Culturing Yeast: Baking Bread in Biology

Loves of bread.

Nice and fluffy loaves of bread requires the generation of bubbles in the dough. This is typically done either with an acid-base reaction (baking soda and an acid) or with yeast. Since we’re doing biology, we made some loaves and focused on how the process of bread making require careful management of the environment for the yeast to produce the carbon dioxide gas that makes the bubbles that makes the bread rise.

We followed the bread-baking recipe I’ve used before for the middle school’s student-run business, but had to shorten rising times to get it all done by the end of our class.

Yeast is a single-celled fungi (typically Saccharomyces cerevisiae). Fungi are heterotrophs, so focusing on what the yeast requires for life and metabolic activity requires consideration of:

  • water (moisture)
  • warmth (but not too warm)
  • energy source (short chained carbohydrates to make the energy more easily accessible)

Yeast produces carbon dioxide bubbles via fermentation (Styurf et al., 2017). It could do it through respiration, but in the bread dough there is not a lot of oxygen available (more info on respiration here).

Fermentation looks something like:

C6H12O6 → 2 C2H5OH + 2 CO2

So, the carbohydrate (glucose) is converted to ethanol and carbon dioxide.

As opposed to respiration (which requires oxygen):

C6H12O6 + 6 O2 → 6 H2O + 6 CO2

Yeast fermenting/proofing.

Bread Baking

A beautiful loaf, nested in a hand-made bread bag.

Our bread-baking enterprise was quite popular last year. In the afternoons, just as the loaves were about to come out of the ovens, we’d get the occasional visitor poking their head into our room for “aromatherapy”.

Consumption in progress.

Students also liked the freshly baked bread. Some favored the crust while others liked the insides; which worked out quite nicely most of the time, but I did on occasion come across the forlorn shell of crust, and once, a naked loaf with the crust all gone.

I liked the bread baking for the ancillary reasons: the biology of yeast; the data collection and analysis for the business; having to graph and problem solve with the oven calibration; the chemistry of cooking; and even the chance to study geographic features (primarily lakes and islands, but also dams and erosion).

Equipment

We’d made loaves two at a time. They were big loaves, and that was as much as the students could comfortably kneed.

Equipment for making bread.

Small equipment:

  • Big mixing bowls: For mixing and kneeding the bread. We used metal ones from the restaurant supply store.
  • Quart sized mason jars: For collecting all the liquid ingredients (honey, milk, water and butter). These can go in the microwave (take the metal lids off) to quickly melt the butter and warm the liquids for the yeast.
  • Bread load pans: I prefer glass because, with metal the bottoms tend to burn in our toaster ovens. We can fit two pans per oven.
  • Two cup measuring cup: For measuring milk and water.
  • One cup measuring cup: For measuring honey. There probably is an easier way of doing this but we have not come up with it yet.
  • Dry measuring cup: One cup size.
  • Measuring spoons: You’ll need the tablespoon, teaspoon and half-teaspoon.
  • Butter knife: For cutting butter.
  • Small, sealable, plastic cups (optional): For collecting and storing enough yeast (4.5 teaspoons) for one batch of bread.
  • Large plastic containers (optional): For storing dry ingredients (flour and salt). They need to be big enough to hold seven cups of flour.

Capital Equipment:

  • Microwave oven: Necessary for quickly warming the liquids (for the yeast to make the dough rise).
  • Oven: We used table-top, toaster ovens. If the loaves rose well, they’d get too large and get burned by the top of the oven. We probably could reduce the recipe to prevent this. The ovens were not always reliable, and we had to do a regular calibration to make sure the set temperatures were accurate.

Recipe

The simple ingredients can be bought in bulk. This recipe makes two loaves.

Making the Dough

Dry ingredients: These can be combined ahead of time and stored in a large plastic container. When you’re ready to make the bread just dump them into a large mixing bowl.

  • Bread flour: 7 cups
  • Salt: 4 teaspoons

Wet ingredients: Combine these in a mason jar. They can be kept in the refrigerator for about a week.

  • Honey: 6 tablespoons.
  • Butter: 4 tablespoons.
  • Milk: 2 cups

Students put together the wet ingredients in the mason jars. The butter is sliced into smaller pieces and put in first (lower right), then the milk is added (left) and finally the honey (middle).

Microwave: Usually, we microwave the mason jar for about two minutes, which melts the butter nicely but gets the jar a little warmer than is good for the yeast. This is usually a good time to talk about density and stratification, because the honey sits at the bottom, the milk above it, and the butter floating at the top.

Cooling it down: So to make the yeast happy, we usually add some cold (tap) water to the mason jar with the other wet ingredients.

  • Water: two thirds (2/3) of a cup (cold from the tap).

Once everything is well mixed and the liquid mixture in the mason jar is at or just above body temperature, add the yeast.

