R Au-Young of Palmerston North asks :-

How is yeast made?

Ralph Bungard, the brewer at the Three Boys Brewery, Christchurch, responded.

Complex organisms like us humans are made up of millions of cells and have complex reproduction systems. Yeast on the other hand live as single cells and can simply reproduce by dividing in two. This is called budding. You can grow yeast yourself by providing them with a suitable environment. Usually putting yeast in warm (around 25C) water that contains sugar is enough to get them started. However, growing them in large amounts, or for a long time, will also require some nutrients. You can buy prepared nutrients from a home-brew shop or let the yeast get them naturally by using something like fruit-juice added to your culture. If you want more yeast, simply add more water, sugar and nutrients. You can start a new culture of yeast by splitting your existing one in two and feeding both – ginger beer cultures contain yeast and can be shared with a friend in this way.

Yeast is involved in making many products that are part of our every day life. And what great products they are – imagine life without bread, beer, wine or marmite! People have used yeast in the manufacture of food products for almost as long as history can recall. Some would even argue – perhaps a little “tongue in cheek” – that "beer is probably the reason for civilization”. Some bakers, brewers and winemakers still use the same yeast culture that has been kept alive and used for centuries! Don’t worry though, you can also buy yeast from a supplier and start afresh if need be!

There are in fact around 1500 different types of yeast that have been identified. Some have beneficial properties. Brewers and winemakers use the genus Saccharomyces in fermentations that digest sugars to produce the alcohol and carbon dioxide (the fizzy gas) found in beer and wine.

Likewise, bakers use similar yeast to produce the flavours and softness of bread, and marmite is made from dead yeast cells.

Others are less desirable. For example, winemakers and brewers, like me, take care to avoid contamination by wild yeasts that can produce bad flavours and aroma. And worse, some yeast are even human pathogens, which mean that they can produce diseases – but we won’t go too far down that route!

Modern time has seen humans extend even further our relationship with our single-celled friends. In scientific laboratories yeast is often used as a model organism in studies that may ultimately increase our understanding of human biology. Also, there is intense current research on yeast that can ferment things like wood chips to produce ethanol as an alternative to petroleum-based fuels.

Julius Caesar described my favourite yeast product, beer, as "a high and mighty liquor" - but he’d probably had a few by then! Think of our single-celled friends when you next have marmite on toast!