Profile of gelatinolytic capacity of raw goat milk and the implications for milk quality

Both endogenous and exogenous proteinases occur in milk, and they can have beneficial or detrimental effects on dairy production. Because the lactation length of dairy goats is shorter and the somatic cell count (SCC) of goat milk is generally greater compared with dairy cows, the objectives of the present study were to investigate the prevalence of major proteinases in raw goat milk, their association with SCC and production stage, and their effects on milk quality. Milk samples were collected from individual goats in consecutive weeks for different durations, covering regular lactation, late lactation, and post-milk stasis. Long-term (monthly) or short-term (weekly) fluctuations of milk fibrinolytic and gelatinolytic capacities of individual goats were revealed chronologically on fibrin and gelatin zymograms, respectively. In a separate trial involving milk samples from 23 goats at random production stages, the percentage of ultracentrifuge force-precipitable casein of total milk protein was calculated to represent milk quality and was assessed to evaluate its correlation with the corresponding proteolytic capacities. The results for regular milk indicate that gelatinase B was more abundant than gelatinase A when they first appeared at SCC of similar to 1 x 10(6)/mL. During the last month before milk stasis, both gelatinases A and B were found to be prevalent and prominent in milk regardless of the broad SCC range recorded there. Fibrinolytic activity and the active form of gelatinase A were only regularly detected in post-stasis secretions and were scarce before stasis. The results of the milk quality trial indicate that milk of relatively high proteinase capacity tended to have a low casein ratio. Correlation analysis confirmed a significant relationship between gelatinase capacity of goat milk and production stage, SCC, or casein ratio. It is suggested that an elevation of gelatinolytic capacity of goat milk coincides with an increase in somatic cell number accompanying the extension of lactation length, which is unfavorable for the production of a more desirable quality of goat milk.