Additions of potassium iodate to milk at 0-05 and 0 -l DIM (10 and 20 ppm) before UHT treatment markedly reduced the rate at which pressure built up during processing. This permitted the use of longer processing times before unacceptable pressures were reached in the heat exchangers. Iodate reduced the amount of protein deposited, particularly in the higher temperature sections of the plant, but had no effect on the deposition of minerals. The more compact nature of the highly mineral deposits offered less resistance to the flow path. Reduction in the amount of protein deposited is likely to be caused by increased denaturation of /?-lactoglobulin and oxidation of heat activated sulphydryl groups by the iodate, thus reducing the formation of high molecular weight polymers of sulphur-containing proteins at the heated surfaces. Increasing the level of sulphydryl groups in the milk through the addition of L-cysteine-HCl caused an increase in the amount of deposit formed during UHT treatment. Whilst little detrimental effect on the quality of the milk resulted from additions of iodate at 005 DIM, milks with 01 mM-iodate became bitter during subsequent aseptic storage. Bitterness was a result of iodate-induced proteolysis of casein.The operating time of an indirect heat ultra-high-temperature (UHT) milk treatment plant is limited by the deposition of milk solids on the heated surfaces. Unacceptable pressures build up in the heat exchangers and the thermal efficiency of the plant is reduced (Burton, 1968). The deposits formed during the heating process have been examined by Ito, Sato & Suzuki (1962a, b) and by Lyster (1965). They found that most of the deposit formed in sections of the heat exchanger operating at temperatures of 100-105 °C and consisted largely of protein (50-60 %) and minerals (30-35%). In higher-temperature sections of the heat exchanger the deposit had an increased mineral content (70%) associated with less protein (15-20%). Small amounts of fat were present (4-8%) in deposits throughout the plant.Fore-warming of the milk at temperatures of 85-95 °C greatly reduced the rate at which deposit was formed, especially in the lower temperature sections of the UHT plant (Bell & Sanders, 1944;Lyster, 1965;Burton, 1966). This effect has been attributed to the denaturation of soluble proteins and the precipitation of Ca phosphate in the milk before it reaches a heated surface.

SummaryIndirectly heated ultra-high-temperature (UHT) processed milk was prepared with initially high, medium, and low dissolved O2 contents of 8·9, 3·6 and 1·0 ppm respectively, aseptically bottled, and tested at intervals during storage at room temperature for 150 d. Flavour acceptability increased to a maximum after a few days, but declined slowly after about 6 d; the increase was associated with less off-flavour described as ‘cabbagey’, and the decrease with more ‘stale’ off-flavour descriptions. Milks with higher initial O2 contents were preferred up to 8–13 d, but thereafter acceptability was independent of initial O2 content. Sulphydryl group (–SH) contents rapidly decreased and O2 levels correspondingly declined in the first few days as the flavour improved. Loss of –SH was lower with lower initial O2 contents, and moderate –SH content remained in low O2 samples for several weeks. Ferricyanide reducing (FR) values did not satisfactorily measure stale flavour development. They were initially high and decreased during the first 13 d at rates dependent on O2 content. After 20 d the FR values began to rise in high O2 samples, but continued to decline slowly in low O2 samples up to 90d although stale flavour was increasing.High initial O2 contents resulted in rapid depletion of ascorbic acid and folic acid during storage. Losses of vitamin B12 were small, but were higher with high O2 contents than with low.The beneficial effect of O2 on flavour, therefore, appears to be so slight and confined to such a short period in the early life of the milk as to be completely outweighed by the adverse nutritional effects.

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