Yeast and Sugar

This is taken from my book, Bread Science. I didn’t know how long it would take me to write up a new post, so I decided just to post this:

Sugar affects the rate of fermentation reactions. A little sugar, up to three percent, speeds up fermentation. The yeast processes the added sugar first, saving the time it would take to break down starch into sugar. With over three percent sugar, however, the fermentation rate no longer increases. [1] Above six percent, sugar actually decreases the rate. This is because the sugar begins to dehydrate the yeast cells. This effect, called crenation, was described in the “Salt and fermentation” section (copied below).

Salt slows fermentation reactions by dehydrating the yeast and bacteria cells. Without the nutrients they need, these cells cannot perform fermentation like usual.

Basically, water molecules are able to pass in and out of cells, a process called osmosis (top left). When there is salty water outside of the cells, the salt interferes with the movement of the outside water molecules (top right). They pass into the cells more slowly. The inside water molecules are unaffected and pass out of the cell at their usual rate. Thus the net movement of water is out of the cells, an effect called crenation. This results in dehydrated yeast cells (bottom).

yeast without sugar yeast with sugar

Dehydrated yeast

[1] Barham, H. N., Jr. and J. A. Johnson. “The influence of various sugars on dough and bread properties.” Cereal Chemistry 28 (1951) 463-473.

6 thoughts on “Yeast and Sugar

  1. Pingback: Notes from “The Science of Yeast” Talk | Food Chem Blog

  2. Neal Jessup

    3% refers to what? Bakers percentage or sugar concentration? Of concern is how sweet can you make a liquid preferment. For example if I have 10oz. of flour and 15oz. of liquid in a preferment (sponge/poolish) , how much sugar or honey can I add that would not lead to yeast crenation in my starter?


    1. Emily Post author

      Hi Neal,

      I always thought the 3% referred to a baker’s percent (i.e., 3% sugar relative to 100% flour). I wanted to re-read the reference, though, to make sure, but I’m afraid I cannot find my copy at the moment, and I don’t have access online. What’s worrying me is that if the “players” in the crenation scenario are yeast, water, and sugar, it seems as if those relative amounts would be more important than the sugar:flour ratio. If the 3% IS baker’s percent, maybe it assumes a “normal” bread recipe (with about 60-70% water and 1% yeast).

      Another thing to consider is that the yeast might eventually “catch up” in a sugary mixture. I do a demo in class with three containers with equal flour/yeast/water but differing amounts of sugar. I use lots of yeast to make it run quickly (and dramatically), so I can’t say it’s equivalent to a preferment with only a little yeast, but it still seems illustrative. The mixture with the most sugar starts very slowly, but eventually it begins to rise and then overflows just like the others. If your preferment is sitting overnight, having extra sugar might not be as much of an issue as it is in a dough, where you want it to rise in a few hours.

      Now I’d like to mix some preferments with different percents and monitor them, to have data on the sugar percent that works best, as well as on if it makes a difference in a preferment. But in order to give you a timely reply, I will post this answer without data.

  3. Neal Jessup

    Thank you Ms. Beuhler. A week ago I somehow completely killed a sponge, not a single bubble. I had made it less hydrated by using some of the liquid for a soaker. I had suspected its demise was from an overly strong honey concentration, and a day or so later when I read that section in Bread Science, my suspicion was confirmed. Whether bakers percentage or solution concentration, I plan to reduce the honey in my sponges, which are now at 133% hydration, and usually ready to use in 4-6 hours.

    Thanks again for your reply. This very topic is being discussed in the Perfect Sourdough group forum on Facebook. I hope you don’t mind that I have posted a link to this page.


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