Published online 2 June 2008 | Nature | doi:10.1038/news.2008.869

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The search is on to stop the chemicals that make a brew taste bad.

Beer might soon withstand the ravages of time more easily. Punchstock

For connoisseurs of fine beers, there is little more highly prized than a crisp, brewery-fresh flavour. And there is nothing worse than a beer that has gone stale or sickeningly sweet from being stored too long on the shelf. For centuries the chemicals behind these flavours have been a mystery, but new research is now revealing the elusive compounds.

As beers lose their freshness they first take on bready flavours, which later shift to flavours known as cardboard, sweet, caramel, wine and sherry. While some of those might sound delicious, when generated unintentionally they can taste revolting.

Throughout history, efforts to keep beer fresh have centred on the concept of keeping it out of contact with air throughout the brewing process. Although that strategy keeps some unpleasant flavours at bay, beer still eventually goes bad over time. This led Adriana Bravo and her colleagues at Polar Breweries and Simón Bolívar University in Caracas, Venezuela, to hunt for other causes of flavour deterioration1.

Getting a reaction

The team first found that a chemical reaction called the Maillard reaction, in which sugars are broken down by reactions with amino acids, was taking place in beer sitting on the shelf. “The Maillard reaction is one that is usually seen in baking and roasting where temperatures are high — that it was taking place during beer storage where temperatures were 15-20 °C was really surprising,” comments Bravo.

Bravo and her colleagues suspected that intermediates of the Maillard reaction, called alpha-dicarbonyls, are involved in the change in flavour, because they had observed a chemical marker of the reaction increasing in beer as it aged and deteriorated. They could not be certain, however, without putting a stop to the Maillard reaction, so for experimental purposes they added the drug aminoguanidine to the beer.

Aminoguanidine has been under clinical trials in diabetics to control the reaction of sugar with proteins and enzymes inside the body. The team added it to beer to shut down the Maillard reaction without making the beer toxic. When they tasted the doctored beer, they found that even though it was old enough to have developed unpleasant flavours, none was present.

The researchers still did not know which particular steps of the Maillard reaction were responsible for the off-flavours, and so they embarked on an in-depth analysis of the ageing process.

Adding a chemical called 1,2-diaminobenzene (1,2-DAB) allowed them to isolate and identify trace levels of particular alpha-dicarbonyls as they formed in the beer. Over 105 days they detected the appearance of 11 different alpha-dicarbonyls, some of which increased in concentration continuously as the beer aged. This left the team wondering if these highly reactive alpha-dicarbonyls were the ones involved in forming the off-flavours, and what compounds were formed when they reacted with the rest of the beer.

Complex nose

To do this, Bravo and her team used a technique called gas chromatography-olfactometry (GC-O), which combines the high resolution of gas chromatography with the sensitivity of the human nose. Beer was aged with and without the addition of 1,2-DAB, and the flavours sniffed by GC-O, which can identify specific chemicals in specific odours. Comparison of the flavour profiles of both beers showed that 1,2-DAB had blocked the formation of two compounds – furaneol, which smells like caramel, and phenylacetaldehyde, which smells like roses.

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“How these compounds, and the many that are not yet identified, are combining to generate the ageing flavours still eludes us,” says Bravo, “but we are hoping to prolong the fresh beer flavour for longer by manipulating the Maillard reaction.”

Probing the Maillard reaction at room temperature and quantifying the intermediates involved could ultimately help prevent flavour deterioration, explains organic chemist Paul Hughes at Heriot-Watt University in Edinburgh, who was not involved in the study. “But whether these specific pathways are the ones that cause the most noticeable flavour changes remains to be seen... there are thousands of compounds in beer and we still have a lot to learn” he adds.