Temperature for yeast in gluten-free bread: what's best?

Yeast has optimal activity at 35C (95F), as shown in the graph above. This graph came from a 2009 article where carbon dioxide was measured in a system of sugar water and yeast, at varying temperatures. Let's explore the optimal ingredient temperature for yeast in gluten-free bread.

A graph of mass of CO2 produced as a function of temperature, it shows an increase until 35C and then a sharp decrease.

It's not a new piece of information that we should use warm/hot water to create a favorable environment for yeast when baking bread. However, as bakers, we should be more cognizant of the why behind this. Dough is a mixture; it contains water, flour, sugar, salt, yeast and so on. We care that the dough be the optimal temperature for yeast activity, not the water itself. This is important because some of us freeze or refrigerate our gluten-free flours, or we bake bread in a cold house. All of these factors will affect the thermal equilibrium temperature of the resulting dough mixture.

Two jars filled with water, gluten-free flour and yeast at different temperature. The left jar has cold water and did not rise while the right jar has hot water and rose more.

Thermodynamics of dough temperature

Here comes the thermodynamics part; we have 2 systems, water and flour, each with a different specific heat capacity. Specific heat capacity is the amount of energy it takes to raise the temperature of a substance (Joule for every 1C of 1g of a substance). Water has a very high specific heat capacity; it takes quite a lot of energy to raise its temperature. Flour less so.

Water: 4.19 J/gC

Flour: 1.80 J/gC (note that I only found this value for wheat flour, in GF baking we use a mix of flours, but for our purpose here, we will use this value)

When we mix our two systems together, they will eventually reach thermal equilibrium. Plainly, if we mix cold flour with hot water, the resulting dough will eventually come to a temperature that’s somewhere in between. The heat gained by the flour will be equal to the heat lost by the water. But it doesn’t mean that if we have water at 100F and flour at 50F that the dough will be 75F! We have different quantities of water and flour, and as seen in the high specific heat capacity of water, water is more resistant to change in temperature.

Short algebra lesson

We can use Q=mcdT for each system, where Qflour=-Qwater to calculate this equilibrium temperature. Note: this will give us an estimate, as we don’t have a closed system. Final equation for equilibrium temperature attached. I have derived this equation so many times teaching AP chem! It’s really fun to use it in something I’m passionate about! If algebra isn't your thang, don't worry. You can read this as: the final temperature of my dough depends on the amounts of flour and water, their initial temperatures, and their specific heat capacity which we discussed in the previous paragraph. Initial temperatures are bolded because that's what we can control in our kitchen.

An equation relating the equilibrium temperature of the dough, with respect to the temperature, mass and specific heat capacity of water and flour.

I have calculated several values of final dough temperature, based on a gluten-free bread with 106% hydration (if there were 100g of flour, there would be 106g of water), with different starting temperatures for dough and water. I did the same exercise with wheat flour using a sandwich bread recipe with 58% hydration. We can see that the optimal ingredient temperature for yeast in gluten-free bread are room temperature flour and hot tap water (~100F). I find it interesting that dough with gluten-free flour results in an equilibrium temperature closest to that of water. Intuitively, it should make sense since gluten-free breads contain more water.

A data table showing initial water and flour temperatures, with a corresponding dough equilibrium temperature: the warm water and flour combo is closest to optimal yeast growth temperature.

Putting it all together

The key takeaways from these thermodynamics calculations are one: if you freeze/refrigerate your gluten-free flours, let them come to room temperature before baking; and two, it is best not to use cold water when mixing your dough. I'm not saying to use a thermometer and obsessively play with your tap water until it is 100-110F! Just use hot water that is less than the yeast kill zone temperature of 140F; a good rule is "comfortable bath temperature". I know my kitchen faucet has a hot water limiter that prevents scalding temperature anyways, maybe yours does too!

Thoughts for the future…

1. Yeast dies at 140F. Does mixing water higher than 140F with flour kill the yeast? Or does it reach thermal equilibrium in time for yeast to survive?

2. I have noticed my gluten-free loaves rise A LOT in the oven! How long does it take for the dough to reach 140F in a 400F oven?

3. How much of the extra rise is due to yeast activity vs. hot air expanding in the hydrocolloid network?

4. Does wheat bread rise as much in the oven as gluten-free bread? My hypothesis is no, since we are comparing a dough with 106% hydration for gluten-free vs ~60% for wheat (the chemistry of gluten affects hydration too!) Less water in wheat bread means less resistance to temperature change, therefore reaching yeast death temperature faster.

Did you enjoy this little thermodynamics exercise, or have some knowledge/questions to add? Leave a comment!