Question 1

When does yeast stop consuming sugar?

Accepted Answer

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.

Question 2

If there is limited sugar (or food for the yeast to consume), there will be a lot of alcohol expelled by the yeast.

Accepted Answer

A) True 
 B)False  False

Question 3

The name of the French chemist and biologist who made significant contributions to chemistry, medicine, and, indirectly, the food and beverage industry that have greatly benefited civilization.

Accepted Answer

A)   Albert Schmid
B)   Robert Mondavi
C)   Napoleon
D)   Louis Pasteur D

Question 4

Fermentation occurs in

Accepted Answer

A)   beers
B)   wines
C)   spirits
D)   all of the above

Question 5

The amount of alcohol from fermentation depends on the amount of sugar available for the yeast to consume.

Accepted Answer

A) True 
 B)False

Question 6

The unintentional natural process of fermentation

Accepted Answer

A)   precedes human history
B)   10,000 B.C.
C)   10 B.C.
D)   none of the above

Question 7

Yeast consumes sugar and produces alcohol and carbon monoxide.

Accepted Answer

A) True 
 B)False

Question 8

In general, wine is produced from grain.

Accepted Answer

A) True 
 B)False

Exam 17: Appendix A: The Science of Fermentation

When does yeast stop consuming sugar?

If there is limited sugar (or food for the yeast to consume), there will be a lot of alcohol expelled by the yeast.

The name of the French chemist and biologist who made significant contributions to chemistry, medicine, and, indirectly, the food and beverage industry that have greatly benefited civilization.

Fermentation occurs in

The amount of alcohol from fermentation depends on the amount of sugar available for the yeast to consume.

The unintentional natural process of fermentation

Yeast consumes sugar and produces alcohol and carbon monoxide.

In general, wine is produced from grain.