Studies On Biological Characteristics Of Strains For Yoghourt Ferment

With the domestic dairy products developed in recent years, the production of yogurt increased rapidly accordingly. The starter was an important of factors in the quality of yogurt. It involved biologic characteristics (including antagonism, hydrolysis of proteases,β-galactosidase activity determination), the rheological behaviour of producing exopolysaccharide and the flavor compounds (acetaldehyde) to study the effects of them in improve the quality of DVS yogurt starter cultures.(1) Gain 6 strains Lactobacillus delbrueckii subsp. Bulgaricus and 1 strain Streptococcus thermophilus by isolated and identification.(2) The interactions on symbiosis between 2 strains delbrueckii subsp. Bulgaricus and 1 strain Streptococcus thermophilus, which has good fermenttation capability separately, were studied. The results indicated that:①The interactions on symbiosis could accelerate the growth of them significantly.②The mixture fermentation of delbrueckii subsp. Bulgaricus and Streptococcus thermophilus could accelerate the production of acid, and the ceiling value increased obviously, but the living cell number and the level of acid were not the same because of the delbrueckii subsp. Bulgaricus and Streptococcus thermophilus combination.③The mixed fermentation behaved high producing viscosity, it could decrease the coagulation time and increase the viscosity. Perhaps this was because of the symbiotic interactions. The producing viscosity of L5 and S fermented together increased compared to LC and S in the same conditions, this included that the symbiotic interactions could enhance the viscosity.(3) The productions of acetaldehyde of L5, LC and S were 17.2mg/L, 14.6mg/L and 27.4mg/L, which showed that S was the major flavor components in yogurt fermentation. The growth curve and acidity curve of all strains showed that the production of acetaldehyde mostly existed in logarithmic phase, which in coincidence with acidity curves. All this included that acetaldehyde producted together with cell growth, and varied to the max after the ferment was finished.The mixed fermentation accelerated the production of acetaldehyde compared to isolated strains, which demonstrated the interactions on symbiosis also. The ration of acetaldehyde increased corresponding to the symbiosis.(4)β-galactosidase activity of 3 strains were assayed, which showed L5 was higher than the other two, while the activity of S was the lowest. By the acidity curve, the capability on acidity production of L5 was highest, while the activity of S was the lowest. So we could say there was a positive relationship betweenβ-galactosidase activity and the production of acidity.(5) By the hydrolysis of proteases assay, the amino acid content of L5 and LC were 101μg/mL and 91μg/mL. After mixed fermentation of L5 and S, LC and S, separately, the content could reach 106μg/mL and 103μg/mL.(6) The results showed that viscosity decreased when the pressure for homogenization increased for Lactobacillus delbrueckii subsp. Bulgaricus, which was opposite to Streptococcus thermophilus and mixed fermentation. The extracting rate of whey increased when the pressure for homogenization increased for Lactobacillus delbrueckii subsp, which was opposite to Streptococcus thermophilus and mixed fermentation. It indicated the producing viscosity strains and mixed fermentation had good determination of shear resistance.In conclusion, there was good interactions on symbiosis between. Lactobacillus delbrueckii subsp. Bulgaricus and Streptococcus thermophilus, The mixed fermentation of them could decreased the ferment time, improve the quality of yogurt. But the interactions on symbiosis was not the same, antagonism was also existed between of them. It was very important in choosing the suitable strain.