Microorganisms producing polysaccharides: differentiation between ropy and thickening strains

Microorganisms producing polysaccharides are recognized as being responsible for a good texture of the yoghurt; it is therefore important to succeed in selecting such strains. Sensory analysis has nevertheless shown that the so-called "ropy strains" do not suit all microorganisms producing polysaccharides. Some strains are able to produce a ropy texture, while others give only a thick clot. A rheological study was performed using a viscosimeter. It was observed that ropy strains showed similar tracings but different appearance to those which are thickening. On the basis of this evidence it was possible to classify strains of Lactobacillus bulgaricus and Streptomyces thermophilus of our collection into two categories: ropy and thickening strains.