Help! Yeast Isn't Decomposing!!

[MMassie]

Please help!!

We've now started my son's science fair project three times but the yeast just won't start decomposing. We are following the instructions from a blueprint we bought online: "Will food exposed to yeast decompose more rapidly when it is in different temperatures?" (7th grade)

We make up 18 bags. Six go onto a plate on a heating pad (low temp setting.) Six go in the fridge, six stay on the dining room table. Half have yeast, half do not. We use a carrot slice, a pear slice and a bread slice.

I am using Fleishman's active dry yeast. I followed the directions for reactivation in warm water and proofed it by adding a tsp of sugar. It reacted with a nice, foamy head so we went ahead and spooned 1/2 tsp. onto each piece of food as directed.

Nothing. Not for hot, cold or ambient.

Three times.

What on earth are we doing wrong???

Thanks for your help,

~Michelle

[Amber_MIT]

The foam makes it sound like the yeast are alive and healthy. The yeast should be producing carbon dioxide as they digest sugars. Have the bags inflated at all?

The yeast should also be in a damp environment. The carrot, pear, and bread should be "soggy." If it gets too dry they will probably go back into "deactivation mode".

There might not be enough yeast there in order to see visible results. You might consider adding a bit more yeast/water to each bag.

We also have some yeast projects on our website that would probably give you more visible results. Even if your son doesn't switch projects, the procedures might still be helpful:

https://www.sciencebuddies.org/science- ... p008.shtml
https://www.sciencebuddies.org/science- ... p005.shtml

Also take a look at our project guide, which you and your son might find helpful as he does the project (read the sections under "How to Do a Science Fair Project"):
https://www.sciencebuddies.org/science- ... ndex.shtml

Let us know if you have more questions.

[MMassie]

Hi, Amber!

No, the bags aren't inflating at all (after several days some of them even seemed to "vacuform" themselves to the bread.)

I'll take a look at the links and try adding more water & yeast to the bags...

Thanks so much!

[Amber_MIT]

If that doesn't work, the yeast might not be able to digest those sugars, so it could be starving. I would add a control group to the experiment to help you figure out what is going on. Scoop some of your yeast into a bag with the sugar you were feeding it and see if the bag inflates. You could also try building the simple contraption in those projects I linked to with the lime juice.

If the project is due soon and you still aren't seeing anything (even with the control), perhaps the yeast are dead (though since they were frothy, it doesn't sound like this is the case). If the control is working but the others aren't, your son should describe why the yeast wasn't able to digest the food. He should research what kind of sugar is in these foods and what the yeast prefer to eat, as well as other reasons in the environment that would hinder their digestion. That may actually be the results.

[MMassie]

Right...I don't think the yeast are dead either because of the foam. Even so, I went and bought new yeast, just to be sure. Same thing; no action. The yeast are being fed a carrot, a pear and a bread slice. (I would think if nothing else the pear would have plenty of sugar.) We started the experiment again last night but it looks exactly the same. We're going to add another 3 ml of yeast tomorrow and if that doesn't work, we may just abandon ship.

I've studied those other links and I'm going to try prepping the yeast according to what they suggest: putting the sugar in first, then the yeast.

Fingers crossed!

Thank you again for all your help...

~Michelle

[MelissaB]

Hi,

How long are you waiting before re-starting the experiment (apologies if you answered this already and I just missed it)? I would expect it to take 4 or 5 days before you saw noticeable results; if you just added the yeast yesterday, they probably haven't had enough time to grow and decompose the items.

I wouldn't expect you to get much of a result with the carrot slice, but unless the bread is drying out on the heater, it should start decomposing/molding within a few days. Try waiting a bit (if you haven't already) and see what you get.

[donnahardy2]

Hi Michelle,

This is an interesting project, and your main task after trying so many times, is to explain why the yeast did not grow. Amber is right to point out that these are your results, and you should write them up and present them, even though the experiment did not turn out as expected. If you have time to try your project again, then perhaps the following information will help you to know what to do differently next time.

Yeast will grow and produce carbon dioxide if they have a suitable source of food, if they are incubated at the right temperature, and if there is enough moisture available. So, here are the possible reasons for your results.

The ambient temperature samples should be growing OK, and the heated samples should be growing if the temperature is less than 37 degrees Centigrade. The refrigerated samples would also grow, but very slowly. So, I don't think temperature is primary source of the problem here. What were the actual temperatures you were using?

The bread, carrot, and pear all have enough sugar content to support yeast growth. How did you prepare the food samples? If the surface area was increased by chopping up the food, or if the pear and carrot were cooked and mashed, this would increase the availability of the sugars to support yeast growth. If the pear and carrot were not cut at all, then the yeast would have a hard time getting access to the sugars. The bread has enough surface area that the yeast could penetrate the pores of the bead slices. Did you see any evidence of growth in the bread sample?

Yeast require lots of moisture to grow. If the surface of your samples is definitely damp with visible moisture on the surface, then there's enough water available. If the samples are dry, then the yeast will not be able to grow. I think this is the most likely cause of the lack of results for all of your samples.

How are you measuring your results? And I have the same question that Melissa had. How long are you waiting before stopping the experiment? If you have an enclosed container and are waiting for the production of gas, it might take a few days. If you are waiting for the visible decomposition of the samples, this might take a week or longer. Please describe your experimental set up if you need more information.

Donna Hardy

[MMassie]

Hi, Donna and Melissa...

On the second & third rounds I waited over a week and a half.

We have added 3 ml of yeast to each bag. All of the foods are sliced into rounds (except bread, of course, which was already sliced.) We see some moisture drops on the heated ones. We may try adding some more yeast liquid tomorrow to see if this helps.

On the heated one (39.9c) we are just now (day 4, try #4) starting to see some bag puffing on the pear and the bread. On the ambient one, the pear is the only one showing a difference by turning very brown but again, instead of the bag puffing it is now clinging to the pear slice. All of the other ambient ones show no change. No change to the refrigerated ones. (I'm assuming that it will be difficult to see any growth on the bread slice since most store-bought breads have preservatives of some type and we are including this in our observations.)

We are measuring our results visually; looking for puffing and decomposition.

Will wait a couple more days to see what happens but at least the pears are showing some browning!

Thank you both so much!

~Michelle

[donnahardy2]

Hi Michelle,

Thanks for the additional details. It sounds like you are getting results now; it's just going to take a little more time. The preservatives in the bread will slow the growth of the yeast, but will not completely stop it. The 39 degrees is slightly above the optimum temperature, and the ambient is slightly below the optimum, so it will take more time for the yeast to grow. Since the carrot and pear are in slices, the surface area is limited and it will take more time for the samples to decompose. You should continue incubating these samples until the deadline for the project. If you do add anything, try adding more water to all of the samples to increasing the moisture content.

Donna Hardy

[GretaStorlie]

dear help yeast isn't decomposing,

I'm sure your yeast will eventually decompose and it is not ready yet. I myself are doing a yeast project but not exactly the same. Stay calm it WILL.