Effect of ‘ropy’ strains of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus on rheology of stirred yogurt

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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.

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