Design an experiment to find the temperature at which yeast works best to
ferment sugar. What materials and equipment will you use? How will you
make sure that your comparisons are fair? (Refer Chapter 4)
Answers
Answer:
Introduction
Enzyme catalysis1 is an important topic which is often neglected in introductory chemistry courses. In this paper, we present a simple experiment involving the yeast-catalyzed fermentation of sugars. The experiment is easy to carry out, does not require expensive equipment and is suitable for introductory chemistry courses.
The sugars used in this study are sucrose and lactose (disaccharides), and glucose, fructose and galactose (monosaccharides). Lactose, glucose and fructose were obtained from a health food store and the galactose from Carolina Science Supply Company. The sucrose was obtained at the grocery store as white sugar. The question that we wanted to answer was “Do all sugars undergo yeast fermentation at the same rate?”
Sugar fermentation results in the production of ethanol and carbon dioxide. In the case of sucrose, the fermentation reaction is:
C12H22O11(aq)+H2O⟶YeastEnzymes4C2H5OH(aq)+4CO2(g)
Lactose is also C12H22O11 but the atoms are arranged differently. Before the disaccharides sucrose and lactose can undergo fermentation, they have to be broken down into monosaccharides by the hydrolysis reaction shown below:
C12H22O11+H2O⟶2C6H12O6
The hydrolysis of sucrose results in the formation of glucose and fructose, while lactose produces glucose and galactose.
sucrose + water ⟶ glucose + fructose
lactose + water ⟶ glucose + galactose
The enzymes sucrase and lactase are capable of catalyzing the hydrolysis of sucrose and lactose, respectively.
The monosaccharides glucose, fructose and galactose all have the molecular formula C6H12O6 and ferment as follows:
C6H12O6(aq)⟶YeastEnzymes2C2H5OH(aq)+2CO2(g)
Experiment
In our experiments 20.0 g of the sugar was dissolved in 100 mL of tap water. Next 7.0 g of Red Star® Quick-Rise Yeast was added to the solution and the mixture was microwaved for 15 seconds at full power in order to fully activate the yeast. (The microwave power is 1.65 kW.) This resulted in a temperature of about 110 oF (43 oC) which is in the recommended temperature range for activation. The cap was loosened to allow the carbon dioxide to escape. The mass of the reaction mixture was measured as a function of time. The reaction mixture was kept at ambient temperature, and no attempt at temperature control was used. Each package of Red Star Quick-Rise Yeast has a mass of 7.0 g so this amount was selected for convenience. Other brands of baker’s yeast could have been used.
This method of studying chemical reactions has been reported by Lugemwa and Duffy et al.2,3 We used a balance good to 0.1 g to do the measurements. Although fermentation is an anaerobic process, it is not necessary to exclude oxygen to do these experiments. Lactose and galactose dissolve slowly. Mild heat using a microwave greatly speeds up the process. When using these sugars, allow the sugar solutions to cool to room temperature before adding the yeast and microwaving for an additional 15 seconds.
Fermentation rate of sucrose, lactose alone, and lactose with lactase
Fig. 1 shows plots of mass loss vs time for sucrose, lactose alone and lactose with a dietary supplement lactase tablet added 1.5 hours before starting the experiment. All samples had 20.0 g of the respective sugar and 7.0 g of Red Star Quick-Rise Yeast. Initially the mass loss was recorded every 30 minutes. We continued taking readings until the mass leveled off which was about 600 minutes. If one wanted to speed up the reaction, a larger amount of yeast could be used. The results show that while sucrose readily undergoes mass loss and thus fermentation, lactose does not. Clearly the enzymes in the yeast are unable to cause the lactose to ferment. However, when lactase is present significant fermentation occurs. Lactase causes lactose to split into glucose and galactose. A comparison of the sucrose fermentation curve with the lactose containing lactase curve shows that initially they both ferment at the same rate.
Explanation:
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