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Measuring the Efficiency of Various Carbohydrate Substrates in Yeast Fermentation

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Measuring the efficiency of various carbohydrate substrates in yeast fermentation.

Cherrishe Brown

October 3, 2007

Dieldrich Bermudez

BSC 2010L Sect# 0560


As expected in the experiment Glucose, Fructose, and Sucrose were all utilized for fermentation. Based on the rate of evolution of CO2 the yeast was most efficiently able to utilize the substrate Glucose, followed by Sucrose and Fructose respectively. Given more time I believe that Sucrose would have surpassed glucose in total rate (ml CO2/hr) as time and energy was taken as the yeasts cells broke Sucrose down into its glucose and Fructose monomers. That being said with all the substrates being of .14M in addition to being the limiting reagent of the reaction Sucrose entered .14M Glucose and .14M Fructose into the glycolytic pathway where as Glucose only entered .14M of glucose into the glycolytic pathway. Thus Sucrose entered more sugar and had more sugar to be used up by the reaction. However this would have taken more time to become apparent in the data because first the yeast cells would have had to break up the a-glycosidic bond found in Sucrose. The differences in the CO2 evolved in the Glucose and Fructose substrates can be attributed to the place they take in the glycosidic pathway. Fructose is farther down the chain of enzymatic manipulations of the glycolytic pathway than glucose in the production of CO2 and alcohol.

In the experiment in addition to the control water, two of the carbohydrate substrates did not evolve any CO2. These two sugars were Lactose and Starch. The amount of CO2 evolved is a measure of the yeasts ability to use the provided substrate for fermentation. Thus it can be said that the yeast was unable to utilize both of these substrates for fermentation. Starch was not efficiently utilized because it was too large of a carbohydrate to enter the yeast cells. Because of this Starch had to first be broken down into its monomers glucose and maltose in order to gain entrance into the yeast cells and begin the fermentation process. That being said the yeast cells were not unable to use the Starch substrate for fermentation rather it just required excess time and energy to allow the fermentation process to begin. Although the yeast suspension of the Starch substrate did not evolve any CO2 during the 80-minute time frame in which the experiment took place I believe given more time to enter the yeasts cells it would have evolved substantial amounts of CO2 at rates even surpassing those of glucose as both maltose and glucose entered the glycolytic pathway. The Lactose yeast suspension on the other hand would not have evolved any CO2 even if given more time. The problem with Lactose was not the size of the molecule as Lactose can readily enter yeast cells. However a problem arises when Lactose is to be broken down into its glucose and galactose monomers. The glucose and galactose in Lactose are bonded together by a b-galactosidic bond that is unable to be broken by the enzymes produced by yeast cells making it unable to enter the glycolytic pathway.

Possible Sources of error for this experiment include the manner in which measurements were taken. The use of small rulers to measure the displacement of CO2 in the arms of the Smith Fermentation tubes is not very accurate and leaves room for approximation. This in combination with marking off the levels yeast-carbohydrate mixture leaves a lot of room for human error and interpretation. It would be recommended for future repetitions of this experiment to implement the use of a more accurate alternative measurement method.

Table 1: Carbon dioxide output from a yeast suspension of various carbohydrate substrates after 20, 40, 60 and 80 minutes at 35oC. The data collected for Rate (ml CO2/hr) at 80 min (1.33hrs) is the total reaction rate for CO2 evolved for each sugar.

Substrate Distance between levels in tube arm (cm) Time (hr) CO2



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