| Fruit bread has a pleasant aroma,?-glucosidase can be used as a flavor enzyme to enhance the flavor of the product,and the two could be combined to improve the bread flavor and baking characteristic through sourdough fermentation technology.In this study,lactic acid bacteria were isolated from four Qu starters,and ?-glucosidase producing lactic acid bacteria were screened by plate colored method and enzyme activity measurement.The enzymatic properties of crude enzyme in the strains were studied and the enzyme activity was located.One strain was selected and used in raisin sourdough fermentation,the basic physical and chemical properties,antioxidant capacity and flavor characteristics of the sourdough was determined,it was found that high ?-glucosidase producing lactic acid bacteria could grow in wheat dough matrix and the enzyme producing activity was preserved.Then the specific volume,texture,antioxidant capacity,flavor and storage properties of the bread were researched,the high ?-glucosidase producing lactic acid bacteria was applied in raisin sourdough making successfully and improved the bread baking properties.The main contents of the research was as followed:A total of 126 lactic acid bacteria strains were isolated from four Qu starters,and 28 ?-glucosidase producing strains were screened by esculin plate colored method.The second screen was done by comparison of enzyme activity,the results showed: the enzyme activity of 28 strains was in the range of 2.2 U/g~10.1 U/g,and the three strains(M12,M24 and J28)with the highest activity were selected for 16 s r DNA gene identification and follow-up study.It was identified that both M12 and M24 were both Lactobacillus harbinensis and J28 was Pediococcus pentosaceus.The results of enzyme activity positioning analysis showed that the three ?-glucosidase producing strains were all intracellular enzymes.Furthermore,the optimum conditions for the enzyme produced by the three strains were studied.It was found that the optimum temperature of M12 and M24 was 37?,while for J28,it was 44?.The optimum pH of the strains was also different,for M24 and J28,it was 4,however the optimum pH of M12 is 4.5.Under the optimum conditions,M12 showed the highest activity and had more practical values.Therefore,M12 was selected as the main fermentation strain of grape raisin sourdough.The growth curve and enzyme activity curve of M12 in liquid medium was determined.The results showed that the colony number of M12 was stable after 12 hours of inoculation,and the highest activity was obtained at the same time,the number of colonies and enzyme activity would decline if continue to ferment.The change regulation of colony number and enzyme activity in sourdough was almost the same as that in fermentation broth.The above studies showed that M12 could grow in the sourdough containing raisin and preserve the original enzyme production ability,it was suitable as a starter for raisin sourdough fermentation.The results of nutrition analysis showed that the total phenol content and DPPH free radical scavenging ability of raisin sourdough fermented with lactic acid bacteria(RSD)was significantly higher than that of common raisin dough(RD)and wheat dough(WD).At the same time,its resveratrol content was increased by 53.7% than RD,which means strong antioxidant capacity.In terms of flavor,the addition of raisin in RD increased the number of volatile flavor substances in the dough by 39.13% compared with WD and the total peak area of flavor compounds was increased by 25.74%.After the fermentation of high ?-glucosidase producing strain M12,the number of volatile flavor substances increased by 56.25% and the total peak area was 34.49% higher than that of RD.Among the compounds,the content of acids,alcohols,ketones and terpenes substance was increased significantly.RSD was used for bread making,and the specific volume,texture,nutrition,flavor and storage characteristics of raisin sourdough bread(RSB),common yeast bread(CYB)as well as raisin bread(RYB)were all analyzed.The results showed: compared with RYB and CYB,RSB could significantly improve the specific volume of bread.As for the texture,RSB showed lower hardness and gumminess,greater elasticity,which means better bread quality,followed by RYB,and the texture quality of CYB was the worst.For the nutrition characteristics,the total phenolic content of all bread samples was greatly lost,the capacity of DPPH free radical scavenging rate was also reduced,while compared with the other two groups,the loss rate of RSB is the lowest.But there was no significant difference in resveratrol content loss between groups.Besides,52 kinds of flavor substances were detected in RSB,and the total number of flavor substances increased by 52.9% compared with CYB,meanwhile,the total peak area was increased by 65.5%.The results of sensory evaluation showed that the raisin sourdough bread(RSB)was superior to common yeast bread(CYB)and raisin bread(RYB)in terms of appearance,flavor and taste.The overall acceptance score sorted from high to low was RSB> RYB> CYB,which confirmed the reliability of the bread baking quality,flavor and other research results of the relative analysis.The colony number,hardness and aging enthalpy of three kinds of bread was researched.The results showed that compared with RYB,RSB could prolong the time of bacterial overproof by 2 days and the time of mold overproof was prolonged by 6 days.At the seventh day of storage,the hardness of RSB was 36.14% lower than that of CYB,31.37% lower than that of RYB,meanwhile the aging enthalpy of amylopectin was lower,which means the raisin sourdough could delay the aging of bread effectively. |
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