Yeast

By Steve Hobman

Yeast is the dark horse of brewing. While brewing, stalwart malted barley and the rather sexier hop often grab the glory and even water (liquor) can be hailed a hero, yeast - essential to brewing - hides its light under the proverbial.

For centuries yeast activity remained rather mysterious, even being regarded as miraculous. That was until boffins of Louis Pasteur's ilk got to grips with it. Then science worked out just what was really going on down there. And it turned out that this little blighter had been having a right rare old time with sex and alcohol.

You see, yeast is a single cell living fungi which reproduces itself. So no need for tacky chat up lines here then. It contains living matter called protoplason. During the vital fermentation process millions of cells merrily thump away and multiply as they turn sugars from the hopped 'wort' into alcohol and carbon dioxide. And they say the sixties were fun!

There's a staggering variety of types and strains of yeast, but for brewing it is basically classified as top-fermenting and bottom-fermenting. With ales and stouts we are only concerned with top-fermenting yeast - Saccharomyces cerevisiae. This is the yeast that likes warmth and first floats to the top of the beer during fermentation. These yeasts produce fruitier, sweeter, ales. Bottom fermenting yeasts work at lower temperatures, creating less fruity esters to give the crisper taste associated with a drink that, I believe, is called lager.

Whichever, yeast quietly and efficiently goes about its task - but with a huge smile on its face. And for cask-conditioned ale that's a big task.

You see the yeasty beastie doesn't just jump on and off like a sex starved matelot in Hong Kong, but keeps stoically plodding on. Way after the ale is unceremoniously dumped in the pub cellar he is still pumping away for several days, making sure we get delicious aromas and flavours in the glass.

Yeast is generally used several times over, but purity is paramount, so it has to be stored in a sterile, refrigerated environment. There's even a national bank where brewers' yeasts are kept in case the curse of infection strikes and new stuff is urgently required. It's all there ready and panting for action.

But don't go thinking this is some sort of insensitive sex and drink fuelled monster. Yeast, it turns out, has feelings too. It doesn't take kindly to disruption and with a dramatic change in a recipe just might go off on one. This is a problem sometimes encountered when ale production has been shifted to another site, through brewery takeover or merger.

Also different strains of yeast have different characteristics and make their own distinct contribution to secondary flavours found in beer. Some appear to work better with certain types of malt and hops - proof that they not merely promiscuous beasts. Yes, yeast, it seems, deserves a much better press.

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