Principles of thermal processing Thermal degradation kinetics

Both the thermal death of microorganisms and thermal degradation of biochemical components of food have been found to obey first order chemical reaction kinetics (with a few exceptions). At constant temperature, therefore, the reaction rate is given by

- — = k.N and - — = k.C [22.1] dt dt where N is number of organisms, C is concentration of biochemical, k is the reaction rate constant and t is time.

In thermal sterilisation technology, equation 22.1 can be rewritten as:

— = 10->D and — = 10 [22.2] No Co where N and C are the number of microorganisms and concentration of biochemical respectively at time t, N0 and C0 are the initial number and concentration and D is the decimal reduction time. The decimal reduction time is the time taken to reduce the number of microorganisms or concentration of biochemical by a factor of 10 (i.e. to a value 1710th of that initially). This decimal reduction time is constant, i.e. for any time interval D, there will be a reduction to one-tenth.

When quantifying the effect of temperature change on the D value, there are two major models used: the traditional 'Canning' (constant-z) model and the Arrhenius model. The former is:

where Dj and D2 are the decimal reduction times at 91 and 92 respectively. The z value is the temperature change required to change the decimal reduction time by a factor of 10.

The Arrhenius equation is:

where A is a constant, the frequency factor, Ea is the activation energy, R is the gas constant and 9k is the absolute temperature.

These two models are actually mutually exclusive but will agree within experimental error over a relatively short temperature range, which is usually the case for death of microorganisms. For larger temperature ranges, which is more usual for biochemical degradations, the Arrhenius relationship has a better theoretical basis and is generally used.

Research work which reports thermal death of different strains of microorganisms or degradation of biochemical components usually gives D and z values at a defined reference temperature (often 121.1°C) or the activation energy Ea and frequency factor, A. There are tables of data given in Holdsworth (1992, 1997) and Karmas and Harris (1988) for microorganisms and biochemical components.

22.2.2 Effect of change in temperature

An examination of the kinetics for microbial death and for degradation of biochemical components shows that the z values for the former are in the region of 10°C, while for the latter they are around 30°C. This difference is the basis of UHT processing: by increasing the temperature of a foodstuff, the microbial death rate increases much faster than biochemical degradation and, for equal levels of sterilisation, higher temperatures will give a better nutritional and organoleptic quality food than lower temperatures. One major disadvantage of this is that some enzymes may survive, especially heat-resistant proteases and lipases. It is important, therefore, that as high a process temperature be attained as is feasible and this is usually in the region of 137°C to 147°C.

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