| To reduce cost,enrich protein nutrition types and contents,enhance flavor and shorten the aroma-producing period of the current milk flavors,soy protein isolate(SPI)was used to partially replace cream to build a mixed substrate,and two novel milk flavors which with strong fermentation aroma and cheese flavor were developed based on the mixed substrate by fermentation in combination with two different enzymatic hydrolysis methods.The flavor differences between the two milk flavors and the reasons for the differences were explored,and the two-step enzymolysis combined with fermentation was regarded as the proper method to prepare the novel cream-SPI flavor;the potential relationship between key volatiles and main metabolites were analyzed for the best milk flavor,and different contributions of lactobacillus and enzymes to flavor formation were discussed;moreover,the best milk flavor was added to bread at different levels to explore its influence on bread quality.Generally speaking,the novel milk flavor which possesses more cost-effectiveness,health benefits and flavor value has great potential to be applied in food industry.Two novel cream-SPI milk flavors were developed by one-step enzymolysis combined with fermentation(adding lactobacillus,protease and lipase to cream-SPI substrate at the same time)and two-step enzymolysis combined with fermentation(first protease and lactobacillus,then lipase),respectively.The flavor differences between these two flavors and the reasons were studied.The two-step enzymolysis combined with fermentation was regarded as the proper method to prepare the novel cream-SPI flavor,which could better promote the growth of Lactobacillus casei LC1,and the milk flavor prepared by this method had lower p H(5.21),higher titratable acidity(2.15(lactic acid,%)),higher content of lactic acid(6.18 mg/g),malic acid(1.06 mg/g)and succinic acid(5.05 mg/g).Also,its protein degradation was more complete,higher concentrations of polypeptides and free amino acids,short-chain volatile acids,3-hydroxy-2-butanone,benzaldehyde,furfural,and δ-dodecanone were accumulated,which led to stronger fermentation aroma and sourness attributes,and gained higher sensory evaluation score;while the flavor prepared by one-step enzymolysis combined with fermentation had higher concentrations of acetic acid(1.59 mg/g)and citric acid(1.23 mg/g),and was more associated with higher levels of free fatty acids and medium-long chain volatile acids,which led to rancid and astringent flavor.Two-step enzymolysis combined with fermentation was regarded as the proper method to develop the target cream-SPI flavor,and different samples were prepared by skipping one or several treatments in the whole process of this method to investigate the effects of L.casei LC1,protease and lipase on the main metabolites and volatiles,as well as on the flavor formation of the targeted milk flavor.The results showed that L.casei LC1 consumed citric acid,serine,glycine,methionine,isoleucine,and promoted the accumulation of lactic acid,acetic acid,polypeptides,small peptides and short-chain free fatty acids.Protease was responsible for the accumulation of arginine,alanine,valine,lysine,tyrosine,phenylalanine and large molecular weight peptides,together with a small amount of polypeptides and small peptides.Lipase promoted free fatty acids,and specifically metabolized histidine and tryptophan.The combination of L.casei LC1 and protease was beneficial to the release of free amino acids,the proliferation and metabolism of L.casei LC1,and thus promoting the production of key volatile compounds including benzaldehyde,acetic acid,3-hydroxy-2-butanone,2,3-butanedione,2-furaldehyde,and 2-pentylfuran;while the treatment of first protease and then lipase,was responsible for the production of key volatiles including butanoic acid,hexanoic acid,octanoic acid,decanoic acid,9-decenoic acid,dodecanoic acid,ethyl caprylate,δ-dodecalactone,as well as for the metabolism of aldehydes,methyl ketones and alcohols into key esters and volatile acids.The flavor prepared by two-step enzymolysis combined with fermentation and cream without any treatment were added into bread in different levels to analyze the effects of cream and cream-SPI milk flavor on bread quality.Cream made little contribution to bread flavor,while little milk flavor could significantly change the composition of volatiles in the bread.The proportion of alcohols,aldehydes,acids,ketones,esters,heterocycles and others in the control bread was 65.33%,18.20%,0.44%,5.87%,6.32%,2.94% and 0.91%,respectively.But that in the bread with the lowest level of milk flavor(2%JP+L),the proportion was changed to 50.76%,11.81%,12.73%,4.66%,16.66%,2.72% and 0.66%;whereas that in the bread with the highest level of cream(8%C),the proportion was only changed to 67.60%,13.94%,0.19%,8.07%,6.11%,3.22%,0.86%,0.43% and 0.45%,respectively.Both cream and cream-SPI milk flavor could increase the specific volume and decrease the hardness of bread,as well as promote gas production of yeast,and the effect became more and more significant with the increase of added content.Moreover,the effect of milk flavor turned to be more remarkable.For the bread added with the highest level of milk flavor(8%JP+L),its specific volume and yeast gas production increased to 4.19 m L/g and 2306 m L from 2075 m L and 336 g of the control bread,and the hardness decreased to 172 g from 336 g.Both cream and milk flavor had no significant effect on the color of bread,but both of them weakened the gas retention ratio of dough.As the addition level increased,the bread added with cream showed a trend of first increasing and then decreasing in G’ and G’’,but no significant change in tan δ;while the bread added with milk flavor appeared a trend of first rising,then falling and finally rising in G’,G’’ and tan δ,which may be due to the establishment of a new cross-linking structure between SPI and gluten protein,and thus improved the dough rheology.It is apparent that the novel milk flavor could improve the bread quality much better than cream. |
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