Screening Of High-producing α-galactosidase Lactic Acid Bacteria And Its Application In Three Legume Flour Sourdough Breads

In this study,a high yielding α-galactosidase enzyme producing lactic acid bacteria(LAB)strain Lactobacillus fermentum C2-8 isolated,screened and purified from Qu Starter,was used as a starter in legume flour sourdough fermentation to reduce the stachyose and raffinose content.In addition,the effect of LAB starter fermentation on biochemical characteristics(cell count,organic acid,raffinose and stachyose degradation,protein hydrolysis and antioxidant properties)of three kinds of legume flour sourdough and baking quality of the legume flour sourdough bread were studied.The main research findings in this study are shown below:Out of 126 presumptive LAB strains isolated from 9 kinds of substrates(Koji,Cheese,Natto,etc.),one high yielding α-galactosidase enzyme presumptive LAB isolate strain was screened by X-α-Gal plate and p-nitrophenol method was selected.The isolate strain C2-8 had an enzyme activity of 30.89 U/g.Using 16 s rDNA,C2-8 strain was identified as Lactobacillus fermentum.At an addition ratio 1:1,raffinose and glucose were the optimal carbon sources for C2-8,with an optimal cultivation time of 18 h,and optimal pH and temperature for α-galactosidase activity at 4.8 and 37°C.The optimal pH,fermentation temperature and time for enzyme production of strain C2-8 were 4.35,37°C,and 24 h,respectively.The strain grew well in the three sourdough,with cell counts of 8.5,8.6 and 8.4 Log CFU/g after 24 h of fermentation in CSD(chickpea sourdough),MSD(mung bean sourdough)and FSD(faba bean sourdough),respectively.Compared to MSD and FSD,the lactic acid and acetic acid content in CSD was highest after fermentation for 24 h,at values of 62.67 and 15.53 mmol/kg,respectively;this also corresponded to the lowest pH value.After 24 hours of fermentation,the raffinose and stachyose content in CSD,MSD,and FSD was completely degraded into glucose,fructose,and galactose.LAB fermentation significantly increased the content of α-amino nitrogen and total free amino acids in legume flour sourdough(p <0.05),while the proportion of high molecular weight peptides of all sourdough declined and that of small molecular weight peptides increased.As shown by the total phenol content,DPPH and ABTS free radical scavenging ability,the antioxidant properties of all sourdoughs were significantly improved after fermentation by LAB strain.The dough characteristics showed that,compared with the control group,LAB fermentation improved the processing ability of bread dough.Addition of sourdough promoted the gluten network extension of gluten network and altered the starch granule structure;this improved the gas production and retention capacity of bread dough.The baking characteristics of bread showed that,LAB fermentation improved the texture and gas cell distribution of legume flour bread.When compared with CYB(chickpea yeast bread),MYB(mung bean yeast bread)and FYB(faba bean yeast bread),the specific volume of CSB(chickpea sourdough bread),MSB(mung bean sourdough bread)and FSB(faba bean sourdough bread)was higher,while hardness,adhesiveness and chewiness reduced significantly(p <0.05).The CD and AF value of bread crumb significantly increased(p <0.05),while quality of chickpea,mung bean and faba bean bread was improved.The nutritional characteristics of bread showed that,sourdough fermentation significantly increased the free amino acid(FAA)content of bread.The order of total FAA content of the six bread types was MYB <CYB <FYB <FSB <CSB <MSB.Compared with CYB,MYB and FYB,the proportion of essential amino acids of CSB,MSB,FSB increased by 2.05,1.47 and 2.08 times,respectively.After sourdough fermentation,the proportion of rapidly digestible starch(RDS)in CSB,MSB and FSB significantly decreased,while that of slowly digestible starch(SDS)and in vitro digestibility of protein(IVPD)was significantly increased(p <0.05).The overall nutritional value of CSB,MSB and FSB improved.The flavor characteristics of bread showed that,54,48,56,48,58,and 49 types of volatile compounds were detected in CYB,CSB,MYB,MSB,FYB,and FSB,respectively.After sourdough fermentation,the types of flavor compounds slightly reduced,but the content,especially of ketone,alcohol,acid and ester flavor compounds increased(p <0.05).The electronic tongue data revealed that the taste of sourness,saltiness,umami,richness in CSB,MSB and FSB were significantly higher than CYB,MYB and FYB,respectively.This shows that LAB fermentation could improve the flavor and taste qualities of three types of legume flour bread.The storage characteristics of bread showed that addition of sourdough delayed the rate of bread hardening and the growth rate of aging enthalpy value,reduce water loss,and effectively delayed the aging of legume flour bread.