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

Abstract

Oxidized flavour developed in whole milk only through the catalytic effect of either Cu or light. The O 2 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 O 2 loss, and could be prevented in stored milk by restricting access of O 2 . In UHT milk with a dissolved O 2 content of 66 mg/1, and in the absence of access of further O 2 , light-induced oxidized flavour did not develop; similarly, O 2 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 O 2 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 O 2 occurred, even during prolonged storage at 5 °C, despite the presence of large O 2 concentrations. However, at 20 °C, a small consumption of O 2 was measured, and this was associated with stale flavour.

Changes in flavour volatiles of whole pasteurized milk as affected by packaging material and storage time

“…The white (TiO 2 ) pigmented bottles were chosen on the basis of consumer preference (White, 1985) with regard to colour and their potential protective effect against harmful incident light. The carbon black pigment has been suggested (Schro¨der, 1982) for maximum protection against UV-VIS radiation, with both pigments being approved by the Food and Drug Administration, USA. Bottle (4) containing inner and outer white coloured layers was white in colour.…”

“…Part of the explanation for this may be the relatively large HS (containing air) which was similar in all packages (50-60 mL). Schro¨der (1982) reported that even a small HS in a container provides a considerable O 2 reservoir relative to the quantity dissolved in milk. In a container with a 15% HS (a common proportion for a commercial milk carton), the ratio between dissolved and gaseous O 2 in milk at 10 1C is approximately 1:5.…”

“…Packaging can directly prevent the development of light-induced off-flavour by protecting the product from both light and oxygen (Schro¨der, 1982;Skibsted, 2000;Vassila, Badeka, Kondyli, Savvaidis, & Kontominas, 2002). Apart from traditional glass bottles and coated paperboard cartons, all-plastic containers have been used for pasteurized milk packaging (Defosse, 2000;Schro¨der, Scott, Bland, & Bishop, 1985).…”

Deterioration of pasteurized milk on storage

“…This problem is now generally well recognized, but even recently indirect contamination has been found through residues of cleaning/disinfectant solutions in processing plants (Van Duin, 1974), or corrosive water which dissolved copper out of water heaters (Bertelsen & Narlid, 1978). Schroder (1982a) found that Cu 2+ -induced oxidized flavour was not directly related to the O 2 concentration of Cu 2+ -adulterated pasteurized milk. Although complete deoxygenation prevented formation of the off flavour, partial deoxygenation produced a greater intensity of off flavour than no deoxygenation.…”

Storage Stability of Milkfat Globule Membrane

“…Since xanthine oxidase is abundant in the MFGMs, it is possible that initiation of lipid peroxidation could be caused by this enzyme (McPherson and Kitchen 1983). It is also important to note that milk normally becomes saturated with 0, between milking and processing, and that this oxygen is not subsequently removed (Schroder 1982).…”