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 the bread
 To enhance the flavor of the bread
 To change the protein structure of the bread to prevent a chewy texture
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)
where:
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.

Calculation of the activation energy in a reaction.
Calculation of the rate constant of a reaction.
Calculation of how the rate constant changes with temperature.


What whould happen if we wanted to make a loaf of bread in, say Denver?
The effects of air pressure on bread.
Return to the Energy page
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[Introduction | Kinetics | Heat Transfer | Mass Transfer | Bibliography]
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.
© 2007 Arizona Board of Regents for The University of Arizona