Effect of oxygen on the keeping quality of milk: I. Oxidized flavour development and oxygen uptake in milk in relation to oxygen availability

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Abstract
SUMMARY: Oxidized flavour developed in whole milk only through the catalytic effect of either Cu or light. The O2 requirement for the 2 processes differed as did the characteristics of the off-flavours produced. Cu-induced oxidized flavour was described as ‘cardboardy’ and light-induced oxidized flavour was ‘painty’. Light-induced oxidized flavour increased in intensity with O2 loss, and could be prevented in stored milk by restricting access of O2. In UHT milk with a dissolved O2 content of 6·6 mg/1, and in the absence of access of further O2, light-induced oxidized flavour did not develop; similarly, O2 uptake of 7·5 mg/1 in in-bottle sterilized milk exposed to fluorescent light did not result in flavour formation. When light-induced oxidized flavour developed consistently in whole milk none developed in skim-milk, indicating the lipid source of the flavour. In contrast Cu-induced oxidized flavour development was not associated with high O2 uptake. Although nearly complete deoxygenation of whole pasteurized milk contaminated with Cu prevented the formation of the flavour, moderate deoxygenation resulted in even greater flavour intensity than non-deoxygenation. The 2 oxidized flavours also differed in relation to ascorbic acid (AA) oxidation. Light-induced oxidized flavour developed only after AA oxidation was complete, whereas Cu-induced flavour developed with AA still present. AA oxidation was greatly accelerated through the effects of both Cu and light. In milk free from Cu contamination and protected from light, after AA oxidation (plus SH group oxidation in the case of UHT milk) was complete, no further loss of O2 occurred, even during prolonged storage at 5°C, despite the presence of large O2 concentrations. However, at 20°C, a small consumption of O2 was measured, and this was associated with stale flavour.