Preparation And Characterization Of Freeze-Dried Highland Barley Sourdough Powder And Its Effect On The Quality Of Sourdough Bread

The use of sourdough is increasingly popular with consumers as it helps to improve the nutrition,flavor and texture of bread.Unlike fresh sourdough,sourdough powder has advantages of high stability and easy usage.In this study,Lactiplantibacillus plantarum LSSR05 was used as a starter to prepare highland barley sourdough.The microbial communities and functional properties were analyzed using metagenomic sequencing technology;the best freeze-drying protectant was screened and the highland barley sourdough powder was prepared by freeze-drying technology,and the physicochemical properties of sourdough powder and its effects on the processing characteristics,nutritional properties and sensory properties of bread were studied.The main results are as follows:(1)Lactiplantibacillus plantarum LSSR05 was used as the starter to prepare highland barley sourdough,and the microbial communities and functional properties of highland barley sourdough were analyzed by combining plate counting and metagenomic sequencing technology.The results showed that the microbial communities of the sourdough changed significantly,with the number of lactic acid bacteria(LAB)reaching9.19 lg(CFU/g)and yeast reaching 3.41 lg(CFU/g)after 12 h of fermentation,while Enterobacteriaceae decreased to less than 1 lg(CFU/g).The metagenomic sequencing results showed that the relative abundance of Lactobacillus increased from 77.11% to96.61%,indicating that Lactobacillus was dominant in the fermentation process,while Enterobacter and Bradyrhizobium decreased significantly,which corresponded to the results of plate counting.In addition,metabolic pathways such as carbohydrate metabolism and amino acid metabolism were dominant during fermentation,with higher abundance of starch and sucrose metabolism,pyruvate metabolism,glycolysis,alanine,aspartate,and glutamate metabolism.(2)Freeze-drying technique was used to prepare highland barley sourdough powder and the effect of freeze-drying protectant on the survival rate of LAB was investigated.The results showed that the survival rate of LAB in the control without the addition of freeze-drying protectant was only 3.75%.When 12% inulin was added,the survival rate of LAB reached 44.43%,which was significantly higher than the survival rate of LAB when trehalose and ice structuring proteins were added.Therefore,12% inulin was used as a protectant for the sourdough powder in subsequent trials.(3)The effects of fermentation and freeze-drying on the α-amylase activity,protease activity and antioxidant activity of sourdough were investigated,as well as the particle surface properties and storage characteristics of highland barley sourdough powder.The results showed that the α-amylase and protease activities of the sourdough were low before fermentation and increased to 11.25 U/g and 12.75 U/g respectively after fermentation,with a small decrease in enzyme activity after freeze-drying treatment.The antioxidant activity of the sourdough increased significantly after fermentation(P(27)0.05)and no significant change was observed after the freeze-drying treatment(P(29)0.05).Scanning electron microscopy results showed that inulin promoted the bonding between the starch particles of the sourdough powder,showing obvious agglomeration of clusters.In addition,the inulin-added sourdough powder maintained a high number and acid-producing capacity of LAB after storage at 4℃ for 60 days.(4)Highland barley sourdough powder was added to the bread dough to explore its effect on the gluten network structure.The results showed that the addition of fresh sourdough(BSB)and sourdough powder(BFB)facilitated the formation of a good gluten network structure in the bread dough compared to highland barley flour(BPB),whose starch granules were uniformly encapsulated in the gluten matrix.The secondary structure analysis showed that BFB had the highest proportion of α-helix and β-fold,with 36.53%and 18.87%,respectively,indicating that the protein structure would be more stable and thus more conducive to the formation of a gluten network structure,which corresponded to the microstructural results.(5)To explore the effect of highland barley sourdough powder on the quality of bread.The results showed that BFB had higher specific volume,fluffier texture and higher water holding capacity,with specific volume,hardness and baking loss of 3.55 m L/g,764.24 g and 8.28% respectively.The addition of sourdough improved the nutritional properties of bread,the total phenolic content in BSB reached 119.72 mg/100 g,and the scavenging rates of DPPH radicals and ABTS radicals reached 42.42% and 68.53%,respectively.In addition,compared with BPB,the in vitro starch digestibility of BSB was significantly reduced(P(27)0.05),and the in vitro protein digestibility was significantly improved(P(27)0.05),and there was no significant difference between BFB and BSB(P(29)0.05).The addition of sourdough also enhanced the sensory and flavor of the bread,with BFB having the highest overall acceptability of 7.50.Characteristic flavor components such as heptanol,acetic acid,acetophenone and ethyl lactate were detected in the sourdough bread,with higher contents in BSB,while the flavor components of BFB were slightly reduced due to the freeze-drying treatment.