| Dendrobium candidum belongs to the genus Dendrobium Sw.of the family Orchidaceae,and is a valuable medicinal herb of homology of medicine and food.It contains polysaccharides,alkaloids,polyphenols,etc.,which has various active effects such as hypoglycemic,regulation of intestinal flora,anti-tumor,antioxidant and enhancement of immunity.Recently,lactic acid bacteria fermentation technology has been widely used in fruit and vegetable juice fermentation due to its significant advantages in preserving quality,improving flavor and enhancing nutrition.With the increasing scale of Dendrobium candidum cultivation,there is an urgent need for in-depth development and research on its products and functionality to meet the demand for health food.In this study,the dynamic changes of viable bacteria count,p H value,total sugar content,total phenolic content,antioxidant capacity and the correlation between active ingredients and antioxidant capacity of Dendrobium candidum juice(DCJ)during 72 hours of fermentation were analyzed by using lactic acid bacteria to ferment DCJ;the fermentation process was optimized by single factor experiments and response surface analysis,and the changes of nutrients and flavor components of DCJ before and after fermentation were analyzed by UPLC-QTOFMS/MS and GC-MS.The research results can provide theoretical reference and data support for obtaining the fermentation broth of Dendrobium candidum with rich nutrition and unique flavor,and provide new ideas for the development and research of other medicinal and edible homologous plant foods.The main results of this research were shown as follows:(1)Preparation conditions of DCJ and screening of fermentation strains.The stems of Dendrobium candidum were used for juicing after cutting and bleaching under different conditions,then,the changes in color and phenolic content of DCJ were analyzed and the optimal sterilization conditions were determined by pasteurization.The results indicated that when the bleaching conditions were set at 100 ℃ for 1 min,the loss of phenolic content of DCJ was the least(33.64mg/L)and the color was relatively better,the chromaticity value(L* value)was 39.23,the a* value was 1.14,and the △E value was 1.80;the best sterilization temperature was at 90 ℃,time for 5min,and the total number of bacteria in DCJ reached the undetected state at this time.The DCJ was prepared by single bacteria fermentation and two-two mixed strains(v/v)fermentation with Lactobacillus plantarum(Lm),Lactobacillus acidophilus(Ls)and Lactobacillus paracasei(Li)in the ratio of 1:1,respectively.Results showed that the mixed bacteria fermentation was better than the single bacteria fermentation in terms of both the number of viable bacteria and acid-production ability.(2)The analysis of the dynamic changes of viable bacteria count,p H value,total sugar content,total phenolic content,antioxidant capacity and the correlation analysis between active ingredients and antioxidant capacity of DCJ during 72 hours of fermentation.All the three mixed strains could grow well in the FDCJ,and the group with the highest viable bacteria count was(Ls+Li),with 8.72 lg(CFU/m L)of viable bacteria.(Ls+Li)consumed the least polysaccharide(24.89%),which was lower than the initial value,followed by(Lm+Li)(40.72%)and(Lm+Ls)(42.31%).The total phenolic content increased continuously during the early stage of fermentation and decreased in the late stage of fermentation,with the largest increase of 23.58% for polyphenols by(Ls+Li).The FDCJ had the greatest effect on the scavenging ability of DPPH radicals,meanwhile,the scavenging ability of hydroxyl radicals and ABTS radicals were also highly enhanced,with the increase in the scavenging ability of(Lm+Li)for ABTS radicals(up to 41.42%),and the increase in the scavenging ability of(Ls +Li)to hydroxyl radicals was increased by 32.73%.The study showed that lactobacillus fermentation could improve the relevant active ingredients and antioxidant capacities in DCJ,and further improve the quality of Dendrobium candidum.(3)Analysis of nutritional components of Dendrobium candidum before and after fermentation.By analyzing the changes in nutrient components such as total phenolic content,polysaccharides,before and after DCJ fermentation,results showed that a total of 18 polyphenol compounds,including 5 hydroxybenzoic acids and their derivatives,3 hydroxycinnamic acids and their derivatives,7 flavonoids and their derivatives and 3 other compounds were identified in DCJ and FDCJ;14 polyphenol compounds were identified in DCJ and 12 polyphenol compounds were identified in FDCJ,among which,a total of 8 compounds were common.The molecular weight of polysaccharides in DCJ was mainly distributed between 1701 and 352904 Da,and their average molecular weight was 175778 Da.The molecular weight distribution of polysaccharides after lactic acid bacteria fermentation was 1448～313601 Da,and their average molecular weight was 91281 Da.(4)Analysis of the nutritional composition of Dendrobium candidum flavors before and after fermentation.By analyzing and comparing the changes of nutrient components such as volatile substances,amino acids and organic acids before and after DCJ fermentation,21 volatile compounds were identified in the DCJ,including 2alcohols(3.96%),3 aldehydes(4.73%),8 olefins(65.16%),1 ketone(2.31%),1 ester(10.30%),and 6 others(13.53%);26 volatile compounds were identified in FDCJ,including 6 alcohols(53.57%),5 aldehydes(7.71%),5 olefins(24.46%),3 ketones(1.85%),3 phenols(3.87%),1 ester(5.04%),and 3 others(3.50%),with the main volatile components changing from olefins before fermentation to alcohols compounds.Furthermore,we found that there were obvious changes in the types and contents of free amino acids in DCJ after fermentation by lactic acid bacteria,9 free amino acids were detected in DCJ at 1.48 mmol/L,however,15 free amino acids were detected in FDCJ at 5.85 mmol/L.Furthermore,3 organic acids were detected in DCJ,namely succinic acid,malic acid and citric acid;and 4 organic acids were detected in FDCJ,namely lactic acid,succinic acid,malic acid and acetic acid,and the total amount of organic acids increased from 1671.9 mg/L to 2045.4 mg/L.(5)Applying response surface methodology to optimize fermentation.The effects of inoculum amount,fermentation time and fermentation temperature on polysaccharide content,total phenolic content,hydroxyl radical scavenging capacity and antioxidant capacity of FDCJ were analyzed by single factor experiments,and response surface optimization was performed on the basis of single factor experiments.The maximum yield of polysaccharide content of 4.57mg/m L was achieved after fermentation time was 46 hour and temperature 33.5℃ with the inoculation amount was 4%. |
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