SOP: Determination of Decimal Reduction Time (D Value)

GIFSQ — Global Institute for Food Safety & Quality

Principle

The death rate of microorganisms when exposed to heat follows a logarithmic course: equal percentages of the surviving cells die in each successive unit of time. The decimal reduction time D is the time required to reduce the bacterial population by 90% at a given temperature. Unlike the TDT, which uses only the two final points of each experiment, the D value is measured at each temperature from the proportion of surviving spores as a function of heating time, and is independent of the initial number of cells.

Materials required

• Semi-log graph paper (linear scale for time, logarithmic scale for survivors)
• Heating data from TDT tube or can runs carried out so that the number of surviving spores can be evaluated after each heating period

Procedure

Carry out the heating studies so that the number of surviving spores can be evaluated — after each heating, count the number of survivors either by subculturing on a solid medium or by dilutions

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If the medium is not suitable for counting by subculture (as with solid products), incubate the tubes or cans directly and take the number of containers showing survival at the longest time of heating as equivalent to the number of surviving organisms (assume each positive tube contains one surviving spore)

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Plot the duration of heating on the linear scale and the number of survivors on the logarithmic scale; draw a straight line to fit the points obtained

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Read the D value from the diagram — it is given by the slope of the line — or calculate it using the formula below

Calculation

  U

D = —————————
log a − log b

Where,

• D = decimal reduction time (death rate) in min
• U = heating time in min
• a = initial number of microorganisms
• b = number of microorganisms which survived the heating time U

Note: the printed layout of this formula is garbled in the available copy; the form above is reconstructed from the book’s own symbol definitions and the worked example in Table 25-6, and should be checked against the printed book.

From the D values at different temperatures, the z value is deduced by plotting D against temperature (see the SOP on plotting thermal resistance curves). The book’s worked values (Table 25-5) are D = 19.65, 10.47, 4.92, 2.56 and 1.23 min at 230, 235, 240, 245 and 250 °F respectively.

Precautions

• The D value depends on the conditions under which the organisms are produced, the medium in which they are heated, and the temperature at which it is determined — but not upon the initial number of cells.
• For direct incubation: initially each tube contained 1,000 spores in 2 ml, and each positive tube is assumed to contain one surviving spore, so the number of positive tubes equals the number of surviving spores out of the 10,000 initially present.

References

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

Rahn, O., Bact. Rev., 9, 1 (1945) — cited in the book.