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Elemental Analysis Manual: Section 2.2 Analytical Sample to Analytical Portion

Elemental Analysis Manual, Version 1 (June 2008)
Authors: Stephen G. Capar and Richard M. Jacobs

Table of Contents

2.2.1 HOMOGENIZATION EQUIPMENT

2.2.2 HOMOGENIZATION PROCEDURES

 

Glossary


This section provides information to assist the analyst on physically preparing the analytical sample prior to taking an analytical portion for analysis. Obtaining representative analytical portions is critical for accurate and reproducible results. EAM methods assume the analyst has made the analytical sample homogeneous with respect to the size of the analytical portion. Homogenized foods will vary greatly in texture and viscosity and may separate into liquid and solid components. Analytical samples that have been stored after being homogenized usually require re-homogenization before acquiring an analytical portion. The analyst must carefully observe the physical characteristics of the homogenate and choose an appropriate means of obtaining a representative analytical portion (e.g., pouring directly from container possibly during active mixing, using a 1, 5 or 10 mL pipette or using a spatula).

Note: If non-homogeneity is known or suspected then replicate analytical portions should be analyzed and the results assessed.

2.2.1. Laboratory Homogenization Equipment  

  • [Food] Blenders 1

    Food BlenderA blender is a kitchen appliance used to blend ingredients or puree food. The term typically refers to a stationary, upright electrical device, which is to be distinguished from a hand-powered or electric mixer that may be used for similar purposes.

    A typical blender is built around a vessel for the ingredients to be blended. At the top of the vessel is a cap to prevent ingredients from escaping when the blender is switched on. At the bottom is a blade assembly, typically removable for cleaning purposes. The bottom seal is most likely watertight. The vessel rests upon a base containing a motor (for turning the blade assembly) with controls on its surface. Most modern blenders offer a number of possible speeds.

  • Food Processors 2

    Food ProcessorA food processor is a kitchen appliance used to facilitate various repetitive tasks in the process of preparation of food. Today, the term usually refers to an electric-motor-driven appliance, although there are some manual devices also referred to as "food processors".

    Food processors are similar to blenders in many ways. The primary difference is that food processors use swappable blades and disks (attachments) instead of a fixed blade. In addition, their bowls are wider and shorter, a more appropriate shape for the solid or semi-solid foods usually worked in a food processor. Usually little or no liquid is required in the operation of the food processor unlike a blender, which requires some amount of liquid to move the particles around its blade.

  • Laboratory Homogenizers 3

    Laboratory HomogenizerLaboratory homogenizers are high-speed, high-shear mixers that reduce samples to uniformed-sized particles through maceration, cutting, and blending. They are used to process liquids, slurries, or granular substances. Product specifications for laboratory homogenizers include media viscosity, capacity, feed rate, motor speed, motor power, pressure range, and operating temperature. There are three basic types of laboratory homogenizers: fluidized bed, rotor-stator, and ultrasonic. Fluidized-bed homogenizers are durable vessels that fluidize the complete product bed. Rotor-stator homogenizers are single-shaft mixers with an impeller that rotates in close proximity to a stationary housing. Ultrasonic or vibrational homogenizers apply ultrasonic waves in a mixed medium to produce a steep gradient of acoustical pressure and, therefore, fluid movement and a very-fine level of mixing action. Homogenizers that produce high shear and ultrasonic waves can disrupt cellular structure of some foods leading to a better homogenization of the material.

  • Cryogenic Mills

    Cryogenic MillCryogenic laboratory mills cool samples to cryogenic temperatures and pulverize them by magnetically shuttling a steel impactor back and forth against two stationary end plugs. The vial is immersed in liquid nitrogen throughout the grinding cycle.


     

  • Mortar Grinders

    Mortar Grinder for foodsMortar grinders force the samples against two hard surfaces (i.e., mortar and pestle) to grind the sample by a combination of pressure and friction. A choice of materials in contact with the sample is available including agate, zirconium oxide, stainless steel, tungsten carbide and porcelain.


     

  • Rotor Mills

    Rotor Mill for foodsRotor mills use a stainless steal beater rotating at a high speed (3000 — 28000 rpm) to cut up the sample by impact and shearing.

