| Soybean is a plant food with very high nutritional value.soy protein is rich in lysine which is commonly lacking in cereals,making it an ideal complementary food to cereals.Steamed bun is one of the traditional Chinese staple foods,and the preparation of steamed buns by fermenting soy milk with lactic acid bacteria(LAB)can not only enhance the flavor and nutrition of steamed buns,but also eliminate the soy smell and anti-nutritional factors associated with ordinary soy flour steamed buns.In this study,high-quality LAB were selected as fermenting strains from sourdough collected in Xinjiang and Yangzhou,and used to prepare fermented soy milk.Then fermented soy milk was used to make steamed buns,and the effects of fermented soy milk with different LAB on the fermentation characteristics of the dough,nutrition and storage characteristics of steamed buns were investigated.The main conclusions are as follows:(1)Isolation,identification and screening of dominant LAB in sourdough.A total of 84 strains of LAB were isolated from the traditional sourdough,all of which belong to Lactobacillus genus.By comparing the results of 16S rDNA sequencing,60 strains were obtained by removing duplicate strains,including 40 strains of Lactobacillus plantarum,12 strains of Lactobacillus fermentum,6 strains of Lactobacillus pontis and 2 strains of Lactobacillus pentosus.Three strains of Lactobacillus were finally selected as fermenting bacteria,L.plantarum grx201,L.fermentum grx202 and L.pentosus grx203,based on acid production and growth ability in dough,acid production and growth ability in soy milk,heat resistance,antibacterial ability and enzyme activity.(2)Effects of soy milk fermented by different LAB on dough characteristics.Three kinds of single LAB and complex LAB(1:1:1)were added into soy milk for fermentation.The fermented soy milk was mixed with flour and water,and then fermented after dough mixing.The rheological properties and microstructure of the dough were measured after fermentation for 1 h.It was found that adding fermented soy milk could weaken the gluten network of the dough,improve its viscoelasticity and make it softer.The contents of organic acid,sugar and a-amino nitrogen of dough after 24 h fermentation were determined.The results showed that the pH values of 6 groups of dough were 4.0-4.7,TTA was 6.99-11.0 mL,and the viable bacteria count was 6.72-8.94 Log CFU/g.Lactic acid and acetic acid contents of dough in 4 groups were higher than those of CG-D and DR-D significantly,and lactic acid content of L.pen203-D was the highest(18.57 mg/g),and acetic acid content of L.f-202-D was the highest(3.81 mg/g).After fermentation,glucose content was significantly decreased,while fructose and galactose contents were significantly increased,and raffinose and hydrothreose were not detected in all doughs.The content of α-amino nitrogen in fermented dough was significantly increased,and the content of L.p201-D was the highest(5.35 μmol/mL).(3)Effects of soy milk fermented by different LAB on the quality and nutritional characteristics of steamed buns.According to the effects of the amount of fermented soy milk on the specific volume and texture characteristics of steamed buns,the optimal amount of fermented soy milk was determined to be 40%.Three kinds of single LAB and complex LAB(1:1:1)fermented soy milk were used to prepare steamed buns,respectively.The sensory quality and nutritional characteristics were measured.The results showed as follows:compared with CG-B,the hardness and chewability of the four fermented soy milk steamed buns were significantly reduced,while the elasticity,viscosity and specific volume were significantly increased.In sensory evaluation,FP-B scored highest in taste,flavor and overall acceptability.The nutritional characteristics showed that the supplementation of fermented soy milk significantly increased the contents of protein and fat,but no significant difference was found in moisture content.Besides,compared with CG-B,the total phenol content of fermented soy milk steamed bun increased by 35.79%,total flavonoids content increased by 105.30%,DPPH free radical scavenging rate increased by 15.5%.Fermented soy milk improved the antioxidant capacity of steamed bread.In addition,compared with CG-B,the contents of condensed tannins and total saponins in FP-B were the lowest(2.34 mg/100g and 0.44 mg/g,decreased by 12.04%and 55.55%).The phytic acid content of L.pen203-B was the lowest(0.64 mg/g,decreased by 43.85%).Compared with CG-B,TFAAs content in the four groups were respectively increased by 10.39%,10.13%,10.13%and 10.73%,and lys increased by 1.29 times from 0.17 g/100g of CG-B to 0.39 g/100g of L.f202-B.Fermented soy milk addition made up for the deficiency of lysine content in cereal foods.In vitro digestibility,compared with CG-B,the glucose content of L.p201-B,L.f202-B,L.pen203-B and FP-B at the end of digestion was reduced by 8.11%,5.79%,9.73%and 13.60%,respectively.The IVPD of the four groups was improved by 5.39%,4.69%,4.28%and 6.51%,respectively.The fermented soy milk slowed down the speed of starch digestion,and promoted the protein predigestion in the steamed buns.(4)Effects of fermented soy milk with different LAB on flavor and quality of steamed buns during storage.A total of 48 volatile flavor compounds were detected in 6 groups of samples by GC-MS(incl.alcohols,aldehydes,esters,acids,ketones and other compounds).There were 30,33,40,35,39 and 47 kinds of flavor substances in 6 groups of samples,respectively,and 25 kinds of compounds co-existed in 6 kinds of steamed buns.The addition of fermented soy milk improved the flavor substances in steamed buns and endowed them with richer aroma.The results showed that the addition of fermented soy milk reduced the water loss,improved the water retention of the steamed bread,delayed the enthalpy growth rate,reduced the aging rate of the steamed bread,and inhibited the microbial contamination to some extent. |
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