Process Optimization And Metabolomics Of Strawberry Juice By Probiotic Fermentation

Strawberry juice is rich in polyphenols,flavonoids and other active ingredients,providing high nutritional and health benefits.Fermented strawberry juice retains the nutrients and active factors in strawberry fruit,enhances the health value of lactic acid bacteria fermented beverage,and meets the increasing demand for natural health foods.In this research,strawberry juice was used as raw material to select lactic acid bacteria strains suitable for fermentation.The optimal fermentation process and magnetic field assisted process of strawberry juice were determined by adding the optimized strain for fermentation.To explore the effects of magnetic field assisted fermentation on nutrients and metabolites of fermented strawberry juice,the main research results and conclusions are as follow:(1)Lactobacillus plantarum(LP121),Lactobacillus acidophilus(LA011),Streptococcus thermophilus(ST021),Lactobacillus rhamnosus(LR026),Lactobacillus reuteri(LR041)and Lactobacillus casei(LC021),were selected and strawberry juice was used as a fermentation substrate to compare the fermentation performance,antioxidant capacity,active ingredient content and aroma components of strawberry juice fermented by different strains.The final reducing sugar content of fermented strawberry juices was 75.7 mg/m L,the total acid content was 18.28 g/L,the polyphenol content was 1.02 mg/m L,and the DPPH radical scavenging rate was 81.03%.Seven unique volatile substances,including ethyl butyrate,ethyl valerate,thymol,linallinol,citronellol,2-amylfuran,and geranial were produced.Aroma components were the most diverse and most abundant.They were significantly different from other lactic acid bacteria and unfermented strawberry juice(P<0.05).After fermentation of strawberry juice,the proportion of irritating and unpleasant odor substances decreased,and some volatile substances with fruity and floral flavor increased.The results show that LP121 has the best fermentation performance and acid production capacity,the highest polyphenol content,and the highest DPPH free radical scavenging rate.(2)Lactobacillus plantarum LP121 was used as the fermentation strain,and fermentation temperature,fermentation time,and inoculation amount were selected for response surface tests.The optimal fermentation process of strawberry juice was determined as a fermentation temperature of 35 ℃,fermentation time of 24 h,and an inoculation rate of 2%.The acid yield of fermented strawberry juice was 16.91 g/L,according to the fuzzy evaluation method,the sensory score was 2.90,and the total phenol content was 1.04 mg/m L、the total flavonoid content was 0.148 mg/m L,DPPH free radical scavenging rate was 81.64 %,and the hydroxyl free radical scavenging rate was 65.85%.(3)The magnetic field assisted fermentation conditions were optimized under the same fermentation temperature,fermentation time,and inoculation quantity.The optimal process of magnetic field assisted treatment was 1 h,and magnetic field assisted intensity was 2.0 m T.The acid yield of fermented strawberry juice was 15.36 g/L,the sensory score was 2.92,and the total phenol content was 1.03 mg/m L,the total flavonoid content was 0.18 mg/m L,DPPH free radical scavenging rate was 84.77%,and the hydroxyl free radical scavenging rate was66.12%.(4)Different metabolites of fermented strawberry juice under two fermentation methods were determined and multivariate statistical analysis was performed.The results showed that the principal component analysis model demonstrated significant separation of metabolites between the magnetic field assisted fermentation group and the static fermentation group.The contribution rate of principal component one and principal component two was 78% and8.50%,respectively,and the total contribution rate was 86.50%.Based on the results of orthogonal partial least squares discriminant analysis,32 metabolites were identified as differential metabolites(with a variable importance projected value >1 and P<0.05),including8 organic acids,4 amino acid,4 sugars and their derivatives,8 alcohols and esters,3 ketones and aldehydes,and 6 other metabolic substances.There were 20 up-regulated substances and12 down-regulated substances,up-regulated substances accounted for 62.50% of the total differential metabolites,and down-regulated substances accounted for 37.50% of the total differential metabolites.Pathway analysis of differential metabolites showed that the top 5metabolic pathways with significant influence were Biosynthesis of amino acid,Glyoxalic acid and dicarboxylic acid metabolism,Biosynthesis of phenylpropanoids,Flavonoid biosynthesis,Sphingolipid metabolism.