Ultra-high-temperature (UHT)-treated milks containing a range of dissolved O2 levels were prepared using both direct and indirect UHT processes. The O2 content of directly processed UHT milk had a significant influence on the changes in the level of cooked flavour in the product during storage over a 12-week period. At storage temperatures of 2 and 20°C, a noticeable improvement in flavour could be achieved by O2 control. In general, samples with a head-space sufficient to yield an initial of 60–100 mm were most preferred. The effect of O2 control on samples stored at 38°C was only minor when compared with the marked decrease in acceptability of flavour occurring due to the high storage temperature.

Directly processed samples stored at 20°C all gelled at 14 weeks while those stored at 2°C showed no evidence of gelation after 36 weeks. Storage at 38°C gave rapid flavour deterioration, bitterness at 7 weeks and gelation at 10 weeks.

All indirectly processed samples were found to be severely cooked under the conditions employed, and any differences in flavour due to differing O2 contents were masked by the intensity of this flavour.