BACKGROUND Trans-cinnamic acid (t-CA) and its derivatives have been shown to be antioxidant and antibacterial, and can be used in food additives and flavors. However, a traditional t-CA production via chemical synthesis with a condensation reaction may not be suitable for its application in the pharmaceutical and food fields as it is not a 'green' route. This study investigated the bioconversion of l-phenylalanine (l-phe) to t-CA using engineered whole-cell Escherichia coli BL21(DE3) expressing phenylalanine ammonia-lyase (PAL, EC 4.3.1.5) as a biocatalyst. To prevent product inhibition, an in situ reaction and separation of t-CA was performed using a pH-controlled biotransformation strategy. RESULTS A coupling reaction and separation system based on pH adjustment was proposed for improving the bioproduction of t-CA. Using this process, 39.61 g L-1 t-CA was bioproduced from 40 g L-1 of l-phe in 8 h and a high purity (>98%) of t-CA product was achieved directly without additional purification. CONCLUSION The bioprocess developed in this study is a highly efficient method for the bioproduction of t-CA and, therefore, offers a promising alternative route for t-CA production. (c) 2019 Society of Chemical Industry