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 CO_2-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 CO_2 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 results indicate clearly that the choice of hop products has a decisive influence on beer flavour stability.
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