Study On The Mechanism Of Lipid Metabolism And Gut Microbiota Regulation In Rats By Sweet Potato Residues Products Fermented With Lactobacillus

Sweet potato（Ipomoea batatas Lam.）is an annual or perennial vine herbaceous plant belonging to the Convolvulaceae family,which is the fifth principal food crop following rice,wheat,maize,and potato.According to FAO statistics,the sweet potato annual planting area and output of China in 2019 ranking first in the world,which were2.374 million hectares and 52 million tons,respectively.In China,sweet potatoes are mainly used for the production of starch and thereof products,a large amount of sweet potato residues generated and often discarded directly,resulting in waste of the high-quality resources.Due to its richness in starch,dietary fiber,protein and other substances,sweet potato residue was a good substrate to support the growth and metabolism of microorganisms.Sweet potato residue can be converted into high value-added products via microbial fermentation,which is of great significance to realize high-value utilization of sweet potato residue,reduction of environmental pollution,and enrichment of sweet potato processed products varieties.Recently,the research has focused on plant-based materials（grains,fruits,vegetables etc.）fermented with microorganism,which raise its quality via improving nutritional value and flavor,removing anti-nutritional factors,enhancing antioxidant activity and hypolipidemic activity,extending shelf life,etc.However,there is no report on the full utilization of sweet potato residue fermented by microorganism.Therefore,this study analyzed the effects of different kinds of strains fermentation on the nutritional and functional composition,physicochemical properties of sweet potato residues（SPR）,and the optimal probiotics were selected.The technological parameters process of SPR fermented with compound Lactobacillus rhamnosus and Pediococcus pentosaceus were optimized,and the SPR fermented under optimal conditions（FSPR）were obtained.In addition,the behaviors of FSPR during in vitro saliva-gastrointestinal digestion and colonic fermentation were investigated,and the mechanism of regulating lipid metabolism and gut microbiota of high-fat dietary rats were studied.This study provides a theoretical basis for realizing the full utilization of SPR,increasing the added value of SPR,and developing fermented SPR products with lipid metabolism and gut microbiota modulation activity.（1）Effect of five kinds of strains on nutritional and functional composition,physicochemical properties of fermented SPR were investigated,and the probiotics suitable for fermenting SPR were selected.The results showed that Lactobacillus rhamnosus（LR）CICC 23119,Leuconostoc membranosus（LM）CICC 21860,Pediococcus pentosaceus（PP）CICC 21862,Streptococcus thermophilus（ST）CICC21728 and Bifidobacterium longum（BL）CICC 6188 fermentation all significantly increased the contents of lactic acid,acetic acid,soluble dietary fibre（SDF）and total polyphenols,and decreased oxalic acid content（p<0.05）.Among them,SPR fermented by Lactobacillus rhamnosus（LRF-SPR）had the highest total polyphenols content（97.35 mg/100g DW）and lactic acid content（67.18 mg/g DW）,SPR fermented by Pediococcus pentosaceus（PPF-SPR）showed the highest SDF content（16.63 g/100g DW）.After fermentation with 5 kinds of strains,sweet potato residues products showed higher viable bacteria counts and antioxidant capacities,in which LRF-SPR had the highest viable bacteria counts of（10.03×10⁸CFU/m L）and antioxidant activities（DPPH free radical scavenging capacity:62.04 mg AAE/100g DW;Ferric ion reducing antioxidant power（FRAP）capacity:71.87 mg TE/100g DW）.The gray correlation analysis of nutritional and functional components and antioxidant capacities showed that the comprehensive nutritional value of LRF-SPR and PPF-SPR were rank first and second,respectively.Fermentation with 5 kinds of strains also increased the storage modulus（G’）,loss modulus（G"）,viscosity,volume-average particle size(D^[4,3]),and area-average particle size(D^[3,2])of the SPR fermented products,which could promote the stability of the fermentation product system.The SPR fermented with 5 kinds of strains showed obvious dietary fiber structure.The intensity and area of the absorption peak increased significantly at 1741cm^-1,and decreased at 3394cm^-1 and 1632cm^-1,indicating that fermentation could increase the content of SDF,and degrade starch and cellulose.The SPR fermented with 5 kinds of strains can improve the umami and sour tastes,and the gas chromatography mass spectrometry（GC-MS）analysis results showed that the proportion of volatile acids increased significantly after fermented with different strains.（2）The single factor experiment design was used to explore the process parameters of SPR fermented with LR and PP,and the characteristics of fermented sweet potato residues products under optimal process（FSPR）were studied.The results showed that the optimum conditions for preparing FSPR were as follows:fermentation time 48 h,solid-liquid ratio 1:10,inoculum amount 1.5%,fermentation temperature 37℃,LR:PP was 1:1,and the organic acids content and viable bacteria count of FSPR were 59.67mg/g and 9.83 lg CFU/m L,respectively.Compared to SPR,the SDF content（15.02g/100g DW）,total phenolic content（95.74 mg/100g DW）and antioxidant activities of FSPR were increased significantly,among which the DPPH free radical scavenging ability and Ferric ion reducing antioxidant power（FRAP）capacity increased by 1.68and 2.23 times,respectively.The intensity and area of the absorption peak increased significantly at 1741cm^-1,and decreased at 3394cm^-1and 1632cm^-1,indicating that probiotic fermentation could increase the content of SDF,and degrade starch,cellulose and hemicellulose in the process of enzymatic hydrolysis and fermentation.Compared to SPR,the storage modulus（G’）,loss modulus（G"）,viscosity,volume-average particle size(D^[4,3])and area-average particle size(D^[3,2])of FSPR were increased significantly（p<0.05）,indicating that FSPR showed higher viscosity and stability.The analysis of sensory characteristics showed that FSPR showed lower brightness（L*）value,higher sour tastes,umami and volatile acids compounds.