred from one mode of transport to another. This method is costly due to slow loading and unloading, rough handling, and high labor costs. Break-bulk marine shipping dominates in certain trade lanes but is slowly losing ground to containerized shipping. In general, reefer ships are more efficient for trade lanes dominated by refrigerated products. Container ships predominate in trade lanes with a large share of dry freight.

In most break-bulk systems, packed products are handled as pallet units. When a pallet load is broken apart, its inner surfaces are exposed to ambient conditions, which can increase product deterioration. These problems are minimized in newer ships that can handle pallet loads and have better refrigeration systems. Packed products in pallet units are sometimes transported part of the way in break-bulk ships, transferred at a port to marine containers, and transported in container ships to final destination ports.

3.3.3. Managing product transit temperatures

3.3.3.1. Loading environment

Ideally, produce should be staged for loading within an enclosed and temperature controlled loading dock. Precooled trailers, with refrigeration units or fans turned off to minimize the intrusion of warm, moisture-bearing air, are backed up to the dock doors for inspection. Padded door seals help prevent leakage of outside air into the loading dock. In open loading docks warm air pulled into the trailer may cause significant condensation on product, carton and trailer surfaces. This condensation may promote microbial intrusion into product and increase contamination. Trailers are inspected for proper precooling for the intended load as well as cleanliness and absence of odors from prior loads.

3.3.3.2. Load patterns and load sizes

The pattern and size of the load often affect temperature maintenance. Loads must be assembled in ways that will maintain their integrity in transit. Stacking packages on pallets not only helps maintain integrity during transit but also facilitates loading and unloading. Occasionally, slip-sheets are still used for more stable produce items, such as potatoes. Various types of gates, braces, air bags, and locking bars (load-locks) are also used to maintain the integrity of loads.

Incentive freight rates offer per-package-freight cost decreases as load weight increases. Larger, heavier, and tighter loads, however, make temperature maintenance difficult, especially with products that are not properly cooled before loading. Overloading also blocks air circulation so that product temperatures increase during transit. Tight loading may reduce air circulation by 90%. Well-constructed pallet loads, adequately precooled and with sufficient side-air channels, provide better assurance of proper transit temperatures in heavy loads. In hot weather, loads should be kept away from sidewalls to prevent warming in wall-abutted rows. Highly perishable products, such as strawberries, are center-line loaded in trailers or vans; the pallet units are braced away from the side walls and the two rows of pallets contact each other along the center line of the trailer.

3.3.3.3. Condition of transit vehicle

The ability to maintain product temperature is affected by the condition of the transit vehicle. Intact side walls and insulation; clean floors and drains; refrigeration units that are properly serviced, maintained, and calibrated; intact air-delivery chutes; and tight, undamaged doors and seals are all essential. The carrier owner/operator is primarily responsible for the equipment's condition. Users (shippers, buyers, brokers, or receivers) or their representatives, however, are responsible for assessing the equipment's condition before loading and for damage to the vehicle caused by their workers during loading or unloading operations

3.3.3.4. Recording thermometers

Many shippers place recording thermometers in each loaded transit vehicle; others place thermometers only in vehicles going to their most distant markets. Buyers are increasingly specifying the use and placement of recording thermometers or data loggers in each load. In truck trailers, the thermometer is generally secured high on a sidewall at about the three-quarters length of the trailer, or on top of the load toward the rear. These recording thermometers measure only discharge air temperature at their specific locations and provide performance records of the operation of the refrigeration unit. If attached to the sidewall the recorded temperature may be slightly above the ambient internal temperature at that location.

Strip charts may be viewed immediately upon unloading, without the need for special download devices or a computer station. They may be helpful to quickly identify temperature management problems during transit but reading the recorded line is sometimes difficult. Data loggers are more costly but far more versatile. On-dock downloading of transit information has become fast, easy, and more affordable. Some models include indicator-light alarms, displayed without the need for electronic data capture, to show that the recording unit was exposed to a low or high temperature outside the range programmed into the unit before the trip. Neither unit will monitor or record product temperatures within loads. Newer models of compact, recording thermometers have dual capabilities. Ambient monitoring is combined with an external sensor probe that can be inserted into the load space to more accurately record product temperatures at the specified location.

