CHARACTERISTICS OF CONCORD GRAPE POLYPHENOLOXIDASE INVOLVED IN JUICE COLOR LOSS

Abstract

ABSTRACT The factors affecting postharvest color losses in Concord grapes were investigated, with emphasis on the extraction, partial purification and characterization of naturally occurring enzymes that may be involved. Color losses were accelerated in Concord juice by the crude enzyme extracted from the grapes. Catechol was oxidized more rapidly than other o ‐dihydroxyphenols, although pyrogallol, caffeic acid and dl‐DOPA were good substrates for the crude polyphenoloxidase (PPO). The optimum activity was in the pH range 5.9–6.3 and at temperatures 25–30°C. Kinetic data for the crude enzyme showed a Michaelis constant of 67 mM of catechol at pH 6.0. Effective inhibitors were Na 2 S 2 O 5 , dithiothreitol, phenylhydrazine, cysteine and others. Peroxidase (PRO) activity of the crude extract was very low in ripe Concord grapes. Anthocyanin pigment degradation by the crude enzyme extract was dependent on maturity of the grapes. The most rapid degradation of pigment occurred when crude enzyme from ripe grapes was added.