Picture yourself living in ancient Egypt and imagine that it
is your job to rise before the sun each day to bake crackers for
your family.
Mixing up ground wheat and honey one afternoon, you are
distracted. Maybe you're watching a pyramid being built just across
the Nile. You forget to cover up the cracker dough. It
sits all night in an open window, caressed by a warm breeze
carrying tiny life forms that are too small to see.
When you wake the next morning, you find the dough puffed up
and overflowing its bowl. Everyone will be awake and hungry
soon and you don't want to get in trouble, so you go ahead and bake
it.
The crackers are not hard and flat like usual, but emerge
from the hearth light, puffy and delicious. You have just baked the
first bread in human history.
No one really knows how the ancient Egyptians discovered yeast,
but we have learned from their writings and artwork that they have
been making bread for over 4,000 years.
How bread rose was a mystery though, until a famous scientist
named Louis Pasteur proved that tiny living organisms called yeast
were responsible for making bread dough puff up.
Bread yeast is a type of fungus and is related to
mushrooms. If you look at yeast cells under a microscope, you
will see that they are shaped like balloons and footballs.
The single-celled organisms reproduce themselves by making tiny
buds that will become new yeast cells.
Bakers' yeast growing
Growing yeast love to eat sugar and starches, like the ones in
bread flour. When they eat these starches, some of the
proteins in the flour, called glutens, swell up. Yeast eating
starch make a gas called carbon dioxide that forms lots of tiny
bubbles in the bread dough. The tiny bubbles pop during
baking, but leave tiny holes where they were. You can see
these holes in the bread you eat.
The yeast you buy at the store is alive, but it is dried and
can't grow until you add water to it.
Here's a fun experiment you can try to see what makes yeast grow
best. All you will need are some zip-lock bags, yeast, salt,
sugar and water.
What makes yeast grow?
How to:
1. Label four bags as follows:
- Sugar + warm water
- Sugar + cold water
- Sugar + salt + warm water
- No sugar + warm water
2. Add a package of yeast (or 2 tsp.) to each plastic
bag. Add 2 tsp. of sugar to each of the bags that say sugar
and 1 tsp. of salt to the bag that says salt.
3. Carefully, add half a cup of water to each bagg. The
warm water should be warm, but not too hot, or it will kill the
yeast. The cold water can be room temperature.
4. Seal the bags, squeezing out as much of the extra air as
possible and let them sit. (The yeast will grow faster in a
warm room than a cold one.)
5. Watch the bags to see what happens. You will know your
yeast is growing if the baggie it is in puffs up. Keep an eye
on your experiment. If a bag gets so puffy that it
looks like it might explode, be sure to open it to let the pressure
out!
What do you think?
Which ingredients help yeast grow best?
Did you find an ingredient that kept them from growing well?
Do yeast grow better in warm or cold water?
What is making the bags puff up and how does this tell you that
the yeast is growing? (Hint: the answer is in the paragraph
above about how yeast makes bubbles in bread!)
Try coating the yeast with oil before adding the sugar and
water. What happens if you add fruit juice to the bags?
Honey?
What else could you try?
Article by Liz Heinecke, Kitchen Pantry
Scientist
