The Relationship Between The Streptococcus Thermophilus Exopolysaccharide Diversity And Fermented Milk Viscosity

Yogurt retains all the nutritional value of pure milk, and also has many other funct-ions such as relaxing bowel, reducing blood press, boosting immunity, keeping the tum-ors at bay, tonifying deficiency appetizing and alleviating malnutrition among children.But the yogurt is associated with low viscosity and serious whey exhalation in the prod-uction process. Although there is no direct relationship between the viscosity and thenutritional value of yogurt, this directly impair the taste of yogurt, reducing the willingfor consumers to buy. To solve this problem, people often add synthetic thickener to theyogurt to improve the texture. This not only covers the real taste of the yogurt, but alsodevaluates the nutritional value of yogurt. So the research of the viscosity of the yogurthas been inevitable. Exopolysaccharides produced by Streptococcus thermophilus ferm-ented milk plays a crucial role in the viscosity of the fermented milk. However, therelationship between these two aspects is not very clear.This paper studied the relationship between the exopolysaccharides produced byStreptococcus thermophilus fermented milk and viscosity, aiming to increase theviscosity of fermented milk by controlling the generation of functional exopolysacchar-ides. This has great significance to improve the texture of yogurt and develop high effic-ient straight investment yogurt starter.This research studied the content of exopolysaccharides of19different Streptococ-cus thermophilus fermented milk and the corresponding viscosity of it. Aanlysis theircorrelation using SPSS software and the correlation is0.841(P<0.001). This shows thatthey were significantly related, in other words, strains which produced large amount ofexopolysaccharides could improve the viscosity of fermented milk.Separated the exopolysaccharides of three Streptococcus thermophilus crudely:zlwTM11-EPS, zlwB9-3-EPS and zlwSP1.1-EPS. These three crude exopolysaccha-rides were added to the S. thermophilus zlwSP1.1fermented milk with five differentconcentrations respectively, that was,0.1mg/g,0.15mg/g,0.2mg/g,0.25mg/g and0.3mg/g. It showed that the contribution of the zlwTM11-EPS on the viscosity of thefermented milk is the largest.Through the DEAE-52cellulose chromatography and Superdex200chromatogr-aphy purification of zlwTM11-EPS crude exopolysaccharides, four major purified com-ponents were obtained: zlwTM11-1, zlwTM11-2, zlwTM11-3and zlwTM11-4. The rec-overy rate were4%,10%,5%and9%respectively and the purity were89.35%,92.49%,81.03%and93.37%respectively. High performance gel permeation chromatographydetected their purity, and they were all relatively symmetrical single peak. This illustrat-ed that these four components were very pure. These four purified components wereadded to the S. thermophilus zlwSP1.1fermented milk with five different concent- rations respectively, that was,10μg/ml,15μg/ml,20μg/ml,25μg/ml and30μg/ml. Thefour purified components could all increase the viscosity of the fermented milk, but theeffect of the zlwTM11-2was significantly higher than the other components. This confi-rmed that the zlwTM11-2palyed a major role in improving the viscosity of fermentedmilk. Fourier transform infrared spectroscopy suggested that the zlwTM11-2wasβ-D-pyranose. The monosaccharide composition of zlwTM11-2was analyzed by Gaschromatography mass spectrometry(GC-MS), and zlwTM11-2was composed of rham-nose, mannose, glucose and galactose with the ratio of9.7:4.87:39.14:46.29.Through single factor experiments, the study determined each factor’s range ofconditions (passage number, content of skim milk, inoculum size, fermentation tempe-rature, fermentation time, ripening temperature and ripening time) impacting theproduction of the functional component zlwTM11-2of the S. thermophilus zlwTM11fermented milk, and by Plackett-Burman experiment screened three main factors: ferm-entation temperature, fermentation time and inoculum size. Through the central compo-site design experiments Central Composite Design (CCD), the optimum culture conditi-ons of high procuction of zlwTM11-2was determined:4.49%of the inoculum size,6.74h of fermentation time,43℃of fermentation temperature. At last, the actual productionof the zlwTM11-2was42.947mg/L which was11.87%higher than before.