SOP: Evaluation of Thermal Process by the General Method

GIFSQ — Global Institute for Food Safety & Quality

Principle

The General Method (also called the Graphical Method), developed by Bigelow et al., integrates graphically the lethal effects of all the time-temperature relationships existing at the cold point during heating and cooling. The rate of destruction of an organism per minute at any temperature is the reciprocal of the time in minutes required to destroy the organism at that temperature. Each temperature on the heat penetration curve is given a lethal rate; the area under the lethality curve is the lethality of the process.

Materials required

• Heat penetration data (cold-point temperatures at recorded times)
• TDT curve of the most heat-resistant spoilage organism likely to be encountered (TDT known at all temperatures attained during the process)
• Coordinate paper for plotting the lethality curve

Procedure

Enter the heat penetration data systematically from the heat penetration curve: column 1 = time, column 2 = temperature

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From the TDT curve, note the TDT corresponding to each temperature in column 2 and enter it in column 3; find the lethal rates by taking the reciprocals of the TDTs (lethal rate = 1/TDT) and enter in column 4

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Plot the data of columns 1 and 4 on coordinate paper and join the points by a smooth curve to form the lethality curve

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Since the product of lethal rate and time equals lethality, express the area below the curve directly in units of lethality; define a unit sterilization area as the area on the lethality curve that just represents complete sterilization (in the book’s example each side of a square represents a lethality of 0.01 and a time of 5 min, so one small square represents a lethality of 0.05, and an area of 20 squares equals the unit sterilization area)

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If the area under the lethality curve is less than the unit sterilization area, the process is inadequate; if more, it is greater than needed

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To determine the process time for unit lethality: shift the cooling portion of the lethality curve to the right or left to give an area equal to 1 (trial and error); alternatively, carry out three or more heat penetration runs for different process periods, plot the lethality curve and find the area in each case, then plot lethality against process time and read the actual time required

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In the book’s example, the area under the lethality curve for a 47-min process was 38.4 squares (192% of the unit sterilization area); cooling curves drawn starting at 25 and 35 min gave areas of 42% and 97% of the unit area, and graphical interpolation showed that processing for 35.5 min gives complete sterilization

Calculation — TDT at any temperature

  250 − T

log(t/F) = ———————
z

or

            (250 − T)

t = F × antilog ———————————
z

Where,

• t = time required to destroy the organism at temperature T (°F)
• T = temperature whose TDT is required
• F = time in minutes required to destroy the organism at 250 °F
• z = slope of the TDT curve in °F

Note: the printed layout of this derivation is garbled in the available copy; the form above is reconstructed from the book’s definitions and verified against its worked example (TDT of Cl. botulinum at 250 °F = 2.78 min, z = 18, gives log(t/2.78) = 10/18, hence TDT at 240 °F = 9.99 ≈ 10 min, as printed), and should be checked against the printed book.

Precautions

• The method is long and laborious and not well suited for routine calculation, but it is simple, versatile, and especially valuable when the heat penetration curve on semi-log paper cannot be represented by one or two straight lines.
• The TDT at every temperature must be calculated; the method cannot readily be used for lethalities based on initial temperature, retort temperature or container size other than those used in the particular heat penetration test (a formula of Schultz and Olson is available for this purpose).

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.

Bigelow, W.D., G.S. Bohart, A.C. Richardson & C.O. Ball, Nat. Canners’ Ass. Bull., 16-L (1920); Schultz, O.T. & F.C.W. Olson, Food Res., 5, 399 (1940) — cited in the book.