Incr-Edible Science: Yeast (Micro Fungi – part 2)

After covering mold type fungi in Part 1, we moved onto the unicellular yeast:

1. This is Leeuwenhoek, the man responsible for the discovery of micro-organisms through his very simple ‘microscope’
2. A labelled diagram of a yeast cell
3. Yeast as we know it

To find out a bit more about yeast we mainly used the same resources as before:

We also spent some time on the internet researching the properties of yeast.  Yeasts, much like their fungal relatives mold, rust and mildew are eukaryotic organisms.  However, unlike their relations they are unicellular, that is each one is made up of only a single cell.  Most yeasts reproduce asexually by mitosis, with many doing so by an asymmetrical division known as budding:

Photo credit

Here are the single yeast cells as shown under a light then an electron microscope with increasing magnification.  Although budding can be seen in picture 2 and 3, it is shown most clearly in picture 4.

We attempted to look at yeast under our own microscope.  Each child prepared their own yeast slide, even A5, and looked on the TV microscope screen:

T12 had the most success at seeing the yeast cells, and that was with our other standard microscope and, although he tried, he couldn’t get a decent photo of them.

One reason that yeasts are considered a fungus is due to their inability to make their own nutrition, thereby feeding off organic matter whenever they can.  Thus they are known as decomposers, much like their fellow fungi mould and rust.  We tested the yeasts ability to decompose by carrying out a simple experiment adapted from one in Apologia’s General Science:

1. We cut up a banana into three equal pieces and run them under a tap.  One was immediately placed in a baggy labelled ‘banana and water’.  The second one was handled by all the children to see if the bacteria from their hands had the same decomposing effect.  This was placed in a baggy labelled ‘banana, water and handled’
2. The final piece of banana was placed in the baggy and a teaspoon of yeast was added.
3. The three baggies were left to sit in a warm kitchen for a day
4. Approximately four hours later there was little change to the baggy containing the handled banana or to the baggy holding the untouched plain banana.  However, as can be seen in the baggy on the left hand side of picture 4 the banana which had been mixed with the yeast was separating, soggy and squishy, with a pool of thick liquid surrounding it.  This suggested that the presence of yeast had a decomposing or breaking down action on the banana.

And these were the results the next day:

I asked the children what they knew about yeast and they said it rose bread by giving off a carbon dioxide gas.  It didn’t surprise me that they were able to deduce this as we had done many experiments to illustrate gasses being used in the rising of various breads in our past Incr-Edible Science:

Click here to read about how bicarb works when mixed with an acid to rise soda bread and then Click here to read about buttermilk substitutes in soda bread:

Click here to read about the experiments we did making our own home made baking powder to rise banana chocolate muffins:

Click here to read about the experiments we did with sour dough, using natural yeasts found in the air:

I thought it would be fun to demonstrate this using a bottle and a balloon and replicating a similar demonstration we did with bicarb and vinegar.  I asked them if they thought the reaction would be as fast as the bicarb and vinegar and if not why not?

The children thought slower because it wasn’t a chemical reaction, rather a biological one, and you don’t often see explosions in nature.  L thought it was interesting that it took a full 60 minutes to inflate the balloon, which ‘is about the time we leave the dough to rise when we are making bread!’  Sometimes I just love home school!

Following this demonstration, I wanted us to investigate the best possible conditions in which yeast was able to maximise its fermentation, thereby producing the most CO2.  This information would be invaluable to L11, who loves to bake as well as useful knowledge for the other three.

Basically we sourced as many test tubes as we could get our hands on and set up two experiment stations.  The first shown is in the red test tube holder.  This station was manned by T12 and L11.  They were testing the effect that heat has on the respiration rates of yeast.  They also tested to see if an absence of food in the form of sugar would have any effect. (Picture 1 below)

The second station was manned by C11 and A5, and contained the yellow test tube holder.  They were testing to see if the pH effected the respiration rates of the yeast.  They used a strongly acidic solution (vinegar), a basic solution (water and salt), and a weakly acidic solution (water and lemon juice).

