Effect Of Rice Bran On The Yogurt Stability And Its Mechanism

As people pay more attention to the intake of bioactive components in food,such as dietary fiber,adding the plant fiber into yogurt to prepare the high-fiber yogurt has become a hot topic in recent years.Rice bran is an important by-product during rice processing.It is rich in dietary fibers and phenolic acids,and thus can improve the nutritional value of yogurt.However,these components of rice bran affect the yogurt stability in different ways.Thus,the effect of rice bran the yogurt stability is unclear.In addition,lactic acid bacteria fermentation may change the composition and microstructure of rice bran.That is,adding rice bran before and after fermentation may have different effects on the yogurt stability.Therefore,this work investigated effects of adding rice bran before and after fermentation on the yogurt stability and further studied effects and mechanisms of key components of rice bran,including the insoluble fiber,the soluble fiber and phenolic acids,on the yogurt stability to provide theoretical guidance for the development of the high-fiber yogurt.The main results are as follows.Firstly,the effects of adding rice bran before and after fermentation on the yogurt stability were investigated.It was found that adding an appropriate amount of rice bran before fermentation improved the stability of yogurt,while adding rice bran after fermentation reduced the stability of yogurt.Taking adding 2.0% rice bran for example,the syneresis of the control group was 7.86%,the syneresis of yogurt with rice bran added before fermentation was 6.86%,and the syneresis of yogurt with rice bran added after fermentation was 11.18%.Correspondingly,scanning electron microscopy and laser confocal scanning microscopy analysis showed that yogurt with rice bran added before fermentation had more compact network structure and smaller whey pores than the control group,while yogurt with rice bran added after fermentation had looser network structure and larger whey pores.In addition,the fluorescence of rice bran in yogurt with rice bran added after fermentation was significantly stronger than that in yogurt with rice bran added before fermentation.High performance liquid chromatography analysis showed that yogurt with rice bran added before fermentation had more free p-hydroxybenzoic acid,p-coumaric acid,ferulic acid,and sinapic acid than yogurt with rice bran added after fermentation,which indicated that lactic acid bacteria fermentation converted bound phenolic acids of rice bran into free ones.Obviously,adding rice bran before fermentation was more beneficial than adding rice bran after fermentation.Therefore,the effect of adding rice bran before fermentation on the p H,texture and rheological properties of yogurt was investigated.It was found that rice bran increased the p H and decreased the firmness,consistency,cohesiveness,viscosity and gel strength of yogurt,which indicated that the stabilization effect of rice bran on yogurt did not result from the thickening effect.Therefore,it was speculated that adding rice bran before fermentation was beneficial for the components in rice bran,such as phenolic acids,to interact with milk protein,making yogurt have a more compact and porous microstructure,thereby improving the yogurt stability.Subsequently,rice bran was separated into the insoluble fiber and soluble fiber,and effects of the insoluble fiber and soluble fiber on the yogurt stability were studied,respectively.It was found that adding the insoluble fiber before and after fermentation significantly reduced the stability of yogurt.Correspondingly,the addition of the insoluble fiber made the gel network structure of yogurt looser and decreased the protein aggregation.The above results indicated that adding the insoluble fiber before and after fermentation hindered the protein aggregation in yogurt and damaged the integrity of the yogurt gel,thus reducing the stability of yogurt.Yogurt with the insoluble fiber added before fermentation had more free p-coumaric acid,ferulic acid,and sinapic acid than yogurt with the insoluble fiber added after fermentation,suggesting that lactic acid bacteria fermentation also converted bound phenolic acids of the insoluble fiber into free phenolic acids.In addition,adding the insoluble fiber before fermentation did not affect the p H but significantly decreased the viscosity and gel strength of yogurt.On the other hand,adding the soluble fiber significantly improved the stability of yogurt.Correspondingly,addition of the soluble fiber resulted in stronger connections between yogurt protein aggregates and fewer whey voids.However,adding the soluble fiber showed no significant effect on either the viscosity or the gel strength of yogurt.Besides,laser confocal scanning microscopy analysis showed that addition of rice bran soluble fiber promoted aggregation of casein micelles and increased the size,which indicated the interaction between casein micelles and soluble fibers.Thus,it was speculated that the stabilization effect of the soluble fiber on yogurt might result from interaction between the soluble fiber and milk protein rather than the thickening effect of the soluble fiber.As mentioned above,lactic acid bacteria fermentation converted bound phenolic acids of rice bran into free phenolic acids,and yogurt with rice bran added before fermentation was more stable than yogurt with rice bran added after fermentation,which indicated that free phenolic acids might improve the stability of yogurt.Therefore,ferulic acid,one of the most abundant free phenolic acids in rice bran,was used as a representative to study the effect of free phenolic acids on the stability of yogurt.Specifically,yogurt without ferulic acid was used as the control group,and 20,100,and 500 μg/m L ferulic acid were added into yogurt before and after fermentation,respectively.The syneresis of yogurt with 100 μg/m L ferulic acid added before fermentation was 4.72%,which was significantly lower than that of the control group.However,the syneresis of yogurt with 500 μg/m L ferulic acid added before fermentation was higher than that of yogurt with 100 μg/m L ferulic acid added before fermentation.Adding ferulic acid after fermentation had no significant effect on syneresis of yogurt.After fermentation for 90 min,the p H of yogurt added with 500μg/m L ferulic acid was significantly higher than that of the control group and yogurt added with 20 and 100 μg/m L ferulic acid.The above results indicated that adding an appropriate amount of ferulic acid before fermentation could improve the stability of yogurt,but adding too much ferulic acid might inhibit the fermentation of lactic acid bacteria,which was not conducive to the stability of yogurt.In addition,adding ferulic acid had no significant effect on the viscosity or the gel strength of yogurt.Intrinsic fluorescence spectroscopy analysis showed that ferulic acid could interact with casein and whey protein through the hydrogen bond and the van der Waals force.Therefore,it was speculated that adding an appropriate amount of ferulic acid before fermentation could improve the stability of yogurt through the interaction between ferulic acid molecules and milk protein molecules.Since the milk protein was gelled in yogurt,it was difficult for ferulic acid to interact with gelled milk protein when ferulic acid was added after fermentation.Thus,adding ferulic acid after fermentation had no significant effect on the stability of yogurt.