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Studies on the heat stability of milk protein: II. Effect of exposing milk to light

Published online by Cambridge University Press:  01 June 2009

A. W. M. Sweetsur  and
J. C. D. White
A. W. M. Sweetsur
Affiliation:
The Hannah Research Institute, Ayr, Scotland, KA6 5HL
J. C. D. White
Affiliation:
The Hannah Research Institute, Ayr, Scotland, KA6 5HL
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Summary

It was found that the increase in heat stability (coagulation time) that occurs when some milks are stored in light was not caused by changes in pH. These labile milks invariably gave a type A heat-stability curve, had a pH in the range corresponding to the coagulation-time minimum of the curve, and showed the expected abnormal 2-stage type of coagulation. The increase in coagulation time following exposure to light resulted from a delay in the occurrence of the first coagulation, the second coagulation time remaining unaltered. The presence of riboflavin in milk and of O2 in the headspace atmosphere during storage were necessary conditions for milk to be labile. Since light, headspace O2 and riboflavin had similar influences on rennet coagulation time it is suggested that in both instances the causative reaction, as proposed by others to account for the increase in rennet coagulation time of H2O2-treated milk, is stabilization of the heat- and rennin-sensitive methionine-phenylalaninebond in κ-casein by photo-oxidation of the methionine to methionine sulphoxide.

Type
Research Article
Information
Journal of Dairy Research , Volume 42 , Issue 1 , February 1975 , pp. 57 - 71
Copyright
Copyright © Proprietors of Journal of Dairy Research 1975

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References

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