Functional Properties Of Lactic Acid Bacteria Peptidoglycan And Its Effect On The Quality Of Fresh-Cut Apples

As a major category of probiotics,lactic acid bacteria have many functions such as anti-tumor,anti-oxidation,immune regulation and balance of intestinal microorganism.Studies have found that the physiological activity of lactic acid bacteria is related to the peptidoglycan(PG)in the cell wall.However,due to the low separation efficiency of peptidoglycan,unique structure and difficult to dissolve,there are relatively few applied studies.Fresh-cut fruits and vegetables often undergo oxidative browning and spoilage during storage.Some natural antioxidant components can be used to improve the storage performance of fresh-cut fruits and vegetables.In this experiment,four food-grade lactic acid bacteria,Lactobacillus plantarum,Lactobacillus casei,Lactobacillus bulgaricus,and Streptococcus thermophilus were used for the separation and preparation of peptidoglycans.And the structural properties,antioxidant,bacteriostatic properties and the functional properties of probiotic of the four peptidoglycans were studied.As for the preliminary explored the effect of peptidoglycan on the quality of fresh-cut apples.The main contents and conclusions of this research are as follows:(1)Four kinds of lactic acid bacteria,Lactobacillus plantarum,Lactobacillus casei,Lactobacillus bulgaricus and Streptococcus thermophilus are fermented and cultured,and peptidoglycan is obtained through the steps of collecting bacteria,deteichoic acid,deproteinization and degreasing.The measured aminohexose content of the four peptidoglycans were 41.86%,38.12%,39.19%,40.35%,the reducing sugar content was 8.86%,7.65%,9.16%,10.34%,and the protein content was 47.06%,49.91%,47.25%,46.82%.HPLC,FTIR,and MALDI-TOF-MS were used to analyze the structural composition of the four peptidoglycans,and the main constituent amino acids of the four peptidoglycans were alanine,glutamic acid,serine,lysine,glycine,and leucine.The side peptide chain composition of acid and aspartic acid are both lysine type.Among them,glycine exists in the side chain of Lactobacillus casei and Streptococcus thermophilus.(2)Part of the physical and chemical properties of peptidoglycan were studied,and the relationship between the properties and structure of peptidoglycan was studied by measuring basic physical and chemical indicators.Thermal stability analysis found that the complete degradation temperature of peptidoglycan is 600?,and the higher degradation temperature is related to its dense three-dimensional network structure.The peptidoglycan was hydrolyzed by lysozyme,and the hydrolyzed peptidoglycan was obtained by ethanol precipitation.Rheological analysis showed that the peptidoglycan had a certain viscosity after hydrolysis,and the solution was weakly acidic,which was related to the amino acid composition of the side peptide chain.Congo red experiment proved that its aqueous solution does not have a triple helix structure,which is related to the hydrolysis of peptidoglycan.SEM analysis revealed that the hydrolyzed peptidoglycan was decomposed from capsules into smaller fragments.SDS-PAGE electrophoresis analysis showed that the molecular weight of the hydrolyzed 12 h peptidoglycan was mainly distributed between 55-70 k Da.(3)The antioxidant function,bacteriostasis and probiotic effect of peptidoglycan were tested.DPPH,ABTS,hydroxyl radical,superoxide anion radical scavenging experiment and metal ion chelating ability were used to analyze the antioxidant capacity of the four peptidoglycans.As a result,it was found that the antioxidant capacity of the four lactic acid bacteria peptidoglycan hydrolyzed solutions gradually increased within the concentration range of 4 mg/m L.Prolonged hydrolysis time can increase the antioxidant activity of peptidoglycan.After comprehensive comparison,it is concluded that the antioxidant capacity of the hydrolyzed Lactobacillus casei peptidoglycan for 12 hours is significantly higher than that of other hydrolyzed products.The inhibitory effects of four hydrolyzed peptidoglycans of lactic acid bacteria on Escherichia coli,Staphylococcus aureus,Bacillus subtilis,Pseudomonas aeruginosa,Micrococcus luteus,Candida albicans,Aspergillus niger and Penicillium chrysogenum were explored.The results showed that PG slightly inhibited the growth of Escherichia coli,Staphylococcus aureus,Bacillus subtilis,and Micrococcus luteus,it has no inhibitory effect on fungi.PG did not have good antibacterial properties as a whole.The results of in vitro probiotic experiments showed that peptidoglycan had no significant effect on the growth of probiotics.(4)Based on the preliminary experiment,the 12 h hydrolyzed Lactobacillus casei peptidoglycan was selected,and the effect of peptidoglycan on the quality of fresh-cut apples was preliminarily explored by measuring the physical and chemical indexes of fresh-cut fruits,the activity of browning-related enzymes,and the MDA content in different storage times.The results showed that compared with the blank group,the processed fresh-cut apples can maintain a certain degree of hardness,organic acid and soluble solid content during storage,and the weight loss rate is significantly reduced,and can effectively delay the increase of PPO and POD activity,and reduce The increase in MDA content and the decrease in total phenol content have no significant effect on CAT activity.According to analysis,the 2 mg/m L peptidoglycan hydrolysate of Lactobacillus casei has a better performance.It slows down the oxidative damage of fresh-cut apples during storage,reduces the consumption of nutrients,delays the senescence and maintain the storage quality of fresh-cut apples to a certain extent.