The Degradation Of Milk Proteins By Lactic Acid Bacteria And Its Application Research

Lactic acid bacteria (LAB) have a very long history of using in the manufacturingprocesses of fermented milk products. The proteolytic system of LAB involved in milkprotein utilization provides cells with polypeptide, oligopeptide and free amino acid duringgrowth in milk and contribute to the development of the organoleptic properties of fermentedmilk and products quality. The influences of incubation temperature, homogenization pressure,and strains on the formation of yoghurt gel had been studied by many researchers; howeverthe internal factors including milk protein, polypeptide, oligopeptide and free amino acid wereseldom investigated.In this study, yoghurts were fermented by mix-starter culture (Lactobacillus bulgaricus/Streptococcus Thermophilus) and single-starter culture (Streptococcus Thermophilus), and thechange of milk protein, polypeptide, oligopeptide, free amino acid and texture of twoyoghurts were compared during low-temperature storage (0d/2d/6d/10d/15d/21d/28d，4℃).Moreover, the relationship between milk protein, polypeptide, oligopeptide, free amino acidand texture was discussed, and establish the regression models between texture andproteolysis.The changes of milk protein, polypeptide, oligopeptide, free amino acid had been studiedby SDS-Urea-PAGE, Tricine-Urea-PAGE and HPLC. The results showed that milk proteinwas still degraded in mixed-culture yoghurt during low-temperature storage; Twopolypeptides(9.4kD，5.8kD) were generated,2-3peptide and3-4peptide were mainoligopeptide (57.63%-83.47%) in mixed-culture yoghurt, oligopeptide in mixed-cultureyoghurt was rich in Ser/Thr, Leu and Glu residue (39.08%-51.13%)，the content of freeamino acids increased by0.4-8.84times respectively in mixed-culture yoghurt, but therelative content of Glu and Pro were decreased; Milk protein was still degraded insingle-culture yoghurt; One polypeptides(7.2kD) was generated,2-3peptide was mainoligopeptide (49.90%-70.11%) in single-culture yoghurt, oligopeptide in single-cultureyoghurt was rich in Ser/Thr, Lys and Glu residue (42.11%-62.48%). Most free amino acidscontent tented to be stable, except Ala/Cys increased by3.53times, and relative content of Glu and Pro were decreased. According to degradation products and amino acid componentanalysis, protease and peptidase of LAB still showed high activity under low-temperature.The texture of yoghurt was test by texture analyser. The result showed that the yoghurttexture characteristics (firmness, consistency, viscosity, cohesiveness) increased sharplybefore6-10d and tended to be stable after6-10d low-temperature storage.The texture of yoghurt was affected by protein degradation in two ways according tostudy on the relationship between protein degradation and texture of yoghurt:(1) theformation of yoghurt gel was effected by milk protein and its degradation directly byparticipating in the formation of yoghurt gel;(2) oligopeptide and free amino acid resultingfrom protein degradation can affected yoghurt texture indirectly in nutrition ways of LAB.