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NMR spectroscopy, and a chromatography sniff test have yielded results that could help brewers improve the flavour and shelf-life of beer thanks to work by scientists in Venezuela. The team has identified alpha-dicarbonyls as important compounds that reduce beer's flavour and point to a new approach to brewing beer that stays fresher, longer. "The characteristic of beer most appreciated by consumers is its fresh flavour," says Adriana Bravo of brewery company Empresas Polar, in Caracas, "This appealing characteristic is progressively lost on the shelf due to flavour changes, which are referred to as aging." Preserving the fresh taste of beer has been high on the agenda for brewers for decades. The centuries old art of preserving water with hops by converting some of the sugar into alcohol, however, has only recently yielded to modern analytical techniques. Bravo and colleagues, using NMR and other techniques, have identified key substances involved in causing beer to develop an "aged" or oxidized" flavour. Bravo and colleagues at the Simón Bolívar University point out that most efforts to preserve the fresh taste of beer focused on protecting the beer from contact with air throughout the brewing process. They say that the return on this investment has been relatively low as there are a group of chemical intermediates, not considered before, that threaten flavour stability as they degrade in beer during aging. They have no revealed these flavour degraders compounds as a group of little-known substances called alpha-dicarbonyls. The team used derivatization with 1,2-diaminobenze to generate detectable quinoxalines in order to identify the alpha-dicarbonyls in stored bottled pilsner lager type beer and found several alpha-dicarbonyls, including four observed for the first time. These compounds were present in fresh beer at very low concentrations, the researchers say, and increased continuously during the storage at 28 Celsius. "The low reactant concentration in fresh beer makes it necessary to use analytical methods that include efficient concentration and isolation steps," the researchers explain, "The diversity of known alpha-dicarbonyls also makes necessary the employment of high-resolution chromatographic techniques such as capillary GC that allow sensitive determinations. A HRGC-MS method also was developed for the quantification of alpha-dicarbonyls as quinoxalines. The mass spectrometric detection was necessary to ensure adequate selectivity." NMR - 1H, 13C, DEPT, COSY, HMQC, and HMBC were used to identify the quinoxalines. They have also figured out how the formation degradation of these compounds might be inhibited by adding to beer, for experimental purposes, reagents that block the alpha-dicarbonyl moiety. In earlier work, the team had found that the addition of aminoguanidine (one of such alpha-dicarbonyls blockers) to beer reduced the deterioration of flavour, in particular it inhibited the production of bread and caramel flavour notes. However, they could not determine whether this effect was due to the inactivation of alpha-dicarbonyls or some other mechanism. In the present study they used 1,2-diaminobenzene (the same derivatizing reagent used for identification purposes) to block more specifically the degradation of alpha-dicarbonyls in beer during storage, and fount it to be four times more effective than aminoguanidine. "This result suggests that 1,2-DAB may be used as trapping reagent to evaluate the role of alpha-dicarbonyls in beer flavor deterioration, without the complication of possible side reactions with sensory-active monocarbonyls," explain the researchers. ?The impossibility of ingesting the beers spiked with 1,2-diaminobenzene to perform sensory analysis was overcome by the use of gas chromatography-olfactometry (GCO).? They used (GCO) - basically a "sniff test" - analysis of the beer headspace in order to determine which aroma compounds were missing in aged beers after spiking with 1,2-daiminobenzene, in other words, to determine which flavour compounds were inhibited after alpha-dicarbonyl blockage. Their GCO analysis of these samples revealed that furaneol and phenylacetaldehyde had been inhibited, all of which affect flavour detrimentally. The authors suggest that inhibiting the initial formation of the alpha-dicarbonyls or their degradation during beer aging may in turn prevent the accumulation of such foul-tasting aldehydes. Of course, the idea is not to add any ingredient to beer to block alpha-dicarbonyls reactions. The challenge is to find out ways to control such reactions," Bravo told SpectroscopyNOW. Until now, a technological solution to the important practical problem of beer aging has remained out of site because of the incomplete knowledge concerning the chemical mechanisms involved. "Until now, most of the approaches considered in the brewing industry have been oriented toward the elimination of oxygen uptake throughout the brewing process to eliminate trans-2-nonenal or its precursors from malt and wort," the researchers explain. Their studies offer an alternative perspective on preserving the fresh flavour of beer. 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 Aged beer, not to be sniffed at |