3.3.3.5. Product temperature as a control point

The increased recognition of the value of applying HACCP principles to the production and processing systems used for marketing minimally processed fresh fruits and vegetables has, over the past 10 years, stimulated an on-going and essentially unresolved debate over the role of temperature as a valid critical control point in food safety plans, required documentation, and regulatory enforcement. Temperature management is a primary component of:

  • retaining product quality of both visual and sensory shelf-life 
  • delaying general microbial growth and spoilage 
  • reducing or preventing the growth of specific decay pathogens with aggressive activity on minimally processed products (Erwinia, Botrytis, Geotrichum)
  • reducing or preventing growth of specific pathogens involved in foodborne illness (primarily Salmonella and pathogenic forms of E. coli

Clearly, product temperature management has an important role in limiting the potential for illness to follow consumption, specifically if pathogens were present on the product at the point of post-processing storage, transportation, distribution, or foodservice or retail handling. Not all pathogens require multiplication to be infectious. Some are not even capable of multiplication on fruits and vegetables (for example, viruses and protozoan parasites). Prevention and reduction of the potential for contamination before shipping are the prerequisite food safety activities that are augmented by good temperature management throughout the distribution chain. Temperature monitoring and documentation is one mechanism for all partners in the distribution chain to evaluate and adjust their practices to achieve the optimal temperature history for the useful life of the product.

Controlling and documenting temperature history, while not perfect, is far more practical, far less expensive, and more informative than microbial indicator monitoring or pathogen testing of product. For each product category it is necessary to determine whether pathogen growth and toxin production as a result of time/temperature abuse is a likely and significant hazard. The key criteria are:
 

  • Levels of pathogens on the product at shipping 
  • Presence of pathogens capable of multiplication on produce. Well-designed on-farm and in-plant sanitation programs (prerequisite programs) will minimize the presence of pathogens, but in most cases it is not reasonable to assume that they will completely prevent their introduction.
  • Unsafe levels of pathogen growth or toxin production, before spoilage
     

Research has repeatedly shown on many inoculated produce items, held at permissive temperatures (above 41°F / 5°C) that bacterial pathogens will grow because:

  • the moisture available to support pathogen growth in the product (water activity) is high;
  • natural openings or wound sites release easily used nutrients; 
  • preservatives are not typically used;
  • the acidity (pH) of most produce, even items such as strawberries, tomatoes, peaches and plums are not inhibitory;
  • modified atmospheres have limited impact on microbial growth retardation;
  • competing spoilage organisms cannot be relied upon for control. 

The above questions should be evaluated in relation to the potential for time/temperature abuse in the absence of controls. The effect of temperature on the growth rate of microbes is a continuum. Over a broad range, there is no sharp cut-off point but rather a gently sloping curve that is, characteristic for each class, type, and sub-type of microorganism. A complex integrative analysis of time and temperature history of the conditioned air and the product itself, the presence or absence and starting population density of pathogens, as well as growth potential and growth rates on specific product components and the interaction of packing on these rates, should be performed. Clearly, this is not practical. Recognition of this complexity has been the basis for relying on air temperature monitoring as a cardinal feature of safe food handling. It is equally apparent that product exposure to elevated temperature during transportation does not automatically result in unsafe food. Extreme temperature abuse typically reduces visual and sensory quality to the point that a reasonable consumer would not eat the product.

Optimal temperature ranges for storage and transportation of fruits and vegetables vary depending on their inherent respiration rates and resistance to chilling injury and freeze damage. The optimum for most minimally processed produce is 32 °F (0 °C) for quality retention. The generally recommended upper limit of 41 °F (5.0 °C) is well supported as an acceptable benchmark for imposing a significant barrier to the growth of bacterial pathogens. Although temperature control is a significant variable that may impact pathogen growth in produce, other factors also determine temperature controls:

  • Some raw commodities are temperature sensitive and will not tolerate storage or more than transient shipping at 41 °F (5.0 °C).
  • Minimally processed produce has not, in general, been associated with foodborne illness, despite the high volume of consumption and the realities of temperature control in transportation, foodservice handling, and retailing.
  • Temperature management beyond the shipping point is rarely controllable by the shipper/processor, the entity responsible for implementing a food safety plan.
  • Temperature management is only relevant for a narrow spectrum of pathogens; However, prevention programs (other than temperature) that are in place to minimize the presence of non-affected pathogens are also effective for temperature responsive ones.
  • Pathogens, such as Listeria monocytogenes, will grow to a potential harmful level, over extended time, even at refrigerated temperatures. In this case, temperature cannot be a control point. Plant sanitation programs are the key preventive control for minimizing the presence of this bacterium. 

A limited exception to the above may be illustrated with the example of minimally processed melons. Revisions to the 1997 FDA Food Code lowered the temperature limit for potentially hazardous foods to 41 °F (5 °C). Minimally processed cantaloupe was, at the same time, placed on the list of potentially hazardous foods that must be transported and distributed at or below this temperature limit (alfalfa sprouts also fall into this category). Failure to maintain this level of cold chain control will result in the melons being classified, from a regulatory enforcement perspective, as "adulterated". The product would legally be required to be destroyed.

