Optimization of preparation process and functional study of high-bioactivity mulberry leaf oligosaccharides

【Objective】To optimize the best preparation process of mulberry leaf oligosaccharides which promoting the proliferation of Lactobacillus rhamnosus GG（LGG）, and analyze its effects on the production of short-chain fatty acids by LGG, which could provide scientific reference for the development and application of mulberry leaf oligosaccharides. 【Method】Using the LGG proliferation rate promoted by mulberry leaf oligosaccharides as an indicator, mulberry leaf oligosaccharides were prepared by enzymolysis, and the optimum hydrolysis enzyme was selected. Then, the enzymolysis process was optimized by a single-factor test and response surface method. Finally, the effects of mulberry leaf oligosaccharides on the fatty acid metabolites of LGG was analyzed by gas chromatography （GC）.【Result】The best proliferation promoting effect of mulberry leaf oligosaccharides was prepared by enzymatic hydrolysis with β-glucanase, and the optimal preparation process was optimized by response surface method, which was as follows: the amount of enzyme addition was 1500 U/mL, enzyme digestion temperature was 51 ℃, enzyme digestion time was 4 h. LGG proliferation rate of mulberry leaf oligosaccharides prepared by this process was 253.16%, which was close to the theoretical prediction （244.80%） and proliferation promotion effect was better than that of mulberry leaf polysaccharides and commercial oligosaccharides （oligogalactose and oligoisomaltose） at the same mass concentration. The order of the effect factors on LGG proliferation rate was enzyme digestion temperature （B） ＞ enzyme addition （A） ＞ enzyme digestion time （C）, and the order of the effect of the interaction between 2 factors on LGG proliferation rate was AC ＞ AB ＞ BC, the interaction of AC and AB had significant effect on LGG proliferation rate （P＜0.05, the same below）, while the interaction of BC did not have significant effect （P＞0.05）. The molecular weight of mulberry leaf oligosaccharides was determined to be 1900Da by gel permeation chromatography. The mulberry leaf oligosaccharides significantly enhanced the short-chain fatty acid-producing capacity of LGG, including acetic acid, propionic acid, butyric acid and valeric acid, which increased by 207%, 39%, 17% and 10% than those of the control group（without polysaccharides or oligosaccharides） respectively.【Conclusion】The oligosaccharides obtained from mulberry leaves through enzymatic hydrolysis exhibit high bioactivity.They improve the ability of LGG to produce short-chain fatty acids significantly, with good application prospect.