Hypothermia protection effect of antifreeze peptides from pigskin collagen on freeze-dried Streptococcus thermophiles and its possible action mechanism

Antifreeze peptides (APP) obtained from pigskin collagen hydrolyzates, with molecular mass distribution among 150-2000 Da, rich in glycine (23%), proline (15%), hydroxyproline (13%) and alanine (8%), showed a high hypothermia protection activity on lactic acid bacteria during freeze-drying process. In this study, we investigated the cryoprotective activity on Streptococcus thermophiles in the presence of APP. The optimum condition for APP to protect the cells was 1 mg/mL under pH 9.0. When the S. thermophiles were dried together with 1 mg/mL of APP, 71% of organism cells survived. The intracellular proteins, nucleic acid and the key enzyme activities were determined to elucidate the possible action mechanism, and the results showed that the intracellular proteins and nucleic acid were more stable in the APP-treated group than those in the groups treated by sugars and skim milk as cryoprotectants. Moreover, the activities of lactate dehydrogenase and β-galactosidase were both higher in the APP-treated group than those in control groups. Under the investigation of SEM, cells of APP-treated group were significantly full and integral, while cells in control group were shrinking. FTIR studies implied that APP had interactions with the cell membrane phospholipids during freeze drying.