The respiration of the yeast was demonstrated by the production of CO2 which was collected in the balloons tightly sealed around the test tube lips:

According to our experiments, yeast grows best in warm conditions, needs a source of energy (sugar) to grow and prefers a slightly acidic environment, although will grow perfectly well in a neutral environment.  None of our results were a surprise, except maybe the boiling water which we thought would kill the yeast.  Either yeast is far more robust than we first thought or the water had cooled down a great deal between boiling the kettle and doing the experiment.

One variable which I hadn’t considered was the moisture level.  If I did this again, I would include a test tube with no liquid, one with minimal liquid and one with an excess, observing the amount of CO2 produced in each test tube.

This was a simple experiment which was largely visual.  A more accurate experiment could be carried out by taking measurements of the balloons at certain time intervals and plotting a graph.  Doing so would have lost all three of my girl’s interest, so I chose the more simple but less scientific visual method!

From their research on-line the children found out that yeast do not require sunlight to grow, using organic compounds (mainly sugars) as their energy source.  They require oxygen for respiration and thrive in a neutral or slightly acidic environment.

As the yeast grows, it converts its food (in the form of sugar or starch) into alcohol and carbon dioxide through the process of fermentation. This property has led to yeast being used widely in the making of wine and beer, as well as the process of baking.

In the same way as the reactions of bicarb with vinegar, I wanted the children to see and understand the equation of the reaction of yeast with a sugar solution.  It is the enzyme, invertase, which is present in yeast which acts as a catalyst to convert the sucrose into glucose and fructose:

invertase
C12H22O11 + H2O ==> C6H12O6 + C6H12O6
Sucrose     + water   =        Glucose +  Fructose

The glucose, C6H12O6, and fructose, C6H12O6, formed are then converted into ethanol and carbon dioxide by another enzyme, zymase, which is also present in yeast.

zymase
C6H12O6 ==> C2H5OH + 2CO2

Simple sugar =>Ethanol   + carbon dioxide

The addition of yeast can make bread rise because the yeast produces carbon dioxide from sugar (C6H12O6). Ethanol, the other product of the reaction, evaporates from the bread dough while it rises or it boils away during cooking.

We’ve been making bread on and off for years and all of the older children know how to do it, C11 at one point could probably have done it without consulting a recipe.  I wanted them to see the difference between a loaf made with yeast, compared with one made with no yeast:

The difference was obvious and expected.  No surprises here….that is until we cooked it:

It was interesting to see that whilst both loaves were put in the same oven on the same baking tray, and the non yeast one was significantly smaller, it was the larger yeast bread which cooked in the allotted time, whilst the non yeast bread remained practically uncooked.  So the air (CO2) in the yeast bread actually has two roles – giving the bread a lovely light texture, but also reducing the time it takes to cook a loaf.  Who knew? (Obviously not me!)

I also wanted them to look over their other Inc-Edible Science experiments done with soda bread and baking powder and answer the question – Why do you have to leave yeast to rise for an hour whereas both soda bread and muffins made with baking powder can be and should be cooked immediately?  They had answered this question, unknowingly whilst we were comparing the balloon experiment done with yeast with the same experiment done with bicarb and vinegar.  I wondered though whether they would be able to transfer this knowledge and understanding to the cooking process, which is the reason I write this curriculum.

It was a bit like pulling teeth, to be honest.  I had all sorts of (ridiculous) answers until a light bulb suddenly went on in T’s head and they suddenly got what I was after!  It was much easier once they had remembered that yeast was living whilst bicarb was not (!).  It took a bit of prodding before T answered the query of why we allowed the dough to rise before putting it in the oven (ie. how come it had to sit at room temperature for an hour whilst the soda bread was put straight in the oven)  Finally I got the answer I was looking  for – that the heat would denature the yeast and stop it from reproducing.  But then again, as T pointed out, our prior experiments did not demonstrate that at all, with the yeast in the boiling water working as well as those in the luke warm water.  We surmised that the water must have cooled enough before putting it into the test tube for death not to occurred.

I’m debating whether or not to take yeast a new level and experiment with fermented goods.  We’ll see, after the sour bread stench episode, I’m at all sure I’m up for it!