Thermal Food Processing

286 Thermal Food Processing: New Technologies and Quality Issues
through its effect on their physicochemical properties. 101 Several changes that can
occur in UHT milk during storage will be discussed in this section.
9.8.3.1 Gelation
Gelation during storage is a common problem of UHT milk, which ultimately
limits its shelf life. 87 A major initiating factor is partial proteolysis of the caseins,
by either plasmin or residual heat-resistant bacterial proteinases. Milk sterilized
by direct heating gels more rapidly during storage than milk treated by indirect
methods. 94 This effect appears to be due to the greater inactivation of the proteinases
and greater stabilization of the casein micelle by complexation with
denatured whey proteins during the more severe indirect heating. The addition
of sodium hexametaphosphate (0.1%) to raw milk before processing delays the
onset of gelation of UHT milk during storage. 101
9.8.3.2 Fat Separation
Despite homogenization of milk in the UHT process, a layer of fat occasionally
develops on the surface of the milk during storage. Less fat separation occurs in
milk processed with direct steam injection than with indirect heating, 102 due to
the additional homogenization effect of the steam injection.
9.8.3.3 Sedimentation
Sedimentation in UHT milk is due to destabilization of casein micelles. The
amount of sediment increases with the time and temperature of heating and with
the time and temperature of storage. 103 The pH of the milk is very important, as
severe sedimentation occurs at pH < 6.5. Sedimentation occurs more readily in
concentrated milk than in normal-strength milk, but less in reconstituted milk. 82
Goat’s milk is particularly susceptible to sedimentation, which has been attributed
to its high ionic calcium content. Sediment volume decreases with increasing
homogenization pressure and addition of chemicals such as trisodium citrate or
disodium hydrogen phosphate (0.025 to 0.1%) to raw milk. 104
9.9 IN-CONTAINER STERILIZATION OF MILK
AND CONCENTRATED MILK
In-container sterilization (e.g., in bottles, cans, or jars), as mentioned earlier, is
one of the oldest thermal processing strategies applied to milk, predating even
the work of Pasteur. Today, most processors producing long-life milk use UHT
systems, due to the advantages of high-throughput and continuous operation. In
addition, the long times for which products are exposed to high temperatures in
sterilization processes result in extensive deterioration in the quality of the products
(e.g., extensive development of brown colors due to Maillard reactions and
creation of strong cooked flavors due to production of volatile sulfydryl compounds
from β-lg). Typical treatment conditions used for in-container sterilization
processes for dairy products are 110 to 120°C for 10 to 20 min. 105