Effect of time of sample collection, milk composition, and stage of lactation on the growth-promoting properties of milk for Streptococcus agalactiae.

Milk samples were taken from noninfected mammary quarters before the AM milking and at intervals after milking. The pH values of the post-AM milk samples were higher than the AM foremilk samples for approximately 3 to 6 hours after milking. When these samples were pasteurized, inoculated with a culture of Streptococcus agalactiae, and incubated at 37 C for 24 hours, there was a greater pH change (delta pH) in the post-AM milks. The higher pH and delta pH values of 3-hour samples coincided with higher concentrations of bicarbonate and chloride, lower concentrations of citrate and phosphate, and lower titratable acidities. Milk samples were taken before the AM milking and 3 to 4 hours later from 2 noninfected quarters of each of 2 cows at weekly intervals for 9 months. The growth of S agalactiae in the milk samples showed that the delta pH and bacterial counts (i) increased as the lactation progressed and milk yield decreased, (ii) were higher in he 3- to 4-hour samples than in the AM samples, and (iii) were higher in the whole milk than in the skim milk of the 3-to 4-hour samples; this difference was attributed to the cream layer and in most instances could be duplicated by layering liquid petrolatum on the surface of skim milk. Also, the bacterial counts in the 3- to 4-hour skim milks were higher than those of the corresponding AM skim milks in 78% of the paired samples, which indicated that another factor, in addition to the cream layer, might be involved in promoting better in vitro growth of S agalactiae in the 3 to 4-hour milk.