Fermented milks were prepared using raw milk with different fat contents, and their flavor was evaluated. The pH trend during fermentation indicated that the change in milk fat content had no effect on the acid production of lactic acid bacteria. Rheological and texture measurements showed that with the increase in milk fat content, the elastic modulus and the viscous modulus decreased, hardness and cohesiveness did not obviously change, but gumminess significantly increased (P < 0.05). By gas chromatography-mass spectrometry (GC-MS), a total of 34 flavor substances were detected from 5 groups of fermented milk samples, including 7 hydrophilic ones and 27 lipophilic ones. Change in milk fat content greatly affected the content of flavor substanc es. Principal component analysis and correlation analysis showed that change in milk fat content caused significant differences in flavor between fermented milk samples, and the trend of flavor contents was related to their hydrophobicity. With the increase in milk fat content, the content of milk fat degradation products such as 2-nonanone and hexanal but not γ-butyrolactone increased first and then decreased. Among other substances, the content of hydrophilic flavor compounds increased first and then decreased, and the content of lipophilic compounds decreased continuously.

The purpose of this investigation was to study the influence of adjuvant starter cultures on the quality of fermented milk. Seven strains of common lactic acid bacteria, including Lactococcus lactis subsp. cremoris (Lcr), Lactococcus lactis subsp. lactis (Ll), Lactobacillus casei (Lca), Lactobacillus fermentum (Lf), Lactobacillus rhamnosus (Lr), and two Lactobacillus plantarum strains (Lp1 and Lp2), were used separately as adjuvants to a commercial starter culture for fermented milk. The results showed that addition of adjuvant starter cultures could shorten the fermentation time and affect the pH value of fermented milk during storage. Moreover, Lcr and Ll could cause significant changes in the oxidationreduction potential during fermentation. Rheological measurements indicated that these strains could increase the viscosity, and reduce the elasticity and fluidity of fermented milk. The texture analysis showed that all the strains could increase the water-holding capacity, and Lr, Lf and Lp1 could enhance the cohesiveness and adhesiveness of fermented milk. Gas chromatography-mass spectrometry (GC-MS) analysis and principal component analysis showed that Lr, Lp1, Lp2 had little effect on the flavor of fermented milk, while Lca could promote the formation of heptanol, benzaldehyde and 2-tridecanone, and Lf, Ll and Lcr could increase the content of flavor substances such as short-chain and medium-chain fatty acids in fermented milk.

In this study, the relationship between milk clotting characteristics by rennet and different protein components was discussed. Addition of whey protein concentrate (WPC), κ-casein (κ-CN), β-lactoglobulin A (β-LgA) and β-lactoglobulin B (β-LgB) to skim milk could reduce the milk clotting time, increase the viscosity after coagulation and result in a more compact microstructure, while the formation of curds was inhibited by the addition of α-La, α-CN, β-CN and total CN and the microstructure of the formed curds was relatively loose. Dynamic light scattering method was applied to determine the particle size distribution of casein micelles during coagulation. The results showed that molecular radius distribution in casein solutions added with WPC, κ-CN, β-LgA and β-LgB changed quickly, indicating rapid aggregation of the protein molecules in solutions. Bidirectional electrophoresis and mass spectrometry identification of raw milk samples with significantly differential milk-clotting properties indicated that the differential protein components related to the rennet coagulation characteristics of bovine milk were mostly bioactive trace proteins affecting breast cell metabolism.