Guide to Inspections of Low Acid Canned Food 14
Manufacturers - 2
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MERCURY-IN-GLASS (MIG) THERMOMETER
Each retort system used for the thermal processing of lacf(s) must be equipped with a MIG thermometer as per 21 CFR 113.40. This is emphasized in the lacf regulations by being repeated for each specific retort system covered (note: currently only aseptic processing systems may have a temperature indicating device other than MIG thermometer as the sole temperature indicator-21 CFR 113.40(g)(1)(i)(a)).
The MIG thermometer is the reference instrument for all temperature readings, including vent temperatures, come-up temperatures and process temperature during the processing of lacf(s).
The regulations require that the MIG thermometer be graduated in divisions which are easily readable to 1° F (1/2° C) and whose temperature range does not exceed 17° F (8° C) per inch (2.54 cm). The thermometer may be graduated in 2° F (1° C) divisions as it is possible to read a MIG thermometer graduated in this manner to the nearest 1° F (1/2° C). The lacf regulations require that MIG thermometers be tested for accuracy upon installation, and at least once per year after that. It is important that the MIG thermometer be tested/calibrated at the operating temperature of the retort system (i.e. 240° F, 250° F, 260° F etc.) and if possible in the heating medium used in the retort. If the retort is operated at more than one processing temperature or over a wide range of temperatures the MIG thermometer should be checked at all of the temperatures normally used for processing lacf(s). The MIG thermometers should be tested against a thermometer that can be traced back to an NIST (National Institute of Standards and Technology) Standard thermometer. The standard thermometer should also be checked for accuracy on a routine basis. Documentation should be available for the calibration of the standard thermometer used and its last check for accuracy. The accuracy of the standard thermometer should be checked at least once every 3 years depending upon how it is handled and stored.
One way that firms using steam for thermal processing can check their MIG thermometers is to use a cross made of 3/4 inch pipe for holding the thermometers with 1/16 inch holes drilled in the fittings to provide for a flow of steam past the thermometer bulb. The cross is then mounted on a retort or steam manifold with a certified standard thermometer in the center and one MIG retort thermometer to be tested in each of the two outside arms. The thermometers are then allowed to heat to equilibrium for 10 to 15 minutes. If an adjustment is needed the face plate screws are loosened and the face plate is adjusted on the MIG thermometers. After the adjustment another reading is taken to insure that the thermometer is now reading the same as the standard MIG thermometer.
The installation of the standard thermometer in the retort next to the MIG thermometer to be checked and the use of a circulating laboratory oil bath are also acceptable methods used to check MIG thermometers for accuracy.
Records should be kept of the accuracy check of all MIG thermometers which include the date of the last check, the person performing the check, the standard used, the method used, and the amount of correction needed to bring the thermometer back into calibration. The regulations recommend that each thermometer be identified with a tag, seal or through some other means that identifies the date on which the last accuracy test was made. The MIG thermometer must at least be identified in a manner that will allow the thermometer to be matched with the thermometer calibration records.
MIG thermometers that have a divided column, that are broken, or which can not be adjusted must be repaired or replaced immediately.
The MIG thermometer must be installed in a manner that it can be easily read, without the operator having to go to extraordinary measures, such as entering a steam flow, touching hot equipment, or climbing a ladder to make the reading. If it is difficult for the MIG thermometer to be read, the operator will be more likely to use the recording chart or some other method to obtain temperature readings, not the MIG as required by the regulations.
TEMPERATURE RECORDING DEVICE
The lacf regulations, 21 CFR 113.40, require that each retort system be equipped with an accurate temperature-recording device. This has been interpreted to mean that the recording device provides a continuous record of the temperature in the retort system during thermal processing. Common systems found use circular or strip charts which are marked with ink pens, electrical sparks, pressure pins, or which are created by graph plotters at the time temperature readings are received.
The regulations require that graduations on the temperature-recording device charts must not exceed 2° F (1° C) within a range of 10° F (5.5° C) of the processing temperature. A working scale of not more than 55° F per inch (12° C per Cm) is required within 20° F (10° C) of the processing temperature. A ruler can be used during the inspection to determine the firms compliance with this part of the lacf regulations. The object of the chart specifications is to provide a chart which can be easily read by the operator and record reviewer.
The regulations require that the temperature
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