Inhibition of enzymatic browning in foods and beverages

Oxidation of the flavonol fisetin by polyphenol oxidase

The present study demonstrates the antiradical efficiency of fisetin, a flavonol widely distributed in fruits and vegetables, by its ability to react with two different free radicals, ABTS; and DPPH;. The polyphenolic nature of fisetin led us to consider whether it might be oxidised by polyphenol oxidase (PPO), and the results reported show that it can be oxidised by PPO extracted and partially purified from broad bean seeds. The reaction was followed by recording spectral changes with time, with maximal spectral changes being observed at 282 nm (increase in absorbance) and at 362 nm (decrease). The presence of two isosbectic points (at 265 and 304 nm) suggested that only one absorbent product was formed. These spectral changes were not observed in the absence of PPO. The oxidation rate varied with the pH, reaching its highest value at pH 5.5. The fisetin oxidation rate increased in the presence of sodium dodecyl sulfate, an activator of polyphenol oxidase. Maximal activity was obtained at 0.87 mM sodium dodecyl sulfate. The following kinetic parameters were determined: Vmax=49 microM/min, Km=0.6 mM, Vmax/Km=8.2x10-2 min-1. Flavonol oxidation was inhibited by selective PPO inhibitors such as cinnamic acid (a classical competitive inhibitor, Ki=1.4 mM) and 4-hexylresorcinol, which behaved as a slow-binding inhibitor. The results reported show that fisetin oxidation was strictly dependent on the presence of polyphenol oxidase.

Kinetics of combined pressure-temperature inactivation of avocado polyphenoloxidase

Irreversible combined pressure-temperature inactivation of the food quality related enzyme polyphenoloxidase was investigated. Inactivation rate constants (k) were obtained for about one hundred combinations of constant pressure (0.1-900 MPa) and temperature (25-77.5 degrees C). According to the Eyring and Arrhenius equation, activation volumes and activation energies, respectively, representing pressure and temperature dependence of the inactivation rate constant, were calculated for all temperatures and pressures studied. In this way, temperature and pressure dependence of activation volume and activation energy, respectively, could be considered. Moreover, for the first time, a mathematical model describing the inactivation rate constant of a food quality-related enzyme as a function of pressure and temperature is formulated. Such pressure-temperature inactivation models for food quality-related aspects (e.g., the spoilage enzyme polyphenoloxidase) form the engineering basis for design, evaluation, and optimization of new preservation processes based on the combined effect of temperature and pressure. Furthermore, the generated methodology can be used to develop analogous kinetic models for microbiological aspects, which are needed from a safety and legislative point of view, and other quality aspects, e.g., nutritional factors, with a view of optimal quality and consumer acceptance.

Inhibition of polyphenol oxidase by copper-metallothionein from Aspergillus niger

Metallothionein from Aspergillus niger was found to be an inhibitor, in a model system, for the enzymic activity of a commercially purified mushroom tyrosinase. The inhibitory effect of metallothionein was higher on catechin oxidation compared with that on chlorogenic acid. The degree of inhibition of enzymic colour formation by metallothionein, using catechin as substrate, was lower than that of oxygen uptake; however, a reverse pattern was observed with chlorogenic acid as substrate. The demetallization of metallothionein did not increase its inhibitory effect on polyphenol oxidase whereas pre-incubation of the enzyme and metallothionein increased this inhibitory effect.

Mechanisms for controlling enzymatic reactions in foods

Enzymes, as biocatalysts, are capable of specific manipulation of all the major biomolecules found in foods, and a wide variety of enzymes are therefore used as supplements for various food processing operations. There are, however, certain enzymes whose activities result in a rapid deterioration of food quality to be controlled in order to maintain quality and to extend product shelf-life. Various mechanisms and techniques for controlling the undesirable activities of such enzymes in foods have been developed ingeniously over the years by food scientists. These include traditional methods like low- and high-temperature treatment, chemical treatments, and control of water activity involving dehydration and salting. These techniques, however, have certain limitations, such as (1) changing the prime quality of certain foods, (2) health risks associated with certain chemical treatments, and (3) consumer preference for certain foods to be raw and "natural". Novel and potentially viable techniques such as pressurization, use of enzymes ("killer enzymes"), ionizing radiation, chemical modification of enzymes, and other naturally occurring protein inhibitors have therefore been developed to help alleviate some of these problems.

Investigation of the stability of apple allergen extracts

To determine optimal conditions for allergen preservation, we investigated the influence of different stabilizing additives and of storage temperature on the allergen activity of apple protein preparations, obtained by extraction in phosphate buffer or by precipitation in diacetone alcohol and resolubilization in phosphate buffer in the presence or absence of enzyme inhibitors. For this purpose, the extracts were stored for 6 months either in frozen state at -20 degrees C or in lyophilized state at -20 degrees C, 4 degrees C, or room temperature and were characterized by SDS-PAGE, immunoblot, ELISA inhibition, and prick test. The highest stability revealed the extracts that were prepared by precipitation in the organic solvent in the presence of enzyme inhibitors, lyophilized, and stored at -20 degrees C. For storage of extract solutions at 4 degrees C, PBS/glycerol and cysteine/sodium citrate/glycerol were found to be the most effective stabilizing additives.

Enzymatic browning reactions in apple and apple products

This review examines the parameters of enzymatic browning in apple and apple products that is, phenolic compounds, polyphenoloxidases, and other factors (ascorbic acid and peroxidases), both qualitatively and quantitatively. Then the relationships between intensity of browning and the browning parameters are discussed, including a paragraph on the methods used for browning evaluation. Finally, the different methods for the control of browning are presented.