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How the Type of Sugar Affects Yeast Fermentation

Experiment Overview


Sugars are carbohydrates and consist in single units known as monosaccharides, like glucose and fructose. These single monosaccharide units, termed monomers, can also be building blocks for larger sugars like disaccharides (two monomers), tri-saccharides (three monomers) or polysaccharides (many monomers). Recall from the chemical equation shown in your handout that one of the byproducts of fermentation by yeast is carbon dioxide. Thus, we can the measure the amount of CO2 gas evolved per unit time to gauge the response of Saccharomyces cerevisiae to different types of sugars.

We will interpret the results of an experiment to determine if the rate or efficiency of yeast fermentation, measured as milliliters of CO2 produced per minute (ml/CO2/min), varies across a range of sugars including glucose, (a monosaccharide), sucrose (table sugar, a disaccharide), maltotriose (a tri-saccharide) and amylose or starch (a polysaccharide) (Figure 2). We will assume that the greater the CO2 production per unit of time, the more rapid or efficient the fermentation of a specific sugar may be. Our basic question for this exercise is: how does the type of sugar affect the efficiency of fermentation by Saccharomyces cerevisiae? Table 1 contains the data from the experiment needed to answer the questions below. Assume that all fermentations were conducted under the same conditions (e.g., temperature, etc.).

1. Rank the four sugars used in this study from the greatest efficiency of fermentation by Saccharomyces cerevisiae to the lowest.
2.  How might the chemical structure (e.g., monosaccharide, disaccharide, etc.) of the four sugars used in this experiment affect the efficiency of fermentation? Explain your answer.
3. If you were to use one of the four sugars from this experiment in baking bread, which would you choose? Explain your answer in a practical manner.
4. If the experiment was conducted over a longer period of time, like an hour, would you expect the results to differ? Why or why not? Explain your answer.
5. Enzyme catalysis of chemical reactions, like fermentation of sugars by yeast, is usually temperature-dependent. Explain how higher and lower temperatures might affect the efficiency of fermentation of different sugars by yeast.
6. Artificial sweeteners like sucralose in the commercial product Splenda are not fermented by yeast. However, some CO2 is produced when Splenda is added to a yeast solution. Why? Search online for the ingredients in Splenda and provide a hypothesis for fermentation of the product by yeast.
7. Peary is an alcoholic cider made from pears. Unlike fermented apple cider, finished peary is sweet. Given what you’ve learned about fermentation of different sugars by yeast, why might this be so?
8. Brewers usually seal their fermentation vessels with an airlock, limiting exchange of gases with their fermenting product and the outside environment, yet bakers generally allow their bread to rise without limiting exposure to outside gasses. Why might this be so?
9. Yeasts allow some sugars to be transported across the cell membrane to be broken down by internal enzymes. This uptake occurs primary by diffusion. The four sugars used in this experiment differ in their solubility in water with glucose > sucrose > maltotriose > amylase. Explain how sugar solubility might have affected the results of this experiment.
10. Yeast cells secret an external enzyme called sucrase or invertase that can break down sucrose into the smaller sugars, glucose and fructose. Why does sucrose need to be broken down before it can be used by yeast?

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