Toward an understanding of mechanisms involved in non-polyphenol oxidase (non-ppo) darkening in yellow alkaline noodles (YAN)

Abstract

Asian noodles prepared from bread wheat flour darken over time due to a combination of polyphenol oxidase (PPO) activity and non-PPO effects. Although the enzymatic mechanism associated with the PPO reaction is well established, the non-PPO component consists of both physical (e.g., changes in surface properties) and chemical reactions. Variations in pH and solvents were used to gain a quantitative estimate of the contribution of physical and chemical components to non-PPO darkening in yellow alkaline noodles (YAN). In a set of five common high-PPO Australian wheat cultivars it was estimated that on average non-PPO darkening accounted for 69% of total darkening, with approximately two-thirds of this due to physical darkening and one-third had a chemical origin. Data from the chemical portion of non-PPO darkening is consistent with the presence of a PPO-like enzyme that oxidizes tyrosine, has a pH maximum of 8.1, and is inhibited by 50% methanol or ethanol but in the noodle is insensitive to PPO inhibitors such as tropolone. Therefore, with low-PPO and PPO-free wheat varieties becoming available, it may be possible to further reduce darkening in YAN by breeding for wheat varieties with low or zero levels of this PPO-like enzyme.