Purpose: By baking a loaf of bread, students will learn how chemical changes can alter physical properties. After analyzing the chemical reactions that occur in breadmaking and observing how they alter the physical properties of bread, students will diagram each chemical process that occurs.

Procedural overview: In this activity, students will read the online Science News article “These chemists cracked the code to long-lasting Roman concrete” and complete the “Concrete physical and chemical changes” discussion activity for homework. Using the chemical and physical changes identified in the discussion activity for reference, students will study how the relationships between yeast, sugar, flour and heat create bread. As students complete each breadmaking step, they will identify the chemical reactions and observe how those reactions change the bread’s physical properties. Students will then create diagrams demonstrating each chemical process.

Approximate class time: 2 class periods

Supplies:

All-purpose flour

Sugar

Salt

Fleischmann’s RapidRise or another brand of instant yeast

Olive oil

Bowls

Cling wrap

Baking trays

Scales

Toaster oven

Microwave or refrigerator

Building bread student worksheet

Directions for teachers:

The setup

To prepare for this activity, have students read “These chemists cracked the code to long-lasting Roman concrete” from Science News online. A version of this article titled “Chemists crack the code to ancient Roman concrete” appeared in the Feb. 11, 2023 print issue of Science News. The reading and the discussion activity can be completed as homework.

Before beginning, decide the order of the activity’s components. In method one, students will learn the basics of breadmaking before making and baking the bread during a second class session. In method two, students will make the bread dough during the first class session before learning the science behind breadmaking. Students will leave their dough to rest overnight before baking their bread during a second class session.

The benefit of method two is that it allows for a longer rise time, which produces lighter and more flavorful bread.

The science behind breadmaking

Review the information below about the chemical reactions that occur during breadmaking.

Yeast and sugar are essential. Yeast metabolizes sugar in a process called fermentation, producing carbon dioxide, ethanol and energy (in the form of ATP). This process forms carbon dioxide pockets inside the bread, causing it to rise.

C₆H₁₂O₆ -> 2C₂H₅OH + 2CO₂ + 2ATP

Sugar -> ethanol (alcohol) + carbon dioxide + energy

Breadmaking also requires gluten formation. To form gluten, bread must include water and flour, which contains proteins called glutenin and gliadin. When these proteins come in contact with water, they uncoil and link together to form a gluten network. Kneading the dough works the small gliadin and the large glutenin proteins together, helping them form strong connections and a gluten network. This gluten network can trap the carbon dioxide produced by fermentation. But the amount of kneading can affect the strength of the gluten network, making it either too strong or too weak for proper rising.

Bread must be baked. In bread, temperatures from 140° to 165° C trigger the Maillard reaction. In this process, sugars and the amino acids in protein undergo multiple chemical reactions to form a variety of new compounds that affect the color, texture, aroma and flavor of bread. The Maillard reaction is also responsible for the browning of meats or fried foods and can occur at a variety of temperatures. Because bread is baked above 165° C, it must be carefully watched while baking to prevent it from burning.

After students learn about fermentation, gluten formation and the Maillard reaction, make sure they know the difference between endothermic and exothermic reactions. They’ll need to understand the difference to answer the questions about the chemical properties of bread.

In breadmaking method one, students should answer the questions before starting to make bread. In breadmaking method two, students should begin making bread before answering the following questions. The students’ bread can rise while they work on their questions.

1. How are the processes of making self-healing concrete and bread similar?

Making Roman concrete and making bread both involve chemical reactions that also change their respective physical properties.

2. How do chemical reactions change the physical properties of bread?

Carbon dioxide production causes bread dough to rise, kneading the bread dough strengthens it by forming a gluten matrix, and baking browns the bread.

3. What is the gluten matrix, and why is it important in breadmaking?

The gluten matrix is formed by bringing glutenin and gliadin together, linking them. This gluten matrix can then trap the carbon dioxide produced by yeast during fermentation, helping the bread rise.

4. What happens to the gluten matrix when the dough is overworked, meaning it was kneaded too long? How could excessive kneading affect the bread’s structure?

The gluten matrix could become so strong that the dough cannot stretch and capture all the carbon dioxide being produced. Because of this, the dough might not properly rise.

5. What happens to the gluten matrix when the dough is underworked, having not been kneaded enough? How could this affect the bread’s structure?

The gluten matrix will not form, and the carbon dioxide will not be captured in the dough. The carbon dioxide might instead bubble out of the dough, preventing the dough from rising.

6. Exothermic reactions release energy to convert one chemical compound to another. Endothermic reactions require energy to convert one chemical compound to another. Was Roman concrete production exothermic or endothermic? Why?

Quicklime was mixed with water to produce Roman concrete. This reaction produced heat, a type of energy, making it an exothermic reaction.

7. Predict whether breadmaking is primarily an endothermic or exothermic process. Explain your answer.

Breadmaking is primarily an endothermic process because the Maillard reaction requires heat to occur. Bread cannot be made without this heat-induced chemical reaction.

Baking the bread

Both breadmaking methods in this activity use the same set of ingredients and combine them in the same way, but the methods for rising differ. Method one takes approximately 50 minutes from start to finish and can be completed during the second class session. Method two is started during the first class session and finished during the second class session. Both methods could benefit from additional rising time as longer rising times result in lighter and more flavorful bread. In both methods, the bread should be shaped into a long, narrow log to ensure even baking and to avoid an uncooked interior.

Be sure students have access to the ingredients and breadmaking directions throughout the activity.

Ingredients for both methods:

• 3.5 grams of Fleischmann’s RapidRise or another instant yeast
• 7 grams of granulated sugar
• 9 grams of salt
• 227 grams of warm water (not over 43° C or ∼110° F)
• 344 grams of all-purpose flour (or more as needed)
• Olive oil

Method one:

1. Mix the yeast, sugar and water together.

2. Mix the yeast mixture with the flour and salt.

3. Knead the dough, rotating the dough 90 degrees with each fold, until it comes together and becomes smooth.

4. Drizzle the dough with olive oil, and cover the dough with cling wrap in a microwavable bowl.

5. Microwave the dough for 25 seconds.

6. Let the dough rest and rise for 5 minutes.

7. Repeat steps 5 and 6.

8. Shape the dough into a narrow log by pulling the corners of the dough and folding them toward the center, creating a narrow log.

9. Repeat steps 5 and 6.

10. Place the dough onto a tray and bake at 230° C or ∼450° F for 25 minutes or until fully cooked.

Method two:

1. Mix the yeast, sugar and water together.

2. Mix the yeast mixture with the flour and salt.

3. Knead the dough, rotating the dough 90 degrees with each fold, until it comes together and becomes smooth.

4. Drizzle the dough with olive oil, and cover the dough with cling wrap in a bowl.

5. Let the dough rest and rise for at least 40 minutes and up to 3 hours.

6. Place the dough in the refrigerator to rise overnight.

7. The following day, remove the dough from the refrigerator.

8. Shape the dough into a narrow log by pulling the corners of the dough and folding them toward the center, creating a narrow log.

9. Place the dough onto a tray and let it rest 20 or more minutes to come up to room temperature.

10. Bake at 230° C or ∼450° F for 25 minutes or until fully cooked.

Ask students to fill in the breadmaking chart in their student worksheet as they complete each step in the breadmaking process. Here’s a breadmaking chart with answers for teachers.