SOP: Construction of Lethal-Rate Paper and Calculation of Sterilizing Value

GIFSQ — Global Institute for Food Safety & Quality

Principle

Schultz and Olson’s special coordinate (lethal-rate) paper removes the need to calculate the TDT at every temperature: the temperature scale is replaced by lethal-rate spacing, so the lethality curve is plotted directly from the heat penetration data, and the F value of the process is calculated from the area under the curve with a single equation. A different paper must be constructed for each z value employed.

Materials required

• Lethal-rate (L) values for the processing range (from the book’s Table 25-17 for z = 18, or Table 25-18 for other z values)
• Plain paper, ruler

Procedure — Construction of the paper

Draw on a sheet of paper two horizontal lines a unit distance apart; consider the bottom line as the zero line and mark the top line as 250 °F (or the maximum temperature likely to be obtained in the processing range)

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For every degree of temperature in the processing range, calculate the L value and draw a horizontal line at a distance of L units above the zero line; number these lines not with the values of L, but with the corresponding temperatures; draw vertical lines in the linear scale for the time

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If the highest temperature reached in the heat penetration test is much lower than 250 °F, the curve would occupy too narrow a band to measure — adjust the top line to the nearest z-value unit above the maximum observed temperature (for example, with z = 18, number the top line 232 instead of 250, or 268, 214, etc.)

Procedure — Calculation of the F value

Mark the points corresponding to the time-temperature points of the heat penetration data on the lethal-rate paper; draw a smooth curve connecting these points

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Measure the area under the curve by any convenient means (as described under the Improved General Method)

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Calculate the F value of the process from the area using the equation below

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To determine the processing time having a given F value: draw cooling curves at different points, calculate the areas under these curves, find the F values from the formula, and by graphical interpolation find the process time equivalent to the desired F value

Calculation

The F value of the process is calculated from the area under the lethality curve using the book’s equation involving the quantities defined below.

Note: the printed layout of this equation is illegible in the available copy — only the symbol definitions are recoverable, so the assembled equation is withheld here. It must be verified against the printed book before use; the retort-temperature/initial-temperature conversion formulas from the same section are likewise omitted for the same reason.

Where,

• v = number of minutes represented by one inch on the time scale
• A = area under the curve in square inches
• n = number of changes in z units that must be made to change the number of the top line from 250 to the desired number (n is positive when the top line is numbered less than 250, and negative when numbered more than 250)
• d = number of inches from the bottom line (zero line) to the top line

Precautions

• A different lethal-rate paper must be constructed for each different z value employed.

References

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

Schultz, O.T. & F.C.W. Olson, Food Res., 5, 399 (1940) — cited in the book.