Hop bitter acids were extracted and from them were produced isomerised alpha acid (IAA), reduced isomerised alpha acid (RIAA), tetrahydro isomerised alpha acid (THIAA) and hexahydro isomerised alpha acid (HHIAA). Beer brewed from unhopped wort was treated with these hop acid extracts by injecting a measured quantity of the acid into the beer after final filtration. The quantities of hop acids used were intended to produce total bitter substance levels of 5, 10, 15, 20 and 25 mg/litre in the different beer samples. As well as pure acid extracts, a blend of 65% IAA and 35% HHIAA by volume was used, and a control sample had water added instead of extract. The bitterness, fresh beer clarity in Formazin turbidity units (FTU) at 32 and 70 degrees F, and the clarity at these temperatures of beer subjected to a six day cycle of alternating periods at 32 and 140 degrees F were measured. Foam quality was assessed by the Rudin head retention value measurement, the NIBEM foam collapse test, and by pouring a measured amount of beer from a fixed height into a clean glass of standard type and measuring the time required for the head to drop by one inch. In this last procedure, foam cling is also evaluated by comparison with pictorial standards. It was found that even in unhopped wort and in beer fermented from such wort by yeast which had never been in contact with hops, there was a residual "bitterness" of 2 to 3 BU arising from non hop derived components. Beer turbidity increased with rising hop extract levels; clarity was acceptable up to an extract addition rate of 10 mg/litre. Foam quality was significantly improved by adding 5 mg/litre of all except RIAA. Added at a rate of 10 mg/litre, THIAA and HHIAA proved to give the best foam quality, followed (in order) by 65/35 IAA/HHIAA blend, RIAA and IAA. It is pointed out that as flavour effects were not studied, the foam and clarity tests alone may not be a valid indication of the relative merits of the different extracts.
Keywords : alpha acid beer beta acid bitter substances bitterness extraction foam stability haze hopping iso alpha acid