| In this manuscript different kinds of direct investment type starter cultures on yogurt quality and the impact of aromaticsubstances content in the yogurt was studied. Firstly five kinds of direct investment type starter cultures were selected, andthe enzyme activity of five kinds of enzymes including β-galactose glucoside enzyme, glucose kinase, pyruvate kinase,lactate dehydrogenase and determination of pyruvate decarboxylase were determined. Then use the five direct investmenttype starter cultures to product yoghourt, and the pH and total acidity during the fermentation of yogurt were determined.Finally the quality of yogurt during cold storage indicators (pH, total acidity, viscosity, a hydraulic) and the content of theyogurt aromatic substances (acetaldehyde and diacetyl) were determined.Test I: Five kinds of direct investment type starter cultures were selected, and the enzyme activity of five kinds ofenzymes were determined. Result follows: five kinds of starter cultures in β-galactose glucoside enzyme differ significantly(P <0.05), enzyme activity of NO.4ups to111084.50U/g while NO.3lows to13355.50U/g; In Glucose, enzyme activity ofNO.5and NO.4were significantly different (P <0.05), and NO.5ups to9.06U/g while NO.4was the lowest2.58U/g; Inpyruvate kinase, enzyme activity of NO.4and NO.3have the significant difference (P <0.05), NO.4ups to969.56U/g whileNO.3was the lowest92.04U/g; In pyruvate decarboxylase, enzyme activity of NO.2and NO.3were significantly different(P <0.05), enzyme activity of NO.2ups to120.73U/g while NO.3lows to57.22U/g; In lactate dehydrogenase, enzymeactivity of NO.4and NO.1have the significant difference (P <0.05), NO.4ups to90.33U/g while NO.3was the lowest7.78U/g.Test II: Yogurt fermentation was used starter cultures of the test I and commercial western spring pasteurized milk. Inyogurt fermentation process, pH and total acidity were determined every1h. Result shows that in the determination of6h, pHof five different starter yogurt reduced gradually and the total acidity increased. At the end of the6h fermentation, pH ofNO.3, NO.4were significantly different with NO.1,NO.2,NO.5(P <0.05), and pH of NO.3, NO.4were higher thanNO.1,NO.2, NO.5; Total acidity of NO.3, NO.4were significantly different with each other, they have significant difference(P <0.05), Total acidity of them were66.42°T and77.17°T respectively.Test III: Determination of yogurt quality and trace of aromatic substance content. Using fermented yogurt of test II, thepH, total acidity, viscosity, hydraulic, acetaldehyde content and diacetyl content were determined at0h,24h,48h,72h,96h,120h,144h point respectively. Result shows that at the end of the cold storage144h, yogurt pH of NO.4and NO.5havesignificant difference (P <0.05), NO.4was2.54%lower than NO.5; Differences between acidity were not significant (P>0.05), NO.1has the lowest acidity95.17°T; Yogurt viscosity between NO.2and NO.3were significantly different (P <0.05),NO.2were41.21%higher than NO.3; Acetaldehyde content aspect have no significant difference between various yogurt (P>0.05), NO.2has the highest levels21.88μg/ml; Diacetyl content between various acid milk have no significant difference(P>0.05), and NO.5ups to18.04μg/ml.After comprehensive comparison, NO.2and NO.5starter cultures slowed the speed of pH decline in refrigerationprocess and also improved the consistency of the yogurt; they increased the content of aromatic matter acetaldehyde anddiacetyl. So NO.2and NO.5starter cultures have obvious advantages. |
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