Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/1/2006
Publication Date: 4/1/2006
Citation: Fuerst, E.P., Anderson, J.V., Morris, C.F. 2006. Delineating the role of polyphenol oxidase in the darkening of alkaline wheat noodles. J. Agric. Food Chem. 54:2378-2384. Interpretive Summary: Color is a key attribute of wheat foods. The enzyme PPO (polyphenol oxidase) has been implicated in discoloration. Tropolone inhibited PPO activity in some wheats but not in two very low PPO varieties. Tropolone also reduced noodle sheet darkening up to 50%. It has been determined that PPO activity correlates with the substantial darkening of alkaline wheat noodles. However, darkening of alkaline noodles is probably due to the cultivar-specific level of PPO activity and the presence of at least one additional darkening mechanism. Results indicate that darkening would occur even at zero PPO activity.
Technical Abstract: This study evaluated the effects of inhibitors on polyphenol oxidase (PPO) activity, the effect of the PPO inhibitor tropolone on noodle darkening, and the correlation of PPO activity with darkening of alkaline noodles. The PPO inhibitors tropolone and salicylhydroxamic acid (each at 1 'M) reduced kernel PPO activity by approximately 50% in three hexaploid wheat cultivars, but did not inhibit PPO activity in the two very low PPO cultivars, durum 'Langdon', and the synthetic hexaploid-derived 'ID580'. Tropolone (100 ug/g flour) inhibited alkaline noodle darkening ('L*) by 13-25% in the low PPO wheat cultivar, 'ID377s', and by 39-54% in the high PPO wheat cultivar, 'Klasic'. Alkaline noodle darkening among 502 wheat samples was correlated with kernel PPO activity (r = 0.64). Results substantiate the hypothesis that PPO plays a major role in darkening of alkaline noodles. However, results also indicate that substantial darkening would occur even at zero PPO activity, as measured in the kernel PPO assay. Therefore, darkening of alkaline noodles is probably due to the cultivar-specific level of PPO activity and the presence of at least one additional darkening mechanism. Further investigation is required to identify the phenolic discoloration agent(s) and to determine the potential roles of non-PPO discoloration mechanisms, both enzymatic and non-enzymatic, in wheat products.