（3）The in vitro simulation experiments were used to investigate the in vitro digestion and colonic fermentation behaviors of FSPR.The viable bacteria counts,total polyphenols content and total dietary fiber structure during in vitro stages were analyzed,and the effects of colonic fermentation on p H,short-chain fatty acids,and gut microbiota were also investigated.The results showed that during the simulated saliva-gastrointestinal digestion stages,the viable bacteria counts of FSPR decreased from6.93 lg CFU/g to 5.86 lg CFU/g（gastric stage）and 5.60 lg CFU/g（intestinal stage）,respectively.The total polyphenols showed no significant difference during simulated saliva-gastrointestinal digestion,but increased significantly during the in vitro simulated colonic fermentation process,after colonic fermentation for 48 h,the total polyphenols content could reach 179.89 mg GAE/100 g DW.In the simulated colonic fermentation stage,FSPR could significantly reduce p H,increase lactic acid,acetic acid,propionic acid and butyric acid contents.After colonic fermentation for 48 h,the relative abundance of bifidobacteria and Lactobacillus could be increased by 38.47%and 153.85%,respectively.Meanwhile,the relative abundances of Megamonas and Lachnoclostridium decreased by 61.33%and 38.47%,indicating that FSPR showed probiotic activity in vitro.The FTIR absorption peak intensity and area of FSPR total dietary fiber at 3394 cm^-1,2920 cm^-1 and 1741 cm^-1 wavenumbers were reduced significantly after colonic fermentation for 48 h.The main diffraction peaks of XRD patterns at different in vitro stages were 22.4°and 16.4°,and the crystallinity of FSPR total dietary fiber was 22.12%at colonic fermentation of 48 h,which was significantly higher than that of FSPR total dietary fiber（16.72%）.The SEM results showed that after colonic fermentation,TDF were broken into short rod-like and granular materials.The above results indicating that the dietary fiber of FSPR were degraded by gut microbiota during colonic fermentation,and the amorphous region was preferentially utilized.（4）High-fat diet-induced obesity model rats were used to study the effects of FSPR on the regulation mechanism of lipid metabolism and gut microbiota.The results of rat body weight,organ index and colon length showed that the body weight and liver organ index of high-fat diet-induced obesity model rats were significantly higher than those of normal rats,while the colon length was decreased significantly（p<0.05）.The body weight of rats in the control group（CON）,model group（MOD）and treatment groups all increased gradually with the prolongation of feeding time,and FSPR treatment could slow down the weight gain of rats with high-fat diet.Meanwhile,FSPR could reduce liver index and increase colon length,which could restore to levels similar to those in the CON group.The analysis of secretory immunoglobulin A（s Ig A）in the colon of high-fat diet rats showed that high-doses of FSPR and its DF could significantly increase s Ig A content in colon（p<0.05）.After intake of FSPR,especially high-dose FSPR,the levels of interleukin-1β（IL-1β）,interleukin 6（IL-6）,tumor necrosis factor alpha（TNF-α）and interferon gamma（IFN-γ）decreased.The results of inflammation and oxidative stress in the liver and colon from high-fat diet rats showed that high-fat diet increased the levels of inflammation and oxidative stress in liver and colon significantly.Mainly manifested in the increase of myeloperoxidase（MPO）levels and decrease of glutathione（GSH）,total antioxidant capacity（T-AOC）and superoxide dismutase（SOD）activities of liver and colon,FSPR could reduce the level of tissue inflammation and oxidative stress,especially the high-dose FSPR group showed the lowest tissue oxidative damage.The serum and liver lipid levels of rats were analyzed,and the results showed that FSPR could decrease the levels of triglycerides（TG）and total cholesterol（TC）in serum and liver were decreased.the levels.Meanwhile,the levels of free fatty acid（FAA）and low-density lipoprotein cholesterol（LDL-C）decreased and the levels of high-density lipoprotein cholesterol（HDL-C）in serum increased after treatment with high-dose FSPR.The pathological analysis of rat liver,colon,ileum and pancreas tissues stained by haematoxylin and eosin（H&E）showed that FSPR and its DF could significantly reduce fat accumulation and steatosis in rat liver.Meanwhile,high-dose FSPR could increase the villus height,crypt depth and goblet cells numbers of colon and ileum from high-fat diet rats,alleviating intestinal tissue damage caused by high-fat diet.In addition,high-dose FSPR could also alleviate the injury and inflammatory response of pancreatic from rats caused by high-fat diet.The results of short-chain fatty acids in the colon from rats showed that high-dose FSPR and its DF could all increased the short-chain fatty acids concentration,especially the contents of lactic acid,propionic acid and butyric acid.The results of gut microbiota of rats showed that the abundance of Lactobacillus in the FSPRH group（11.88%）was significantly higher than the positive control（PCON）group（4.82%）.Compared to MOD group,the abundance of Subdoligranulum in FSPRH group were increased by82.20%.FSPR can also promote the proliferation of Faecalibacterium,indicating that FSPR could promote the growth of beneficial bacteria and improve the gut microbiota structure.The results of this study showed that FSPR could alleviate the inflammation caused by high-fat diet,and improve the structure of gut microbiota.