Isolation Of A Yeast That Promotes Flocculation Of Starch By Lactic Acid Bacteria And Preliminary Research On Its Flocculation-Promoting Mechanism

The syrup method had settled starch in China for a long time.The physalis was a pale yellow and sour liquid which was made by mung bean or sweet potato with water and grinding into a slurry,and was fermented naturally by an acid-producing microorganism.In the production of traditional starch,the syrup caused the starch granules to aggregate immediately,so that the starch granules sink quickly due to the increase of gravity.So the physicochemical method was still used widely in actual production.In this study,a strain of yeast that promoted the flocculating starch of Lactobacillus was isolated from natural fermented sweet potato physalis.And the growth conditions of the target strain were optimized in the study.The stronger the coaggregation of lactic acid bacteria and yeast,the better the flocculation effect on starch.Then,based on the coagulation rate,several physical and chemical factors affecting the growth of strains were analyzed,and the mechanism of coagulation was preliminarily explored.1.Screening and identification of yeasts that promote flocculation.The cultivation was carried out in a PDA solid medium by a plate coating method.A yeast strain having a flocculating starch of Lactobacillus paracasei L1 was selected from the naturally fermented sweet potato physalis by the coagulation rate.According to the individual and population morphological characteristics of the cells,physiological and biochemical indicators and the 26 S rDNA molecular sequencing results of the strain,the yeast was determined to be Meyerozyma guilliermondii and named M.guilliermondii 616.2.Optimization of growth conditions and determination of growth curve of M.guilliermondii J616.According to the OD value of the bacterial liquid,the formula of the sweet potato juice glucose medium was optimized by single factor experiment: the carbon source was glucose,the addition amount was 3%;the nitrogen source was beef extract,and the added amount was 2%.The conditions were optimized by response surface experiment: culture temperature 31?,inoculum 6%,initial pH 5.0.The yeast entered a stationary phase after 28 h and then entered the decay phase after 44 h.3.The effect of physical and chemical factors on the coagulation efficiency of M.guilliermondii 616 and L.paracasei L1.According to the co-aggregation efficiency,four factors were determined to promote coagulation of M.guilliermondii J616 and L.paracasei L1 by single factor experiments.The four factors were as follows: sterile distilled water as buffer and ultrasonic dispersion treatment before culture;or the pH was 5.0 and the temperature was 35 ? during culture.Different growth stages of yeast had insignificant effect on coagulation.4.The heat treatment,protease digestion,potassium periodatecould,malonic anhydride,the cell walls of yeast and mannose could reduce the coagulation efficiency of L.paracasei L1.And the heat treatment reduces the coagulation efficiency of M.guilliermondii 616.It was speculated that the agglutination factor of L.paracasei L1 was a substance that contains protein and polysaccharide chains,possibly glycoproteins.Mannan might be an agglutination factor receptor of M.guilliermondii 616.