Study On The Application Of Tannase-producing LAB In Legumes Sourdough Steamed Bread

Steamed bread is a popular traditional fermented wheat-based food product in China.Legumes are characteristically high in protein,low in fat,possess multi-fiber and considered of high nutritional value.In this regard,use of legumes in wheat flour to make steamed bread can enhance protein content of steamed bread,while also increasing the overall nutritional value of the bread.However,legumes usually contain ant-nutritional factors(ANFs)such as tannin,phytic acid,and oligosaccharides,among others,which negatively impact nutrient digestion and absorption in the human body.In addition,presence of legume flour in steamed bread lowers the overall quality of the bread.Lactic acid bacteria(LAB)can produce enzymes that degrade anti-nutritional factors.Use of these LAB strains during sourdough fermentation may improve the quality and effectively reduce the anti-nutritional factors in legume flour based steamed bread,while improving its nutritional value.In this study,a high tannase enzyme producing LAB strain isolated from naturally fermented faba bean flour was used as starter in the fermentation of red bean,lentil and faba bean sourdough;the sourdoughs were used to prepare steamed bread.Growth characteristics of the LAB strain,biochemical changes during the LAB fermentation process of legume flour sourdoughs,degradation of anti-nutritional factors in legume flour sourdough fermentation,steaming characteristics,storage characteristics and nutritional characteristics of the legume sourdough steamed bread were studied.The main findings and conclusions are as follows:Four tannase enzyme producing presumptive LAB strain isolates were screened by the chromotropic circle method.Of the 4 strains,D23 isolate had the highest(P<0.05)tannase enzyme activity(8.77 U/m L),and was selected for use in further experiments.Using 16 S r DNA gene sequence,D23 isolate was identified and confirmed as Lactobacillus fermentum.Strain D23 exhibited stable growth and reached a microbial cell count of more than 9 Log CFU/m L when cultured at 37? after 8 hours.The tannase exhibited mainly extracellular enzyme activity with an optimum temperature and p H of 32? and 4.5,respectively.Strain D23 adapted and adequately grew in red bean(RS),lentil(LS)and faba bean(FS)sourdough,with a cell count of 7.83 Log CFU/g,7.88 Log CFU/g,7.64 Log CFU/g,respectively,after 4 hours of logarithmic growth phase.After sourdough fermentation by strain D23,the condensed tannin content decreased significantly by 57.87%,53.54% and62.95% in RS,LS and FS,respectively(P<0.05).The content of other anti-nutritional factors also decreased(P<0.05).The free total phenol content increased by 75.70%,66.03% and169.12% in RS,LS and FS,respectively.The scavenging rate of DPPH free radical and ·OH free radical also increased.Moreover,content of high molecular weight peptide decreased and small molecular peptide increased,this was most significant in FS.Compared to the controls,total free amino acids(FAA)content increased by 2.48 times,2.22 times and 2.13 times,to reach 691.92 mg/100 g,788.21 mg/100 g and 859.87 mg/100 g in RS,LS and FS,respectively.Similarly,the essential FAA content increased by 429.31%,358.20% and 485.19%,in RS,LS and FS,respectively.At the same time,the content of soluble dietary fiber(SDF)increased by2.01 times,2.87 times and 1.90 times in RS,LS and FS,respectively,after sourdough fermentation.While the content of insoluble dietary fiber(IDF)decreased by 26.43%,19.11%and 20.23% in RS,LS and FS,respectively.The legume sourdoughs were used to prepare steamed bread.The sourdough steamed breads were compared with legume flour steamed bread prepared without sourdough.Based on the dynamic rheological and microstructural results,addition of sourdough moderately weakened gluten network and homogenized the internal structure of the dough,reduced its viscoelasticity property which made the dough softer and enhanced its processing performance.In the prepared steamed bread,specific volume and height diameter ratio of each legume sourdough steamed bread increased,while hardness of red bean(RSB),lentil(LSB)and faba bean(FSB)sourdough steamed bread decreased by 6.91%,23.49% and13.43%,respectively.Sensory evaluation of RSB,LSB and FSB was higher than that of control group.During storage,compared with the control,after 72 hours,hardness of RSB,LSB and FSB decreased by 14.67%,13.71% and 16.02%,while the enthalpy of aging decreased by 14.93%,17.94% and 20.77%,respectively.Also,the relaxation time T21,T22 and T23 fluctuated less,the soluble starch content decreased more slowly,and the contaminating microorganisms grew less in sourdough groups.Through legume sourdough fermentation by D23 strain,water holding capacity of steamed bread were enhanced to an extent which reduced water loss during the storage period and thus delay the aging of steamed bread and extend the shelf life.Compared with the control(RB,LB and FB),the contents of alcohols,acids,esters and other volatile flavor compounds increased after addition of the respective sourdoughs.For instance,the esters content increased by 76.16%,37.07%,and 145.69%,in RSB,LSB,FSB,respectively.This made the sourdough steamed bread a richer aroma.In addition,the in vitro digestibility of starch and protein in steamed bread prepared with sourdough was effectively improved;total FAA content increased from 129.75 mg/100 g,125.61 mg/100 g,146.20 mg/100 g to 320.27 mg/100 g,205.61 mg/100 g,199.01 mg/100 g,and condensed tannin content decreased by 58.71%,55.87% and 70.51% in RSB,LSB and FSB,respectively.The nutritional value of protein in the steamed bread prepared with legume sourdoughs fermented by LAB strain was superior and higher than the controls.