Contribution of Enzymic Browning to Color in Sugarcane Juice

Abstract

The contribution of polyphenol oxidase (PPO) and peroxidase POD) to enzymic browning in sugarcane juice was investigated. Inactivation of these enzymes with heat resulted in juice of lower color (absorbance measured at 420 nm), but POD was found to be more heat stable than PPO. Salicylhydroxamic acid (SHAM) completely inhibited PPO activity and markedly reduced juice color but had no effect on POD activity. Removal of oxygen in the presence of the substrate chlorogenic acid also stopped color formation. Upon subsequent addition of oxygen, browning continued, indicating that the process was oxygen dependent. Color development in juice was complete after 20-30 min even though PPO was still active. Addition of chlorogenic acid at this point restarted browning, suggesting that color development was limited by the availability of phenolic substrates. Varietal differences were observed in levels of PPO activity, phenolics, and color. There was a correlation between juice color and phenolic content but not between juice color and PPO activity. The sugarcane PPO enzyme was most active with chlorogenic acid. It was not active with p-diphenols and was inhibited by SHAM, suggesting that it is a catechol oxidase-type enzyme (EC 1.10.3.1)and not a laccase (EC 1.10.3.2). It is atypical in that it was inhibited by SDS. The results suggest that enzymic browning contributes significantly to color formation in sugarcane juice and that PPO is the major enzyme involved.