2.2.2. Homogenization Procedures  

 

2.2.2.1. General Procedures

Examples of homogenization equipment used for specific types of foods are listed in 2.2 Table 1.

 2.2 Table 1.
Suggested Procedure for Preparing Analytical Samplea
Food ProductSuggested Procedure
Apple (red), raw (with peel)Food Processor
Applesauce, bottledBlender
Apricot, rawBlender
Asparagus, fresh/frozen, boiledFood Processor
Avocado, rawFood Processor
Bagel, plain, toastedFood Processor
Banana, rawFood Processor
Beans, kidney, dry, boiledFood Processor
Beef chow mein, from Chinese carry-outFood Processor
Beef stew, homemadeBlender
Beef stroganoff with noodles, homemadeFood Processor
Beef, ground, regular, pan-cookedFood Processor
Beef, meatloaf, homemadeFood Processor
Beef, roast, chuck, oven-roastedFood Processor
Beef, steak, loin, pan-cookedFood Processor
BeerMix by Hand
Beets, fresh/frozen, boiledFood Processor
Beverage, carbonated, cola, low-calorieMix by Hand
Beverage, carbonated, cola, regularMix by Hand
Beverage, carbonated, fruit-flavored, regularMix by Hand
Beverage, fruit drink (10% juice), canned or bottledBlender
BF, applesauceBlender
BF, bananasBlender
BF, beef and broth/gravyBlender
BF, carrotsBlender
BF, chicken and broth/gravyBlender
BF, chicken noodle dinnerBlender
BF, corn, creamedBlender
BF, fruit dessert/puddingBlender
BF, green beansBlender
BF, infant formula, milk-based, high iron, RTFBlender
BF, Infant formula, milk-based, low iron, RTFBlender
BF, Infant formula, soy-based, RTFBlender
BF, juice, appleMix by Hand
BF, juice, orangeMix by Hand
BF, macaroni, tomato and beefBlender
BF, mixed vegetablesBlender
BF, peachesBlender
BF, pearsBlender
BF, peasBlender
BF, rice cereal with applesBlender
BF, rice infant cereal, instant, prepared with milkBlender
BF, spinach, creamedBlender
BF, split peas with vegetables & ham/baconBlender
BF, squashBlender
BF, sweet potatoesBlender
BF, teething biscuitsFood Processor
BF, turkey and riceBlender
BF, vanilla custard/puddingBlender
BF, vegetables and beefBlender
BF, vegetables and chickenBlender
BF, vegetables and hamBlender
Bologna (beef/pork)Food Processor
Bread, biscuits, refrigerated-type, bakedFood Processor
Bread, cornbread, homemadeFood Processor
Bread, cracked wheatFood Processor
Bread, muffin, fruit or plainFood Processor
Bread, ryeFood Processor
Bread, white rollFood Processor
Bread, white, enrichedFood Processor
Bread, whole wheatFood Processor
Broccoli, fresh/frozen, boiledFood Processor
BrownieFood Processor
Brussels sprouts, fresh/frozen, boiledFood Processor
Butter, regular (salted)Food Processor
Cabbage, fresh, boiledFood Processor
Cake, chocolate snack with chocolate icingFood Processor
Cake, chocolate with icingFood Processor
Cake, yellow, with white icing, prepared from mixesFood Processor
Candy, caramelSeparately weigh an equal amount of deionized water and candy. Heat water to boiling in a 2 - quart stainless steel bowl. Add candy with continuous stirring using large stainless steel spoon. After candy has melted, pour into appropriate containers. [MCF = 0.5]
Candy, hard [Hard candy: Based on sugars cooked to the hard-crack stage, including suckers (known as boiled sweets in British English), lollipops, jawbreakers (or gobstoppers), lemon drops, peppermint drops and disks, candy canes, rock candy, etc.] 4Separately weigh an equal amount of deionized water and candy. Heat water to boiling in a 2 - quart stainless steel bowl. Add candy with continuous stirring using large stainless steel spoon. After candy has melted or dissolved, pour into appropriate containers. [MCF = 0.5]
Candy, milk chocolate bar, plainFood Processor. Avoid inducing elevated temperatures that cause phase separation.
