Dry heating a freeze-dried whey protein powder: Formation of microparticles at pH 9.5

We investigated the effect of dry heating (DH) a whey protein powder (water activity 0.24) at pH 3.5, 6.5 and 9.5 for up to 36 h at 100 °C on the structural properties of proteins. Powder browning, particle structures in suspensions reconstituted from the dry-heated powders (DHP), residual native protein content and Maillard reaction markers were studied. The browning index of the DHP rapidly increased after 6-10 h of DH and then levelled off at all pH values. β-Lactoglobulin, α-lactalbumin and bovine serum albumin were denatured by DH at all pH values. In contrast to pH 3.5 and 6.5, DH at pH 9.5 led to abundant production of microparticles. The highly heterogeneous size of these particles was related to the initial powder particle size. One g of DHP in suspension led to 20 g of hydrated microparticles made from 0.5 g of dry material. Maillard reactions at pH 9.5 were probably involved in microparticle formation.