Indirectly 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.