Effects of pH during dry-heat preparation on the physicochemical and emulsifying properties of rice starch and whey protein isolate mixtures

Dry-heat and pH-shifting are simple and effective methods for the modification of starch and protein. To improve the ability of native rice starch (RS) to form stable emulsions, whey protein isolate (WPI) was incorporated into RS samples (RS/WPI mixtures) under different pH conditions (pH 3.0, 5.0, 6.5, 9.0, and 11.0, and pH-shifting) using a dry-heat treatment (130°C, 6 h). The physicochemical properties and emulsifying abilities of the RS/WPI samples with and without dry heating were investigated. Both particle aggregation and contact angle of the dry-heated samples increased with increasing pH. The particle size of D (4, 3) and the contact angle for the sample prepared at pH 11.0 were 34.78 μm and 95.06°, respectively. Pickering emulsions stabilized by the dry-heated RS/WPI samples prepared at pH 6.5, 9.0 and 11.0 showed lower amounts of creaming. Micrographs showed that the RS/WPI-stabilized Pickering emulsions were packed with numerous starch granules adsorbed spherical droplets and a lot of free starch granules and aggregates filling in the droplet gaps. The results suggest that a combination of pH-shifting and dry heating is an effective strategy in improving the emulsifying ability of RS/WPI mixtures.