Fermentation of Sugar in Bread by Yeast
Kinetics is the study of chemical and physical changes over time.
Physical changes can take place through changes in temperature, pressure,
and concentration. A chemical reaction is a form of chemical change that
converts one substance into another. One such chemical reaction is the
fermentation of sugar into alcohol and carbon dioxide by yeast. This
process is the fundamental step to making bread dough. Before most bread
doughs get baked, they require time to rise. This is also the time when
fermentation takes place.
The fermentation process serves three primary purposes:
- To produce carbon dioxide gas to create a light and airy texture in
- To enhance the flavor of the bread
- To change the protein structure of the bread to prevent a chewy
The fermentation process is the time during which the yeast converts the
sugar present in the flour and the dough into carbon dioxide and alcohol.
Carbon dioxide is the compound that humans breathe out and plants
consume. In bread, carbon dioxide provides the light, airy texture of
bread by leaving gas pockets inside the bread dough. The alcohol, in
addition to more complex compounds, produces the main taste in the
finished bread. However, don't worry about consuming the alcohol since it
is evaporated off during the baking process. Likewise, the high
temperatures during the baking process kill any live yeast cells left in
the bread dough.
Yeast cells are a collection of single-cell fungi that will rapidly
reproduce in the right conditions. Yeast requires a form of sugar or
starch as food and a moist environment to grow in. The best temperature
for growth is from 110°F to 115°F, but the best products for making bread
are formed from 80°F to 95°F although the growth rate achieved is
smaller. If bread dough is kept colder than this temperature range, the
yeast will not grow sufficiently. On the other hand, if water that is
too hot is added to yeast, the yeast cells will not grow since the high
temperatures will have killed them. A temperature of 140 ° F will
kill most yeast cells. Obviously, the temperature at which bread dough
rises is important to the overall results. Arrhenius' equation describes
the changing yeast growth rate constant based on the temperature:
k = A*exp(-E/R*T)
k is the growth rate constant
A is called the pre-exponential factor (another constant)
E is the activation energy
R is the gas law value (yet another constant)
T is the temperature
Here are some examples to work involving growth rates.
What whould happen if we wanted to make a loaf of bread in, say
The effects of
air pressure on bread.
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This project was funded in part by the National Science Foundation and
is advised by Dr. Masel and
Dr. Blowers at the
University of Illinois.