Action of a Ring-Cleaving Enzyme in Preventing Oxidative Darkening of Juices

¹ Western Regional Research Laboratory, Agricultural Research Service, U. S. Department of Agriculture, Albany, California 95710.

Catechol type compounds such as chlorogenic acid are the natural substrates for polyphenol oxidase in fruit juices, and these compounds are converted by the enzyme to polymers that result in a dark juice. When apple juice, in the presence of ascorbate, was treated with the enzyme protocatechuate 3,4-dioxygenas, darkening was permanently prevented since the enzyme oxidatively cleaves the benzene ring of the substrates to yield branched chain derivatives of muconic acid. The muconic acid derivatives cannot participate in the polyphenol oxidase browning reaction.