Starch phosphates produced by extrusion: Physical properties and influence on yogurt stability

Starch phosphorylation with sodium tripolyphosphate (STP) was performed using a single-screw extrusion process. The extrusion variables studied were temperature (from 99.5°C to 200.4°C), water content (16.3% to 19.7%, w/w), and STP content (0.82% to 4.18%, w/w). The resulting phosphorylated starch was evaluated with respect to expansion, water solubility index (WSI), water absorption index (WAI), degree of substitution (DS), and viscosity. The phosphorylated starch was tested as a stabilizing additive in yogurt formulations, with respect to the syneresis index (SI) and the syneresis susceptibility coefficient (SSC). The results show that the expansion increases with increasing STP content, and decreases with increasing temperature and water content employed in the process. Increasing extrusion temperatures and decreasing water content resulted in increasing WSI and decreasing WAI. The DS was dependent on STP content, increasing as STP content increased, but appeared to be insensitive to changes in water content (between 16% and 20%, w/w). The highest level of yogurt stability, as measured by the syneresis index (SI) and the syneresis susceptibility coefficient (SSC), were obtained when the yogurt was formulated with the starch of the highest DS (0.018), which had been obtained at 150°C, 18% water content, and 4.12% (w/w) of STP.