Prevention of enzymatic darkening in frozen sweet potatoes [Ipomoea batatas (L.) Lam.] by water blanching: relationship among darkening, phenols, and polyphenol oxidase activity

Enzymatic darkening in sweet potato [Ipomoea batatas (L.) Lam.] is a result of phenol oxidation catalyzed by polyphenol oxidase (PPO). Water blanching prevents darkening in frozen sweet potatoes by significantly decreasing the PPO activity but does not reduce phenol levels. The effect of curing on darkening was indirect and cultivar dependent. Compared to Centennial, the cultivar jewel contained lower phenols but higher PPO activity. Generally, a blanch treatment at 100 degrees C for 3 min or at 94 degrees C for 5 min is required to produce products with minimal darkening. The results suggest that the phenol concentration should be used as an indicator for the potential enzymatic darkening in green sweet potatoes, whereas the residual PPO activity is a better predictor of darkening in the blanched or processed products