This study investigated the effect of adding whey protein isolate (WPI) as a complementary drying aid of maltodextrin (MD) on spray drying of soy sauce powders. Soy sauce powders were prepared by spray drying soy sauce liquid adding 5%, 10% and 15% of WPI respectively together with MD as the drying aids. Tests were conducted to evaluate the powder properties relevant to the caking issue of the soy sauce powders. Results showed that addition of just 5% WPI could significantly increase the product yield for the spray drying process. At the same time, the caking strength of the spray dried soy sauce powders were significantly reduced during storage with WPI addition. XPS results indicated that WPI have preferential migration to the surface of the soy sauce powder. The over-expression of WPI on powder surface after spray drying might explain the improved stability for soy sauce powders during the caking test. (C) 2013 Elsevier Ltd. All rights reserved.