What is yeast?
While the tangible nature of barley and hops makes their contribution to beer easier to comprehend, the role of yeast is more mysterious. Yeast is a single-celled fungal organism which can be cultivated in laboratories but which also occurs naturally.
Aside from brewing it’s a key element in baking bread, where it contributes to making bread ‘rise.’
In brewing, it is used to turn the sugars formed during the mashing process into alcohol and carbon dioxide, the process known as fermentation. As well as producing alcohol and CO2, the action of enzymes in the yeast on the sugar help to create some of the common flavour compounds found in beer.
These are known as secondary flavours as opposed to a flavour given to a beer by hops or other raw ingredients, which are primary flavours. A yeast cell is composed of walls of cellulose surrounding living matter called protoplasm. It is capable of reproducing itself, which it does during the fermentation process, meaning that a batch of yeast can be used more than once.
Yeast has to be cultivated and carefully stored to keep it in its best condition, which is why cleanliness is paramount in breweries. Contaminated yeast has to be replaced.
Yeast banks in major brewing countries keep samples of most strains that are in use, which brewers can use to replenish stocks or buy different strains to make different styles of beer.
Yeast in brewing
Although brewers have been using yeast for centuries the science behind it was only unfolded by Louis Pasteur in his books Etudes sur la Biere, published in 1876. In modern brewing, there are two principal methods of fermentation using different types of yeast strain. One is top fermentation (or warm fermentation), used in making ale, stout, wheat beers and other European specialities such as the German altbier style. Top fermenting, or ale, yeast has the latin name Saccharomyces cerevisiae, although there are hundreds of strains which come under this umbrella.
This type of yeast has to be ‘pitched’ into the cooled wort (the liquid produced by heating the mash with hops in the brew kettle) at temperatures of between 15ºC and 20ºC.
As the yeast begins to work away breaking down the sugars it creates heat which increases the temperature to almost double this. Bubbles float to the top and a thick white crust builds up over time on the top of the beer. Many traditional ale breweries use open fermenting vessels which allow visitors to see this happening and to breath in the ester aromas released during fermentation, though changing fashions and stricter food manufacturing regulations make this less likely as time goes. The vessels increasingly tend to be made of stainless steel, though some ale brewers still use wood fermenters. Ale yeasts tend to be less efficient at turning sugars from the wort into alcohol and carbon dioxide that many ales have a residual sweetness to them.
Bottom-fermentation (or cool fermentation) is used principally in lager production. The yeast is pitched at a cooler temperature – somewhere closer to 10ºC, – and fermentation takes place over a longer period of time. From the fermentation tanks – which are either upright or horizontals conicals with a spout to drain off the yeast residue at the bottom – the beer goes into lagering (or maturing) tanks, in which the remaining yeast performs a secondary fermentation, turning more of the sugar into alcohol and CO2.
This gives lagers their fizziness but also ensures the ale-like sweeter flavour notes are removed, leaving a drier, crisper beer.
Lager yeast goes under the name saccharomyces uvarum, or sometimes saccharomyces carlbergensis, after the Carlsberg brewery in Copenhagen were the strain was first isolated.
The brewing world has one significant and magical variation on the two main types of fermentation. Spontaneous fermentation uses airborne yeast strains that occur naturally to perform the fermentation process.
This is a key element in the production of the fantastic sour-tasting lambic beers of Belgium, on which many of the country’s fruit beers are based. The brettanomyces yeasts that perform this process are encouraged to feed on the wort overnight by using open-topped fermenting vessels and leaving brewery windows open.
Spontaneous fermentation is not confined to Belgium. The Melbourn Brothers brewery, based in Stamford, Lincolnshire, produces a range of fruit
beers using spontaneous fementation, principally for the United States market. Real ale and bottle-bonditioning Cask-conditioned ale – or real ale – describes a beer in which a second yeast fermentation takes place after the beer has been put into casks.
This is done using a fresh dose of yeast replacing the exhausted one that was in the fermenting vessel. Isinglass finings are added to casks to attract yeast cells to the bottom of the cask, allowing a clear pint to be drawn from the cask.
Bottle-conditioning is similar to cask ale, with a secondary fermentation taking place in the bottle. Although there are hundreds of smaller bottle-conditioned beers on the market, few have have made significant commercial in-roads into the market.
Arguably, this is because of a lack of drinker understanding of the nature of the product, which has a small amount of yeast sediment in the bottom of the bottle, which can be off-putting to the uninitiated, but is in fact harmless. Famed bottle-conditioned beers such as Courage Imperial Russian Stout and Guinness Extra have fallen victim over the years to the commercial pressures of the modern brewing industry.
Worthington White Shield has undergone a revival under new owners Coors, which brews it at its microbrewery at its museum in Burton on Trent.
This savoury spread on which millions of Brits are weaned from an early age is made from waste yeast from the brewing industry, and is a rich source of B vitamins found in yeast. The tower of the Marmite factory is a famous landmark on the skyline of the British brewing capital, Burton on Trent.