Probiotic Properties Of Streptococcus Thermophilus ST1and Application In The Low-fat Fermented Milk

Aimed of this study were to investigate some probiotic properties and functionalproperties of Streptococcus thermophilus ST1(ST1) and Streptococcus thermophilusKS1(KS1). The tolerances of ST1and KS1strains to low pH and0.3%bile salt werestudied. Results showed that: After the incubation at pH3.0for3h, ST1and KS1could survive as a rate of67.17%-67.49%. Strains were able to grown in pH4.0or0.3%bile salt showed high viability. In antimicrobial activity assays, strains ST1andKS1showed inhibitory properties toward pathogenic bacteria, particularly against S.aureus, E. coli, S. typhimurium and S. flexneri. In antimicrobial susceptibility assays,ST1were resistant to Streptomycin sulfate, Kanamycin sulfate and Gibberellin. KS1were resistant to Streptomycin sulfate, Kanamycin sulfate, Gibberellin, Rifampin,Polymyxin B sulfate and Erythromycin. The study found that two strains had highcholesterol-reducing ability. The cholesterol-degrading rates by the two strains were65.14%and74.49%, respectively.We investigated the effect of starter culture ropy exopolysaccharide-producingStreptococcus thermophilus ST1and combine with stabilizer on textural andmicrostructural attributes of low-fat yogurt. ST1had a high capacity to produce acidsand EPS. Within4.5h, the pH dropped to4.5, the acidity reached80°T, and EPSreached65.27mg/L. ST1was more effective at increasing viscosity as well as waterholding capacity and reducing spontaneous whey separation compared with controlsample. ST1combined with carrageenan had the highest viscosity and the best waterholding capacity as well as the lowest spontaneous whey separation. ST1combinedwith whey protein concentrates showed better results than with xanthan，but worsethan with carrageenan. The microstructure of the fermented milk was examined usingscanning electron microscopy. ST1combined with carrageenan resulted in compact and uniform structure, which is agreed with quality improvement of fermented skimmilk that used ST1combine carrageenan as fat substitute.Reduction of fat in cheese milk may cause textural and organoleptic defects ofcheese products. This study aimed to use an exopolysaccharide (EPS)-producingstrain of Streptococcus thermophilus ST1in combination with stabilizers such ascarageenan, xanthan or whey protein concentrate to improve the textural and sensoryproperties of lo63w-fat soft cheese. The results showed that the use of EPS-producingculture could significantly increase the moisture retention and decrease the hardnessof the low-fat soft cheese, and further addition of carrageenan (0.05%, w/v), but not ofxanthan or whey protein concentrate, to cheese milk resulted in cheese products withimproved texture and sensory properties similar to those of the control full-fat softcheese. Observation by scanning electron microscopy of the cheese samples showedthat the protein matrix of the low-fat soft cheese made with the EPS-producing culturepossessed an open structure with porosity; the protein matrix of the low-fat softcheese made with the EPS-producing culture together with carrageenan appeared tobe layered and more open with spaces occupied mainly by the moisture, thussoftening the low-fat cheese structure to resemble the full-fat type.