Plant Bioactives Group, Post-Harvest & Agro-Processing Technologies, Agricultural Research Council (ARC) Infruitec-Nietvoorbij, Private Bag X5026, Stellenbosch 7599, South Africa; Department of Food Science, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa. Electronic address: [Email]
Phenolic compounds of Aspalathus linearis (rooibos) are susceptible to oxidation during "fermentation", a process characterized by the formation of a red-brown leaf color. The role of enzymes in this process is not yet understood. An experiment with dried green rooibos plant material pre-treated at 170 °C for 30 min to denature and "inactivate" endogenous enzymes was conducted to confirm the role of oxidative enzymes. The phenolic composition of "enzyme inactivated" plant material was not significantly (p ≥ .05) affected by simulated fermentation, compared to control samples, as determined using piece-wise multivariate analysis of variance for successive time intervals. This proves that rooibos enzymes participate in the oxidation of phenolic compounds during fermentation of the plant material. A kinetic modeling approach was subsequently used to establish reaction kinetic parameters for selected rooibos phenolic compounds. The degradation of aspalathin and nothofagin and formation of eriodictyol glucosides during simulated fermentation at four temperatures from 37 to 50 °C were best described by the fractional conversion model based on first-order kinetics (r2 > 0.98), which allows for non-zero equilibrium concentrations. The extent of degradation for other compounds was too low to enable kinetic modeling. Reaction rates for the degradation/formation of phenolic compounds during fermentation followed the Arrhenius law. Less phenolic degradation (higher equilibrium concentration), but a higher reaction rate constant, was observed at higher temperatures, which could possibly be attributed to inactivation of enzymes.