Effect Of Starter Cultures And Soy Protein Hydrolysates On Yoghurt Quality

In order to increase its viscosity and improve its sensory properties,protein or thickener is applied in yoghurt industry,but destabilization,such as whey separation,is usually observed at late storage period.Enzymatic hydrolysates,such as soy protein hydrolysate(SPH),is an efficient nitrogen resource that can increase production of lactic acid and yoghurt viscosity.Fermentation and storage characteristics of yoghurts fermented by 187,495 and 885 starter cultures and by SPH hydrolyzed by Alcalase 2.4L for 24 h(SPH-A24)were studied.SPH-A24 had growth stimulation effect that decreased quickly the p H value of yoghurts fermented by three starter cultures.During storage,SPH-A24 increased the exopolysaccharide content and viable count of Streptococcus thermophiles(ST),inhibited whey separation and decreased the viable count of Lactobacillus delbrueckii(LB).Particularly,SPH-A24 improved significantly the rheological,textural and sensory properties of yoghurt fermented by 495 starter culture.Correlation analysis was applied to investigate the relationship between sensory properties and physicochemical properties,and 11 indicators were chosen for evaluation of yoghurt quality.The correlation analysis of sensory properties showed that sourness,cohesiveness,sum of textural scores and synthesis score were significantly correlated.The correlation analysis of physicochemical properties showed that rheological and textural properties were correlated with sensory properties.Based on fluid characteristics and correlation analysis,rheological and textural properties were classified into three categories,which indicate fluid characteristics in the Power’s Law zone,the First Newtonian zone and the Second Newtonian zone respectively.Io further analyze the relationship between metabolism of lactic acid bacteria and yoghurt quality,sourness,cohesiveness,K,viscosity,η,Je,whey separation rate,pH,exopolysaccharide content and viable counts of ST and LB were chosen as evaluation indicators.Nine kinds of SPHs were prepared by different enzymes(Alcalase 2.4L,papain,trypsin)under different hydrolysis time(3,12,24 h),respectively.During fermentation,SPHs significantly shortened fermentation time,and SPH hydrolyzed by Alcalase 2.4L for 12 h(SPHA12)greatly shortened fermentation time for 1.3 h,indicating best growth stimulation effect.During storage,SPH-A12 greatly inhibited post-acidification of yoghurt,and improved its rheological and sensory properties.Yoghurt with SPH-A12 showed the highest viscosity,exopolysaccharide content and cohesiveness,167.66 mPas,339.26 mg/kg yoghurt and 7.87 respectively.Correlation analysis indicated exopolysaccharide content was significantly correlated with cohesiveness,viscosity,K and Je.SPHs could increase exopolysaccharide content of yoghurt so as to improve rheological and sensory properties and yoghurt quality.An ultrafiltration membrane of 4 kDa was applied to separate SPH-A12 and two component(Mw > 4 kDa and Mw < 4 kDa)were obtained(SPH-H4 and SPH-L4).The addition of SPH in yoghurt indicated that SPH-L4 was better than SPH-A12 in promoting the production of exopolysaccharide by lactic acid bacteria,inhibiting post-acidification of yoghurt,decreasing whey separation rate and viable count of LB,increasing viable count of ST,improving its rheological and sensory properties.Therefore,ultrafiltration is an efficient tool in extraction of active component in SPH-A12.