Study On The Formation And Improving Methods For Off-flavor Of Soymilk And Soy Milk Fermented Liquid

Soymilk is a kind of vegetable protein beverages.Western and Asian consumers have different sensitivity to beany flavor,and flavor quality is an essential factor affecting the consumption level of soymilk products.Compared with soymilk,fermented soymilk has more acceptable flavor quality,but the residual beany flavor is still a quality defect.Fermented soymilk beverage,as a new soy-based beverage,is obtained by fermented soymilk added with water,sugar and then treated with sterilization,however,the effect of sterilization on flavor quality is not clear.Therefore,the intrinsic relationship of soybean components and soymilk flavor quality was first investigated to establish an evaluation method based on soybean cultivars to predict soymilk flavor characteristics;the effects of hot-water blanching and grinding,cooking temperature,cyclodextrin addition on critical soymilk flavor compounds were then analyzed to provide the parameter ranges for improving the flavor quality of soymilk;subsequently,the flavor quality of soymilk fermented liquid before and after heating were compared,the condition for the change in flavor quality were elucidated,the reason for the residual beany flavor in the soymilk fermented liquid was explored,and the improving methods were accordingly proposed.The results were listed as follows:According to the clustering analysis for the critical physicochemical indexes of soyebean that significantly affected soymilk flavor,67 soybean cultivars could be classified into three classes.The results of principal component analysis showed that,the second class was characterized by intense soymilk flavor,and the third class had weak flavor,while the first class fell in between.Two discriminant functions based on the critical physicochemical indexes were established by discriminant analysis.For unknown or new soybean cultivars,the soymilk flavor characteristics could be predicted by applying the critical physicochemical indexes of soybean into discriminant functions.The effects of different processing methods on soymilk flavor were analyzed.For hot-water blanching and grinding,when the temperature was>80?,beany flavor was significantly decreased,and the temperature higher than 90? also led to significant loss of non-beany flavor,while 70? was sufficient to improve the flavor quality of soymilk.If the cooking temperature was>110?,the contents of beany flavor compounds and non-beany flavor compounds tended to decrease,and the flavor quality of soymilk was improved;?0.5%?-cyclodextrin added at 60? during soymilk heating or after heating could effectively decrease beany flavor,and when 0.75%?-cyclodextrin was added after heating,non-beany flavor compounds didn't suffer great loss,and the flavor quality of soymilk was best.Hexanal(the critical beany flavor compound)in soymilk or SPI solution existed as free hexanal and bound hexanal,and the latter mainly bound with soy globular proteins(especially 1IS).During fermentation,lactic acid bacteria could metabolize all the free hexanal and a little bound hexanal.A moderate hydrolysis of soymilk or SPI solution with neutral protease would result in the conversion of a little bound hexanal into free hexanal,and the generation of more bound hexanal which could be metabolized by bacteria,which helped bacteria metabolize more hexanal,thus decreasing the residual amounts of this beany flavor compound.Besides,for the soymilk or SPI solution in the acidic condition caused by fermentation or acid adjustment,heating(?70?)would make a small amount of bound hexanal released and become free hexanal,therefore,a certain amount of hexanal was newly detected in the soymilk fermented liquid treated with heating.On the other hand,rancid sour odor was also generated in the soymilk fermented liquid treated with heating,but there was no significant difference in the flavor profiles of this fermented liquid before and after heating.For the heat-induced rancid sour odor,2,3-butanedione and 2,3-pentanedione(produced by lactic acid bacteria in soymilk whey),soy protein with>0.6%protein content in acidic condition,and heating(?80 ?)were necessary factors.During fermentation,Streptococcus thermophiles ST-M5 and Lactobacillus casei YMC1069 were able to produce 2,3-butanedione and 2,3-pentanedione,while L.bulgaricus Lb-12,L.acidophilus La28,L.plantarum LP45 hardly produced the two ketones,so they could be applied as starter cultures to avoid the generation of rancid sour odor.The addition of valine and isoleucine(1%:1%or 2%:1%)into soymilk could effectively inhibit the formation of two ketones during fermentation,but this also blocked the metabolism of bacteria,leading to the increase of pH.?0.25%cysteine,?0.5%arginine,?0.71%lysine,or ?0.3%NH3 H20 added into soymilk fermented liquid could help decrease heat-induced rancid sour odor,however,cysteine itself had off-flavor,while arginine and lysine caused alkaline flavor,the increase of pH and distinct browning,and NH3 H2O also gave rise to a pungent alkaline flavor and the increase of pH.