A key ingredient of beer debated by many to be nothing more than a catalyst, is in fact an important contributor to the flavour profile of your beer. As well as converting the sugars from the malts into alcohol, yeast also adds a natural carbonation to the beer.

To brewers, this special ingredient is of particular fascination. Yeast happily eats away at the simple sugars that are present in the kilned malt, and then turns it into alcohol. But who would have thought that such a small thing could make such a big impact on the texture, flavour and aroma of the beer you drink? The right yeast can bring about hints of spice, pears, nutmeg, banana and bubblegum and are most noticeable in wheat beers where a yeasty trace is very desirable.

In Germany and Bavaria in 1516, the German Purity of Brewing Law: also know as the Reinheitsgebot was introduced. This law stipulated that the only three ingredients that could be used in the production of beer were water, barley and hops. In part, this helped ensure that brewers did not use rye and wheat to brew with as it was reserved for bread making, but it was also was the first food regulation to be imposed.

You can tell where we're going with this... where is the yeast in the list of approved ingredients? And the answer to that is short: before 1500, brewers weren’t aware of yeast or the essential process that it is a part of, so it was left off the list.

"But if brewers in the 16th Century had no idea about yeast, how could they brew beer with any kind of alcohol content?" we hear you ask. Well they would often use some of the sediment from a previous batch of beer or leave their brews to 'open ferment' with no idea that these two techniques exposed the beer to yeast which started the fermentation. So, whilst they knew that something was going on that transformed their mixture into alcohol, they really didn't have the tools to truly understand what it was, and so called it a “gift from God” or chalked it up to white witchcraft.

The discovery of the true role that yeast plays in beer production is largely attributed to Louis Pasteur in the 1800’s, though of course there were many other brewers and scientists that were also coming to realise there was more than met the eye when it came to brewing. As they realised that it was such a crucial ingredient, the Purity Law was later amended to allow its use. 

Since the 1800’s, brewers have come to understand the impact that yeast has in terms of its flavour, texture and aroma characteristics. They have experimented, studied and learned over time, and today we are able to separate yeast strains into two broad categories: lager yeast, and ale yeast

The two main strands of yeast are characterised by their behaviours; lager yeast work slowly, and at cooler temperatures over a longer period of time – typically a lager would be fermented at 12-18° over a period of roughly 28 days and work their magic at the bottom of the fermentation vessel; ale yeast prefer warmer temperatures of about 22-24°, and therefore work much faster, often completing fermentation within one week at the top of the fermentation vessel. Each type imparts it’s own flavour, lagers are light and biscuity, whilst ales are warm, fruitier and more bready.

*Side note - there is an alternative fermentation process, which includes ale yeast as part of a complex microbial culture and is similar to the some of the first beers brewed. It is a process known as spontaneous fermentation, and uses a mixture of wild yeast and bacteria, which today is still used to create another group of beers called lambics. More on that to come!

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