Cantaloupe, raw/frozenBlender
Carrot, fresh, peeled, boiledFood Processor
Cauliflower, fresh/frozen, boiledFood Processor
Celery, rawFood Processor
Cereal, corn flakesFood Processor
Cereal, cream of wheat (farina), enriched, cookedFood Processor
Cereal, crisped riceFood Processor
Cereal, fruit-flavored, presweetenedFood Processor
Cereal, oat ringFood Processor
Cereal, oatmeal, plain, cookedFood Processor
Cereal, raisin branFood Processor
Cereal, shredded wheatFood Processor
Cheese, American, processedFood Processor
Cheese, cheddar, natural (sharp/mild)Food Processor
Cherries, sweet, rawBlender
Chicken breast, oven-roasted (skin removed)Food Processor
Chicken nuggets, fast-foodFood Processor
Chicken potpie, frozen, heatedFood Processor
Chicken, fried, fast-foodFood Processor
Chicken, fried - homemadeFood Processor
Chili con carne with beans, homemadeFood Processor
Coleslaw with dressing, homemadeFood Processor
Collards, fresh/frozen, boiledFood Processor
Cookies, Chocolate chipFood Processor
Cookies, sugarFood Processor
Cookies, sandwich with crème fillingFood Processor
Corn, cream style, cannedBlender
Corn, fresh/frozen, boiledFood Processor
Corn, hominy grits, enriched, cookedFood Processor
Corn/tortilla chipsFood Processor
Cottage cheese, 4% milk fatBlender
Crackers, butter-typeFood Processor
Crackers, grahamFood Processor
Crackers, saltineFood Processor
Cream cheeseFood Processor
Cream substitute, non-dairy, liquid/frozenBlender
Cream, Half & halfBlender
Cucumber, peeled, rawFood Processor
Cucumber, pickles, DillFood Processor
Cucumber, pickles, sweetFood Processor
Doughnut, cake-typeFood Processor
Egg, cheese, and ham on English muffin, fast-foodFood Processor
Eggplant, fresh, peeled, boiledFood Processor
Eggs, boiledFood Processor
Eggs, friedFood Processor
Eggs, scrambled with oilFood Processor
English muffin, plain, toastedFood Processor
Fish sandwich on bun, fast-foodFood Processor
Fish sticks or patty, frozen, oven-cookedFood Processor
Frankfurter (beef/pork), boiledFood Processor
Frankfurter on bun, fast-foodFood Processor
French fries, frozen, heatedFood Processor
Frozen meal - Salisbury steak, heatedFood Processor
Frozen meal - turkey, heatedFood Processor
Fruit cocktail, canned in light syrupBlender
Granola with raisinsFood Processor
Grapefruit, rawBlender
Grapes, red/green, rawFood Processor
Green beans, fresh/frozen, boiledFood Processor
Ham, cured (not canned), bakedFood Processor
Hamburger, quarter-pound on bun, fast-foodFood Processor
Hamburger, quarter-pound with cheese on bun, fast-foodFood Processor
HoneyMix by Hand
Ice cream, light, vanillaBlender
Ice cream, regular, vanillaBlender
Jelly, any flavorBlender
Juice, apple, bottledMix by Hand
Juice, grape, frozen concentrate, reconstitutedBlender
Juice, grapefruit, bottledBlender
Juice, orange, frozen concentrate, reconstitutedBlender
Juice, prune, bottledMix by Hand
Juice, tomato, bottledMix by Hand
Lamb chop, pan-cooked with oilFood Processor
Lasagna with meat, homemadeFood Processor
Lemonade, frozen concentrate, reconstitutedBlender
Lettuce, iceberg, rawFood Processor
Lima beans, immature, frozen, boiledFood Processor
Liver (beef/calf), pan-cooked with oilFood Processor
Luncheon meat (ham)Food Processor
Luncheon meat, salami, (not hard)Food Processor
Macaroni and cheese, prepared from box mixFood Processor
Macaroni, boiledFood Processor
Margarine, regular (salted)Food Processor
MartiniMix by Hand
Mayonnaise, regular, bottledFood Processor
Milk shake, chocolate, fast-foodBlender
Milk, chocolate, low fat, fluidBlender
Milk, evaporated, cannedBlender
Milk, low fat (2%), fluidBlender
Milk, skim, fluidBlender
Milk, whole, fluidBlender
Mixed vegetables, frozen, boiledFood Processor
