Science of bread making – how yeast works

So, being nosey as usual, after making a batch of spelt walnut loaf I thought I ought to learn a bit more about yeast and the bread fermentation process. I remembered some stuff about yeast from A-Level biology (yeast is one of the most researched and written about organisms going) and I’ve had a batch of sourdough starter yeast on the go for some while, which is fascinating to look at and use. This post is an amalgam of some of the descriptions of yeast and fermentation I’d found both in books, e-publications and websites. I’d urge you to go and look up any specific aspects of this you find interesting for more in-depth information than I’ve given. There’s tons of detail out there (in fact I got rather overwhelmed by the amount of specific information) and it’s really interesting! Oh, and go give your own ‘pet yeast’ project a try by growing a starter dough – I’ll post about this too soon.

What is yeast?

Yeasts are single cell fungi and there are apparently around 1,500 species of yeast. They are prevalent everywhere, even in the air, which is why anyone can make a sourdough starter.

Only one of those 1,500 species is used for baking and beer making though, and this is Saccharomyces cerevisiae (pronounced sak-ka-roh-my-sees serra-viss-ee-i). The name comes from combined Greek and Latin origins of saccharo for sugar, myces for mould and cerevisiae for ‘pertaining to beer’. Its name represents the way that it thrives off sugar and can be used in the fermentation process for beer.

Although it’s same species as Baker’s Yeast, Brewer’s Yeast is a slightly different strain. In the past (and you could still do this, if you had access to a brewery!) bakers used to skim off the yeast from the brewing process and use that for their bread.

How yeasts do what they do

Yeasts normally ‘breathe’ (respire) oxygen and reproduce by budding. If they come into contact with sugars (carbohydrates) and are starved of oxygen they start to behave very differently – fermentation.

We can start off fermentation in bread when we mix and knead the dough, as the yeast becomes trapped within the gluten structures and is deprived of air. The dough also provides the carbohydrates the yeast needs. The lack of oxygen and presence of sugars is the perfect environment for yeast fermentation and making bread.

During the fermentation process the yeast’s two enzymes (amylase and invertase) break down the complex carbohydrate molecules in the flour into simpler sugar molecules. The yeast then consumes the sugar and carbon dioxide and alcohol (ethanol) are produced as ‘waste’. If there is more sugars than the yeast needs the result is a sweeter bread – this is controlled by the type of grain used (some grains have more carbohydrate than others) and with the additional of extra sugars in sweet dough, eg Kugelhopf or Chelsea buns etc.

The ‘waste’ carbon dioxide of course isn’t really waste to us at all. This is what causes the rise in leavened bread. The carbon dioxide is caught as bubbles within the gluten strands and puffs up the dough. The alcohol that is given off at the same time gives the dough flavour – a slow rising process (in a cooler environment) will result in a more mellow flavour.

So should we be worried that bread has living organisms, carbon dioxide and alcohol? Absolutely not – not only would you not get (leavened) bread without them, but during the baking process bread the heat will kill off the yeast and evaporate the alcohol and carbon dioxide. All that will be left is the holes where the gas and alcohol had been – which gives you the lovely spongy, holey texture of a well-risen loaf.

Do we need to knock-back anymore?

A growing movement in baking, spearheaded by a number of bakers including  Dan Lepard, Richard Berninet, Mark Bittman etc, is to treat bread with less intervention but more reverently. This means that vigorous kneading is reduced and the knock-back is eliminated. Dough is left to rise by itself with only brief kneading (a typical recipe of this type says to left rise for 10 min then knead for one minute, repeating this three times) and/or a folding technique which introduces more air. The theory is that not only is the yeast left to do its work with less disturbance but why would you knock back all the air that you’ve so lovingly tried to incorporate? The one thing I’ve not been able to find out about as yet is how this affects the original explanation of reducing the increased amount of carbon dioxide via the knock-back. As explained below, knocking-back is supposed to get rid of any very large holes – but it seems that without knocking back you don’t necessarily get giant holes anyway. Maybe less kneading means the yeast works slightly less vigorously producing more even (and therefore desirable) holes in the first place? And if you get very large holes, does it matter? Perhaps the carbon dioxide releases during baking or that cutting the loaf and exposing the pockets of gas to air simply eliminates this problem or, simply, it wasn’t such an issue as people thought in the first place?

Why was knocking-back bread used if we want the yeast to make the dough rise?

The thoughts behind using knocking-back are mainly based around the pockets of carbon dioxide gas that fermenting yeast can create during the initial proving process. Huge holes aren’t seen as desirable in a loaf – not great for toast or sandwiches, but lots of mall and medium-sized holes are very de-rigueur now (during the second world war in the UK getting a holey slice of bread felt to people like they were getting cheated out of their bread allowance, so it became imprinted on the UK psyche that holes = bad). Knocking back bursts these larger pockets of aire and helps distribute the carbon dioxide, the alcohol, the yeast and any sugar molecules left through the dough. The process is also supposed to re-activate the yeast, giving it a ‘second wind’ to go on and ferment the remaining sugars, but this is becoming seen as less important now the trend is to left the dough rise sufficiently (but not over rise) in the first place. Typically, the second proving stage after knocking back is shorter because the yeast is exhausted and less vigorous so there is little chance of large holes developing at this stage – but equally less chance of a decent rise if you’ve bashed the hell out of your loaf. Still, done correctly and with a big less vigour the traditional way of making bread still produces wonderful loaves – the trick is to pick the technique to match the recipe and give the no-kneading approach a go to see how it works for you.

What affects yeast

Overwhelming the yeast with salt

Adding salt directly onto the yeast can inhibit or even kill it in extreme cases. So, best to add salt into the bowl after the flour and/or water has already been put in. You can easily reduce the amount of salt in a bread recipe if you are trying to cut down, as salt isn’t part of the fermentation process – it’s only there so you can taste salt in your bread!

High temperatures

A warm room and the yeast will become nice and active. However, too hot and the yeast can’t cope. If you’ve had a loaf that won’t rise chances are you used milk or water (dependant on your recipe) that was overly hot. You can use cold liquids (rather than the warm usually specified) – the kneading process will create friction, and therefore heat, to activate the yeast anyway. Yeast will die at anything around or above 50C (122F). An ideal temperature for yeast is around 30-35C (86-95F).

Cold temperatures

Actually, cold doesn’t kill yeast, but it will slow fermentation right down (keep your sourdough starter in the fridge to slow it down, if you’re not making bread that often). Fresh yeast can actually be frozen as the extreme low temperatures make the yeast dormant and, after raising to the right temperature, it’ll spring back into life.

You can safely let a bread rise somewhere fairly cool but it’ll just take longer (probably overnight). Some recipes call for the dough to be placed in a fridge/cool place anyway to slow the fermentation process down and create a more mellow yet deeper flavour.

If you’ve got a cold environment but can’t afford to wait overnight, there are some excellent tips on Epicurious’s Bread Recipes and Tips page – see section 4.Proofing about getting round the cold and speeding up fermentation.


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