Thermal Food Processing

Thermal Processing of Dairy Products 281
and sparked research activity in several laboratories. 76 Progress has been slow
because of difficulties in growing the organism; it is extremely slow growing
(taking up to 4 months to culture for counting purposes 77 ), and hence can be
overgrown by any organisms whose growth is not inhibited during incubation.
Trials to determine the resistance of this bacterium to pasteurization have yielded
mixed results, with some laboratories reporting complete destruction under
simulated commercial conditions, while others have reported survival of some
organisms in some trials. 47,78–80 Inactivation during pasteurization appears to
depend on the strain of the organism and the number of organisms present. Some
strains can be inactivated completely by classical HTST conditions (72°C for
15 sec) at levels of 100 cfu/ml, but small numbers (4 to 16 cfu/ml) have been
shown to survive higher-temperature (90°C for 15 sec) pasteurization of milk
with an initial high (10 6 cfu/ml) MAP count. 47
Increasing the holding time during pasteurization processes has been found
to be more effective in inactivating MAP in milk than increasing the pasteurization
temperature. Homogenization before pasteurization has been shown to increase
the lethality of the heat treatment on MAP. These findings suggest that heat
penetration is more important than the intensity of heat applied; this may be
related to the tendency of the organism to form clumps. 79
9.8 UHT PROCESSING OF MILK
9.8.1 TECHNOLOGY OF UHT AND EFFECTS ON MILK
CONSTITUENTS AND STABILITY
UHT processing of milk involves heating milk in a continuous process to temperatures
higher than 135°C for a few seconds, cooling rapidly, and aseptically
packaging the milk into sterile containers. 81,82 The sequence of steps in UHT
processing is shown in Figure 9.3.
UHT treatment must be sufficient to produce a commercially sterile product
(one in which bacterial growth will not occur under normal storage conditions),
but not severe enough to cause chemical changes that result in an unacceptable
flavor, color, and nutritive loss. In general, the heating should be equivalent to
a minimum of 9 log reduction of thermophilic spores (sometimes referred to
as a B* value, >1) and a maximum reduction of 3% in the level of thiamine
(sometimes referred to as a C* value,