RT Journal Article SR Electronic T1 Action of a Ring-Cleaving Enzyme in Preventing Oxidative Darkening of Juices JF American Journal of Enology and Viticulture JO Am J Enol Vitic. FD American Society for Enology and Viticulture SP 221 OP 226 DO 10.5344/ajev.1969.20.4.221 VO 20 IS 4 A1 S. H. Kelly A1 B. J. Finkle YR 1969 UL http://www.ajevonline.org/content/20/4/221.abstract AB 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.