From Equation (4.4), it is clear that a plot of the log of the number of
surviving cells at a given temperature against time should give a straight
line with negative slope,k(Figure 4.1). As the temperature increases, so
the slope of the survivor curve increases.
From this relationship we can derive a measure of an organism’s heat
resistance that is useful in calculating the lethality of heat processes. The
D value or decimal reduction time is definedas the time at a given
temperature for the surviving population to be reduced by 1 log cycle, i.e.
90%. The temperature at which a D value applies is indicated by a
subscript,e.g.D 65.
A D value can be obtained from a plot of log 10 survivorsversustime,
where it is the reciprocal of the slope, 1/k. [You can confirm this by
substitutingN 0 ¼ 10 Nandt¼Din Equation (4.4).]
Alternatively, it can be calculated from:
D¼ðt 2 t 1 Þ=ðlogN 1 logN 2 Þð 4 : 5 ÞwhereN 1 andN 2 are survivors at timest 1 andt 2 respectively.
One consequence of Equation (4.5) is that one can never predict with
certainty how many decimal reductions a heat process must achieve (its
lethality) for a product to be sterile since there is no logN 2 forN 2 ¼0.
When the initial microbial population in a batch of product is 10nand a
heat process producingndecimal reductions (nD) is applied, there will be
one surviving organism in the product (log 1¼0). If you apply a more
severe heat process, (nþ1)D, (nþ2)D or (nþ4)D say, then the number
of survivors will be 10^1 ,10^2 or 10^4 respectively. Physically, it is
meaningless to talk of a fraction of an organism surviving, so these
figures are interpreted as a probability of survival, corresponding to there
being a 1 in 10, 1 in 100 and a 1 in 10 000 chance respectively that an
organism will survive the heat process.
To give an example: if the D 72 ofSalmonellaSenftenberg 775W (the
most heat-resistant salmonella) in milk is 1.5 s, then HTST pasteurization
(15 s at 72 1 C) will produce a 10D reduction in viable numbers. If we
Figure 4.1 The D value
Chapter 4 67