Impacts of Different Types of Exopolysaccharides on the Physical and Rheological Properties of Yogurts

Abstract Text: Exopolysaccharides (EPS) produced by some starter cultures are often used to modify yogurt texture. Our goal was to study the impact of different EPSs on yogurt properties. Reconstituted skim milk was inoculated with 8 strains of S. thermophilus; ST1-UWM (non-EPS producer used as a control) and 7 EPS producing strains. Due to the different acid producing ability of each strain, milks were supplemented with different amounts of peptone, and the inoculation rates were varied in order to achieve a similar acidification rate for all strains. Milks were fermented at 40°C until pH 4.6 was reached. Small amplitude oscillatory rheology was performed to monitor yogurt gel formation. Gel permeability and whey separation were determined. Microstructure of the yogurt gels was studied using fluorescence microscopy. EPS from each strain were isolated for molar mass determination using size exclusion chromatography – multi angle laser light scattering (SEC-MALLS), and for repeating unit structure determination using nuclear magnetic resonance spectroscopy (NMR-spectroscopy). Gelation pH of each yogurt was significantly different. The yogurt made from the control strain had significantly lower gelation pH (pH ~ 5.10) than yogurts made from EPS-producing strains (pH ~ 5.17 – 5.29). Storage modulus (G′) of the control gel at pH 4.6 (G′~ 97 Pa) was significantly lower than the other yogurts made from EPS-producers (G′~ 151 – 191 Pa) except the yogurt made from ST-143 strain (G′~ 67 Pa). Permeability and whey separation of each yogurt were significantly different with the control yogurt having higher permeability and whey separation than other yogurt gels. The microstructure of the control yogurt showed finer and smaller pore sizes compared to other gels. The yields of EPS produced from each strain varied between 34 and 95 mg dry material/L with ST-143 being the highest EPS producer. The molar mass of the isolated EPS ranged from 0.13 Χ 10⁶ to 1.74 Χ 10⁶ g/mol. The structures of the repeating units showed that all EPS were different in terms of sugar compositions, linkages and conformations. Since all samples had similar milk composition, heat treatment, and rate of acidification, this study shows that EPS produced during yogurt fermentation modifies the gelation mechanism and physical properties of the yogurt gels.

Keywords: EPS, Yogurt, Rheology