Instrumental texture of set and stirred fermented milk. Effect of a ropy strain of Lactobacillus delbrueckii subsp bulgaricus and an enriched substrate

Texture profile analysis (TPA) of stirred and set cultured milk were evalua ted, using an exopolysaccharide producing strain of Lactobacillus delbrueck ii subsp. bulgaricus (NCFB 2772) and a protein enriched substrate (retentat e). In both cases, samples were compared respectively with a nonropy strain (NCFB 1489) and reconstituted skim milk substrate. The retentate fermented products were firmer in comparison with skim milk products, both at 10% of total solids. A loss of structure occurred when the retentate products wer e stirred causing a nine-fold decrease in hardness, whereas in cultured mil k products the loss was of around three-fold. The main differences were fou nd in adhesiveness and fracture characteristics. Set retentate ropy product was five times more adhesive than the nonropy one, whereas in stirred rete ntate and set skim milk products the difference was two-fold. No difference s in adhesiveness were detected when skim milk products were stirred using either strain. ne fracture force on ropy retentate samples increased after fracture while the nonropy ones showed a decrease, implying more structural breakdown in the latter. On set products, cohesiveness increased slightly due to the ropy strain. Changes in texture observed between ropy and nonrop y strains can be attributed to exopolysaccharide attachment to the casein m atrix being increased when the protein content is higher.