yeast growth experiment

The fermentation of sugars using yeast: A discovery experiment

Charles Pepin (student) and Charles Marzzacco (retired), Melbourne, FL

Introduction

Enzyme catalysis 1  is an important topic which is often neglected in introductory chemistry courses. In this paper, we present a simple experiment involving the yeast-catalyzed fermentation of sugars. The experiment is easy to carry out, does not require expensive equipment and is suitable for introductory chemistry courses.

The sugars used in this study are sucrose and lactose (disaccharides), and glucose, fructose and galactose (monosaccharides). Lactose, glucose and fructose were obtained from a health food store and the galactose from Carolina Science Supply Company. The sucrose was obtained at the grocery store as white sugar. The question that we wanted to answer was “Do all sugars undergo yeast fermentation at the same rate?”

Sugar fermentation results in the production of ethanol and carbon dioxide. In the case of sucrose, the fermentation reaction is:

\[C_{12}H_{22}O_{11}(aq)+H_2 O\overset{Yeast\:Enzymes}{\longrightarrow}4C_{2}H_{5}OH(aq) + 4CO_{2}(g)\]

Lactose is also C 12 H 22 O 11  but the atoms are arranged differently. Before the disaccharides sucrose and lactose can undergo fermentation, they have to be broken down into monosaccharides by the hydrolysis reaction shown below:

\[C_{12}H_{22}O_{11} + H_{2}O \longrightarrow 2C_{6}H_{12}O_{6}\]

The hydrolysis of sucrose results in the formation of glucose and fructose, while lactose produces glucose and galactose.

sucrose + water \(\longrightarrow\) glucose + fructose

lactose + water \(\longrightarrow\) glucose + galactose

The enzymes sucrase and lactase are capable of catalyzing the hydrolysis of sucrose and lactose, respectively.

The monosaccharides glucose, fructose and galactose all have the molecular formula C 6 H 12 O 6  and ferment as follows:

\[C_{6}H_{12}O_{6}(aq)\overset{Yeast Enzymes}{\longrightarrow}2C_{2}H_{5}OH(aq) + 2CO_{2}(g)\]

In our experiments 20.0 g of the sugar was dissolved in 100 mL of tap water. Next 7.0 g of Red Star ®  Quick-Rise Yeast was added to the solution and the mixture was microwaved for 15 seconds at full power in order to fully activate the yeast. (The microwave power is 1.65 kW.) This resulted in a temperature of about 110  o F (43  o C) which is in the recommended temperature range for activation. The cap was loosened to allow the carbon dioxide to escape. The mass of the reaction mixture was measured as a function of time. The reaction mixture was kept at ambient temperature, and no attempt at temperature control was used. Each package of Red Star Quick-Rise Yeast has a mass of 7.0 g so this amount was selected for convenience. Other brands of baker’s yeast could have been used.

This method of studying chemical reactions has been reported by Lugemwa and Duffy et al. 2,3  We used a balance good to 0.1 g to do the measurements. Although fermentation is an anaerobic process, it is not necessary to exclude oxygen to do these experiments. Lactose and galactose dissolve slowly. Mild heat using a microwave greatly speeds up the process. When using these sugars, allow the sugar solutions to cool to room temperature before adding the yeast and microwaving for an additional 15 seconds.

Fermentation rate of sucrose, lactose alone, and lactose with lactase

Fig. 1 shows plots of mass loss vs time for sucrose, lactose alone and lactose with a dietary supplement lactase tablet added 1.5 hours before starting the experiment. All samples had 20.0 g of the respective sugar and 7.0 g of Red Star Quick-Rise Yeast. Initially the mass loss was recorded every 30 minutes. We continued taking readings until the mass leveled off which was about 600 minutes. If one wanted to speed up the reaction, a larger amount of yeast could be used. The results show that while sucrose readily undergoes mass loss and thus fermentation, lactose does not. Clearly the enzymes in the yeast are unable to cause the lactose to ferment. However, when lactase is present significant fermentation occurs. Lactase causes lactose to split into glucose and galactose. A comparison of the sucrose fermentation curve with the lactose containing lactase curve shows that initially they both ferment at the same rate.

Fig. 1. Comparison of the mass of CO 2 released vs time for the fermentation of sucrose, lactose alone, and lactose with a lactase tablet. Each 20.0 g sample was dissolved in 100 mL of tap water and then 7.0 g of Red Star Quick-Rise Yeast was added.

However, when the reactions go to completion, the lactose, lactase and yeast mixture gives off only about half as much CO 2  as the sucrose and yeast mixture. This suggests that one of the two sugars that result when lactose undergoes hydrolysis does not undergo yeast fermentation. In order to verify this, we compared the rates of fermentation of glucose and galactose using yeast and found that in the presence of yeast glucose readily undergoes fermentation while no fermentation occurs in galactose.

Fig. 2. Comparison of the mass of CO 2 released vs time for the fermentation of sucrose, glucose and fructose. Each 20 g sugar sample was dissolved in 100 mL of water and then 7.0 g of yeast was added.

Fermentation rate of sucrose, glucose and fructose

Next we decided to compare the rate of fermentation of sucrose with that glucose and fructose, the two compounds that make up sucrose. We hypothesized that the disaccharide would ferment more slowly because it would first have to undergo hydrolysis. In fact, though, Fig. 2 shows that the three sugars give off CO 2  at about the same rate. Our hypothesis was wrong. Although there is some divergence of the three curves at longer times, the sucrose curve is always as high as or higher than the glucose and fructose curves. The observation that the total amount of CO 2  released at the end is not the same for the three sugars may be due to the purity of the fructose and glucose samples not being as high as that of the sucrose.

