Can Yeast Be Re-used After the Process of Fermentation?
by Randy Abend and Martin Duran
Introduction:
    From the experiments we conducted in class involving fermentation and distillation, I became curious about what happened to the yeast. Was the yeast dead? Could it be used again? I figured that the yeast would be killed after fermentation. I thought the yeast would be killed because the fermentation process eventual ends. If the yeast were still active, then the solution would continue to bubble.
Methods and Materials:
    After handing in my initial proposal, it turned out that Martin Duran had thought of the same question. Mr. Reiblein had paired the two of us together. Martin and I sat together and brainstormed on how we were going to perform our experiment. We decided that we were going to take leftover fermentates from our fellow classmates.

    Using these fermentates we would take a standard sample of yeast from these fermentates and set the fermentation process up from scratch; substituting of course the old yeast. We knew that for each experimental fermentation bottle we were to make a matching controlled bottle. When we did fermentation experiments as a class the setup was: 5 grams of yeast, 100 grams of sugar, and the remaining of the mixture was water until the entire solution was 500 ml. Once the solution was placed into a one liter soda bottle, the bottle was to be shaken thoroughly. Next the bottle was to be stopped with a rubber stopper connected to a tube leading into a small vial filled with water (3/4 filled). This vial is then supposed to be attached to the liter bottle.

    For every one set of data two soda bottles were created: one using new yeast; the other, using yeast from past fermentates. The way Martin and I separated the yeast was by using the centrifuge machine. This machine separated the yeast from the rest of the solution, and we massed out 5 grams of yeast and added it to the experimental bottle. Once the two bottles are set up, we let them fermentate for a few days. When the fermentation process is finished, we take 250 ml of the fermentate and put it through the distillation process. Once the process is finished, we take the distillate and find its density(M/V). After finding out the density, we look at the Ethyl Alcohol chart and find the percentage of alcohol for the solution.

Materials:
  1. large supply of soda bottles(plastic one liter)
  2. rubber stoppers with whole for tubes
  3. glass tubing
  4. rubber tubing
  5. small vials
  6. sugar
  7. active yeast
  8. used yeast
  9. water
  10. centrifuge machine
  11. centrifuge vials
  12. balance
  13. ml measuring vials
  14. distillation kit
Results:
    Because there was an abundance of fermentates to start out with we used three experimental bottles and one control. These bottles were made on 4/18. By the time they were done fermenting and distilling, the controlled bottle contained a 49.6 % alcohol solution. The experimental contained very small alcohol quantities. One had 5.28%, the other two had barely a trace. On 4/28 the distillate using new yeast was an 80% solution. The old yeast solution was only 24%.
Discussion:
    After coming to the fast conclusion that yeast cannot successfully be re-used after fermentation, Mr. Reiblein suggested that we determine what it is that kills or inhibits the yeast. Martin suggested that the alcohol might kill the yeast . Next we decided that a way to test if this was true was by starting out with a solution of alcohol. By relating the end percentage to the initial percentage of alcohol we would be able to determine the effectiveness of the alcohol. In order to maintain the 500 ml solution we used a 79.4 ml alcohol solution that was 50 ml alcohol. We got this information by taking a 25 ml sample of distillate.

    22grams/25 ml = .88 density = 63% alcohol
    50ml/x = 63/100
    x = 79.4ml.

    We began to create fermentation bottles with this distillate. After a few days we began to count the bubbles in the vials for 60 seconds each. The results were as follows:

    control: 25 bubbles
    exper #1: 14 bubbles
    exper #2: 10 bubbles
    exper #3: 10 bubbles

    The next day we counted the bubbles again. We found a problem in the way we were collecting data. We determined that depending on how high or low the tube was placed in the small vial was directly related to the number of bubbles. The lower the tube, the less bubbles because the more pressure that exists.

    Once the fermentation process was over we set up the distillation. Once distilled we found that the experimentals had a lesser percentage of alcohol than when we started. They went from 63% to about 45%.

Conclusion:
    By the time we finished our experiment, we came up with several different conclusions. First, that yeast cannot be re-used after fermentation. Next, that the main reason why yeast cannot be re-used is that it is inhibited by the alcohol once it arrives in the solution. This conclusion was very clear. Not only did the yeast prove not to add any alcohol to the fermentation process, but it actually decreased the percentage of alcohol by 18%.

    We ran into several problems along the way. When using the centrifuge machine, we approximated that one gram of yeast had existed in each vial. This number was only an approximation, therefore the data might be off by a little. Because we were pressed for time, we sometimes had to start distillation before the fermentation process had ended. This may have caused for some slightly larger alcohol percentages. I do not think this was a major problem because it worked relatively for both the experimental and controlled aspects. Overall, I think that the experiment worked rather well, and the conclusion is clear enough that the errors had minimal effect.

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