The Bread Dough That Would Not Rise
A student investigates yeast respiration in dough. She mixes flour, water and yeast in three bowls. Bowl A is kept at 35 degrees Celsius, bowl B at 5 degrees Celsius, and bowl C at 35 degrees Celsius but with a boiled yeast suspension. After 60 minutes, bowl A dough has doubled in height, bowl B has barely risen, and bowl C has not risen at all. She then adds a large amount of sugar to a fourth bowl D at 35 degrees Celsius and finds it rises more slowly than bowl A. She concludes that temperature is the only factor controlling yeast activity.
1. Use the results to evaluate her conclusion, including an explanation for bowl D.
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The results show temperature matters: bowl B at 5 degrees Celsius rose little because enzyme activity is low at low temperature, so respiration and gas production are slow. Bowl C shows that boiling denatured the yeast enzymes, so no respiration occurred even at the optimum temperature. However, the conclusion that temperature is the only factor is not supported, because bowl D differs only in sugar concentration yet rises more slowly than bowl A. A very high sugar concentration draws water out of yeast cells by osmosis, dehydrating them and slowing metabolism; high solute concentration can also inhibit enzymes. So substrate concentration is a second controlling factor. The fair comparison is A versus D, which isolates sugar concentration, and A versus B, which isolates temperature. Bowl C is a negative control confirming the rise is due to living yeast, not to the flour or water alone.
2. Explain what gas causes the dough to rise and how the design could be improved to measure yeast activity quantitatively.
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Yeast respires anaerobically in dough, converting sugar to ethanol and carbon dioxide. The carbon dioxide bubbles become trapped in the gluten network, inflating the dough and making it rise. To measure activity quantitatively, the student could collect the gas in an inverted tube or gas syringe over a set time and record volume, or measure dough height at fixed intervals with a ruler and plot a time course. She should control variables: identical masses of flour, water and yeast, identical bowl size and position, and the same starting temperature, with replicates to reduce random error. She should also measure the actual temperature in each bowl rather than assuming it, and run each condition at least three times to calculate a mean. This converts an observation of rising into a measured rate of gas production.