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Abstract
UHT processing of milk and its subsequent storage causes several changes which affect the shelf-life of UHT milk although it remains `commercially sterile'. These changes include whey protein denaturation, protein-protein interaction, lactose-protein interaction, isomerisation of lactose, Maillard browning, sulphydryl compound formation, formation of a range of carbonyl and other flavoursome compounds, and formation of insoluble substances. They ultimately reduce the quality and limit the shelf life of UHT milk through development of off-flavours, fat separation, age gelation and sedimentation.
The extent of these changes depends on many factors, a major one being the type of UHT heating. This review compares the effect heating milk by direct and indirect modes on various aspects of processing and quality of UHT milk.
Aust. J. Dairy Technol. 57, 211-227
Introduction
UHT processing of milk denotes a continuous heating process at temperatures higher than 130degC (usually 140-150'C) for a holding period of a few seconds (usually 2-10 s) followed by aseptic packaging to produce a `commercially sterile' product (Mehta 1980; Kosaric et al. 1981; Burton 1988). All microorganisms capable of growing under the normal conditions of storage of the product are destroyed and under these conditions (very high temperatures for a short time), chemical, physical and organoleptic changes are kept to a minimum. This is in contrast to in-container sterilisation processes which involve heating at lower temperatures for longer times (110-120degC for 30-15 min). The reason for the difference is illustrated in Table 1 showing the changes with temperature, for the same bactericidal effect, in the relative rates of heat-induced chemical changes and times for equivalent destruction of bacteria. These data are based on a Q^sub 10^^sup 1^ of 10 for spore destruction and 3 for chemical change (Burton et al. 1977). A graphical representation of the relative chemical effect versus temperature (see Figure 1) shows that, for a given bactericidal effect, less chemical change, and hence a better quality product, results when the heating process is as rapid as possible at an appropriately high temperature.
The time-temperature combinations used in UHT systems are determined by the need to inactivate heat-resistant bacterial endospores such as those of Bacillus stearothermophilus (Burton 1969), and the need to limit chemical changes which decrease the sensory and nutritional qualities of...