Purwandari, Umi (2009) Physical properties of functional fermented milk produced with exopolysaccharide-producing strains of Streptococcus thermophilus. PhD thesis, Victoria University.

Abstract

This thesis focused on the study of the influence of different exopolysaccharide types produced by two strains of Streptococcus thermophilus on the physical properties of fermented milk. First, the fermentation factors affecting EPS production were studied to ascertain required carbon source and environmental conditions which would support their production. Higher fermentation temperature (42°C) resulted in a greater cell growth and EPS production. EPS production was growth associated in glucose or lactose-containing M17 medium. The examined strains appeared to be able to utilize galactose for the EPS assembly and produced comparable amounts of EPS, albeit restrictive cell growth. The EPS production of the two strains was comparable, ranging from ~100 to ~600 mg/L. Secondly, the EPS were rheologically characterized to show their resistance to deformation. Influence of temperature, pH and concentration on the flow behaviour of these EPS was also assessed. Under acidic conditions, capsularropy EPS was less responsive to temperature with a higher zero shear viscosity ηo (14.36 to 150.82 mPa s) than capsular EPS (93.72 to 9.24 mPa s), and slightly higher relaxation time τ (0.43 to 15.82 s for capsular-ropy EPS and 0.72 to 9.36 s for capsular EPS). The opposite behavior was observed under neutral pH. EPS concentration did not give significant effect (P&gt0.05) on ηo and τ. The second study examined the effects of types of EPS on yoghurt texture under selected conditions. Fermented milk made using capsular-ropy EPS showed greater resistance to flow with less solid-like behaviour. It also had greater water holding capacity although the milk gel was less compact and brittle compared to fermented milk with capsular EPS. The EPS production in milk during fermentation between the two strains was comparable with maximum concentration was 840 plus/minus 47.5 mg EPS/kg fermented milk. Syneresis was lower in fermented milk incubated in low temperature, was ranging from 4.1-2.4 g/100 g fermented milk with capsular-ropy-EPS, and 10.9-26.6 g/100 g in fermented milk with capsular EPS. G’ was 23.8-365.1 Pa and 57.6-1040 Pa for fermented milk with capsular ropy and capsular EPS, respectively. The third study examined the involvement of EPS in the texture creation of fermented milk supplemented with calcium and/or sucrose, or calcium and whey proteins. Calcium addition to milk base resulted in increased acidity and greater syneresis (~20-30 g/100 g in fermented milk with capsular-ropy EPS and ~30-50 g/100 g in fermented milk with capsular EPS) and thixotropy of fermented milk, as compared to fermented milk without added calcium. Sucrose affected the parameters in opposite manner. EPS production did not differ from that of the control fermented milk. Storage modulus (G’) was 96-230.4 Pa, and 502.8-1143.5 for fermented milk with capsular ropy and capsular EPS, respectively. The effect of heat-untreated whey protein isolate or whey protein concentrate on calcium-fortified fermented milk was studied using capsular ropy EPS producer. Result showed that combined effect of both supplement was detrimental to texture of fermented milk to make it resemble that of drinking yoghurt. Syneresis was up to ~50 g/100 g, while G’ was only around 4 mPa. The next experiment studied the effect of heat-treated whey protein isolate addition on fermented milk texture. Results showed that heat-treatment applied to added whey protein preserved the G’ and syneresis with the values close to those of normal fermented milk. However, at high concentration of added heat-treated whey protein (whey protein:casein 3:1), the texture became very hard with 0 m2 permeability. Gelation was started very early in fermented milk added with heatdenatured whey protein. Whey protein addition induced the beginning of gelation. Supplemented fermented milk made using capsular-ropy EPS producer consistently showed lower G’, lower syneresis, and more shear-resistant compared to that made using capsular EPS. In conclusion, capsular ropy EPS, both in dispersion and in fermented milk with or without different supplementation, exhibited less solid-like properties and more shear-resistant behavior compared to capsular EPS.

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