The protein composition and digestion characteristics of goats’ milk were studied using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and in vitro simulated digestion. The results showed that goats’ milk proteins mainly included casein (αS1-casein, αS2-casein, β-casein and κ-casein, accounting for 23.10%, 30.39%, 38.09% and 8.42% of the total casein, respectively) and whey protein (α-lactalbumin, β-lactoglobulin, lactoferrin, serum albumin and immunoglobulin, accounting for 24.59%, 57.50%, 4.35%, 8.69% and 4.88% of the total whey protein, respectively). Casein was mainly digested in the intestinal juice, and the digestibility reached more than 99.00% after 120 min in the gastric juice, while it only took 2 min to reach more than 99.00% digestibility in the intestinal juice. The digestibility of lactoferrin, serum albumin and immunoglobulin in the gastric juice and intestinal juice was significantly higher than that of α-lactalbumin and β-lactoglobulin (P < 0.05), and α-lactalbumin and β-lactoglobulin were not easy to digest in the gastric and intestinal juice. The digestibility of α-lactalbumin and β-lactoglobulin was 67.53% and 50.82% in the gastric juice at 120 min, and 58.16% and 64.04% in the intestinal juice at 30 min, respectively. Gastrointestinal juice digestion also showed that casein in goats’ milk was more digestible than whey protein in goats’ milk.

The composition and digestion characteristics of proteins in donkeys’ milk and mares’ milk were investigated by sodium dodecyl sulfate polyacrylamide gel electropheresis (SDS-PAGE) analysis. The results showed that the digestibility of casein in donkeys’ milk and mares’ milk was 97.9% and 96.7% respectively after digestion in simulated gastric juice for 120 min, and was 98.6% and 97.7% respectively after digestion in simulated intestinal juice for 30 min. It took 30 min to almost completely digest proteins in both milks in simulated gastrointestinal juice, and mares’ milk proteins were easier to digest than donkeys’ milk proteins. Taken together, it was found that donkeys’ milk and mares’ milk proteins were mainly digested in simulated intestinal juice rather than simulated gastric juice and the latter was more digestible than the former.