POLYPHENOL OXIDASE IN WHEAT GRAIN: WHOLE KERNEL AND BRAN ASSAYS FOR TOTAL AND SOLUBLE ACTIVITY

Authors: FUERST, E - WASHINGTON STATE UNIV; Anderson, James; Morris, Craig

Submitted to: Cereal Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/10/2005
Publication Date: 1/1/2006
Citation: Fuerst, E.P., Anderson, J.V., Morris, C.F. 2006. Polyphenol oxidase in wheat grain: whole kernel and bran assays for total and soluble activity. Cereal Chem. 83(1):10-16.

Interpretive Summary: Color is a key aspect in the quality of wheat, and through this study it was determined that the predominant insoluble fraction of PPO (polyphenol oxidase) is responsible for discoloration in wheat products. The testing consisted of examining whole kernel and bran extracts for total and soluble activity. Four wheat varieties were studied and the results indicate that PPO activity is the same in whole-kernel and kernel-leachate tests, and that insoluble PPO contributes to darkening and discoloration.

Technical Abstract: Color is a key quality trait of wheat products, and polyphenol oxidase (PPO) is implicated as playing a significant role in their 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 varieties, 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 to 95% of total PPO activity remained insoluble, associated with the bran particles. The kernel assay detected < 2% of PPO measured in an equivalent amount of homogenized bran. However, total PPO activity was two-fold higher in ‘Klasic' than in ‘ID377s' both when measured in the kernel assay and in homogenized bran, indicating that the kernel assay was an accurate predictor of relative total extracted PPO activity in these two varieties. Adding detergents (0.1% SDS plus 0.2% NP-40) to the bran extraction buffer increased both soluble and insoluble PPO activity. Results indicate that relative PPO activities among wheat cultivars are similar in whole-kernel and kernel-leachate assays, and that the predominant insoluble fraction of PPO, which is relatively uncharacterized, may largely be responsible for wheat product discoloration.