Exam 17: Appendix A: The Science of Fermentation

Correct Answer:

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Yeast stops consuming sugar when one or more of the following conditions are met:

1. Depletion of Sugar: Yeast will stop consuming sugar when there is no more sugar available in the environment to metabolize.

2. Ethanol Toxicity: As yeast ferments sugar, it produces ethanol (alcohol) as a byproduct. At a certain concentration, ethanol becomes toxic to the yeast cells, inhibiting their ability to function and eventually causing them to die. The tolerance level for ethanol varies among different yeast strains.

3. Anaerobic Conditions: Yeast can consume sugar both aerobically (with oxygen) and anaerobically (without oxygen). However, in the absence of oxygen, yeast will switch to anaerobic fermentation, which is less efficient and leads to the production of ethanol. If the environment becomes too anaerobic, yeast growth and sugar consumption can be inhibited.

4. Temperature Extremes: Yeast has an optimal temperature range for growth and sugar consumption. If the temperature falls too low, yeast activity slows down and can eventually stop. Conversely, if the temperature is too high, it can kill the yeast cells, halting sugar consumption.

5. pH Changes: Yeast prefers a slightly acidic environment, typically with a pH around 4 to 6. If the pH becomes too acidic or too alkaline, it can inhibit yeast activity or kill the yeast cells.

6. Nutrient Depletion: Yeast requires various nutrients, such as nitrogen, vitamins, and minerals, to grow and metabolize sugar. If these nutrients are depleted, yeast growth and sugar consumption will slow down or stop.

7. Osmotic Stress: High concentrations of sugar can create an osmotic pressure that is detrimental to yeast cells. This can cause water to flow out of the cells, leading to dehydration and cessation of yeast activity.

8. Stationary Phase: In a closed environment, such as a fermentation vessel, yeast will eventually reach a stationary phase where the population stabilizes due to the depletion of nutrients or the accumulation of waste products. At this point, yeast cells will stop dividing and their metabolic activity, including sugar consumption, will decrease significantly.

Understanding these conditions is crucial in various industries, such as brewing, winemaking, and baking, where yeast is used to ferment sugars to produce alcohol, carbon dioxide, and other flavor compounds. Controlling these factors allows for the management of yeast activity and the desired outcome of the fermentation process.