Effects of starter culture and its exopolysaccharides on the gelation of glucono-δ-lactone-acidified bovine and caprine milk

The effects of the starter culture biomass material and its exopolysaccharides (EPS) on the structure of bovine and caprine acid gels were investigated. Inactivated cell material and/or extracellular polysaccharide, which had been separated from yogurt starter cultures, were added to both types of milk. The milk systems were acidified using glucono-δ-lactone (GDL) and the gelation processes were monitored using dynamic (oscillatory) rheological testing. The addition of cell material had no effect on the bovine acid gel, whereas the addition of polysaccharide produced a weaker gel structure. The addition of cell material (> 5 × 10⁸ cells/mL) and polysaccharide (0.35% w/v) weakened the caprine acid gel system. The treated gels were some three to eight times weaker than the untreated samples, suggesting that the starter culture metabolites and especially the EPS may act by interfering with the gel structures, particularly in the cases of milk systems such as caprine yogurt or low protein content bovine yogurt.