To our knowledge all the studies about laccase kinetics and its inhibition have been performed with substrates and conditions very different from those of real grape juice. Moreover, none of these researches really measure enzymatic browning, since they have not taken into account what happens after the oxidation of o-diphenols in o-diquinones and their subsequent polymerization to form melanins.
For that reason, the aim of this research was to develop a new model to measure the kinetics of browning caused by Botrytis cinerea laccase under conditions much closer to those of grape juice and using the substrates naturally present in it.
A grape juice model solution containing 100 g/L of D-glucose, 100 g/L of D-fructose and 4 g/L of tartaric acid adjusted to pH 3.5 was used for all the browning assays. Five phenolic compounds including one triphenol: gallic acid; three orthodiphenols: caftaric acid, (+)-catechin and (-)-epicatechin; and one monophenol: 4-hydroxybenzoic acid were used at concentrations between 0 and 0.8 mM. Laccase from Botrytis cinerea was purified. Browning reaction was started by adding 2 units of laccase activity/mL and absorbance at 420 nm was measured at time 0, 15, 30 and 45 minutes. The slope of the regression straight line was determined in order to express the intensity of browning. The Michaëlis-Menten and Hill plots were depicted for each substrate in order to determine the kinetic parameters of browning: Vmax, K0.5 and Hill number. All the experiments were performed in triplicate.
The results indicate that o-diphenols are better substrates for laccase browning than triphenols and that monophenols, or at least
4-hydroxybenzoic acid, do not appear to be reactive. Moreover, of the o-diphenols, (+)-catechin showed the greatest browning intensity, followed in decreasing order by (-)-epicatechin and caftaric acid.
This research proposes a synthetic model for measuring laccase browning in a matrix close to real grape juice that makes it possible to study how laccase browning acts in the presence of different possible substrates. Further studies are needed to verify the efficiency of the proposed model on other laccase substrates such as anthocyanins, flavonols and proantocyanidins, and also to determine the inhibitory effect toward laccase browning of the most frequently used antioxidants – sulfur dioxide, ascorbic acid and glutathione – and other possible inhibitors of laccase browning such as oenological tannins.
Poster presented at Macrowine virtual (June 23-30, 2021)
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