  • Yeast: 4.5 teaspoons (which is equivalent to two of the small packets you buy from the store).
    • Yeast is much, much cheaper if you buy it in bulk. Even the small, 4 ounce jars at the supermarket are around $4, while a 1 pound bag is about $7. We get ours from Sam’s Club, and store the yeast we have not used yet in a mason jar in the refrigerator.

Stir the yeast in well. Don’t stress if there are still some small clumps.

Combine wet and dry: Dump the contents of the mason jar into the large mixing bowl with the dry ingredients. Do it quickly, otherwise the yeast will settle to the bottom of the jar and not all come out.

A hand shaped lake in a land of flour.

Now, kneed the dough. We usually use our hands and kneed in the mixing bowls. You may need to add a little more flour as you’re kneeding it if the dough is too sticky. Alternatively, you can add a bit of water if it’s too dry, but I’ve found it much easier to start with the dough too wet and add flour than doing it the other way around.

You can tell when the moisture is right, and the dough is ready, when it stops sticking to your fingers.

This dough seems a little too wet. They'll sprinkle a little flour on the top and kneed it in. When the dough is ready, it won't stick to your fingers. The last time I said something about their dough needing a bit of flour, the student told me that they knew very well and I should go away because I was just causing trouble. I consider this a success.

I’ve not had any student who was unable to manage the dough, but the quality of the end result depends on the amount of care and effort the students put into it. Unsurprisingly, the more tactile oriented students tend to produce some magnificent dough.

The kneeding is done, and the dough is ready to rise.

Rising and Baking

Once you have a nice dough, it needs to rise for about an hour, although we’ve found that 45 minutes works better since we prefer slightly smaller loaves. Drape a damp towel over it to keep it moist. Use a big enough towel, because if you’ve done everything right, and the yeast is happy, the dough should double in size.

If the dough is left too long it will expand to fill the entire bowl and begin to collapse in on itself.

After it’s risen, punch the dough down, split it into two, roll each piece into the shape of a loaf, and place them into loaf pans.

Now let it rise again for another hour, or 45 minutes in our case (don’t forget the damp towel).

After the second rise (in the pans), place the loaves into the oven at 350 degrees Fahrenheit for 45 minutes. It usually takes the ovens about 10 minutes to preheat to the correct temperature.

And then, you’re done. Enjoy.

Hot out of the oven, a loaf of bread with the school logo. We set an aluminum foil cutout of the logo on top of the bread while it was baking to imprint the shapes in the crust.

Time

Managed well the entire process can fit nicely into the afternoon schedule. We mixed and kneeded the bread during the half hour of Personal World just after lunch (around 12:30).

With the dry and wet ingredients already measured out ahead of time (once a week during the Student Run Business period) our expert bakers could kneed the dough and clean up after themselves in less than 15 minutes.

Then, all that’s left is to transfer the dough to the bread pans, which takes about 5 minutes (including washing up); put the bread in the ovens an hour later (1 minute); and then taking them out of the oven and washing the big mixing bowl (another 5 minutes). Timed right, the bread is finished just in time for everyone to get to their classroom jobs. It helps that everything, except the mixing bowls, can go into the dishwasher.

Rising bread

Yesterday, one of our experimental loaves of bread failed to rise, so re-tried it today and had a discussion about all the things we can do encourage it to rise. Since yeast is an organism, and we talked about the role of yeast in baking bread yesterday, this was a chance for the students to take what they’d learned and extrapolate into a new situation.

These types of situations pop up all the time in the student run business, especially when we try something new. It gets to the critical thinking skills adolescents need to practice. It is the reason Maria Montessori advocated for a boarding house middle school that ran a business. It is one of the reasons I insist that we start at least one new business every year in addition to our core pizza business.

Baking bread; the yeast question

Although I’m pretty sure I’d explained this before, I had a student ask me today what makes the bread rise. He’d been combining the ingredients to make bread for the student run business with a rather thoughtful look on his face. So I told him that yeast is a fungus that “eats” the sugar in the honey and “releases” carbon dioxide bubbles, which get trapped in the dough causing the bread to rise.

I could see the look of disgust racing across his face at the mention of fungi, so I asked, “Would you like to look at it?” He did, and he was not the only one. So after lunch I broke out the microscope, which we have not used much this year since we’re doing the physical sciences this year. A slide, a cover slip, a drop of the residue from the glass jar we used to mix the liquids for the bread, a quick (so very quick) demonstration of how to use the microscope and whallah.

Under 10 times magnification you could see hundreds of cells moving across the field of view. The students were impressed by how many there were. Under 40 times magnification you begin to see cell structures.

Image from Wikimedia Commons, but yeast under the 40x microscope objective looks pretty similar.

We’ll look at yeast again next year when we’re focusing on the life sciences, but when I think of the Montessori axioms that the role of the teacher is to prepare the environment and to follow the child, I think of situations like this. At this time, in this place, after kneading dough for half a year, the student asked the question, and everything was ready for him to answer the that question and whet his appetite for more.