Reviews of the progress of dairy science Thompson (30). Simple rapid methods for the measurement of the yellow colour of milk by means of a 'lactochrometer'(3i) and of carotene in butter fat by visual comparison with potassium bichromate standards (32) have been described. Effern(33) has applied the reaction of Rosenthal & Erdely(34) to the photometric estimation of vitamin A in butter, but the value of this method is not proven and the calculations are open to criticism.In previous reviews (35,36,37) papers dealing with the apparently abnormal behaviour of vitamin A and carotene in butter were discussed, but evidence is now growing that the vitamin A . activity of milk or butter calculated from proper chemical and physical measurements of vitamin A and carotene agrees with results of biological assay (25,38). The recent results of Fraps et al. {39) correcting their earlier findings also point in the same direction, while Shrewsbury & Kraybill(40) now report that carotene dissolved in butter fat devoid of vitamin A activity has the same availability for rats as when dissolved in cotton-seed or maize oil. On the other hand, Koehn(27,28) is still of the opinion that the factor for converting colorimetric readings into international units of vitamin A is appreciably higher for milk than for cod-liver oil.Normally the fat-soluble pigment of milk consists mostly of /? carotene and the ratio of 94 % j8 carotene and 6 % xanthophylls put forward by Gillam(4i) has been generally accepted. It has been known, however, for some time that the exact proportions vary according to the composition of the carotenoids in the fodder of the cow(37). More recent work in which the analysis was done by chromatography instead of by phase separation indicates that even in normal milk the contribution of /3 carotene may be smaller than hitherto believed, while special feeding of the cow may result in the appearance in the milk of much inactive carotenoid. Thus Johnson et al. (42,43) found in the butter of cows receiving molasses lucerne silage only 68 % of the total carotenoids in the form of /} carotene, the rest consisting of xanthophylls, 24 %, and 'acidformed' pigments(44), 8 %. Phosphoric acid lucerne silage yielded 65 % ft carotene and a higher proportion, 12 %, of pigments formed by the action of acid. Even pasture butter had 20 % of xanthophyllic pigments, but artefacts were absent and carotene amounted to 80 % (42,43). Similar values for pasture butters are reported by Thompson et aZ.(45) who also observed that Guernsey cows are less selective than Shorthorns and pass into their milk under comparable conditions a larger proportion of inactive carotenoids. This is doubtless another expression of the known peculiarities of carotenoid metabolism of the Channel Island cattle. There are further examples of these peculiarities. Thus, Gillam & Kon(46) showed that the Shorthorn cow effectively excludes lycopene, an isomer of carotene, from her milk, even when large quantities of it are fed in the form of tomato puree. Under similar...
