The effects of β-cyclodextrin (β-CD) on polyphenol oxidation catalyzed by apple polyphenol oxidase (PPO), endive PPO, or mushroom tyrosinase have been compared. β-CD forms a complex with phenolic substrates of PPO by inclusion. Assuming a 1:1 β-CD/phenol stoichiometry, and assuming that PPO is inactive on the complex β-CD/phenol, K D values were similar when determined kinetically by inhibition of apple PPO or endive PPO. However, the experimental velocities found during inhibition of mushroom tyrosinase by β-CD were higher than the values predicted by this model. In this latter case, it was assumed that mushroom tyrosinase is able to act on the complex β-CD/phenol. A new model based on this assumption allows experimental and calculated velocities to be fit in presence of β-CD. Keywords: Enzymatic browning; polyphenol oxidase; apple; endive; mushroom; inhibition; cyclodextrin

In order to find nonsulfite antibrowning agents for the food industry, cyclodextrins (CD) have been tested. CD are able to form inclusion compounds with polyphenols and inhibition properties have been studied in model solutions containing one or two phenols and purified apple polyphenol oxidase (PPO). The dissociation constants (K D ) of a series of phenolic compounds with α-CD, β-CD and hydroxypropyl-β-CD have been determined both by enzymatic kinetic analysis and NMR experiments in solution with one phenol. Large differences have been found among the phenols tested resulting in wide variations in the inhibitory properties of CD. In the mixtures of two phenolic compounds, addition of β-CD can result either in a large inhibition, a slight inhibition, or even a slight activation of oxygen uptake, depending on the kinetic constants and of the phenols present in the solution. Moreover, since β-CD induced a modification of the balance of free substrates in a multiphenolic solution, the colors obtained after enzymatic oxidation catalyzed by PPO can also be modified in different ways by addition of β-CD as demonstrated on the 3 types of mixtures of two phenolic compounds that we have tested, namely chlorogenic acid / (-)-epicatechin, chlorogenic acid / (+)-catechin and 4-methylcatechol / caffeic acid.Enzymatic browning which occurs during handling, storage and processing of fruits and vegetables is mainly initiated by the enzyme polyphenol oxidase (odiphenol : oxygen oxidoreductase EC 1.10.3.1) or PPO (7, 2). The loss of cell integrity results in the decompartmentation of phenolic substrates and enzymes and then, in the presence of molecular oxygen, the oxidative production of colored quinones (3). Brown pigmentation following this enzymatic reaction and subsequent non enzymatic 0097^156/95/0600-0295$12.00/0

The oxidation of different phenols [4‐methylcatechol, chlorogenic acid, (−)‐epicatechin and (+)‐catechin] by endive polyphenoloxidase (PPO) was investigated in the presence of an extract from Carica papaya. The occurrence of cysteine and another ‘quinone‐trapping’ substance in the extract was demonstrated. The unknown substance was purified, as a 4‐methylcatechol conjugate form, by a combination of Bio‐gel Pz chromatography and semipreparative high‐performance liquid chromatography (HPLC). Use of liquid chromatography/tandem mass spectroscopy (LC–MS/MS) equipment and an amino acid analyser allowed us to identify this agent as a dipeptide cysteine‐glutamic acid, commercially available as γ‐Glu‐Cys. γ‐Glu‐Cys formed one adduct compound with 4‐methylcatechol and chlorogenic acid, and two with the flavan‐3‐ols. The thiol adducts were not substrates for endive PPO but, in the case of the 4‐methylcatechol conjugate, they acted as competitive PPO inhibitors.