SOP: Estimation of Process Lethality by the Count-Reduction Method

GIFSQ — Global Institute for Food Safety & Quality

Principle

In the count-reduction system proposed by Yawger, cans inoculated with known concentrations of microorganisms are removed from the processing line at different intervals and subcultured to determine the number of surviving bacteria. The data so obtained are used to calculate the lethality of the process — without requiring any heat penetration data. The same principle can be applied to determine the residual enzyme concentration and the chemical degradation taking place during the process.

Apparatus required

• Retort or processing line from which cans can be withdrawn at intervals
• Subculturing equipment and media for the test organism

Reagents

a. Inoculum of the test microorganism at known concentrations.

Procedure

Inoculate the cans with known concentrations of the microorganisms

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Take the inoculated cans out from the processing line at different intervals of processing time

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Subculture the contents of each withdrawn can and determine the number of surviving bacteria

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From the survivor counts at the different intervals, calculate the lethality of the process

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The same principle may be applied to determine the concentration of residual enzyme and the chemical degradation taking place during the process

Precautions

• This method is specially suitable for evolving a thermal process by flame sterilization, where it is difficult to get reliable heat penetration data.

References

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

Yawger, cited by Leonard, S., G.L. Marsh, R.L. Merson, G.K. York, J.R. Heil, S. Fryer, T. Wolcott & A. Ansar, J. Food Sci., 40, 250 (1975) — cited in the book.