Soymilk-based yogurt has emerged as a popular alternative to traditional dairy-based yogurts as interest in the health-promoting benefits of soybean consumption increases. Soymilk yogurts are known to contain functional phenolics, but it is unknown whether the contents of these bioactive phenolic compounds change during yogurt culturing. Phenolic antioxidant mobilization during yogurt production from soymilk was investigated using active probiotic Kefir cultures. Total soluble phenolic content and DPPH radical-scavenging antioxidant activity were measured every 8 h for 48 h. Further, the activity of several enzymes (beta-glucosidase, laccase, peroxidase) associated with the microbial degradation of polymeric phenolics and lignin and previously linked to phenolic mobilization from soybean during solid-state bioprocessing by dietary fungi were also investigated. Soluble phenolic content increased with Kefir culture time and was strongly correlated with total peroxidase and laccase activity. However, the phenolic content dropped sharply at 48 h. Antioxidant activity increased with Kefir culture time and was strongly correlated to decreased soluble phenolic content over the same time period. This research has important implications for the optimization of functional phytochemicals in commercial soymilk-based yogurt production strategies. (c) 2004 Elsevier Ltd. All rights reserved.