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SOP: Calculation of Sterilizing Value by the Equal Time Interval Procedure

GIFSQ — Global Institute for Food Safety & Quality

Principle

Patashnik’s simplification of the General Method removes the need to plot the lethality curve once the z value is known: heat penetration readings are taken at equal time intervals, the L value for each reading is taken from the tables, and the integration under the lethality curve is accomplished by simple addition of the lethality values.

Materials required

  • Heat penetration data read at equal time intervals
  • Table 25-17 of the book (lethal rates when z = 18) or Table 25-18 (lethal rates for other z values)

Procedure

Take the heat penetration readings at equal time intervals

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Corresponding to each equal time interval temperature reading, note the L value from Table 25-17 or 25-18 for the required z value

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Enter the data in columns: time in the first column, temperature in the second column, and the L value corresponding to the temperature of column 2 in the third column

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Add the L values in column 3 and enter the sum in column 4

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Multiply the summed L value in column 4 by the time interval used for noting the container temperature readings during heat penetration — the product gives the F value of the process

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In the book’s example, a time interval of 4 min is used and ΣL = 0.105, giving F = 0.105 × 4 = 0.42 min

Precautions

  • Deviation with increase in time interval is not considerable, but using shorter time intervals gives a considerably higher degree of accuracy.
  • The method holds good where heat transfer is primarily by convection.
  • By this method it is possible to note the L value at each temperature during heating and cooling and to calculate the F value of the process almost immediately thereafter; Pflug and Esselen have shown that the F value so obtained is almost the same as that obtained by the graphical method.

References

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

Patashnik, M., Food Technol. (Chicago), 7, 1 (1953); Pflug, I.J. & W.B. Esselen, Food Processing Operations, Eds. J.L. Heid & M.A. Joslyn, The AVI Publishing Co., Westport, USA (1963) — cited in the book.