Infusion of walnut (Juglans regia L.) shell tea: multi response optimization and antioxidant potential

Abstract

Walnut shells are byproducts of walnut processing which are considered as a valuable source of bioactive compounds. This study investigated the effect of infusion conditions (solvent to sample ratio, temperature and time) on color values (a* and b*), total phenolic content (TPC) and total tannin content (TTC) of walnut (Juglans regia L.) shell tea (WST). Response surface methodology (RSM) and Box-Behnken design with 5 center points were performed in the optimization process. The derived equations were remarkably significant (p < 0.0001) and showed good fits (R-2 >= 0.894). Optimum infusion conditions for the infusion of WST were determined as 4:1 of solvent to sample ratio, 77.28 degrees C of temperature, 30 min of time. At optimum infusion conditions, a*, b*, TPC and TTC of WST were determined as 0.56, 2.31, 866.01 mg gallic acid equivalent (GAE)/L of WST and 53.23 mg tannic acid equivalent (TAE)/L of WST, respectively. Additionally, antioxidant activities performed by DPPH and FRAP assays were found as 0.88 +/- 0.02 and 6.26 +/- 0.99 mmol Trolox equivalent (TE)/L of WST, respectively. The polyphenol composition of WST was mostly based on phenolic acids and vanillic acid (371.792 mu g/L of infusion) was prominent. The mineral amounts detected in WST were in the following order of K > Mg > Na > Si > Mn > Cu and Cr, Fe, Ni, P, Pb, Se and Zn were trace elements.