Science Buddies: "Ask an Expert"

Hi there
I am conducting an experiment right now, testing whether immobilized yeast needs more or less time to completely ferment a sugar solution of a high concentration. I conducted the experiment once by putting immobilized yeast into sugar solutions of the concentrations 1, 3, 5, 7, 9 and 11% and measuring the time until all yeast balls went up to the surface. It did not quite work out how I wanted it to as no time pattern could be made out. Before I repeat the whole experiment I wanted to know what to expect, whether the times would be shorter or longer when the sugar concentration is higher. From research in the internet and books I could not find a clear answer for this. From what I know I would say I would take longer to ferment all the sugar as there is more energy for the yeast to transform, but I also read that with more sugar the yeast is more active...

Thank you in advance, you would help me by a great deal to answer as quickly as possible
Greetings,
therealcora

Hi,
I'm not sure what happened in your experiments but I'll try to help. What I am thinking first is that the amount of yeast that you added is too small relative to the amount of sugar. If the lowest concentration of sugar is more than that amount of yeast can metabolize then putting them in higher concentrations won't make them work faster.

You can think about this like an experiment in enzymology. Yeast contains enzymes that digest the sugar but they can only operate at a certain rate. That rate depends on the temperature and the concentration of the substrate, the thing that the enzyme acts on--sugar in this case. But if you give the enzymes more sugar than they can handle then the rate is maximum. We say that the enzyme is saturated with substrate. It can't go any faster.

You could test this by increasing the amount of yeast relative to the sugar. You could do this by adding more 'balls ' and by reducing the volume of sugar that you put the yeast in. Are you keeping the yeast (I am assuming that it is bread yeast ) warm? It should be at least 90 F for it to work well.

Let us know what you think about this and if you have more questions.

Good luck!

Sybee

Hi Sybee,
thanks for your quick answer.
I produced immobilized yeast beads by mixing 2.5g baker's yeast with sodium alginate solution and dropping it into calcium chloride solution. That would mean that most of my sugar solutions had a higher concentration than the yeast. This was conducted in a water bath so the temperature should not be the problem but your argument that the sugar concentration was too high for the yeast sounds very logical to me. So if I would conduct the experiment again with a higher concentration of yeast, do you think it would take longer for the yeast to ferment at a higher sugar concentration than a lower?
Thanks a lot for your answer,
therealcora

Hi again.

You are correct--the more sugar you add, the faster the yeast will produce carbon dioxide gas. Once the enzymes are saturated with sugar, however, the reaction will stay at that rate no matter how much more sugar you add. Here is a site that shows you what it would look like if you plotted yeast activity against sugar concentration: http://www.ucl.ac.uk/~ucbcdab/enzass/substrate.htm

I really have no idea how much sugar is a saturating amount for a given amount of yeast. You could do a search for this information but it would be simpler to just try it. Start by choosing a fixed amount of your yeast beads in a relatively small volume--maybe 2 mL.

Make a strong sugar solution (by 'sugar' I assume we are talking about sucrose, right?) like 20% by dissolving it in hot water. Do you have a digital scale and a way to measure volume in milliliters (mL)? When you make a sugar solution in the lab it is usually expressed as weight of sucrose in a given volume of water, w/v. For 20% sucrose, then, you need to have 20 g of sugar in a FINAL volume of 100 mL. That means you need to add the sugar to a volume that is less than 100 mL then add water until you reach the 100 mL mark.

Now, to test your yeast you will need a small dropper that delivers about 0.1 mL. If you have a pipetter that you can use that would be better but a school lab may not have one. Add one drop of the 20% sucrose to the yeast in a volume of 2 mL and use a stopwatch to time the reaction. This amount of sugar gives a final concentration of about 1%. I don't know how fast the yeast will work, but if you don't see much activity try another tube of yeast with 2 drops of sugar and keep doing that until you get a good volume of CO2 produced. You should eventually be able to determine the saturating concentration of sugar for this amount of yeast.

I hope this isn't too complicated or difficult for you to do with the materials you have. You could just make up a bunch of sucrose solutions of lower concentrations and add the yeast to them, but you really don't know where to start. I'm guessing 1% sucrose, but I could be wrong.

Let us know what you think and we'll help you plan it out.

Good luck!

Sybee