Yeast Experiments and Bread in a Bag

Overview: Making “bread in a bag” is a popular classroom activity (see resources section). In addition to the fun and satisfaction inherent in making something you can eat, bead baking is an excellent introduction to food science, measurement, and following multistep instructions. This activity takes it a step further, asking students to investigate yeast and what yeast needs to grow. These activities can be part of an introduction to cells, and a bridge between discussing single-celled organisms like yeast and the more complex systems of our bodies.

Notes: The activity can be done as a kit for remote learning. You should check with your school about policies on cooking and eating recipes in class or at home. You may need to purchase ingredients in individually sealed packages, modify recipes based on food allergies, or do the yeast experiments as a class and suggest the bread recipe as an option for families to use at home at their discretion.

(Amounts will vary depending on how many experiments you do. A suggested per-student amount is given, assuming two rounds of 5 experiments. Equivalents are given for a remote learning kit.)

Necessary per student:

Yeast: Per-student: at least 5 teaspoons (Kit: 4-5 packages, each packet is 2.25 teaspoons. Bulk: approximately 160 teaspoons in a pound of active dry yeast.)

Flour: Per-student: at least 4 cups, ideally more. (Kit: 2-pound bag per kit. Bulk: approximately 3 ⅓ cups per pound.)

Sugar: Per-student: 10 teaspoons (Kit: 10 teaspoon packets. Bulk: approximately 108 teaspoons in a pound of sugar.)

Salt: Per-student: 10 teaspoons (Kit: 60 packets, or one salt shaker. A teaspoon of salt equals 12-15 salt packets. You can also reduce the amount of salt in the recipe by half. Bulk: approximately 79 teaspoons in a pound of salt.)

Zip close bags: Per-student: at least 10 (“sandwich size” or one-quart capacity. Test the brand to make sure they are strong and easy to close. )

Measuring spoon: ½ Teaspoon

Measuring cup: 1 cup (with ½ cup marking)

Optional (you can substitute or workaround):

Permanent marker for writing on bags

Aluminum pie plate 8”, or small loaf pan for baking

Oil (for greasing the pan, but can also be an ingredient to experiment with adding to the recipe)

Yeast are single-celled organisms (a fungus) and reproduce rapidly when they have the right environment and food. (When we talk about yeast growing, we are really talking about them multiplying: more cells, not larger cells.) Like us, yeast needs food. Like us, they give off carbon dioxide as a waste product. Our bodies have complex systems for digesting food, bringing food to the cells, and removing carbon dioxide. Yeast are much simpler: each cell absorbs food and gives off carbon dioxide directly. The bubbles of carbon dioxide make bread rise as they get trapped in the sticky dough. The carbon dioxide will also puff up the zip bags in our experiments -- showing us how much the yeast in each bag have grown.

Give out the bread recipe in the next step (you can bake some yourself to show). Go over the background information and have students think about what the yeast in the recipe might need to grow. Go through each ingredient and discuss whether students think the yeast might need it. If it does not seem like something yeast would need, what other roles might this ingredient play in the recipe? The science behind bread baking is actually quite complex, at this point just come up with some good hypotheses -- we will test them in a moment.

How can you test to see what ingredients the yeast need? You can mix just a few of the ingredients in a bag and watch what happens. Discuss with the students what combinations to try. For now, keep to the amounts given in the recipe and only change the combinations of ingredients (in later iterations students may want to try varying the amounts.) Design a data sheet with the students (or use our sample) to have each student keep track of their results, either separately or on one shared document. Ideally, each student should do several experiment bags and one full recipe which they can then bake and eat.

Ingredients:
Yeast: ½ Teaspoon

Sugar: 1 Teaspoon

Salt: ½ Teaspoon

Flour: 1 cup

Water: ½ cup

To make the bread:

1: Put all the dry ingredients in the bag. Close the bag and shake to mix well.

2: Add the water. As you close the bag, lay the bag flat so as to trap as little air as possible. Gently mush it around until everything is well mixed and there are no lumps.

3: Put the bag in a tray or pot that will not be damaged if the bag leaks, and keep it out of the cold. Turn the bag occasionally, and observe what is happening over the day.

4: After about 4 hours, remove the dough from the bag and put it on a baking sheet, pie plate, or in a muffin pan (ideally grease the pan with a little oil, butter, or shortening).

5: Bake at 350 degrees for approximately 45 minutes (you can bake the bread in a regular oven or a toaster oven). When done, the bread should be a bit brown on the sides and top. Get an adult to help with the baking.

The result is a very simple bread. How could we improve the recipe?

• Since the yeast needs time to grow, start the experiment in the morning, and discuss the results in the afternoon. The full recipe should be ready to bake in 4 hours, the dough will be bubbly, and the bag puffed up. If not, you can let it sit longer.
• Not everyone needs to try every combination, but do have several students try each combination. The results may vary based on additional variables such as temperature, small differences in amounts, or mistakes. Discuss the different results. Scientists usually try many replications to refine their experimental approach and results.
• Include one bag with just the dry yeast. It is a good control to compare with the other bags.
• Make sure students observe their bags often: there is lots to see as the yeast starts to grow. Record observations and times on the datasheet.
• Students will ideally come up with other variables to test based on their observations over the first day. For example, the temperature may affect how fast the yeast grow; if the water is too hot it can kill the yeast; what would happen if you vary the quantity of the ingredients; what happens if you add more sugar after the yeast have stopped growing? Plan on following up on these investigations over subsequent days.
• Students will probably not get all the variables perfectly sorted out at first -- for example, if students change two variables at once, say sugar and temperature, they will not know if it was the heat and/or the sugar that affected the results. Let students make some mistakes and discuss them -- these are good ways to learn about how science works!
• Using what you have learned, try improving the recipe. Have students talk to their parents, grandparents, or others to get more ideas (there are different bread traditions in every culture). Share the ideas. Have everyone come up with their own improved bread recipe.
• You may need to modify the recipe if a student has food allergies, and baking should be done with adult supervision.

This Exploratorium page provides a summary of bread science and will lead you to many further references, as well as some follow up experiments for yeast, and other food science investigations:

https://www.exploratorium.edu/cooking/bread/bread....

Another Bread in a Bag recipe (there are many others on the web, including no-knead and kneaded versions):

https://www.instructables.com/No-Knead-Bread-in-a-...

Math and bread (and notes on why the amount of water matters):
https://www.instructables.com/Metric-Bread/

Video of budding yeast cells:

https://www.youtube.com/watch?v=GFEgB_ytDZY

This work is made possible by support from STAR, a Biogen Foundation Initiative. The team at Lesley supporting this initiative includes faculty and staff in the Lesley STEAM Learning Lab, Science in Education, the Center for Mathematics Achievement, and other related Lesley University departments and programs.