Particles as large as several /an diam. have been observed occasionally in normal milk and commonly in prepartum and postpartum colostrum. These particles can be dissociated by EDTA and their appearance closely resembles that of normal casein micelles. However, they are often too large to have been completely formed within the Golgi vesicles of mammary epithelium and hence some degree of postsecretory aggregation of caseins is thought to occur. Two possible mechanisms of post-secretory aggregation of caseins are: (1) a continuation of the normal processes of micelle assembly in the alveolus and (2) aggregation as a result of limited proteolysis of the caseins during the time the milk is in the mammary gland. Incubation of milk with fibrinolysin, however, failed to produce aggregation of normal micelles.Large natural variations in the average size of bovine casein micelles have been observed in normal milks. Differences in average micelle size of the order of a factor of 2 have been found in the milks of different cows and in the milk of the same cow at different times (Holt, 1975;Holt & Baird, 1978). A study of bulked milks from creamery silos in south-west Scotland revealed a marked seasonal trend in average micelle size with smaller particles in the summer months than during the rest of the year (Holt & Muir, 1978). These investigations were conducted using lightscattering methods which are most sensitive to changes in the proportion of large micelles in the size-distribution curve. The size distribution of casein micelles in 2 normal milks was determined by electron microscopy using 2 different methods of specimen preparation and compared with the average size found by 2 different light-scattering methods . There was a pronounced large-particletail-region in the size-distribution curve with small numbers of particles having diam. up to 600 nm. It can readily be shown that a small change in the number fraction of these large particles will have a profound effect on the average size as measured by light-scattering methods. It is possible therefore that post-secretory aggregation of caseins can account for the large natural variations in the average size of casein micelles. This phenomenon can be demonstrated more clearly in the prepartum mammary gland.
MATERIALS AND METHODSFull details of experimental methods and the collection of milk, colostrum, prepartum colostrum and mammary tissue have been described elsewhere (Brooker,