Mushrooms, rawFood Processor
Mustard, yellowMix by Hand
Noodles, egg, enriched, boiledFood Processor
Nuts, mixed, no peanuts, dry roastedBlender
Oil, olive/safflowerMix by Hand
Okra, fresh/frozen, boiledFood Processor
Olives, blackFood Processor
Onion, mature, rawFood Processor
Orange (navel/Valencia), rawBlender
Pancakes from mixFood Processor
Peach, canned in light/medium syrupBlender
Peach, raw/frozenBlender
Peanut butter, creamyFood Processor
Peanuts, dry roasted, saltedFood Processor
Pear, canned in light syrupBlender
Pear, raw (with peel)Food Processor
Peas, green, frozen, boiledFood Processor
Peas, mature, dry, boiledFood Processor
Pepper, sweet, green, rawFood Processor
Peppers, green, stuffed, homemadeFood Processor
Pie, apple, fresh/frozenFood Processor
Pie, pumpkin, fresh/frozenFood Processor
Pineapple juice, frozen concentrate, reconstitutedBlender
Pineapple, canned in juiceBlender
Pinto beans, dry, boiledFood Processor
Pizza, cheese and pepperoni, regular crust, from pizza carry-outFood Processor
Pizza, cheese, regular crust, from carry-outFood Processor
Plums, rawFood Processor
Popcorn, popped in oilFood Processor
Popsicle, fruit-flavoredBlender
Pork and beans, cannedBlender
Pork bacon, oven-cookedFood Processor
Pork chop, pan-cooked with oilFood Processor
Pork roast, loin, oven-roastedFood Processor
Pork sausage (link/patty), oven-cookedFood Processor
Potato chipsFood Processor
Potato, baked (with peel)Food Processor
Potato, boiled (without peel)Food Processor
Potato, french-fried, fast-foodFood Processor
Potatoes, mashed, from flakesFood Processor
Potatoes, scalloped, homemadeFood Processor
Pretzels, hard, saltedFood Processor
Prunes, driedFood Processor
Radish, rawFood Processor
[Alternative: Food Processor with separately weighed amounts of deionized water and food product. [MCF = 0.5]]
RaisinsFood Processor
[Alternative: Food Processor with separately weighed amounts of deionized water and food product. [MCF = 0.5]]
Rice, white, enriched, cookedFood Processor
Salad dressing, French, regularMix by Hand
Salad dressing, Italian, low-calorieBlender
Salmon, steaks/fillets, bakedFood Processor
Sauerkraut, cannedFood Processor
Sherbet, fruit-flavoredBlender
Shrimp, boiledFood Processor
Soup, bean with bacon/pork, canned, condensed, prep with waterBlender
Soup, chicken noodle, canned, condensed, prep with waterBlender
Soup, clam chowder, New England, canned, condensed, prep with whole milkBlender
Soup, mushroom, canned made with whole milkBlender
Soup, tomato, canned, condensed, prep with waterBlender
Soup, vegetable beef, canned, condensed, prepared with waterBlender
Sour creamBlender
Spaghetti with meat sauce, homemadeFood Processor
Spaghetti with tomato sauce, cannedBlender
Spinach, fresh/frozen, boiledFood Processor
Squash, Summer, fresh/frozen, boiledFood Processor
Squash, winter (Hubbard/acorn), fresh/frozen, boiledFood Processor
Strawberries, raw/frozenFood Processor
Sugar, white, granulatedMix by Hand
Sweet potato, fresh, bakedFood Processor
Sweet roll/Danish pastryFood Processor
Syrup, chocolateMix by Hand
Syrup, pancakeMix by Hand
Taco/tostada with beef and cheese, from Mexican carry-outFood Processor
Tomato catsupBlender
Tomato sauce, plain, bottledBlender
Tomato, rawFood Processor
Tomato, stewed, cannedBlender
Tortilla, flourFood Processor
Tuna noodle casserole, homemadeFood Processor
Tuna, canned in oilFood Processor
Turkey breast, oven-roastedFood Processor
Turnip, fresh/frozen, boiledFood Processor
Veal cutlet, pan-cookedFood Processor
Water, tapMix by Hand
Watermelon, raw/frozenBlender
WhiskeyMix by Hand
Wine, dry table, red/ whiteMix by Hand
Yogurt, low fat, fruit-flavoredBlender
Yogurt, plain, low fatBlender