The reasons for this rare produce safety regulation include the facts that:

  • Cut cantaloupes have been associated with outbreaks of Salmonella on several occasions, including strong evidence for contamination at the source of production in a few cases.
  • The surface of cantaloupe is hard to clean, even with disinfectants, prior to processing. Residual pathogen populations may carry over into processing.
  • Contaminated, minimally processed cantaloupes, as with produce in general, are difficult to surface disinfect after cutting.
  • Salmonella has a restricted growth rate above 41°F (5 °C).A rapid growth rate at the higher temperatures are too often experienced in current distribution, foodservice, retail display, and consumer handling. 

The duration and magnitude (either in a continuous or cumulative exposure) of temperatures above this limit can result in an elevated risk of illness. Pathogen multiplication has been shown to reach hazardous levels prior to degradation of product appearance and quality, which would ordinarily be a sufficient deterrent to consumption by the average individual.

Producers of minimally processed cantaloupes, and melons in general, are advised to incorporate these factors into the design of their food safety program, as well as their carrier interactions and contracts.

3.3.4. Mixed load management

Selecting and maintaining optimum temperatures in mixed loads is difficult, especially when several commodities are involved. Commodities are generally packed in different sizes and shapes of packages that are then loaded in different load patterns in various parts of vehicles. With currently used shipping containers, these variations often result in the blockage of air circulation. When products with different optimum temperature requirements are shipped together, compromise temperature settings are used that are designed to protect the most perishable, or the most valuable, commodity in a load. Charts, tables, and matrix guides are available to assist in arriving at a mixed load management decision.

Vertical partitions or dividers that are used to separate wet from dry parts of the load should extend no lower than the top deck-boards of the pallets or racks (in trucks). Any part of a load that sits directly on the floor effectively blocks the air circulation under the entire load.

3.3.4.1. Product compatibility

In mixed loads certain product compatibility factors must be considered. These include the following:

Temperature compatibility. Differences in temperatures needed for various products in a load must be considered. For example, products that must be kept near 0 °C (32 °F) should not be shipped with products sensitive to chilling injury below about 12.5 °C (55 °F). For example, mature-green tomatoes will likely sustain chilling damage after a 5-d transit mixed with iceberg lettuce at a temperature of 5 °C (41 °F), a combination too often made when truck availability is limited.

Ethylene production and sensitivity compatibility. Care must be taken not to ship commodities that produce large amounts of ethylene (for example, apples, pears, avocados, and certain muskmelons) with commodities that are sensitive to ethylene (for example, broccoli, carrots, lettuce, kiwifruit, and most ornamentals). The incidence of russet spotting on lettuce (caused by exposure to ethylene) is about three times greater in mixed loads than in straight loads in truck shipments. Continuous regulated ventilation with outside air in marine containers can allow mixing of ethylene incompatible commodities in loads under certain conditions. Approved ethylene blocking agents or ethylene "scrubbing" or adsorption units may be used to protect sensitive commodities.

Product odor compatibility. Some commodities (for example, onions, garlic) produce odors, which can be absorbed by other products, causing the latter to have an objectionable odor and less market appeal. The sensory quality of some products, such as apples, is more readily affected by absorbed odors.

Moisture compatibility. Most products benefit from a high relative humidity in the transit atmosphere. Other commodities (for example, garlic, dry onions) benefit from intermediate humidity levels. Although humidity control at high levels is important during long transit periods, it is difficult to achieve. The use of large evaporator coils on the refrigeration unit helps to accomplish this. Water vapor adsorbing materials, commonly mineral zeolites, are in use to attempt to maintain a lower humidity for some commodities.

3.3.5. Special issues

Beyond the technical aspects of refrigerated transport management, produce shippers and buyers must factor in risk related to transportation shortages, an inevitable seasonal reality. There has been an increasing trend for major carriers away from handling highly perishable fruits and vegetables. Smaller carriers and individual owner/operators carry much of the horticultural products, especially in tight transportation markets. At approximately $55-65,000 for a new trailer and refrigeration unit, independent carriers often have old equipment. With high fuel costs and low margins, truckers may not adequately maintain equipment or engage in cost-saving practices to save a few dollars. Temperature management may be more difficult, ultimately impacting market quality.

