SOP: Correction for Heating Lag in Thermal Inactivation Time Determination

GIFSQ — Global Institute for Food Safety & Quality

Principle

The thermal inactivation time (TIT) as measured needs correction for the delay in bringing the contents of the tube to the temperature of the water bath. The extent of inactivation during the come-up period is calculated by the graphical method of Bigelow et al., as developed by Nath and Ranganna: the inactivation rate (1/TIT) is plotted against time, and the area under this inactivation-rate curve during come-up is compared with the area for an instantaneous temperature rise. The ratio gives the effectiveness of the come-up time, from which the correction is subtracted.

Materials required

• Semi-log paper having 3 cycles (for the TIT curve)
• Coordinate graph paper (for the heat penetration and inactivation-rate curves)

Procedure

Plot the TIT as determined (without correction for the come-up time) on the semi-log paper: mark temperature on the linear scale and time on the log scale

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Plot the heat penetration into the tubes during the come-up time on a coordinate graph sheet

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Find the TIT corresponding to the various temperatures at time intervals from the TIT curve; calculate the inactivation rate using the expression 1/TIT, and plot the inactivation-rate curve

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Measure the area under the inactivation-rate curve (analogous to the lethality curve of microorganisms) QRSQ — this represents the extent of inactivation contributed during the come-up time; measure the area under OPRSO, which represents the total inactivation if the temperature rise had been instantaneous (for measuring the area, see the Improved General Method)

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Calculate the effectiveness and the corrected TIT using the expressions below

Calculation

  Area under QRSQ

Effectiveness = —————————————————
Area under OPRSO

Correction = Come-up time × (1 − effectiveness)

Corrected TIT = Experimental TIT − Correction

Where,

• QRSQ = area under the inactivation-rate curve during the come-up time
• OPRSO = area under the inactivation-rate curve for an instantaneous temperature rise
• Effectiveness = portion of the come-up time having lethal effect on the enzyme
• Come-up time = time required for the tube contents to reach the bath temperature
• Correction = time to be subtracted from the experimental TIT
• Corrected TIT = the actual thermal inactivation time

Note: the printed layout of the effectiveness fraction is garbled in the available copy; the form above is reconstructed from the book’s labels, and should be checked against the printed book.

Precautions

• Since the cooling is very fast and the inactivation contributed by it is negligible, it is not necessary to include cooling in calculating the correction.
• An example of the corrections at several temperatures is given in the book (Table 25-8).

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); Nath, N. & S. Ranganna, J. Food Sci., 42, 1306 (1977) — cited in the book.