The characteristics are given of a Chaddar cheese defect known as ‘tallowy discoloration’ or ‘white streak’. In the tallowy portions of the cheese the copper content is lower and the iron content higher than in the adjacent normal cheese. The copper probably migrates in cheese following oxidation of the sulphydryl compounds in the tallowy area while the unexpected distribution of iron may be explained by changes that occur during salting of the curd. The proportion of ferrous to ferric iron tends to increase as the cheese matures and tallowy portions of cheese do not contain less ferrous iron than adjacent portions.

The addition of iron salts to cheese milk did not increase the incidence of tallowy discoloration, but the addition of sufficient copper salts reduced it. Although the sulphydryl content of cheese appeared to be related to tallowy discoloration, little evidence of the part it plays was obtained. The oxidation of sulphydryl groups and the formation of peroxides occurred more readily at lower temperatures.

The significance is discussed of ferrous iron in a fat-oxidizing system where copper does not increase oxidation.