During brewing, yeast changes sugar to alcohol. A beer's characteristic flavour depends on the strain of yeast used to make it. While some small breweries buy in their yeast, most have their own proprietary strains - which they culture themselves.
At the end of the brewing process the yeast cells clump together, or aggregate, and drop out of the beer, leaving it clear and free of impurities. The yeast is then re-used, usually going through 12 fermentations. The efficiency of the yeast during fermentation depends entirely on its physical condition.
'Yeast cells are living things. Even a few old or sick cells can hinder fermentation and stop the yeast culture from aggregating, leaving a cloudy beer that does not taste right,' said Katherine Smart of the Brewing Yeast Physiology research group at Oxford Brookes University, which devised the test.
'By monitoring the cell surface we can detect sick cells and alter the brewing conditions quickly to maintain ideal conditions.'
Dr Smart's test measures hydrophobicity - a property of the cell surface that makes it resistant to mixing with water. This varies according to how well fed and how old the yeast is. The more hydrophobic the yeast, the more it aggregates. This means the test can be used to check the yeast before fermentation and to monitor and feed it the right amount of sugar during fermentation.
As well as affecting the flavour or producing a cloudy beer, unhealthy yeast cells can affect the rate of fermentation. 'The delay may be two hours to two days, and sometimes the process stops altogether in what is known as a sticking fermentation,' Dr Smart said. 'This is very expensive, not only in terms of the time in the fermenter, but also because all the raw materials are wasted.'
If a proprietary strain of yeast developed a hereditary problem it could be destroyed, costing a brewery millions.
The test devised by Dr Smart takes two hours to complete and can be carried out in a test-tube. The yeast is mixed with a dye and then shaken in an oil-and-water mixture. If the yeast is hydrophobic, it is attracted to the oil. The degree of hydrophobicity is determined by measuring how much dye is left in the water.
Dr Smart explained that although this was not a new technique, it had never been used before for yeast. She is now working on a method of separating out unhealthy cells to extend the life of the culture. The technique could also be applied in other industries - pharmaceuticals, flavourings and aromas - all of which use yeast in their production processes.
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