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Guide to Inspections of Low Acid Canned Food 18

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and processing times.

In 1948 Taylor Instrument Co. supplied the lacf industry with the first fully automatic controlled retort systems. Other manufacturers have introduced similar systems since that time. These systems use electronic/mechanical clocks, stepping switches electronic relays, pressure switches, solenoid valves and temperature sensors to provide for automatic/pneumatic control of vent valves, bleeder valves, cooking times, over-pressure, cooling water, cooling times, and draining of the retort. The operators function is to set the process time, the thermal process temperature, and the vent times to meet those of the operating thermal process. In order to start the process the operator only has to push a start button. The control of the retort thermal process is then through the controller. The system may be equipped with a series of lights which indicate when the steam is turned on, when the process is up to temperature (start of thermal process) and when the retort is in the cooling phase. The operator is still required to observe and manually record MIG thermometer temperature readings, processing times, vent times and other factors critical to the process. At the end of the process the timing clocks are automatically reset to the preselected times. The system is then ready for the next thermal process.

Because these systems are fully automatic, including the operation of the bleeder valves, maintenance of the system is often neglected. Failure to adequately maintain the system can result in failure of bleeder valves, vent valves and system controls to operate properly. When these types of retort control systems are encountered the investigator must observe the operation of the system through several cycles to insure that all control valves are operating properly, that bleeder valves are fully open and functional during the venting and thermal processing cycles, and that scheduled process time and temperature parameters are being met.

Cam operated control systems have been used by U.S. and European equipment manufacturers to automatically control retort processing including processing temperature, pressure, processing times and cooling times. In these systems a cam is cut out of plastic, metal or some other durable material. As the cam is turned by an electrical/mechanical clock a cam follower rides upon the cam and operates a series of electrical sensing switches. The electrical switches are used to operate electrical and/or pneumatic control valves which supply steam and air to the retort or serve other functions such as venting of the retort. Care has to be taken to cut the correct profile in the cam to provide the correct thermal process. As the cam wears the profile may change, changing the thermal process.

European manufactures such as LaGarde and Stock have used card readers to control retort temperature, retort pressure and process timing. In these systems a card made of plastic, metal or paper is either cut or punched to a certain profile for each thermal process. The card is then placed in a card reader. In the LaGarde system the card is read by a light source. As the card passes through the reader the light source activates limit switches. In the Stock system the card comes into contact with the limit switches as is passes through the card reader. The limit switches control the signals sent to the control valves which may be either electronically or pneumatically controlled. The profile of the cards are very important. Care has to be taken to punch or cut the correct profile for each thermal process. If the profile is not cut correctly for the process the correct process will not be provided. The cards also become worn or damaged during use and care must be taken to maintain the cards in good working condition.


In recent years digital electronic controllers have replaced the relays and sensing switches of mechanical/electrical control systems. Digital control systems may range from the single-loop controller to complex high-end computer systems.

If the process to be controlled consists of numerous sequential (logical) steps, the controlling device can be a first-level computer device called a logic controller. The logic controller may be set up as a single loop controller.

A single loop controller would be responsible for controlling one function such as temperature in the retort. The controller loop would be programmed to control the retort temperature within set temperature parameters. The loop would consist of the microprocessor controller, a sensor, and actuator for the steam valve and a digital/analog signal converter. The simple single loop controller contains Read Only Memory (ROM) which is manufactured into the controller or programmed into the controller by using Programmable Read Only Memory (PROM), Erasable Programmable Read Only Memory (EPROM), or Electronically Erasable Programmable Read Only Memory (EEPROM). PROM is field programmable by the manufacturer or customer one time only by burning out fuses in the PROM microprocessor chips. EPROM is electronically programmed by the manufacturer or user. EPROM microprocessor chips can be reprogrammed by exposing the chip to an ultraviolet light source which resets the original chip configuration. EEPROM microprocessor chips can be

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