Effect Of Four Strains Of Probiotics On Quality And Flavor Of Fermented Yogurt During The Storage

Probiotics are beneficial microorganisms to health,and milk is the best carrier of probiotics for intaking into the human body.In this study,fresh milk was used as raw material to prepare solidified yoghurt using the traditional culture co-fermented with Bifidobacterium animalis(A-Ba),Lactobacillus plantarum(A-Lp),Lactobacillus casei(A-Lc)and Lactobacillus acidophilus(A-La),respectively.The objectives of this study were to obtain the optimal combination of cultures for the production of probiotic yoghurt with the best viability of lactic acid bacteria,structure,taste and flavor,to explore the changes of the physicochemical properties,viability,structure,protein oxidation,lipid oxidation,and the volatile flavor of the probiotic yoghurt at 4 ? for 28 days,and to provide the basis for the development of probiotic yoghurt.1.The physicochemical properties and viability of four strains of probiotics yoghurt were studied.The yoghurt cultures were co-fermented with Bifidobacterium animalis(A-Ba),Lactobacillus plantarum(A-Lp),Lactobacillus casei(A-Lc)and Lactobacillus acidophilus(A-La)to prepare solidified yoghurt,respectively.Viability,acidity,diacetyl contents,acetaldehyde contents and water holding capacity(WHC)were measured and the changes of physicochemical properties and viability were evaluated for the yoghurt stored at 4 ? for 28 days,and the correlation between the measurements was analyzed.The results showed that the viability of L.casei was the highest,followed by B.animalis,L.plantarum,and L.acidophilus(P<0.05)in the storage period.The survival rates of S.thermophilus in the treated group and the control group elevated first and then decreased(P<0.05).Titration acidity increased during the storage period,the acidity of the A-La group was the highest,and the A-Ba group was the lowest.Moreover,A-Lc had relatively higher diacetyl content and better butter flavor during storage period.After 7 days of storage,the acetaldehyde content of ALc was better,which was better than other treatment groups.The WHC of yoghurt showed an increase trend,and the WHC of each group was not much different.When yoghurt was stored for 28 days,L.acidophilus had largest capacity for acid production,followed by L.casei and L.plantarum,and the weakest was B.animalis.According to the correlation analysis,the acidity was negatively correlated with the viability and diacetyl content,while positively correlated with acetaldehyde and WHC.2.The quality structure of four probiotics yoghurt strains was studied.The changes of pH value,texture and rheological properties of yoghurt stored at 4 ? for 28 days were detected,and the changes of microstructures were observed.The results showed that during the storage process,the sensory evaluation of A-Lc group and A-Lc group had better taste.The hardness and cohesiveness of the ALc group were both higher in the middle of storage(P<0.05).According to the rheological properties of the detection,the elastic modulus(G')were greater than the viscous modulus(G''),G' of A-Lc group in the storage process was higher,the shear scan found that A-Lc group had high apparent viscosity,through observing the microstructures,A group,A-La group and A-Lc group had more chain,three-dimensional structure were more stable,but the difference of microstructure of each yoghurt in d 14 and d 28 was not obviously.Therefore,the hardness,cohesiveness,elasticity,apparent viscosity and stability of fermented yoghurt in A-Lc group were better.3.Protein oxidation and lipid oxidation of four probiotics were studied.The pH value of each group of yoghurt showed a downward trend during the storage period,and the pH value of the A-La group was significantly lower than the other groups.By measuring the content of carbonyl group and sulfhydryl group of yoghurt,it was found that the protein oxidation of A-La group was enhanced due to the strong post-acidification during storage.The activity of protease in A-Lp group was significantly higher than the other groups at the early storage stage(P<0.05),and the content of free amino acids in A-Lp group was higher at the early storage,but lower at the late storage.According to SDS-PAGE,there was no significant difference in casein banding among the four probiotics during storage,but ?-lactoglobulin and ?-whey banding were significantly deeper on d 28 than on d 1.POV value of yoghurt showed an upward trend during the storage period,and the POV value of yoghurt in A group was significantly higher than the other groups at the later stage of storage(P<0.05),and then was A-La group.The content of malondialdehyde in A group and A-La group was higher than other groups.It could be concluded that,because L.acidophilus has strong acid discharge and strong postacidification,protein oxidation and lipid oxidation in A-La group were higher during storage.4.The volatile flavor compounds of four probiotics yoghurt strains were studied.The volatile flavor compounds in yoghurt were detected by Solid Phase Microextraction-Gas Chromatograph-Mass Spectrometer(SPME-GC-MS).The main contents were acids,ketones,aldehydes,alcohols,esters,aromatic heterocyclic and hydrocarbons substances,the content of ketones was the highest.And the content of acids in A-Lc group was higher in d 1 to d 7,mainly due to the higher content of caproic acid.In d 14 to d 28,there was no significant difference in the acid substances in yoghurt.Although the content of ketones in the A-Lc group only reached the maximum on 7 and 21 d,but the content was not low at other time points,the main flavor component 2,3-butanedione in A-Lc group was higher than other groups during storage.The content of aldehydes in each yoghurt were not high,no more than 10%,but A-Lc group had the most kinds of aldehydes,including 12 kinds.In the storage process,the esters content of yoghurt showed a decreasing trend.The hydrocarbon substances detected in A-Lc group were lower than that in A group only on d 1,and the other storage time points were all higher than that in the other groups.For aromatic heterocyclic substances,A-Lc group were not detected on d 1,but the other time points were all higher than the other groups.PCA analysis showed that the principal components of A-Lc group contained a variety of substances that contributed to the flavor of yoghurt.5.In summary,the A-Lc group was better than the other groups in the comprehensive evaluation of the number of viability,hardness,viscosity,three-dimensional structure and flavor after 28 days of storage at 4 ?,Therefore,the co-fermentation of L.casei and traditional starter was the best probiotic yoghurt combination.