Tomato (Solanum lycopersicum) leaf juice induced whey protein gelling: Unveiling the potential of endogenous proteases in novel applications

Summary

Tomato leaves are by-products of tomato production and a potential source of proteins. Part of these proteins are proteases that carry out important functions for the plants. These endogenous proteases can bring challenges in downstream protein extraction, and can alter protein functionality. The aim of this study was to characterize the endogenous proteases in tomato leaf juice and to investigate the hydrolysis by these proteases on both endogenous leaf proteins and WPI. In addition, we characterized the gelation behavior, rheological properties and structural and thermal properties change of WPI by the juice proteases. 38 different proteases were identified in the juice, along with 4 protease inhibitors. The proteases hydrolyzed both endogenous leaf proteins and WPI, with predominantly an endopeptidase manner for the latter. The protein hydrolysis was positively affected by the incubation time and temperature. The protease activity in juice could not be fully eliminated by heat deactivation or the addition of protease inhibitors. The protein hydrolysis however remained limited (max. 43 %) under all conditions below 60 °C. This hydrolysis resulted in small changes in both secondary structure and thermal properties of WPI. Limited hydrolysis led to WPI gelation at lower protein concentration. Gel formed with WPI after limited hydrolysis had higher gel strength and more brittle structure than the gel with intact proteins. To conclude, the complex and active nature of proteases in tomato leaves poses challenge for downstream protein extraction. Alternatively, juice from tomato leaves could be used as natural gelling agent for WPI.