Lactic acid bacteria exopolysaccharides have the characteristics of increasing the viscosity of fermented milk,preventing the precipitation of whey,and enhancing the strength of curd.It is currently one of the research hotspots in the field of food science.In this study,the basic medium M17 was used as a control,and the compound nitrogen source of M17 liquid medium was replaced with soy peptone,tryptone and casein peptone to explore the effect of different nitrogen sources on the yield,molecular weight and structure of the extracellular polysaccharide of Streptococcus thermophilus IMAU20561.The results showed that the yield of extracellular polysaccharide in M17 medium with soy peptone as the sole nitrogen source for Streptococcus thermophilus IMAU20539 reached 480.7 mg/L,which was much higher than the yield of polysaccharides obtained from the other three mediums.Among them,the M17 medium with soy peptone as the sole nitrogen source had the highest production of extracellular polysaccharides,reaching 480.7 mg/L.Much higher than the yield of polysaccharides from the other three media.The EPS fractions of the crude EPS obtained from different nitrogen sources were different after purification,and the molecular weight of each EPS fraction was also significantly different.The M17 medium with casein peptone as the sole nitrogen source had the highest EPS (5.964×105 u),followed by basic medium M17 (1.260×105 u),soybean peptone medium (6.635×104 u) and tryptone medium (1.192×104 u).There were significant differences in the structure of polysaccharides obtained from different nitrogen sources.The EPS obtained from soy peptone medium is mainly composed of galacturonic acid,galactose and arabinose;the EPS obtained from the tryptone medium mainly consists of glucuronic acid,glucose and galactose.The EPS obtained from casein peptone medium and basic medium M17 had similar monosaccharide composition,mainly composed of mannose,glucose and galactose,and the percentages were 10.46:1.62:87.37 and 20.37:2.77:74.91,respectively.