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.

19 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?

    Thanks.

    Reply
    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.

      Reply
  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.

    Reply
    1. Emily Post author

      Hi Karen, Sorry for my delayed reply. If the sugar gets over 6% (relative to the weight of the flour), it will slow the yeast activity, even if you add more yeast. All of the yeast will be affected. So you might see a slight increase in rising speed with doubled yeast, but the high sugar percentage will still control the dough’s rising rate. A better solution is to use “osmotolerant” yeast that is a strain of yeast that can withstand the higher sugar percentage. Or, if all you have is regular yeast, if you wait long enough, the dough will eventually begin to rise and rise enough; it just has a delay.

      Reply
    1. Emily Post author

      Hi Connie, Even if you do not add sugar to bread dough, yeast fermentation will occur. The flour contains a tiny amount of sugar, but it also contains starch. The flour and yeast contain the enzymes needed to break down the starch into sugar. This sugar is then fermented. (If there is no sugar at all—for example, in a substance other than bread dough—I do not think fermentation can occur.) I hope this answers your question, and I am sorry for my delayed reply. Emily

      Reply
    1. Emily Post author

      Hi Jasoooon, I’m afraid I have not put any time into finding an answer. I’m sorry to not be more helpful.
      Emily

      Reply
    2. Paul Galloway

      The 3% refers to flour ‘on weight’
      Example….if you have 3kgs of flour, you will use the equation 3,000 (Flour weight) x 3% (the weight of sugar you need) = 90gms

      Reply
  4. Ana

    Hello!
    If the sugar is converted in alcohol and CO2 during the fermentation. How much of that 3% sugar is still remain in the dough after the fermentation?

    Thanks

    Reply
    1. Emily Post author

      Hi Ana, I think it would be very hard to calculate this, because while the original sugar is being used up, new sugar is being produced (from the starches breaking down). When you bake the bread (and thus stop the yeast activity), it’s like you halt the process in the middle. But, even if you let the dough ferment until the yeast stops producing gas, there might still be sugar in the dough that was simply not near a yeast cell, and therefore not “eaten.” It’s such a complicated process!

      Reply
  5. Tony English

    How long will 2 quarts of 100 degrees water 2 1/2 cups of sugar and .25 oz of yeast Produce CO2? I use this Mixture in my diy bed bug traps And need to know how often I need to change this Mixture to keep Attracting bedbugs with the CO2.

    Reply
    1. Emily Post author

      Hi Tony, It’s been a while (decades!) since I calculated something like this. I’d have to dust off my general chemistry textbook and refresh my memory, and I’m afraid I don’t have the time for that. I think, though, that you don’t need someone with specialized bread knowledge to solve the problem. If I remember right, the calculation is done with the concentrations of the reactants, and the rate constant for the particular reaction, at the reaction temperature. (It’s tempting to try to figure it out, but I have deadlines to meet for work.) I hope you can find the answer you need.

      Reply
  6. Christopher

    Hi Emily, I am a beginner baker and few days ago, when I add more than 3% of sugar to my recipe, I noticed the fermentation rate has slowed down compared to some other recipe I made without adding sugar, and then I came across your post and finally realises too much sugar will slow down the rate of fermentation. Thank you for this post! Appreciated.

    Reply

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