EFFECT OF ROPY STRAINS OF LACTOBACILLUS-DELBRUECKII SSP BULGARICUS AND STREPTOCOCCUS-THERMOPHILUS ON RHEOLOGY OF STIRRED YOGURT

The TA-TX2 Texture Analyser and the Brookfield RVT Viscometer have bee n used to investigate the contribution of ropiness to the texture of s tirred yogurts made using ropy strains of bacteria. Back extrusion and texture profile analysis, not commonly used to quantify rheological p roperties of semi-solid foods, have been found useful in distinguishin g the contribution of exopolysaccharides to different texture attribut es (Toba et al., 1990). Thus ropiness, a characteristic which is impar ted to the product as a result of fermentation with particular polysac charide-producing strains, contributes to 'adhesiveness', while 'firmn ess' and 'elasticity' are likely to be influenced more by the protein matrix of the yogurt than by secretion of the polysaccharide by the ro py strains. Effects on viscosity and ability to recover viscosity afte r disruption were apparent, although the contribution of ropiness was not always positive. Ropy strains increased viscosity of stirred yogur ts when compared to yogurt made with son-ropy cultures. But, whilst a ropy Lactobacillus delbrueckii ssp. bulgaricus (Lb r(+)) combined with a non-ropy Streptococcus thermophilus (St r(-)) produced a viscous pr oduct which recovered its viscosity well, a yogurt made by combining b oth 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 inc lusion of a ropy strain will not always lead to improved texture attri butes, that while ropy strains may increase viscosity they may not inf luence 'firmness' and lend support to the view that this latter attrib ute is more influenced by protein-protein interactions.