One of the most common questions I get asked at the Cookery School is about the different types of yeast and how they work in relation to bread baking. Well, Ladies and Gentlemen, I’d like to introduce you to Saccharomyces Cerevisiae. You want to make bread? You need to get to know her a little.

I say 'her' for a good reason. Let me paint you a picture; millions of years ago when the dinosaurs walked the Earth, some plant species developed an ingenious method of spreading their seeds a widening distance.

They wrapped them in a sweet tasting flesh which birds loved to eat, seeds and all. The seeds were then deposited a set number of hours later a good distance away. The birds were kind enough to leave an appropriate amount of fertiliser as part of the deal to help the seeds get started.

Stay with me here...

Not long after that an entrepreneurial fungus realised that it could latch on to the fruit and feed away devouring the natural sugars on its skin using oxygen to convert the sugars to carbon dioxide. Should no oxygen be available then it fed anyway, but transformed the sugars into alcohol.

This clever little fungus multiplied by 'budding', or creating daughter cells. Hence, 'her', she's lexically female.

Sometimes this budding process enabled them to create alcohol whether oxygen was present or not. As alcohol is a poison, this helped the fungus to kill off its rivals and therefore maintain a serious advantage.

That's why decomposing fruit smells alcoholic. Could this be why humans have a penchant for alcohol? Our ancestors associated it with sweet, heavily ripened fruit, the world's first alcopop!

In more contemporary times, this fungus' ability to create alcohol has been used to brew beer and its ability to to give off carbon dioxide to leaven bread. This amazing fungus is what we refer to today as 'yeast'.

Yeast is truly remarkable stuff, a fungus which exists around, on and in us. There are thousands of recognized types and is arguably the world's first domesticated organism.

So how have humans put it to use?

The first breads leavened by yeast would have been through pieces of dough left over from the previous batch of bread in which the natural yeast in the air had started to ferment naturally. The word 'ferment' derives from the Latin 'fervere' which means 'to boil' or 'seethe', which is exactly what yeast appears to do when activated.

Throughout history there have been many different approaches to harnessing yeast to leaven breads. The ancient Egyptians first used left over dough then progressed to using beer froth or 'barm' which of course had yeast in it. You now know the etymology of the word 'barmy'. Barm was so important in the middle ages that it was referred to as 'goddisgoode', because it was made with the blessing of God.

The Romans would mix grape juice with wheat bran, allow it to ferment and form into small cakes which were dried in the sun. These cakes could be soaked in water to reactivate the yeast when needed.

Basically, a sloppy mix of natural yeast, flour and water was used to leaven breads for centuries. This is what we know as 'sour dough starter' today. No one really knew why this worked or that yeast was in fact, alive.

Right up until 1859 when Louis Pasteur proved how yeast operates.

Pasteur was the first to show how yeast is a living organism. This led to the ability for scientists to cultivate the cells, concentrating them into a block and then taking the water content away from the original sludgy mixture. This block or 'cake' is what we would recognise as 'fresh yeast' today.

However, fresh yeast has a very limited shelf life. During the second world war there was a need for a yeast with greater longevity. It was reduced to small granules, dried, and a natural coating was allowed to form around the granule, thus enabling it to be kept indefinitely. Much like the ancient Roman version it is reactivated in water. This is what we know as 'dried yeast' today.

To make life even easier, these granules were reduced in size so that they could be added straight to the dry mix of flour with no 'pre - rehydrating' necessary. This is what we know as 'fast acting yeast' today.

So how does yeast actually work?

It's a living organism. Like all living organisms, it feeds. Yeast feeds predominantly on sugars which can be found naturally in flour. When water is added to the flour, enzymes immediately break down the starches and release sugars which the yeast feeds on like Cookie Monster with the munchies.

As the yeast feeds on the sugars it produces alcohol and carbon dioxide in the form of bubbles which raise the dough. The gluten in the dough forms around the bubbles creating the texture of the final bread. This stage is called 'proving', 'proofing' or sometimes 'blooming' the dough.

According to Harold McGee, the optimum temperature to raise dough is 27° C, much lower than most recipes suggest. In fact, the longer the yeast takes to raise the dough the tastier your final bread will be.

Yeast will work faster at warmer temperatures but will also secrete more unpleasant by-products. So give it some time and prove at a cooler temperature. This is called 'retardation' and often used by skilled artisan bakers.

Yeast is a tough little thing too. The only thing that will kill it is a temperature of 140° C or the 'Thermal Death Point'. When frozen it becomes dormant and simply awaits reactivation.

When you put your loaf in a hot oven you will see it immediately rise, this is called 'oven spring' and is the result of the yeast's swan song, its farewell performance before it dies. This is essential for your bread to have a decent crumb and one of the reasons why your oven needs to be hot enough to get a good rise.

Much is discussed and written about whether dried or fresh yeast is better. In my experience it is far more about the other ingredients, baking technique and hydration levels which affect the quality of the finished loaf.

There are four main types of yeast that I use for baking bread. Let's have a look at your options.

Fast acting dried yeast

This is the quickest and easiest yeast to use. No need to start it up in any way so when you need to knock up a loaf quickly it works a treat. It doesn’t bring any extra flavour the way sour dough starter can so I use it with loaves containing flour with some flavour of its own such a nice malted wholemeal.

Simply sprinkle the yeast straight into your dry ingredients before adding your liquid. That’s it, couldn’t be easier.

Dried yeast

Larger granules than the fast action yeast, this needs to be activated in water before use. Put the yeast in a bowl and cover with some of the water you have measured out for your bread. Remember not to use extra water on top of the specified amount in the recipe or it will affect the hydration balance.

Fresh yeast

Usually sold in a block, fresh yeast should have a 'fresh' smell, be moist, soft and crumble easily. The light ivory colour should not have any dark patches or discolouration. Once opened, fresh yeast needs to be used up quickly. However, I buy in small packets and immediately freeze. Crumble the yeast into a bowl and cover with some of the water you have measured according to your recipe.

Sour dough starter

Using the natural yeasts in the air, sour dough starter is the most natural method for leavening breads. It takes time to create, look at my post 'Sour Dough Rules' for a more in-depth description of the process. It will keep in the fridge under the right conditions for years. The trickiest to master but brings fantastic results.

So there you have it. All you need to know about yeast.

Time to get in the kitchen and start baking!