Exopolysaccharide production by Streptococcus thermophilus SY: production and preliminary characterization of the polymer
ABSTRACT Aims : To evaluate the effect of yeast extract (YE) concentration, temperature and pH on growth and exopolysaccharide (EPS) production in a whey-based medium by Streptococcus thermophilus SY and to characterize the partially purified EPS.
Methods and Results : Factorial experiments and empirical model building were used to optimize fermentation conditions and the chemical composition, average molecular weight (MW) and rheological properties of aqueous dispersions of the EPS were determined. Exopolysaccharide production was growth associated and was higher (152 mg l–1) at pH 6•4 and 36°C with 4 g l–1 YE. High performance size exclusion chromatography of the partially purified EPS showed two peaks, with a weight average MW of 2 × 106 and 5 × 104, respectively. The EPS was a heteropolysaccharide, with a glucose : galactose : rhamnose ratio of 2 : 4•5 : 1. Its water dispersions had a pseudoplastic behaviour and showed a higher viscosity of xanthan solutions.
Significance and Impact of the Study : The fermentation conditions and some properties of an EPS produced by Strep. thermophilus , a dairy starter organism, were described.
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ABSTRACT: The exocellular polysaccharide of Streptococcus thermophilus OR 901, isolated from partially deproteinised whey, is a heteropolymer of D-galactopyranose and L-rhamnopyranose residues in the molar ratio 5:2. The structure was established by methylation analysis and 1D and 2D NMR spectroscopy of the native polysaccharide, in combination with characterisation of oligosaccharide fragments, obtained by partial acid hydrolysis, using methylation analysis and 1D 1H NMR spectroscopy. The polysaccharide has a branched heptasaccharide repeating unit with the following structure: [sequence: see text]Carbohydrate Research 07/1997; 301(1-2):41-50. · 2.04 Impact Factor
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ABSTRACT: The TA-TX2 Texture Analyser and the Brookfield RVT Viscometer have been used to investigate the contribution of ropiness to the texture of stirred yogurts made using ropy strains of bacteria. Back extrusion and texture profile analysis, not commonly used to quantify rheological properties of semi-solid foods, have been found useful in distinguishing the contribution of exopolysaccharides to different texture attributes (Toba et al., 1990). Thus ropiness, a characteristic which is imparted to the product as a result of fermentation with particular polysaccharide-producing strains, contributes to ‘adhesiveness’, while ‘firmness’ and ‘elasticity’ are likely to be influenced more by the protein matrix of the yogurt than by secretion of the polysaccharide by the ropy strains. Effects on viscosity and ability to recover viscosity after disruption were apparent, although the contribution of ropiness was not always positive. Ropy strains increased viscosity of stirred yogurts when compared to yogurt made with non-ropy cultures. But, whilst a ropy Lactobacillus delbrueckii ssp. bulgaricus (Lb r+) combined with a non-ropy Streptococcus thermophilus (St r−) produced a viscous product which recovered its viscosity well, a yogurt made by combining both ropy strains did not recover its viscosity as well as yogurt made by combining two non-ropy cultures and lost its structure more rapidly during the destructive testing. These results show therefore that inclusion of a ropy strain will not always lead to improved texture attributes, that while ropy strains may increase viscosity they may not influence ‘firmness’ and lend support to the view that this latter attribute is more influenced by protein–protein interactions.International Journal of Food Science & Technology 10/2003; 32(3):213 - 220. · 1.24 Impact Factor
- Analytical Chemistry - ANAL CHEM. 04/2002; 37(12).