Differences in Protein Profiles of Kefir Grains from Different Origins When Subcultured in Goat Milk.

Proteins not only serve as a nitrogen source for microorganisms but are the main skeleton of kefir grains. After subculturing in goat milk for 4 months, proteins and peptides in three kefir grains from China, Germany, and the United States were analyzed. Except for the S-layer protein from special Lactobacillus sp. , α s1 -casein, α s2 -casein, and β-casein from goat milk were found in kefir grains. These proteins could form aggregates through a covalent interaction with polysaccharides to maintain the morphological stability of the grains. Furthermore, they were highly related to the microbiota in kefir grains. Additionally, a number of hydrophilic/hydrophobic peptides that were hydrolyzed by extracellular proteases were found from kefir grains. A correlation may exist between peptides and Lactobacillus sp . in kefir grains. Bioactive peptides, including DKIHPF, LGPVRGPFP, and QEPVLGPVRGPFP, were found from these kefir grains. The results indicated that goat milk as a substrate affects the protein and peptide composition of kefir grains.