| Nine strains of L. plantarum with broad-spectrum antibaterial activity have studies proved were used as research object, and the milk-coagulating, autolytic and proteolytic activity were measured to screening target strains. Then yogurt starter culture (YF-L904) was inoculated with L. plantarum which were screened out for fermented milk, both at level of 5×106cfu/g, and stored at 4℃ for 28d. The pH, titratable acidity (TA), proteolytic activity, syneresis, viscosity, texture profile, volatile compounds, viable counts and sensory quality were determined every week. At the same time the principal component analysis for data. The results showed that:(1) In skim milk with the addition of 0.25% yeast extract, the curd time of L. plantarum IMAU70095、IMAU80106 and IMAU10216 was 9.3h,8.5h and 8h, were lower than other strains significantly(P<0.05). The reduction of OD600 in 48h was 0.652±0.015,0.425±0.02 and 0.557±0.018 respectively. When the fermentation ended, the free amino nitrogen concentration was 5.14mmol/L,4.58mmol/L and 2.92mmol/L. The autolytic and proteolytic activity were significantly higher than other strains (P<0.05).Make L. plantarum as a target, and further study its effect on the quality of fermented milk.(2) In 28 days storage period at 4℃, the differences of yoghurt on pH value and titratable acidity (TA) between the control group and the experimental group were not significant (P>0.05). The free amino nitrogen concentration of control group has declined and in each experimental group has increased and higher than that in control group. The free amino nitrogen concentration increased largest is blend L. plantarum IMAU80106 with YF-L904 to make yogurt, was 6.88mmol/L. L. plantarum has a greater impact on the physical properties of fermented milk. It may give products higher viscosity, gumminess, chewiness, resilience and adhesiveness in 1-7d. And with the prolonging of storage time it has large impact on products hardness and springiness. Meanwhile, the physical characteristics of fermented milk showed different variations due to with different strains of L. plantarum. L. plantarum strains can improve the flavor quality of fermented milk and exhibited higher content of the 3-hydroxy-2-butanone, cyclohexanol and acetaldehyde. Among them, the content of acetaldehyde which is characteristic flavor in fermented milk in control group was 0.152-0.166μg/ml, and inoculated with L. plantarum MAU80106、IMAU10216 and IMAU700953 was 0.159-0.179μg/ml、0.195-0.213μg/ml and 0.193-0.216μg/ml. In yogurt, the viable counts of L. plantarumIMAU80106、IMAU10216 and IMAU70095 were not significantly changed (P>0.05).At the end of storage, the counts were (1.55±0.005)×107cfu/g, (6.53±0.32)×107cfu/g and (6.83±0.05)×106cfu/g, showed the good stability and viability during the shelf life. The counts of yogurt starter culture in experimental group were 1.23×109cfu/g、 1.15×109cfu/g and 1.24×109cfu/g. Slightly higher than control group (1.07×109cfu/g), but the differences were not significant (P>0.05). At the same time, the difference of sensory scores between the control group and experimental group were not significant (P>0.05). Through the principal component analysis, there were correlations between sensory quality, physical properties and volatile compounds. |
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