a Based on instructions used in FDA's Total Diet Study. BF: Baby Food.

2.2.2.2. Candy Procedures

Some types of candy products contain small particulates as sources of contaminants. Traditional homogenization procedures may not achieve a sufficiently homogenous analytical sample for the relatively small portions analyzed. This inhomogeneity causes greater measurement variability than expected and is difficult to discern from other sources of measurement variability. The high variability of the results, while reflecting the true inhomogeneous nature of the contaminant, may discredit the quality of the analysis. In an effort to overcome this problem, the analytical sample is blended with dilute nitric acid and heated to solubilize the contaminants 5-6. The objective of the procedure given below is to produce a homogeneous analytical sample from which replicate analytical portions can be taken with the expectation of accurate and precise analytical results. Supporting information for the procedure is available 7.

The potential application of this procedure includes varieties of hard and soft candy, powdered candy products, and fruit candy products, especially candies that have chili or other ingredients that may be the source of the contaminant of interest. However, because of the diversity in the physical nature of candy products this procedure may not be adequate for all confectionary products.

  • Laboratory Sample. To reduce inherent within lot composition variability of candy products, a relatively large laboratory sample is collected for preparation of the analytical sample. A laboratory sample should consist of a maximum of 10, 2 lbs (0.91 kg) subsamples. The subsamples should be sealed retail units from the same manufacturing code. If the candy is packed in a sealed bag of at least 2 lbs, then the each of the 10 subsamples is a bag of candy. Otherwise, each subsample is the number of bags that provide at least 2 lbs of candy.
  • Analytical Sample. The amount of the laboratory sample subjected to homogenization to prepare the analytical sample should be approximately 1 kg. Take equal portions of each of the 10 subsamples to obtain the 1 kg.
  • Equipment. A food processor is required and laboratory homogenizer may be needed. The procedure described below uses portions that can be contained in about a 6 - liter stainless steel bowl of a food processor. The volume of sample plus the volume added water and acid used to aid homogenization should be limited to approximately 2 liters to allow proper blending action. The analytical sample must be contained in a commercial food processing vessel or a beaker that may need to accommodate an equivalent weight of water or acid.
  • Method Blanks. Prepare duplicate method blanks in an identical fashion as preparing the analytical sample.
  • Mass Correction Factor. An analytical portion for analysis is obtained from the treated analytical sample preparations described below. In the analytical method's calculation to obtain the analyte concentration this analytical portion mass must be corrected for the added solvent (water or nitric acid) using a mass correction factor (MCF) (see §3.4.6). To calculate MCF both the mass of the analytical sample and the mass of all added solvent(s) are required. Therefore, these masses must be determined and recorded during the homogenization procedures described below. Determine the mass of the materials at the end of any heat treatment cycle in order to account for the losses of liquid through vaporization.

Candy Product Types

Type I:Semi-solid or soft candy (e.g., chocolate-based, fruit-based, including tamarind, tejocote, apple, etc.) with or without visible particulate ingredients.
Type II:Hard candy (including suckers) with or without visible particulate ingredients either in the candy or on the surface of the candy.
Type IIA:Hard candy (including suckers) with supplemental (but separate) ingredients (e.g., salt, chili).
Type III:Powdered sugar or flavored salt products with or without other particulate ingredients (e.g., chili).

