Dark discoloration of barley-based food products

Barley is a source of beta-glucans and has a variety of potential food uses. However, the dark discoloration of the final barley-based food products negatively influences consumer acceptability. Brightness (L* value) of dough sheets was the best indicator of discoloration potential of barley. Large variation in the brightness of dough sheets was observed among different classes and among genotypes within classes. L* value of dough sheets was lightest in hulled proanthocyanidin-free (72.2--78.1), darker in hulled proanthocyanidin-containing (65.3--69.6) and darkest in hulless barley (59.0--63.9). Total polyphenol, protein and ash content negatively correlated with the discoloration potential of barley.;Genetic factors contributed more to the variability of total polyphenol content, polyphenol oxidase (PPO) activity and brightness of dough sheets than environment. Variation attributed to environment was similar to genetic variation for protein and ash content. Across environments, proanthocyanidin-free barley brightness of dough sheets was consistently higher (73.3--76.3) than proanthocyanidin-containing barley (59.3--69.5). Total polyphenol content of abraded grains was highest in barley produced in a dry region at 0.18%, lower in high rainfall areas at 0.13% and lowest in irrigated areas at 0.12%. Although significant, the magnitude of the G x E interactions was generally small and primarily due to changes in magnitude rather than in rank.;Lowering total polyphenol oxidase content (PPO) activity through abrasion effectively diminished the discoloration potential of barley. Discoloration of barley could also be retarded by heat inactivation of PPO, by limiting PPO activity using low temperature, exclusion of oxygen and use of 4-hexylresorcinol (an enzyme inhibitor). Ascorbic acid also significantly improved the brightness of dough sheets.;In abraded barley, phenolic acids, catechin and six major proanthocyanidins were identified and quantified. The major proanthocyanidins were prodelphinidin B3 and procyanidin B3 and four trimeric proanthocyanidins including procyanidin C2. Fractionation of a proanthocyanidin extract indicated that although monomeric proanthocyanidins (mainly catechins) were present in relatively low amounts in barley, they contributed heavily to the discoloration of dough sheets. Enzymatic oxidation of dimeric proanthocyanidins by PPO and autooxidation and polymerization of trimeric proanthocyanidins also contributed to the discoloration potential of barley.