Anyone that has baked bread, made beer or wine has used yeast, either dried or live, but I wonder if we know what it really is.

Well brace yourselves if you didn’t know - it is a fungus. Yeast is a single-cell organism, called Saccharomyces cerevisiae, which literally means ‘sugar-eating fungus’, that needs food, warmth, and moisture to thrive. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constitute 1% of all described fungal species. Well, fat lot I knew – the only fungi I knew about were mushrooms, and perhaps truffles.

Where is yeast found in nature? Apparently everywhere – it makes little spores, and those spores are just everywhere — on tree sap, on grape skins, on fallen fruits. They drive the process of decay, helping to break down plant material.

Where it proliferates is on rotting vegetative matter and rotting fruit. It likes sugar. For a long time, people used to lump plants and fungi together, but they’re biologically different, and they’re eating the stuff that other organisms have left behind, whereas plants make their own food through photosynthesis.

Saccharomyces cerevisiae and other yeast species eat sugar and produce by-products including carbon dioxide (responsible for making your bread dough rise) and alcohol (think wine and beer).

Where does commercial yeast come from?

My question was: how is yeast grown commercially? That stuff you just pick up off the shelves in a packet – where does it come from? Baker's yeast is commercially produced on a nutrient source which is rich in sugar (usually molasses: a by-product of sugar refining), and the fermentation is conducted in large tanks. Once the yeast fills the tank, it is harvested by being spun in a centrifuge, giving an off-white liquid known as cream yeast.

What's the Difference Between Active Dry, Instant, and Fresh Yeast?

Active dry yeast is the most commonly available form for home bakers and is available in small packets or jars. The yeast is dormant; it needs to be re-hydrated and proofed before using.

Instant yeast is a dry yeast that comes in smaller granules than active dry yeast, absorbs liquid rapidly, and doesn’t need to be rehydrated or proofed before being mixed into the flour. This one is perfect for bread-making machines because it dissolves ‘instantly’.

Fresh yeast, also known as compressed or cake yeast, is active yeast. It's sold in tiny cakes in the refrigerated section of many supermarkets. Fresh yeast does not keep well; it will last about two weeks if refrigerated, but freezes well and will keep for up to 2 years, once wrapped several times first.

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Making your own yeast for bread making

During the COVID-19 pandemic, some folks had difficulty finding yeast in stores, as people stuck at home with time on their hands took to baking their own bread, and it transpires that you can actually make your own yeast, believe it or not, using fruits, potato water, flour or old bread! Once cultivated, you can dehydrate it into dry yeast if you wish or just use the starter to make your own bread.

Making yeast is not an exact science - you can learn the basic ideas of how to make it, but you’ll find it’s not foolproof. If you’ve followed the basic recipe and it didn’t turn out, it’s likely something in the ingredients. Experimenting is okay - It’s how cooks have developed their abilities over the years.

Personally, I would rather keep buying the little packets – or better still, buy some delicious Portuguese pão already baked!


Marilyn writes regularly for The Portugal News, and has lived in the Algarve for some years. A dog-lover, she has lived in Ireland, UK, Bermuda and the Isle of Man. 

Marilyn Sheridan