3.3.6. Industry response to food safety concerns

The Food Safety Task Force of the National Perishable Logistics Association (NPLA) and Refrigerated Transportation Foundation (RTF) have undertaken the effort of developing guidelines for the refrigerated transportation industry involved in highway transportation and logistics management. The following uniform practices have been proposed;

  • Equipment 
  • 30% excess cooling capacity 
  • High airflow flooring 
  • Pressure bulkheads 
  • Intact air chutes 
  • Food quality interiors (wood interiors no longer acceptable) 
  • RTF "Combined Equipment Identification" plates for refrigeration and insulation ratings 
  • Independent electronic data recorder - Minimum one-hour intervals 

Preload Preparation

  • Remove all visible debris 
  • Wash and sanitize interior 
  • Maintain wash records - minimum one year 
  • Clean drains and floor channels 
  • Repair or replace damaged panels and door seals 
  • Electronic pre-trip routine for refrigeration unit 
  • Remove excessive water and condensate 

Loading

  • Precool to transit temperature 
  • Carrier maintains prior load data for inspection - minimum three previous 
  • Verify pulp temperature of load against transit temperature (refuse load if necessary) 
  • Load directly from temperature controlled storage 
  • Verify "Free Airflow" on six sides of load 
  • Verify operational start of data logger 
  • Verify box temperature at one-hour intervals 

Other

  • Institutionalize food safety responsibility 
  • Driver and Owner/Operator awareness and training programs 
  • Annual Food Safety Audits and third party inspections
     

3.3.7. Conclusions

Successful transport of horticultural products to markets depends upon products being cooled to, and loaded at, their desired transit temperatures. Users and carriers must be well informed about the capabilities and limitations of each type of equipment and the condition of the specific transportation equipment supplied. The responsibility for sanitation and preventive food safety programs directly involving equipment resides with the transport provider. Minimizing the risk of microbial contamination from other direct and indirect sources is a shared activity among shipper, carrier, and receiver.

 

Table II-6. Common Produce Transportation Equipment Capacities
Air Containers Reference Nomenclature Maximum Gross Weight Load Volume Refrigeration Source
  LD3 1,591 kg (3,500 lb) 3.5 m3 (140 ft3) None
  E 218 kg (482 lb) <than 0.4 m3 (12 ft3) Gel-ice, dry-ice
  PIP pallet 3,860 kg (8,510 lb) 10 m3 (341 ft3) None
Railcars Mechanical reefer car 130,000 to 166,000 lb 4,269 to 4,498 ft3 Mechanical refrigeration
Refrigerated trailers Reefer trailer     Mechanical refrigeration
  40 ft 22,680 kg (50,000 lb) 62 m3 2,188 ft3  
  45 ft 22,680 kg (50,000 lb) 66 m3 2,328 ft3  
  48 ft 22,680 kg (50,000 lb) 80 m3 2,825 ft3  
  53 ft 22,680 kg (50,000 lb)    
Refrigerated van container Reefer van     Mechanical refrigeration
  20 ft 19,050 kg (42,000) lb 23.8 m3 842 ft3  
  40 ft 20,866 kg (46,000 lb) 56.72 m3 2,003 ft3  
  40 ft "high cube" 20,866 kg (46,000 lb) 58.14 m3 2,053 ft3  
Marine containers Reefer Containers     Mechanical refrigeration
  20 ft 20,990 kg (46,270 lb) 26 m3 919 ft3  
  40 ft 27,620 kg (60,890 lb) 55.5 m3 1959 ft3  
  40 ft "high cube" 27,660 kg (60,960 lb) 64.8 m3 2289 ft3  

Additional Reading

[CA] Intl Controlled Atmosphere Research Conference. 1997. CA technology and disinfection studies. In: Thompson JT, Mitcham EJ, editors. CA '97 Proceedings: Seventh International Controlled Atmosphere Research Conference; Davis (CA). University of California, Dept. of Pomology. p 157. (Post Harvest Horticulture Series No. 15).

Heap R, Kierstan M, Ford G, editors. 1998. Food Transportation. London; New York: Blackie Academic and Professional. 242 p.

How RB. 1991. Marketing fresh fruits and vegetables. New York: Van Nostrand Reinhold. 357 p.

Kasmire RF, Hinsch RT, Thompson JT. 1996. Maintaining optimum perishable product temperature in truck shipments. Davis (CA): University of California, Dept. of Pomology.

Thompson JF, Brecht P, Hinsch T, Kader A. 2000. Marine container transport of chilled perishable produce (ANR Publication 21595). Davis (CA): University of California, Division of Agriculture and Natural Resources. 32 p. Available from: <http://anrcatalog.ucdavis.edu/merchant.ihtml>.

[USDA] U.S. Dept. of Agriculture, Agricultural Marketing Service. 1995. Protecting perishable foods during transport by truck. Washington: USDA, AMS, Transportation and Marketing Division. Report nr Hbk-669. Available from: <http://www.ams.usda.gov/poultry/publications/sea%5Fpubs.htm>.


 


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