Summary Rheological and sensory properties of yoghurts prepared from commercially available cultures that had ropy and non‐ropy characteristics were compared. Our results from both instrumental and sensory data suggest that it may not be the amount of polysaccharide that is important to rheological properties, but the type of exopolysaccharide (EPS)‐producing strains and consequently the interaction of the polymer with the milk proteins during the fermentation. Data also suggest that the interaction and co‐operative growth that occurs in mixed cultures also influences the yield of EPS production in the fermentation, as combining two ropy strains does not lead to an increase in total polysaccharide, although the viscosity can be improved. Texture measurements for viscosity correlated with sensory evaluation of viscosity and with slipperiness.
Introduction and historical background Nomenclature and classification Taxonomic information Morphology Cell wall structure Carbohydrate utilization Ureasc activity Deoxyribonucleic acid (DNA) homology New Old
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 ϩ ) combined with a non-ropy Streptococcus thermophilus (St r Ϫ ) 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.
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