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What is yeast?

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Canto: So we’re going to explore yeast now, for practical purposes. Using a bread machine, I’ve been trying to make, not so much a perfect loaf as an edible one. With beginner’s luck, my first loaf turned out as perfect as this little machine made it, but that has been followed by three failures.

Jacinta: To be fair, none of those failures was inedible, they just didn’t rise satisfactorily.

Canto: Yes, to varying degrees, and the principal and perhaps only culprit, I suspect, was the yeast.

Jacinta: So we need more detail, and then we’ll investigate yeast.

Canto: So I followed the instructions – first some 200-250 mls of lukewarm water, then some butter, salt, sugar, then some prepared flour for a linseed loaf, then some ‘bread improver’, then yeast, all in correct proportions according to a recipe, into the pan of the bread machine, switch to the desired setting, and after 4.5 hours approximately, out came a pretty-well perfect loaf. So, it can be done.

Jacinta: Proof of bread machine concept perhaps, but the experiment needs to be replicated. I believe that in science this doesn’t happen enough, because there’s little kudos in replicating someone else’s experiment – or, in this case, even your own. 

Canto: Well, although I cannot live on bread alone, my desire to replicate the experiment was definitely based on my stomach. So my next effort was perhaps a week later, and I repeated all the steps, or so I believed, but what came out was a shrivelled, concentrated lump. More or less edible as you say, but far from optimal. In fact, a failure. So I went back over my steps and realised I’d forgotten to warm the water. I was thinking the yeast, which I’d taken from the fridge, might’ve needed some warming to get started. 

Jacinta: Well, that’s a hypothesis. So what about the next attempt?

Canto: Well, to be honest, I didn’t try again for some months. But what with the state being in lockdown recently, out of sheer boredom, more or less, I tried again. This time I did all the required steps correctly… well, not quite – I forgot the bread improver.

Jacinta: And I told you it wasn’t a really necessary ingredient, though now I’m not so sure. 

Canto: Yes, because this effort was the most disastrous. The bread didn’t really rise at all – it was flat as a very dense pancake. Or not quite – it was about an inch and a half thick – about as compressed as all those ingredients could be. And that was when I really started thinking about yeast. I’d used the same yeast, from a package I’d opened before the first bread-making attempt. I couldn’t see any use-by date, and I knew that yeast was some kind of living organism. Maybe it was now dead yeast? 

Jacinta: Right. Are we ready to explore yeast in a general way now? 

Canto: Well not quite. So I tried again, this time using a new yeast package – vacuum sealed, but kept in the fridge. So, cold. I did it all correctly this time, but again without the bread improver. Need to know what ‘bread improver’ actually is. Anyway, it kind of half-worked, it definitely rose, but only by half of the tin. So, either the cold state of the yeast, or its only half-aliveness, perhaps, or the lack of bread improver, was responsible. More experiments required. 

Jacinta: Right. And again, the last experiment did result in edible, indeed tasty bread, but a little too compressed. Which brings us to yeast, which is a single-celled egg-shaped fungus, Saccharomyces Cerevisiae (‘sugar-eating fungus’). There are some 20 billion cells in a gram of yeast. These cells derive their energy from the consumption of sugars – remembering that you’ve added brown sugar to the mix (but maybe not enough?), and there is sugar in the pre-mixed linseed flour that you used. Flour contains maltose, a kind of starch, which binds two glucose molecules together. It’s important in brewing. So when the yeast consumes the sugars in your mix, it releases useful end products, such as carbon dioxide and ethyl alcohol. The released carbon dioxide gas becomes trapped in the dough and causes bubbles, which expand as the CO continues to be released, causing the dough to rise. And according to my source:

The ethyl alcohol (and other compounds) produced during fermentation produce the typical flavor and aroma of yeast-leavened breads.

Canto: I haven’t particularly noticed this aroma, but the mention of fermentation is interesting.

Jacinta: Yes, Louis Pasteur did a lot of work on fermentation, in the narrow sense of the byproducts, or end-products, of yeast activity, for example noting that carbon dioxide and ethyl alcohol weren’t the only by-products, and it was later also found that other glucose-consuming organisms and tissues, for example muscle tissue, also engage in a form of fermentation, which we call glycolysis. 

Canto: Right, so there are many types of yeast – for example ‘baker’s yeast’ and ‘brewer’s yeast’. 

Jacinta: Oh yes, there are many forms of Saccharomyces Cerevisiae, the general term for both brewers and bakers. It’s a good source of B vitamins (but not B12), and it’s a favourite ‘superfood’ for those who believe in such things…

Canto: Okay, what is ‘bread improver’?

Jacinta: Well, it’s more yeast. And according to one site: 

It usually also contains emulsifiers, which help to make the loaf soft and fluffy. It may also contain an enzyme which can improve the texture of the bread as well as help it to last longer, or asorbic acid (vitamin C), which can help the dough to rise.  

So it’s likely that using the bread improver would help. Make sure you use it next time. Although  observing use-by dates is probably important for these things. There’s of course a lot more to say about the biochemistry of yeast and the processes of glycolysis and fermentation, and their importance for the energy pathways of all sorts of organisms including humans, but that’s the thing. You start with one simple question and it eventually leads you to how the whole world works. Or at least the living world. But that leads you eventually to the non-living, matter and all that matters. So that’s enough for now.


What is Yeast?





Written by stewart henderson

July 30, 2021 at 11:51 am

Posted in bread, glycolysis, yeast

Tagged with , , ,

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