Anti-oxidants and pro-oxidants in preprocessed brewing ingredients.

van Waesberghe, J.

Abstract
Oxidative reactions play a major role in the deterioration of beer flavour in storage. The possibilities for using natural antioxidants present in brewing materials to counteract these reactions and thus improve flavour stability are increasingly being studied. However, brewing ingredients may contain constituents with oxidant properties as well as antioxidants. The application of the Swift test, in which samples are classified as oxidant, oxidation neutral or antioxidant according to their effects (if any) on the rapidity of the oxidation of maize oil under controlled conditions, to malt extracts and hop products is described and the results discussed. It appears that the oxidant or antioxidant character of malt depends on the conditions during the malting process, especially during kilning. Samples of the same batch taken from different regions of the kiln can differ considerably in this respect. Pale lager malt is a particularly problematical product, since many brewers of pale lager, in their determination to produce the palest possible beer, insist on malt colour specifications which compel the maltster to kiln the malt so lightly that lipoxygenase, which catalyses the formation of some off flavour precursors, is not inactivated, so that these undesirable compounds are formed during mashing. However, some pale lager malts do give an antioxidant reaction to the Swift test. Darker malts are usually richer in antioxidant constituents (e.g. melanoidins) and more or less free from lipoxygenase activity, but nevertheless do not necessarily show an antioxidant Swift test response. Freshly picked hops have strong antioxidant properties, but dried hops (especially those which were harvested in a fairly wet state and dried rapidly and intensively) and their processed products often show a well marked oxidant response to the Swift test, except for isomerized products. However, it has been found to be possible to prevent this shift from antioxidant to oxidant character by drying the hops under carefully controlled conditions.
Keywords : antioxidant beer flavour hops malt off flavour oxidation properties stability test