322 Produce Degradation: Reaction Pathways and their Prevention
loss of vitamins and minerals, formation of thermal reaction components, and,
especially, the loss of fresh appearance, flavor, and texture could occur (Kyzlink,
1990; Ohlsson, 2002).^ The rate of destruction is a first-order reaction, dependent on
the log of surviving microorganisms (or spores) and the time of heating at constant
temperature (death rate curve). The time needed to kill 90% of surviving microor-
ganism (1 log reduction) is decimal reduction time D. Destruction of microorganisms
is temperature-dependent. The empirical dependence of D on temperature is the
thermal death time curve (TDT). The slope z of the TDT curve is defined as the
number of degrees Celsius required to bring about a 10-fold change in decimal
reduction time. D and z values of selected microorganisms with similar data for the
thermodestruction of selected food components are given in Table 10.10. There are
two important consequences from the above principles (Kyzlink, 1990):^ (1) the
higher the number of microorganisms in raw material, the longer the duration of
heating necessary to reduce the contamination to a desired level, and (2) because of
the logarithm of concentration in the kinetic equation, it is theoretically possible to
destroy all present cells in infinite time.
The former consequence should be especially considered during processing.
Handling processes that improve conditions for the growth of microorganisms must
be eliminated. Efficient washing, disinfection, and other pretreatments leading to
the reduction of microbial counts can allow the use of less intensive heat treatment
with a lower negative effect on the processed matter.
The efficiency of heat treatment depends on various factors (Fellows, 2000b):^
- Contaminating microflora. Different species and strains differ in their heat
resistance. Molds, yeasts, and vegetative cells are relatively sensitive. They
are more resistant, but bacterial spores are more heat resistant than the
cells. - Incubation conditions. An increase of heat resistance can be developed
by selection and growing of resistant individuals surviving inside the
equipment due to insufficient sanitation treatment (e.g., resistant Lacto-
bacillus strains surviving in ketchup-producing equipment). Generally,
cells or spores produced or held at sublethal temperatures are more heat-
resistant. The heat resistance is also affected by the age of cultures^ (Bibek,
1996). - Conditions during heat treatment. Lowering the pH value increases the
efficiency of heat treatment and also the sensitivity of bacterial spores.
The effect of organic acids, including chemical preservatives such as
sorbic and benzoic acids, on the heat resistance of ascospores of Neosar-
toria fisheri has been studied by Rajashekhara et al. (2000). Palop et al.
(1999) described the effect of pH on heat resistance of Bacillus coagulans
spores in various food items. Lowering of water activity aw by addition
of osmoactive compounds or fat generally increases the heat resistance
of microrganisms. Food preservatives can increase the sensitivity of
microorganisms to heat treatment.