Microbial growth, EPS concentration and textural properties of fermented milk supplemented with calcium and whey protein analysed using response surface methodology

The fermented milk gels were acidified by a ropy-EPS-producing strain of Streptococcus thermophilus ASCC 1275 at 42°C, stored at 4°C, 21 days. A central composite design containing two factors at five levels each: CaCl2 (1-10 mM) and whey protein concentrate (WPC) or whey protein isolate (WPI) (1-5% w/w). The total solid content of yoghurt bases was 14% (w/w). All fermented milk was analyzed for the culture growth, EPS concentration, viscoelastic and flow behavior, firmness, and water holding capacity (WHC). Second order polynomial models estimated response variables with R² ranging from 0.704 - 0.943. WPI or WPC had no effect on the cell growth during storage, but increased the EPS concentration. Calcium reduced cell growth especially in WPI-supplemented yoghurt. Calcium disrupted gel structure by lowering G’ and firmness, increasing flow behaviour and WHC. WPC addition increased G’, consistency index, and firmness significantly (P 0.05). The interaction between calcium and either whey protein preparation weakened the structure further. These results showed that the texture of yoghurt was modulated by Ca and whey proteins supplementations and the extent and directions of the magnitude depended on the Ca concentration and type of whey protein preparation.