Natural variations in the average size of bovine casein micelles: II. Milk samples from creamery bulk silos in south west Scotland

Summary Samples of bulk silo milk from 5 creameries in the south west of Scotland were taken over a 16-month period. The average radii of casein micelles were determined by measuring the wavelength dependence of the turbidity of skim-milk diluted with its own ultrafiltrate. In addition, the concentrations of casein, soluble and colloidal Ca and P 1 were measured. The average size of casein micelles followed a pronounced seasonal trend with smaller average sizes in the summer compared to the winter period, confirming earlier observations made on milks from individual cows. The average size of casein micelles correlated positively ( r = +0·777, P < 0·001) with the amount of colloidal P 1 per unit weight of casein and negatively ( r = −0·77, P < 0·001) with casein-bound Ca. Average micelle radius was negatively correlated with heat stability at the natural pH of milk ( r = −0·61, P < 0·001), but there did not appear to be any causal relation between these 2 variables.