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The Influence of Hop Products on Beer Flavour Stability
Zufall, C., Wackerbauer, K. and Brandt, C.

The use of reduced iso-alpha-acids in brewing has become more widespread in past years, although their characteristics during beer ageing are not completely known. During our investigations, important differences in ageing characteristics were detected not only between the categories of reduced and non-reduced hop extracts, but also within the group of beers containing reduced hop products. Forced ageing in the absence of light had the strongest impact on beers hopped with CO2-extract, followed by iso-alpha-extracts. Rho, Tetra and Hexa showed a significantly better stability with the tendency to improve from Rho- over Tetra- to Hexahydro-isohumulone. Under light exposure, besides showing lightstruck flavour, beers hopped with CO2-extract were the first to show cardboardy oxidation aroma, while beers containing only reduced hop products were virtually unaffected. Extended periods of light exposure, however, led to the formation of methyl-furfuryl-disulphide (MFDS) off-flavour, also in light-stable beers, as previously reported [1]. The findings from sensory analysis could be confirmed by instrumental analysis of chemiluminescence behaviour. The results indicate clearly that the choice of hop products has a decisive influence on beer flavour stability. The use of reduced iso-alpha-acids in brewing has become more widespread in past years, both to achieve full light-stability and also as additional ingredients in beers brewed with conventional hop products. The partial or complete replacement of conventional hop products represents an important modification of beer characteristics in itself, but furthermore, beer ageing behaviour is most likely affected by this measure. As is commonly known, so-called "light-stable" formulations for beer are only focused on the avoidance of the formation of prenyl mercaptan (3-methyl-2-buten-1-thiol) in beer by the influence of light. The main purpose is to make it possible to bottle the product in transparent glass. There is no protection against any other alterations of the beer's flavour during ageing under the influence of light [2]. In general, hop bitter acids share a chemical structure which makes them function as redox systems and thus, it is obvious that they are able to play a role in beer ageing reactions. In this context, Hashimoto and Kuroiwa [3] showed in 1975 that isohumulone could be oxidatively degraded in model solutions. The magnitude of the sensory impact of this oxidation, however, remained unclear. Our earlier studies showed that ageing characteristics of beers hopped with reduced iso-alpha-acids contrast significantly from those of conventional beers. Some specific well-known stale flavours do not differ between beer types in terms of the flavour impression perceived. However, it was possible to detect flavour notes in light-stable beers which cannot be found in con- ventional products at all [1]. This paper deals with the latest findings about stale flavour intensity and characteristics of beers brewed with the most important types of reduced hop extracts compared within this group and against conventional hop products.Samples of hop products have been provided by two different commercial suppliers. Sensory evaluations of staling beers were carried out by an expert taste panel specifically educated for this purpose. Flavour stability characteristics have been assessed instrumentally by chemiluminescence analysis (CLA) of radical reactions.Ageing of the bottled beer samples in the dark took place at a temperature of 28 ?C. Light exposure took place in a chilled environment (6 ?C) using a 500 W halogen lamp (7,700 lm) placed at a distance of 120 cm to the samples bottled in flint (transparent) glass bottles. Reflective aluminium foil behind the samples provided for even illumination from the back side.

Descriptors: flavour stability, light-stable beer, reduced iso-alpha-acids, sensory analysis, flavour descriptors

BrewingScience - Monatsschrift fr Brauwissenschaft, 61 (May/June 2008), pp. 113-120