A total of 15 food-grade Lactobacillus strains were evaluated for their antagonism against gastrointestinal pathogens using Bifidobacterium animalis subsp. lactis BB-12 as a control. The results showed that nine of the strains possessed excellent antibacterial properties against five indicator bacteria tested, and two of the remaining six strains were antagonistic against four of the indicator bacteria, Staphylococcus aureus, Salmonella, E. coli and Bacillus cereus. According to their antagonistic effect, acid producing ability and milk curding ability, four strains were selected and each of them was combined with Streptococcus thermophilus ST-21 for use as bio-preservative starter cultures. Streptococcus thermophilus ST-21 + strain Z-38 at a ratio of 2:1 could produce acid rapidly, have potent post-acidification capacity and produce a strong milky aroma, being suitable for the production of flavored yogurt. Streptococcus thermophilus ST-21 + strain Z-40 at a ratio of 2:1 resulted in the best texture of yogurt, being suitable for the production of fermented dairy drink. Yogurt fermented by Streptococcus thermophilus ST-21 + strain Z-12 at a ratio of 1:1 showed the lowest consistency and cohesiveness.

This study describes the optimization of the formulation of cryoprotectants for Bifidobacterium lactis Z-1 using response surface methodology (RSM). Using the Plackett-Burman design, trehalose, sucrose, L-cysteine hydrochloride concentration were identified as factors with a significant influence on the viable count of lyophilized Bifidobacterium lactis Z-1. Subsequently, the optimal response ranges of the three factors were determined using the steepest ascent method. The levels of these factors were further optimized using RSM with Box-Benhnken design. As a result, a regression model was fitted into the experimental data. The optimal levels of trehalose, sucrose and L-cysteine hydrochloride were determined as 200.51, 46.16 and 2.31 g/L, respectively, while the other ingredients were the same as those in the traditional recipe (skimmed milk powder 100 g/L, whey protein powder 20 g/L, glycerol 4 g/L, sodium isoascorbate 10 g/L and sodium glutamate 8 g/L).

A total of 30 food-grade Lactobacillus strains isolated from naturally fermented products from Tibetan, Xinjiang and Qinghai regions including Lactobacillus rhamnosus LGG (as a control) were evaluated for acid and bile salt tolerance, hydrophobicity, surface charge and auto-aggregation ability. Out of these, 16 strains were selected and further evaluated for tolerance to artificial gastric and intestinal fluids. The results showed that the survival rates of 4 of the 16 strains were greater than 14.44% after 60 min of incubation in artificial gastric fluid and were 3.91%–100.00% after 30 min. For all 16 selected strains, the survival rate was 31.73%–112.83% after 5 h, and Z-1 had the strongest tolerance to artificial gastric fluid of these strains with a 93.07% survival rate after 60 min. The survival rate of Z-1 was 106.5% after 5 h of incubation in artificial intestinal fluid, hydrophobicity 97.52%, surface charge 97.35% and 24.67% in the presence of chloroform and ethyl acetate, respectively, and auto-aggregation ability 11.26%.