| Strains of Streptococcus thermophilus and Lactobacillus delbrueckii subsp.bulgaricus isolated from yogurt products were screened for commercial direct-vat-starters (DVS). The physiology of yogurt strains, media for their proliferation, protectants for the viability of cells, and the optimal technique for producing lyophilized cells were investigated to improve the quality of commercial yogurt starters. One of the purposes of the present study was to stop a gap between China and developed countries in the production of direct-inoculated-starter cultures used for yogurt products.The interactions, including the symbiosis and antagonism between 4 Streptococcus thermophilus strains and 8 Lactobacillus delbrueckii subsp. bulgaricus strains were studied. The results proved that among the tested strains, ST1 and LB1, STland LB2, ST1 and LB3, ST1and LB4, ST1 and LB5, ST1 and LB6, ST1 and Y-5, and ST1 and Y-6 were symbiotic in media. The strains of streptococci and lactobacilli were paired for further screening based on their abilities to produce lactic acid, viscosity, viable counts, and the levels of amino acid and acetaldehyde. It was seen that the paired strains ST1 and LB1, STland LB2, ST1 and Y-6 were more suitable for the production of yogurt.The optimal media for growing the above-mentioned yogurt strains were investigated. It was found that the strain LB2 required the following ingredients for the optimal growth: that is 6.0% of tomato extract, 0.6% of yeast extract, 2% of whey powder, and 11% of malt extract. The optimal medium for the growth of strain LB1 was tomato extract (6.0%), yeast extract (0.4%), whey power (1%) and malt extract (10%), respectively. The optimal medium supporting strain Y-6 was 7.5% of tomato extract, 0.6% of yeast extract, 3% of whey power, and 10% of malt extract. When incubated at 37℃ for 16 h, the populations of the 3 lactobacilli strains reached 1.76 x 109cfu/mL, 0.97 x 109cfu/mL, and 1.88 x 109cfu/mL, respectively. Meanwhile, medium supporting the optimal growth of streptococci strain ST1 was tomato extract (7.0%), yeast extract (0.3%), CaCO3 (0.9%) and peptone (2.0%). When incubated at 37℃ for 16 h, the live cells of the strain ST1 were 1.05 x 109cfu/mL.The optimal medium for the paired yogurt strains LB2-ST1 was tomato extract (4.0%), yeast extract (0.8%) and peptone (1.0%). That for the paired yogurt strains LB1-ST1 was tomato extract (2.5%), yeast extract (0.5%) and peptone (1.5%). That for the paired yogurt strains Y-6-ST1 was tomato extract (1.0%), yeast extract (0.3%) and peptone (0.5%). The live cells of these yogurt strains, when grown in their respective medium for incubation at 37℃ for 16 h, were 1.59 x 109cfu/mL, 1.22 x 109cfu/mL, and 1.64 x 109cfu/mL, respectively.If they were cultivated at 42℃ for 6 h, the 3 yogurt starters required different components in media to support their maximum growth. For strains LB2-ST1, the best medium was 1.0% of tomato extract, 1.0% of lactose, 0.5% of yeast extract and 1.0% ofpeptone. For strains LB1-ST1, that was 2.5% of tomato extract, 0.5% of yeast extract and 1.5% of peptone. For strains Y-6-ST1 that was 1.0% of tomato extract, 0.3% of yeast extract and 0.5% of peptone. The cell population was up to 1.64×109cfu/mL, 1.34×109cfu/mL, and 2.04×109cfu/mL, respectively.The optimal cryoprotectants to improve the viability of strains ST1, LB2 and LB2-ST1 were investigated. It was seen that the combination of trehalose (1.0%), glycerin (0.5%), glutamate-Na (15%) and 1.0% of sorbitan mono-oleate offered the optimal protection to the viability of the strain STL The mixture of trehalose (2.0%), glycerin (0.3%), glutamate-Na (5%) and Sorbitan mono-oieate (0.5%) favored the cells of strain LB2 to survive best in stress conditions. A combination consisting of trehalose (0.5%), glycerin (0.3%), glutamate-Na (5%) and Sorbitan mono-oleate (0.3%) produced a very positive effect on promoting the viability of the paired strains LB2-STI. Their viable cells after freeze-drying were 4.72 x10 11cfu/g, 1.34×10 11cfu/g and 3.57×10 11cfu/g, re...
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