SOP: Evaluation of Thermal Process by the Improved General Method
GIFSQ — Global Institute for Food Safety & Quality
Principle
Ball’s improvement of the General Method constructs a hypothetical TDT curve passing through the point of 1 min at 250 °F (F = 1), so that the area under the lethality curve, expressed in unit sterilizing areas, gives the sterilizing (F) value of the process directly. The lethal rate L at any temperature is calculated from the TDT-curve slope z, and the L values over the whole process time are summed by integration under the L-versus-time curve.
Materials required
- Heat penetration data (cold-point temperatures at recorded times)
- Table 25-17 of the book (lethal rates when z = 18) or Table 25-18 (lethal rates for other z values)
- Coordinate paper for the heat penetration and L-value curves
Procedure
Enter the heat penetration data in two columns showing the temperatures corresponding to given times
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Plot the temperature at the cold point in the can against time to give the heat penetration curve
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Draw a second curve with the same time scale, but with the appropriate L values from Table 25-17 or 25-18 substituted for temperature — for instance, if after 50 min the cold-point temperature is 220 °F, plot the L value 0.0216 at time 50 min; repeat at various time intervals and join the points to get the L value versus time (lethality) curve
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Measure the area under the lethality curve by any one of the following methods: (i) by counting small squares; (ii) by using a planimeter; (iii) by cutting the area lying beneath the curve from the paper and weighing it on an accurate analytical balance — cut a piece of known area from the same paper, weigh it, and from these data calculate the area under the curve
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This area is proportional to the sterilizing efficiency of the process; express it in terms of equivalent time at 250 °F by drawing a rectangle whose area is equivalent to 1 min at 250 °F on the scales used for the lethality curve (for example, 5 min on the time scale with an L value of 0.2, or 10 min with an L value of 0.1) and determining its area
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Divide the area under the lethality curve by the area representing F = 1 to give the F value of the process — in the book’s example this gives F = 5.5 min
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The symbol F′ introduced by Ball designates the equivalent in minutes at 250 °F of the combined lethality of all time-temperature relationships at the point of slowest heating, represented by the heating and cooling curves for a product during the process — for instance, if the entire contents of a can could be heated instantly to 230 °F, held at that temperature for 36 min, and then instantly cooled, the process would be equal to 2.78 min at 250 °F and has an F value of 2.78 min (36 × 0.0774 = 2.79)
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To determine the process having a given F value: draw the cooling curves at different points as in the General Method, calculate the areas under these curves, find the F values by dividing with the area representing F = 1; plot the F values so obtained against the corresponding process time on linear graph paper, draw a line through the plotted points, and find the process length having the desired F value by interpolation — the precise value is that where the F value of the process equals the F value of the organism, i.e., 100% lethality
Calculation — Lethal rate
(T − 250)
L = antilog ———————————
z
Where,
- L = lethal rate — the time at 250 °F that is equivalent in sterilizing value to 1 min at some other temperature T
- T = any lethal temperature in °F
- z = number of °F required for the slope of the TDT curve to pass one log cycle
Note: the printed layout of this equation is garbled in the available copy; the form above is reconstructed from the book’s definitions and verified against its example (at 240 °F, z = 18: 1 min is equivalent to 0.278 min at 250 °F, as printed), and should be checked against the printed book.
Precautions
- The symbol F′ introduced by Ball designates the equivalent in minutes at 250 °F of the combined lethality of all time-temperature relationships at the point of slowest heating, represented by the heating and cooling curves for a product during the process.
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.
Ball, C.O., Ind. Eng. Chem., 35, 71 (1943); Schultz, O.T. & F.C.W. Olson, Food Res., 5, 399 (1940) — cited in the book.