Research On The Fermentation Of Mulberry Leaf Lactic Acid Bacteria And Its Quality Characteristics

Mulberry leaf（Folium Mori）has a wide range of cultivation,easy survival,large annual output and rich nutritional value.Therefore,it is a high-quality and inexpensive functional food,which reflects extremely high edible and medicinal value.However,in the process of developing mulberry leaf food,a common problem was discovered.The volatile components of mulberry leaf are complex and there is a heavier grassy smell,which makes it difficult for some people to accept it,thus hindering its extensive development in the food field.Therefore,research on mulberry leaf processing technology is helpful to the development and utilization of mulberry leaf,and provides a certain reference for the further development of mulberry leaf related products in the future.In this paper,five kinds of lactic acid bacteria were used to ferment mulberry leaves,and the strains that can grow well,quickly produce acid and have good sensory quality in the mulberry leaf medium were screened.Secondly,the fermentation process parameters were optimized;the basic components,active substances.The main research results are as follows:（1）Through the determination and analysis of the total acid,pH value,OD₆₀₀and other indicators in the fermentation process of different strains of mulberry leaves,three strains suitable for fermentation of mulberry leaves were initially screened out（leuconostoc mesenteroides,Lactobacillus plantarum,Lactobacillus brevis）).The Three strains were used for the compound fermentation test,and the best combination of fermentation strains was obtained:Leuconostoc mesenteroides and Lactobacillus brevis were mixed in equal proportions,and the fuzzy sensory score of mulberry leaves in this compound group was the highest,which was 82.8;Lactic acid bacteria grow well,the total acid content is 8.72±0.42 g/kg,and the OD₆₀₀value is 1.397±0.03.On this basis,the single factor and response surface optimization experiments were carried out,and the response surface model of fermentation conditions was obtained:Y=-145.39+10.23A+1.46B+10.63C+4.54D-8.333e-003AB-0.045AC-0.113AD+9.583e-003BC+0.067BD+0.025CD-0.778A2-0.027B2-0.178C2-0.69D2.The optimal process conditions were determined to be 5%glucose addition,culture temperature31℃,culture time 39h,and inoculation volume 5%.（2）Use GC-O-MS combined with odor activity value（OAV）analysis.According to the analysis of OAV value,the flavor compounds with OAV>1 in unfermented mulberry leaves in descending order of OAV value are1-octen-3-ol>myristaldehyde>eugenol>octanol>nonanal>trans-2-Octenal>decanal>β-c yclocitral>cis-3-hexenol.The grassy and fishy substances identified by GC-O smell are:cis-3-hexenol,1-octene-3-ol,trans-2-octenal,nonanal,decanal,Myristaldehyde.Combining the OAV value and the GC-O olfactory identification results,it is preliminarily identified that the grassy and fishy flavors in unfermented mulberry leaves are cis-3-hexenol,1-octen-3-ol,and trans-2-octenal,Nonanal,decanal,and myristic aldehyde.After lactic acid bacteria fermentation of mulberry leaves,the content of substances showing grassy and fishy flavors is significantly reduced（p<0.05）,showing strong floral and fruity aromas such as linalool,nonanol and other alcohols and ethyl caproate and ethyl decanoate.And other ester compounds increase.（3）Use Principle Component Analysis（PCA）to analyze and screen the types of volatile flavor substances in the fermentation process of mulberry leaves.PCA converts 7 types of volatile components into 3 principal components,and the cumulative contribution rate is It is 96.210%,which can retain the original data to a large extent.Phenols and acids as the main volatile substances are highly positively correlated with PC1（load coefficients are 0.952,0.814,respectively）;aldehydes and ketones are highly negatively correlated with PC1（load factor is-0.949）;alcohols are highly positively correlated with PC2（Load factor is 0.971）;olefins are highly positively correlated with PC3（load factor is 0.975）;esters have a certain degree of positive correlation with PC1 and PC2（load factors are 0.629,0.757,respectively）.Through the load distribution of volatile substances in samples at different fermentation times,it can be concluded that aldehydes,esters,alcohols,acids,olefins,and phenols are volatile substances with significant changes in the content of samples at different fermentation stages.（4）The dynamic changes of 9 indicators including total phenols,total flavonoids and tannins during the fermentation process were analyzed and determined.The study found that after lactic acid bacteria fermentation of mulberry leaves,the total phenol content increased by 14.07%and the total flavonoid content decreased by 8.01%;in terms of nutrients,the retention rate of polysaccharide content was 77.56%,and the retention rate of crude protein content was 92.17%.There was no significant change in fiber content（p<0.05）;in terms of anti-nutritional factors,the content of phytic acid and tannin decreased significantly（p<0.05）,down 20.57%and 67.40%,respectively.（5）The four in vitro antioxidant indexes of DPPH free radical scavenging activity,ABTS+free radical scavenging activity,iron ion reduction ability and total antioxidant capacity in the fermentation process of mulberry leaf lactic acid bacteria were measured,and the metabolites in the fermentation process and in vitro Correlation analysis of antioxidant capacity.The results showed that the in vitro antioxidant capacity indicators all showed a trend of a short-term decline and then a gradual increase.The total phenol content was significantly correlated with DPPH free radical scavenging rate and ABTS+free radical scavenging rate（p<0.01）,and significantly correlated with iron ion reduction ability and total antioxidant capacity（p<0.05）.The reduction ability of iron ions was also significantly correlated with total acid content（p<0.05）,and was extremely significantly correlated with phytic acid content（p<0.01）.