Homogenization of Candy Type I Products

  1. Water treatment — Add the analytical sample, approximately 1 kg, to the food processor bowl. Add a mass of water (ASTM Type I) approximately equal to the mass of the analytical sample except for chocolate-based products. Some chocolate-based products may have too much fat to blend with water. Chocolate should be blended first without water and then an equal mass of water added. At this stage, the mixture can be left over night to soften or partially dissolve. If the sample will not disperse by blending, heating may be required (e.g., on a hot plate). Blend the sample in the food processor until a visually homogenous composite is obtained. This mixture needs to have paste-like consistency in order to disperse particles and to permanently suspend all ingredients. This paste-like composite must be stable (i.e., no phase separation) until an analytical portion(s) is removed for acid treatment. If blending with water fails to form a paste-like consistency for Type I product, then treat the product as you would for a Type II product.
  2. Concentrated nitric Acid treatment — Weigh duplicate portions equivalent to about 40 g of product into beakers. Nitric acid washed glass beakers, polypropylene (translucent) or polymethylpentene (transparent) beakers can be used (heat stable to ~150° C). To prevent deformation a water bath should be used to heat plastic beakers. Add concentrated nitric acid (Metals grade) (20 ml initially) and heat to approximately 100° C until the product has been largely solubilized. Be careful not to heat the mixture too rapidly for this may cause excessive foaming and loss of mixture. The beakers should be covered with watch glasses to minimize contamination. More nitric acid may be added to achieve a more complete dissolution.

Homogenization of Candy Types II, IIA, and III Products

10% Nitric Acid Treatment — Add the analytical sample, approximately 1 kg, to a beaker. For analysis of supplementary packets from Type IIA candies (typically a mixture of salt and chili), combine the number of packets (separately from the candy) that would be equal to the numbers of candy used to make the composite. Add an equal mass of 10% nitric acid. Heat the mixture (to approximately 100° C) until the sample mass is largely dissolved. This step may need to be left over night to maximize dissolution. The remaining particulate materials should be subjected to further homogenization with a laboratory homogenizer such as a laboratory homogenizer. This final mixture should be highly acidic and should be largely a solution with respect to the analyte. Many candy ingredients and some natural product may not completely dissolve after the solubilization procedure. Silicon dioxide, chili flakes, and titanium dioxide common additives will not dissolve in this type of procedure. However, lead metal and other metallic ingredients should be completely dissolved. This liquid should now be in a satisfactory state for removing the analytical portion(s). If particulates persist in the product then stir vigorously while removing the analytical portions.


References

  1. Wikipedia contributors. Blender (device) [Internet]. Wikipedia, The Free Encyclopedia; 21 July 2006 11:38 UTC [cited 23 July 2006 14:21 UTC]. Available from: Wikipedia.  disclaimer icon
  2. Wikipedia contributors. Food processor [Internet]. Wikipedia, The Free Encyclopedia; 21 July 2006 13:37 UTC [cited 23 July 2006 14:47 UTC]. Available from: Wikipedia.  disclaimer icon
  3. GlobalSpec. Homogenizers, Laboratory [Internet]. GlobalSpec Inc.; [cited 23 July 2006 14:21 UTC]. Available from: GlobalSpec.  disclaimer icon
  4.  Wikipedia contributors. Confectionery [Internet]. Wikipedia, The Free Encyclopedia; 2006 Aug 25, 05:15 UTC [cited 2006 Aug 26]. Available from: Wikipedia.  disclaimer icon
  5. Jones, J. W., and Boyer, K. W. (1979) Sample Homogenization Procedure for Determination of Lead in Canned Foods, J. Assoc. Off. Anal. Chem. 62, 122 – 128.  
  6. Suddendorf, R. F., Wright, S. K., and Boyer, K. W. (1981) Sampling Procedure and Determination of Lead in Canned Foods, J. Assoc. Off. Anal. Chem. 64, 657 – 660.
  7. Jacobs, R., Castro, C., Peiffer, G., Shippey, K., Wong, J., Yee, S., and Palmer, P., Determination of Lead in Mexican Candy and Flavored Salt Products, FDA Laboratory Information Bulletin 4346, 21(4), April 2005, (8 pages). (PDF)