SUMMARY The characteristics of polyphenoloxidase in Bartlett pears were investigated. In a citrate‐phosphate buffer containing 0.03M catechol as the substrate, activity of the pear polyphenoloxidase was greatest in the pH range 5.8‐6.4, being optimum at pH 6.2. The Michaelis constant of the enzyme was 0.048M at pH 6.2 in a citrate‐phosphate buffer. It was active only on phenolic compounds having an ortho‐diphenolic configuration. Neither the meta‐ nor para‐dihydroxy phenolic compounds nor phenol was attacked. The energy of activation for pear polyphenoloxidase on catechol was 4.9 kcal per mole. Oxygen was necessary for browning of catechol to take place in the presence of pear polyphenoloxidase, and the activity was greatly decreased when the concentration of oxygen in the reaction mixture was lowered. Diethyldithiocarbamate, a copper‐chelating agent, and phloroglucinol, a competitive inhibitor, reduced browning markedly, but ascorbic acid was most effective of all. It was noted that ascorbic acid acts as an antioxidant rather than as a true enzyme inhibitor. Iodoacet‐amide, a sulfhydryl inhibitor, had no effect on rate of browning. Methods for preventing brown discoloration in canned pears are discussed.
Inhibition of pyrazine formation by natural antioxidants and the foods containing them was measured in a microwaved glucose/glycine model system. Inhibition of lipid oxidation by the same materials was assayed in both bulk and emulsion systems. Pyrazines were determined by solid-phase micro extraction followed by GC. Lipid oxidation volatiles were assayed by polyamide fluorescence produced by either a bulk oil display or a hematin-or 2,2'-azobis-(2-amidino-propane) dihydrochloride-accelerated lecithin or fish oil emulsion. It was shown that (i) the inhibition of pyrazine formation depends on high concentrations of water-soluble antioxidants; (ii) such antioxidants occur naturally in some foods and are usually polyphenols; (iii) during pyrazine inhibition, oxidized polyphenols show enhanced nonfluorescing browning similar to enzymic browning products; (iv) monophenols, which structurally cannot form quinone polymers on oxidation, inhibit pyrazines with less browning; (v) during the final pyrazine-forming phase of the Maillard reaction, polyphenolics and reducing agents such as glutathione and ascorbic acid are partially consumed with some nutritional loss; (vi) fruit powders of grape seed, grape skin, and red wine are highly pyrazineinhibitory, steeped blueberry strongly so, but plum purees are moderately pro-pyrazine, and freeze-dried vegetables strongly pro-pyrazine; and (vii) black and green tea infusions are highly inhibitory, whereas spices have mixed effects.Paper no. J11186 in JAOCS 83, 697-705 (August 2006). KEY WORDS:Front-face solid sample polyamide fluorescence, Maillard reaction, oxidation inhibition, pyrazine inhibition, solidphase micro extraction, water soluble antioxidants.Pyrazines, which are responsible for the nutty flavor from Maillard sugar-amine browning reactions, may be produced by what appears to be a free radical phase of the reaction (1-3), although this has been challenged by some studies (4-6). Their production can be inhibited by high concentrations of watersoluble antioxidants (7-14). When present with creatinine in cooked meats, however, pyrazines produce mutagens of the imidazoquinoline type (8,10,15-18). The extent of the health risks of the latter is still under study (3,16). Although there have been several studies of pyrazine inhibition by antioxidants, none seems to have been conducted under microwave conditions, although Ji and Bernhard (19) showed that the pyrazines produced by microwaving, and presumably the taste and flavor as well, will be quite different from those compounds, tastes, and flavors produced by heat alone. The aims of this study were (i) to assess the antioxidant effectiveness of several natural antioxidants contained in common foods, in aqueous media under high-temperature conditions similar to those favoring Maillard browning [lecithin emulsions initiated by hematin (iron-releasing) or 2,2′-azobis-(2-amidino-propane) dihydrochloride (AAPH; non-iron-releasing)], by monitoring with front-face polyamide fluorescence (2,20) and (ii) to assess their effectivene...
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