Guide to Inspections of Low Acid Canned Food 33
Manufacturers - 2
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from the pressurized upper drum is dropped into the lower processing drum through a connecting pipe and valve. At that time the circulation pump begins to circulate the water by drawing the water out of the bottom of the retort through a suction manifold (also called the "circulation channel") and returning the water through a distribution manifold in the top of the lower drum. The returns located in the bottom of the retort shell should be screened to prevent debris from entering the water circulation system. As the water is circulated it passes through a steam injection chamber where live steam is injected into the water to maintain processing temperatures.
When water enters the lower drum, air must be expelled to make room for the water. This is done through a purge valve (some times called a vent valve by the firm). If the purge valve is not open for a long enough time period, air will remain in the lower drum and the lower drum will take a longer time to fill with water. If the purge valve is open too long, the hot water will flash to steam, and water and steam will be lost out of the purge valve lowering the temperature in the processing vessel. This may cause the come-up period to be extended to reach processing temperature and has been reported in some cases to affect temperature distribution in the retort system. The length of time that the purge (vent) valve is open is determined by the processing authority and normally programmed into the retort controls as a set time period.
The come-up time (CUT) prior to processing is very important in this type of retort. During the CUT the retort is brought up to processing temperature and the water temperature throughout the retort is stabilized at or above the scheduled thermal processing temperature. This may require a CUT which extends beyond the time when the processing temperature is first reached, as indicated by the retort instrumentation. Usually both a time and temperature are required to be met to fulfill CUT requirements. If CUT is used in establishing the scheduled process it must be filed as part of the process and controlled as a critical factor. If CUT is only critical based on the temperature(s) and time(s) required to achieve adequate temperature distribution it is not filed as part of the process, but still must be controlled and recorded as part of the processing record. CUT varies by retort model and make, and for different products.
In the sterilization (hold) phase the product is held for at least the minimum time at the minimum temperature specified in the filed scheduled process.
When the thermal process is complete cold water is introduced into the lower drum through the water distribution system or through a separate pump. This cold water forces a portion of the hot processing water back up into the top storage drum where it is captured and reheated for the next process.
The water circulation pump is the principle device for insuring adequate temperature distribution in the retort. Temperature distribution and heating programs are normally established at the maximum flow rate. If the water distribution system becomes clogged or the pump is damaged or worn the flow rate may decrease. Severe drops in flow rate may be indicated by temperature drops in the retort, however this may not be the case for less severe flow rate changes. Most of the systems noted are equipped with at least a pilot light that indicates that the pump is in operation. Some of the newer systems also incorporate a pressure differential alarm, which measures the difference in pressure from one side of the pump to the other, and alarms if the pressure differential falls outside of preset limits. Neither of the above two methods measure the actual water flow in the system. A flow-indicting device is recommended for this purpose.
Rotation in the retort may be fixed at one speed, allow for selection of several speeds, or may be variable over a wide range of speeds. The newer systems may allow for rotation to be in either direction, to provide a rocking motion agitation, and provide for the baskets to be in different positions during come-up and processing. The systems are normally operated between 6 and 46 RPM. For those systems which do not take agitation into consideration when establishing the process it is still recommended by some authorities that the reel be rotated at a minimum speed to enhance mixing of the processing water and to maintain proper temperature distribution. Temperature distribution must be documented for those systems operated in the still mode. Timing of the rotation is done either through the use of a stopwatch and observation of rotation of the driving mechanism on the rear of the retort or through the use of a recording tachometer. If a recording tachometer is used the accuracy of the tachometer should be checked on a routine basis with a stop watch. The rotational speed of each retort load must be recorded.
Overpressure during processing (to maintain container integrity) is supplied to the majority of these systems by using compressed air. The majority of the retorts are equipped with both a valve for supplying additional air and a valve for releasing excess pressure (small vent or purge valve). This allows for more uniform control of over-pressure processing. The Stock systems normally use steam pressure in the upper drum to provide over-pressure through a connecting valve to the
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