| Polyphenol Oxidase in Wheat Grain: Whole Kernel and Bran Assays for Total and Soluble Activity Cereal Chemistry 83:10-16 |
| Fuerst,E.P.,Anderson,J.V.,Morris,C.F. |
| 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 LDOPA
(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 with the bran particles. The kernel assay detected
<2% of PPO measured in an equivalent amount of homogenized bran.
However, total PPO activity was 2-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 cultivars. 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 be largely responsible for wheat
product discoloration.
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