Yeast 101: tiny but mighty
Yeast. Not the most glamorous ingredient in beer but pretty darn essential to the whole brewing process and quite a miraculous one at that. Where hops and malt might be the high profile celebs of beer brewing, yeast is the behind-the-scenes assistant who might not look like much but really gets shit done and often doesn’t get the credit for it. So following on from our Hops 101 and Malt 101 sessions, we give you Yeast 101.
What is it?
Yeast is a microscopic, single-celled fungus (like we said – not glamorous). It’s been around for a long time – as in hundreds of millions of years. Luckily for you and me, our existence on the planet has happily coincided with the discovery of yeast as an essential part of creating alcohol so that’s something to be pleased about. Yeast has been used for baking and brewing long before it could actually be observed and its function really understood – it was a kind of invisible miracle – and it wasn’t until the mid 19th Century that Louis Pasteur proved its power as a fermenting agent.
What does it do?
Used in the production of beer, yeast ferments the wort i.e. the malted and mashed grain mix, gobbling up its sugary carbohydrates and converting them into alcohol and carbon dioxide. We made that sound pretty simple but the processes involved at the microscopic level are super complex and sophisticated and have a significant impact on the flavour profile of a beer (more on that later). The enormous numbers of cultivated strains of yeast now available mean that it has the potential to be a very powerful and creative ingredient in the hands of a good brewer. While a couple of breweries are having a go at producing very high alcohol content brews with “super yeast” strains, alcohol, ironically, is actually toxic to yeast and there will come a point in the fermentation process when the yeast succumbs to the concentration of alcohol it has produced and dies. Not a bad way to go, you might say. And noble too – sacrificing itself for the greater good i.e. beer. Thanks yeast. Respect.
Types of yeast
There are now many, many varieties of yeast used in beer production with brewers keeping their particular yeast strains under lock and key – that’s how valuable they can be. But generally speaking, beer yeast can be divided into two main categories – ale yeast and lager yeast.
Lager yeast is often referred to as bottom-fermenting yeast and it works its magic in the deep quiet cold – usually between 5 and 10 degrees Celsius. It attenuates much more slowly than ale yeast and needs 6-8 weeks to do its stuff. The result is usually a super clear, full-bodied, clean-tasting mellow beer with very few esters. Think Pilsners and Bocks.
Ale yeast is described as top-fermenting (it’s the one that creates those big lumpy creamy foamy yeast heads you see in pictures of fermentation tanks) and likes to munch away at a much balmier temperature. It gets the job done faster than lager yeast and produces much more in the way of esters and phenols.
Esters and phenols are just two of the by-products produced by yeast fermentation that give beer some of its distinctive flavours and aromas. In fact, yeast contributes more to the flavour profile of a beer than you might have imagined. That’s because we all get blinded by the glamour of hops and malts and forget our little fungus friends. Esters tend to produce fruity flavours and aromas – think banana, strawberries and apples. Phenols come in a wide variety but the most familiar will be a clove-like spiciness. Try a Hefeweizen for a good example of those banana and clove flavours. The alcohol itself can be a strong presence with ethanol providing a warming and/or astringent effect. Just as with hops and malt, the subtlety and variety of yeast flavouring is enormous.
We hope this gives you a little more appreciation for the humble yeast. Without it we would be sad creatures indeed. Invisible it may be, but mighty is its power.