4-Hydroxy-furanone derivatives as new indicators of malt quality
About this project
In 1991, work in Germany using aroma dilution analysis showed that 2,5-dimethyl-4-hydroxy-3 (2H)-furanone (DMHF) was an important contributor to the sweet/caramel flavour of dark larger. In 1996 Japanese workers found that the concentration of this compound correlated well with the perceived sweetness of a series of experimental light lagers. The aim of this project was to establish the occurrence of DMHF, and the closely related methyl (MHF) and ethyl methyl (EMHF) derivatives, in British malts and determine the important factors influencing their formation. The overall aim was to establish the connection between the concentration in malt and beer so that malt furanone concentration could be used as a quality indicator.
From a chemical viewpoint, the furanones are typical intermediate products of the Maillard reaction which occurs on heating sugars and amino acids together and we found, not too surprisingly, that the compounds could not be detected in dry barley, soaked barley or green malt but that kilning was essential for their formation. Both MHF and DMHF were formed but never EMHF. At this point, the degree of heating was critical. Lengthy drying in the 50-60oC range did not produce furanones but in the 90o-110oC range easily detectable amounts formed within a few hours although only DMHF reached concentrations above the aroma threshold in water. The highest concentrations were obtained with malts that had been stewed at 70oC before drying. The same relationship to heat input was seen on examination of commercial malt samples; lager malt and high enzyme malt were free of furanones, ale malt contained a little but crystal malt had a high concentration of DMHF. Significant amounts were found in a sample of wheat malt. The formation pattern of furanones was more complicated than originally expected. At the final stage of drying after an initial increase, the concentration of furanones declined and then rose again to peak after a few hours. Examination of ten barley varieties under identical malting conditions yielded a six-fold variation in DMHF concentration.
Analysis of ten commercial beers showed that all contained MHF and DMHF but only one had significant amounts of EMHF. With one exception, the five beers having more than two flavour units of DMHF were reported by a taste panel to have a sweet/caramel aroma. In laboratory fermentations crystal malt contributed directly to beer DMHF concentration but fermentation of both ale and lager malts resulted in beers with much higher concentrations than predicted. These malts appear to contain compounds, as yet unidentified, which can be converted into both DMHF and EMHF by yeast and this is particularly true for ale malt.
The compound, DMHF, has been shown to contribute a sweet/caramel/malty/fruity note to the aroma of some malts and beers. Controlling aspects of the malting process have been identified and it is now possible to set, empirically, a malt DMHF specification for a particular beer. However, more research is needed into the final kilning and fermentation conditions to understand formation and transformation of furanone precursors to allow a generalised, theoretical approach to quality assurance.
The results presented in this report will be summarised and set into a practical crop management context in the HGCA Wheat Disease Management Guide, due for publication in 2000.
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