Insight into peroxidase-mediated Pinus massoniana resin browning, and browning inhibition strategy

Pine resin is an important non-wood forestry product and has been postulated as an ideal candidate to replace petroleum-derived chemicals and fuels. During crude pine resin storage, peroxidase-mediated enzymatic browning leads to significant color deterioration and quality degradation. This study aimed to discover the browning mechanisms in crude pine resin and improve its quality. Peroxidase (PNmPOD) was extracted from the Pinus massoniana needle and characterised for its browning kinetic properties. The enzymatic browning mechanisms in crude pine resin were analyzed. This investigation employed a simulated system containing hydrogen peroxide, ferulic acid, epicatechin, PNmPOD, and crude resin. The results revealed that the peroxidase activity was optimal at pH 5.6 and 65 °C, with an activation energy of 193.41 kJ/mol. The simulated storage system revealed that epicatechin and ferulic acid undergo peroxidase-driven reactions: oxidation/dismutation-isomerization, demethylation-free radical coupling, and dehydration condensation. These processes correlate directly with chromatic shifts in the experimental model; pine resin's main constituents (resin acids and terpenoids) remain inert in enzymatic browning reactions. To effectively deactivate peroxidase activity with minimal resin chromatic alteration, heat treatment at 75 °C/20 min or adding l-cysteine as an inhibitor emerged as the optimized protocol. This study establishes fundamental strategies for enzymatic browning control and quality enhancement in post-harvest pine resin preservation.