Effects of Various Carbohydrate Substrates on Yeast Fermentation.

glucose molecule fermented. "The products from one glucose molecule are two molecules of ethanol and two molecules of carbon dioxide (Jorgensen, 1911)". A rapid reaction rate and increase in the products released depend on the carbohydrate's compatibility with the yeast's enzyme-based transport systems. This transport system, associated with the cell membrane, allows yeast to bring selected carbohydrates into their cells. In this experiment we tested the effect of several carbohydrate substrates on yeast fermentation. Our focus was carbon dioxide gas as a waste product of fermentation. Carbon dioxide accumulation is an indicator of the rate of substrate degradation in an organism. We hypothesized that the carbohydrates efficiency to bind to the yeast's enzyme and ease of entry to the yeast's cell and glycolytic pathway would increase the rate of evolution of carbon dioxide. We examined the efficiency with which yeast used certain carbohydrates, the rate of evolution of carbon dioxide and the rate of reaction between various carbohydrates on yeast fermentation.

Materials and Methods

Six Smith fermentation tubes were set up for carbon dioxide collection. 15 ml of yeast suspension was placed in each of six tubes. Five five-percent-carbohydrate solutions were prepared. Five tubes were labeled to a specific carbohydrate; glucose, fructose, lactose, sucrose, and starch. Fifteen ml of each of the prepared carbohydrate solutions was poured into its corresponding tube. In the sixth tube, 15 ml of distilled water was added and labeled control. The opening of each tube was sealed with parafilm and each tube was mixed vigorously. Carefully, each sealed tube was tilted to ensure a gas bubble fill the tip of the arm. With all bubbles approximately the same size; a colored pencil was used to mark the levels of yeast-carbohydrate. The same was done to the control tube. All marked tubes were then incubated at 35 degrees C. Measurements along the arm of each tube were taken every thirty minutes, up to one and a half hours, of the amount in length the solution was displaced by carbon dioxide in the tube. A new colored line was made and findings during each interval were recorded (in centimeters). Using the volume of a cylinder formula and the values: distance between levels in the tube arm and time between measurements, the volume of carbon dioxide was calculated and recorded.

Results

Figure one shows the carbon dioxide evolved (read in ml) of each of the six experimental tubes compared to time in hours. This is an indication of the rate at which yeast-carbohydrate solution is being turned into carbon dioxide in each tube at 35 degrees C. We see a steady increase in carbon dioxide in glucose, fructose, and sucrose. The control group (not shown), lactose and starch showed little or no development of carbon dioxide.

Figure two shows the calculated reaction rates in ml/hr of yeast fermentation on each of the carbohydrates: glucose, fructose, lactose, sucrose, and starch at 35 degrees C. Sucrose had the highest reaction rate, followed by glucose and fructose. Lactose and starch were included but did not have a calculable reaction rate. The control was left out.

Discussion

The results of the experiment supported the hypothesis that the reaction rate increases with a carbohydrate's ability to enter the yeast's cell and glycolytic pathway within the allowed time period of an hour and a half. We observed that the sucrose tube had the highest level of evolved carbon dioxide over time. Since the reaction rate is the measure of the carbon dioxide evolved over time, the greatest level of conversion was also found in the sucrose tube. The high ratio of yeast-starch to carbon dioxide (see Figure 1) caused the reaction rate to grow rapidly as well (see Figure 2).

Sucrose follows the hypothesis because it enters the cell and glycolytic pathway. Sucrose is able to do this because it "consists of a glucose and fructose molecule... and its bond is easily broken by an enzyme produced by yeast (Vilet, 1993)". However, the ratio yeast-starch to carbon dioxide is greater than expected. The yeast's efficiency to use sucrose and thus the ability to break its bond is greater than any carbohydrate tested. The yeast's efficiency to use sucrose was expected to be greater than lactose (Figure 1) because yeast lacks the enzyme required to break the molecule bonded by b-galatoside in lactose, and starch (Figure 1) because of its slow rate of fermentation due to its size and inability to enter the yeast's cell efficiently. It wasn't however, expected to be greater than yeast-glucose or yeast-fructose given the fact that "glucose and a slight modified form of fructose are directly used by the glycolytic pathways (Vilet 1993)" (Figure 1).