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.

When heat treatments of the same sporicidal effectiveness were given, directly heated ultra-high-temperature sterilized milk gave twice as much sediment as indirectly heated milk after storage at room temperature for 100 days. Both types of process reduced the rate of clotting of the milk with pepsin and rennin, but the effect of the indirect process was markedly greater than that of the direct process.

An experimental ultra-high-temperature milk sterilizer, of 1140 1/h capacity and capable of operation as a plate-type indirect heating plant or as a steaminto-milk direct heating plant, has been installed for comparisons of the milk produced by the direct and indirect processes. The sterilizer and its ancillary equipment are described, together with methods of plant sterilization and milk processing. Time-temperature profiles of the plant are given for both modes of operation.Methods for the ultra-high-temperature (UHT) sterilization of milk fall into 2 general groups, the direct and the indirect. In the direct method, milk is heated to the final sterilizing temperature by mixing it with steam: the condensate produced is then removed during the cooling stage when the hot milk is injected into a vacuum chamber for expansion cooling. With the indirect method, heat is transferred from the heating medium, which may be steam or pressurized hot water, to the milk through a conducting wall so that the heating and heated fluids never come into contact.Plants employing these alternative methods have specific practical advantages and disadvantages. However, it has also been widely claimed that the milk produced by direct UHT processes is of better quality than that produced by indirect processes in that there is less change in the organoleptic and nutritional properties of the milk. There are theoretical reasons for supposing that the direct process might have these advantages, because the high rates of heat transfer obtainable by mixing with steam and by evaporative cooling allow higher milk processing temperatures for shorter effective processing times. These higher temperatures and shorter times in turn increase the sporicidal effect of the process relative to the chemical change produced (Burton, 1965).It has never been determined conclusively to what extent, if at all, this theoretical benefit is realized in practice. True comparisons between milks processed on different plants have not been possible, even when the plants have been installed in the same country. Analytical methods have differed, and organoleptic comparisons are not valid unless they are made by the same taste panel. Even where a single group of workers has used the same methods for the examination of milk from different UHT

SummaryA comparison was made of the effects of direct and of indirect ultra-high-temperature (UHT) processing of milk, under standardized operating conditions giving equal sporicidal effects, on some of the more labile water-soluble vitamins and on vitamin A and carotene. The effects of processing per se were negligibly small, and the method of processing was important only in so far as the presence of residual oxygen in the sterilized milk has been found to cause losses of folic acid and ascorbic acid during storage subsequent to sterilization. The incorporation of a de-aerator vessel, to reduce the oxygen level in the indirectly heated milk and so eliminate the adverse effects of oxygen during storage, had no effect on the vitamin loss occurring during heat treatment. It is concluded that milk produced by indirect heating plant incorporating a de-aerator should be similar in vitamin content to milk produced on a direct heating plant, both immediately after processing and after storage.