The effect of oxygen content on flavour and chemical changes during aseptic storage of whole milk after ultra-high-temperature processing

Abstract

Summary Indirectly heated ultra-high-temperature (UHT) processed milk was prepared with initially high, medium, and low dissolved O 2 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 O 2 contents were preferred up to 8–13 d, but thereafter acceptability was independent of initial O 2 content. Sulphydryl group (–SH) contents rapidly decreased and O 2 levels correspondingly declined in the first few days as the flavour improved. Loss of –SH was lower with lower initial O 2 contents, and moderate –SH content remained in low O 2 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 O 2 content. After 20 d the FR values began to rise in high O 2 samples, but continued to decline slowly in low O 2 samples up to 90d although stale flavour was increasing. High initial O 2 contents resulted in rapid depletion of ascorbic acid and folic acid during storage. Losses of vitamin B 12 were small, but were higher with high O 2 contents than with low. The beneficial effect of O 2 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.