In a previous lesson, you learned that yeast needs warm water and food to awaken and grow. In that experiment, we used dehydrated dormant yeast from the grocery store. Before grocery yeast was available, bakers used wild yeast. Remember that yeast is widely dispersed throughout our environment. Some yeast is good for baking; some isn’t.
How do you capture your own wild yeast? Here’s one simple way.
Activity: Buy organic grapes. Or pick wild grapes (even more fun!). Crush the grapes with the back of a spoon. Place in a plastic container in a sunny spot and cover with plastic wrap. After a a day, or even a couple of hours if the conditions are right, it should start to foam. That means yeast is actively growing and exhaling carbon dioxide gas.
Questions:
Where is the water in this experiment? Remember yeast needs water to grow.
Why do you need to put the plastic container in a sunny place?
Swipe your hand through the air. Now look at it. Chances are you have brushed against thousands of yeast particles in that brief sweep because yeast, a living organism, exists on most surfaces and in the air all around us. It covers your skin. It is usually dormant…you can think of it as a sort of deep sleep or hibernation. While dormant, yeast doesn’t actively grow. It’s waiting and it can wait a long time. Scientists have found yeast in Egyptian ovens dating over 3000 years old. They woke up the yeast and used it to produce loaves of bread.
How do you wake up yeast? Warm water and sugars is the quick answer.
Why would you care about yeast? Here, we’re talking about culinary yeast. Yeast makes bread fluffy, and some drinks fizzy. That’s because, when yeast is growing, it exhales air and makes bubbles.
In fact, all living things respire to create and release energy. Alternatively, if an organism breathes, it is alive.
Your Breathing:
Activity 1: Take a big breath and then exhale. Do this a number of times, thinking about why we must breathe. This is one of the big differences between living and nonliving objects. If it breathes, it is alive. Rocks, for instance, do not breathe. Breathing involves two basic steps: taking air into our bodies and letting the air pass out.
Activity 2: Put a straw into a glass of water and blow into it. Watch bubbles form. Try blowing soft and slow; then fast and hard. How do the bubbles change? When you blow into the straw, you are exhaling. This simple activity makes our breathing more visible.
Observing yeast respiration (yeast breath!)
Activity 3: Sprinkle a teaspoon of baking yeast over a bowl of lukewarm water. Wait thirty seconds to see what happens. How did the yeast change?
Activity 4: Add 1 tablespoon brown sugar to the bowl and stir gently to dissolve the brown sugar. Now watch what happens to the yeast.
Activity 5: Start with a new bowl, this time with ice water. Sprinkle yeast over the top and observe.
Activity 6: Starting with another bowl, sprinkle yeast over boiling hot water and observe.
What happened:
Warm water activates dormant yeast, which will start to grow. Bubbles and even movement might be observed in the yeast. However, like other living organisms, yeast needs energy to grow and water doesn’t provide energy. Brown sugar is an excellent source of energy. The yeast will start to foam and bubble. How is this similar to breathing activity #2? Observe closely and you can see yeast moving throughout the liquid, and sometimes you can even feel the air being exhaled by the yeast (if you get close enough). Yeast breathes oxygen and exhales carbon dioxide just as we do.
Yeast sprinkled over the ice water will take a long time to activate, even with brown sugar added. Yeast sprinkled over boiling hot water will die. Yeast needs just the right temperature (warm to your hand) to grow. That’s why some bakers have trouble making yeast bread: they don’t get the temperature just right.
Why do bakers use yeast?
The bubbles make bread dough rise. Baked bread is fluffier. Yeast is also used to make some fizzy drinks.
How is commercial yeast made?
Yeast buds are grown in factories, just as you did in activities 3 and 4, except they are grown in large vats and molasses is used for energy. Brown sugar contains molasses, too. Then the growing yeast is scraped off the top of the vats and gently dried. Without water, the yeast stops growing and goes dormant. It is put into pouches or jars for storage. Refrigerating the yeast at home helps keep it viable for much longer.
There are many types of yeast
Some yeast is used for baking, others for nutrition, for making beer or champagne. Another type of yeast that grows on human bodies can cause problems, but mainly when the yeast comes in contact with warm moist regions that are covered by clothing.
Fun for teachers:
Use a clear, narrow glass to grow the yeast. Spread paper towels below the glass. Fill the cup 3/4 full of warm water; then add 1/2 tsp yeast and 1 tsp brown sugar and stir. The point here is to make the yeast grow and foam over the top of the glass, but don’t tell your students in advance. Students will watch the yeast slowly grow…then faster and faster until a huge foam head forms and spills over the side. You should experiment with the proportions and glass beforehand so you get the right effect. Glorious, especially if the teacher acts as surprised as the students.
Science Without a Laboratory 