High molecular weight compounds generated by roasting barley malt are pro-oxidants in metal-catalyzed oxidations

Abstract

The roasting process and color development have been related to an increase of the antioxidant activity of roasted malts. However, roasting is also responsible for the development of high molecular compounds with a pro-oxidant effect, leading to increased levels of radicals in systems based on iron- and copper-catalyzed Fenton reactions. For this reason, the overall antioxidant and pro-oxidant properties of three malt types with different roasting degrees (pilsner, melano and black) were evaluated in a Fenton-based model beer system (5.8 % ethanol, v/v). Black malt exhibited 50 % lower radical quenching capacity compared with pale and melano malts, as determined by spin trapping and electron spin resonance detection. These differences were related to the degree of roasting and the development of high molecular weight browning compounds. High molecular weight compounds isolated from black malt wort (molecular mass in the range of 4 × 106 and 108 g mol−1) were responsible for an increase of radicals (approximately 40 %) in a Fenton reaction and were able to accelerate metal-catalyzed oxidation in a beer model, as shown by a decrease of almost 11 % of the dissolved oxygen. Although black malt was able to reduce the overall levels of radicals generated by the Fenton reaction, high molecular weight compounds had an opposite effect due to the reductive redox-cycling of the catalytic amounts of iron.