Take the guesswork out of sour beer brewing

Sour beers have been gaining popularity in Australia over the years with brewers as well as beer drinkers.

A lot of research has been done on the components in finished sour beers, but not much is known about how acids and other flavours evolve while the beverage ages in a hop.

Now, the University of Redlands’ chemistry researchers Teresa L Longin and David P Soulsby have studied how components evolve during ageing. The information could enable brewers to gain more control over the taste of their sour beers.

The researchers collected beer samples, from Bryan Doty, a master brewer at Sour Cellars, every 2–3 weeks to track changing concentrations of organic acids and trace components during ageing in barrels.

Soulsby and undergraduate student Alexis Cooper examined each sample using NMR spectroscopy coupled with a new analysis method for quantitating the data. They used this approach to track the levels of acetic acid, the main component of vinegar; lactic acid, which gives sourdough bread its distinctive taste; and succinic acid, which is found in broccoli, rhubarb and meat extracts. They found that each acid stabilised at similar concentrations in the different batches, though some batches had greater variability.

“These organic acids give sour beers a lot of their flavour, and the balance of organic acids produces very different types of sour beer,” Longin said. “It can be more like balsamic vinegar, which has a sweet/sour flavour, or it can be ‘puckery’ sour. So, the mix of organic acids is really important for understanding the flavour profile.

“This is a work in progress, but I’m definitely seeing some trace compounds that are changing over time,” Longin said. Some compounds start off at high concentrations and then disappear; they might be sugars that are being consumed by yeast as they produce ethanol and carbon dioxide, and by bacteria as they form organic acids. Others ‘grow in’ over time. They could be additional organic acids, health-promoting antioxidants known as phenolics or vanillin, which lends a hint of vanilla to beer.

The researchers will use the mass spectrometry data to identify the trace compounds and determine whether they come from the barrels or from by-products of yeast or bacteria metabolism. “In addition, if a brewer knows a particular combination of yeast and bacteria produces a desirable flavour profile, they can culture more of it,” Longin said. “Or if they know that a beer with a specific combination of acids is especially pleasing, they’ll know when to stop ageing the beer so it doesn’t lose that balance.”

Results from the study were presented at the American Chemical Society (ACS) Fall 2020 Virtual Meeting & Expo.

Image credit: Bryan Doty/Sour Cellars