Polyphenol Oxidase in Wheat Grain: Whole Kernel and Bran Assays for Total and Soluble Activity

ABSTRACT Color is a key quality trait of wheat products, and polyphenol oxidase (PPO) is implicated as playing a significant role in darkening and discoloration. In this study, total and soluble PPO activities were characterized in whole kernel assays and bran extracts. In whole kernel assays similar to AACC Approved Method 22–85, four wheat cultivars were ranked the same for both total and soluble (leached) PPO activity with L-DOPA (diphenol) as the substrate. Total kernel PPO activity was much greater than soluble PPO activity in three hexaploid wheat cultivars, indicating that insoluble PPO was the major contributor to kernel PPO measurements. Tyrosine (monophenol) was an excellent PPO substrate in kernel assays as expected but had no activity as a substrate for soluble PPO. However, soluble PPO activity with tyrosine was activated by the addition of the diphenols chlorogenic acid and caffeic acid. When PPO was assayed in homogenized bran, 89–95% of total PPO activity remained insoluble, associated wit...

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