How much sugar does yeast consume?

Question

I recently made a challah bread and wondered how much of the sugar is consumed by the yeast and whether this quantity is big enough to make a difference in the perceived sweetness. Essentially if I make a somewhat sweet bread or a cake dough that contains yeast do I need to add more sugar than I would add for a comparable dough without yeast to reach the same level of sweetness or is the amount of sugar consumed by the yeast so small that it has a neglible effect on the sweetness. This could also be useful if you are trying to accurately compute the calories of your food.

One could approach this from the pure chemical perspective, the yeast transforms sugar plus oxygen into water plus carbon dioxyde. Say 1kg of bread doubled in volume, so 1 liter of carbon dioxyde was produced and then compute the weight of the sugar from there but I'm not sure whether this gives even the right order of magnitude for the actual sugar consumption.

Answer 1

I'm very happy to see the long answers explaining the background in depth. For completeness, I'll give a short answer on the practical side.

This is not a consideration in baking. You don't need to make any adjustments.

There are two main reasons for that.

1. The scale on which the sugar consumption happens is negligible. No doubt, there are quantitative changes in the amount of sugar (in both directions, as other answers note). But they don't translate to a different sweetness perception when eating the bread, because the changes are tiny.

2. Recipes are already designed to produce the desired taste. If you have a bread recipe that specifies 1 tbsp of sugar, this means that it produces optimal taste (whatever "optimal" is for the recipe author) when made with 1 tbsp of sugar, not when made with 1-tsp-of-sugar-plus-compensation-for-what-the-yeast-ate.

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I make [...] a cake dough that contains yeast do I need to add more sugar than I would add for a comparable dough without yeast

You shouldn't be in this situation in the first place. Yeast doughs and non-yeast-doughs are fundamentally different, and not easily translatable into each other. But if you do find yourself in it, you'll likely have to make some major changes to the sugar content for the recipe to work at all, and these changes will put considerations both for yeast usage and for level of sweetness in the background.

Answer 2

Bread is a complex mixture, with lots of different factors that alter the rates of growth and respiration of the yeast used in the bread in odd ways, so we can't be sure. Probably much less than a quarter of the sugar will be consumed would be my guess.

Sweet perception is very complex. I've not made sweet yeast-leavened and compared it to a chemically-leavened recipe for bread before, but I doubt that you will notice much difference. This would of course also depend on how much sugar you are adding to each. Chemically-leavened breads will have more salt (from the bicarbonate, this is usually sodium and/or potassium salts), so you might add more sugar to cover this taste. Fermented breads will have additions of flavour such as esters produced by the yeast that will often taste sweeter.

Anyway, on to a more scientific answer.

How much sugar might be used by the yeast depends on a number of factors. Those factors are related to the growth and respiratory rates of the yeast in your bread. These include things like available water, sugar concentration, salt concentration (also related to water availability), how long a ferment/rise you have done, and temperature at which your bread has been fermenting. You should also note that the flour contains starches that the yeast can feed on in addition to added sugars.

In a pure culture situation, such as when you are making beer or wine, you can calculate the final alcohol concentration of the brew by assuming the sugar is completely consumed and converted to alcohol. This uses the equation:

Alcohol Content = (Sugar concentration * 0.136) + (Sugar concentration² * 0.011)

Where alcohol content is percentage (%) and sugar concentration is in grams per litre (g/L). However, it isn't so simple, even for simple cultures such as wine, to calculate the points before the end-point. This is because the growth and consumption curves aren't simple linear, they end up being logistic or some sort of more complex curve that is difficult for us mathematically-challenged mere mortals to derive a calculation from, even if we were able to observe and plot the exact values directly.

A plot of the sugars and yeast growth looks something like the following one I got from Slideshare.net. Image attribution: Vladimir Jiranek, University of Adelaide. You want to look at the green line for sugar content (R/G colourblind - that's the solid line starting high on the Y axis) and the blue dotted line starting at 0,0.

As you can see, it's quite complex. It's even more complex in bread because of all the extra additives you have, but will follow the same trends. Yeast will replicate in a logistic curve (probably quite a lot slower than the one in the graph), sugars will come down in some sort of negative logistic curve, but the durations of the phases for each and what sources of sugars are being utilized at each point make it hard to do any calculations.

You should note that bread making doesn't generally last anywhere near as long as wine/beer brewing, so the yeast won't reach the plateau you see in the graph, you'll be baking the bread somewhere (well) below the point at which the growth line for the yeast hits the pH line in the plot, with concomitant use of sugars.

Answer 3

Yeast does consume sugar (primarily glucose), but when you make bread enzymes in the dough are going to break down the starch into simple sugars (glucose and maltose), and that alone will make the bread sweeter. The longer you ferment the more sugars yeast will eat but also the more sugars that are created from the break-down of the starch.

Also, yeast prefers glucose to fructose, so when the sucrose is broken down by enzymes a lot of the fructose is likely to remain, and fructose is much sweeter than glucose or sucrose, so it will taste sweeter.

Cookies that are aged longer will taste sweeter and will brown more because of the catabolism of the starches and proteins.

The short answer is, don't worry about it. You don't need to add more sugar.