Fermentation rate and sugar concentration

Next, we decided to investigate how the rate of fermentation depends on the concentration of the sugar. Fig. 3 shows the yeast fermentation curves for 10.0 g and 20.0 g of glucose. It can be seen that the initial rate of CO 2  mass loss is the same for the 10.0 and 20.0 g samples. Of course the total amount of CO 2  given off by the 20.0 g sample is twice as much as that for the 10.0 g sample as is expected. Later, we repeated this experiment using sucrose in place of glucose and obtained the same result.

Fig. 3. Comparison of the mass of CO 2  released vs time for the fermentation of 20.0 g of glucose and 10.0 g of glucose. Each sugar sample was dissolved in 100 mL of water and then 7.0 g of yeast was added.

Fermentation rate and yeast concentration

After seeing that the rate of yeast fermentation does not depend on the concentration of sugar under the conditions of our experiments, we decided to see if it depends on the concentration of the yeast. We took two 20.0 g samples of glucose and added 7.0 g of yeast to one and 3.5 g to the other. The results are shown in Fig. 4. It can clearly be seen that the rate of CO 2  release does depend on the concentration of the yeast. The slope of the sample with 7.0 g of yeast is about twice as large as that with 3.5 g of yeast. We repeated the experiment with sucrose and fructose in place of glucose and obtained similar results.

Fig. 4. Comparison of the mass of CO 2 released vs time for the fermentation of two 20.0 g samples of glucose dissolved in 100 mL of water. One had 7.0 g of yeast and the other had 3.5 g of yeast.

In hindsight, the observation that the rate of fermentation is dependent on the concentration of yeast but independent of the concentration of sugar is not surprising. Enzyme saturation can be explained to students in very simple terms. A molecule such as glucose is rather small compared to a typical enzyme. Enzymes are proteins with large molar masses that are typically greater than 100,000 g/mol. 1  Clearly, there are many more glucose molecules in the reaction mixture than enzyme molecules. The large molecular ratio of sugar to enzyme clearly means that every enzyme site is occupied by a sugar molecule. Thus, doubling or halving the sugar concentration cannot make a significant difference in the initial rate of the reaction. On the other hand, doubling the concentration of the enzyme should double the rate of reaction since you are doubling the number of enzyme sites.

The experiments described here are easy to perform and require only a balance good to 0.1 g and a timer. The results of these experiments can be discussed at various levels of sophistication and are consistent with enzyme kinetics as described by the Michaelis-Menten model. 1  The experiments can be extended to look at the effect of temperature on the rate of reaction. For enzyme reactions such as this, the reaction does not take place if the temperature is too high because the enzymes get denatured. The effect of pH and salt concentration can also be investigated.

• Jeremy M. Berg, John L. Tymoczko and Lubert Stryer,  Biochemistry , 6th edition, W.H. Freeman and Company, 2007, pages 205-237.
• Fugentius Lugemwa, Decomposition of Hydrogen Peroxide,  Chemical Educator , April 2013, pages 85-87.
• Daniel Q. Duffy, Stephanie A. Shaw, William D. Bare, Kenneth A. Goldsby, More Chemistry in a Soda Bottle, A Conservation of Mass Activity,  Journal of Chemical Education , August 1995, pages 734-736.
• Jessica L Epstein, Matthew Vieira, Binod Aryal, Nicolas Vera and Melissa Solis, Developing Biofuel in the Teaching Laboratory: Ethanol from Various Sources,  Journal of Chemical Education , April 2010, pages 708–710.

More about April 2015

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Grow yeast experiment

Follow FizzicsEd 150 Science Experiments:

You Will Need:

• 4 packets of dry yeast
• 4 water bottles, chilled in the fridge (we use Thank You Water, a social enterprise that works to get clean water & sanitation to people in need)
• 1 large jug.
• 4 measuring cups.
• 4 thermometers (one will do if you don’t have a class set).
• Access to boiling water plus adult supervision.
• 1 stopwatch.
• A pen to mark the water temperature on each water bottle during the experiment.
• A shelf to leave the science experiment to run.
• A notebook for your observations.

• Instruction

Pour out the 4 chilled water bottles into the large jug and discard the rest of the water (maybe water your  school garden !)

Carefully measure out the water into the four measuring cups as per the measurements below;

Cup 1 – 200mL of chilled water

Cup 2 – 150mL of chilled water

Cup 3 – 100mL of chilled water

Cup 4 – 50mL of chilled water

Use the thermometers to take a measurement of the water temperature in each cup (write this in your notebook).

With an adult, boil a jug of water and then top up cups 2, 3 and 4 so that they too have 200mL of water as per cup 1. You will be testing the effect of temperature on the growth of yeast by measuring how much gas is released by the yeast under 4 different temperature conditions ( variable testing ).

Using a funnel, carefully pour each cup of water into the four separate water bottles. Use the pen to mark the starting temperature of each water bottle.

Add a spoonful of sugar per water bottle and then swirl the bottle to dissolve the sugar.

Add a yeast packet into each bottle and quickly stretch a balloon of the opening of each bottle.

4 yeast growth experiments started, showing a distinct change already!

Start the stopwatch and take notes of when each balloon rises!

OPTIONAL: you could also keep each bottle in the yeast experiment at the same temperature and vary the amount of sugar added instead.

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What is going on?

Your experiment was testing the effect of water temperature on the growth of yeast. Yeast are egg-shaped microscopic cells of fungi that are dormant whilst kept in dry and cool conditions. However, yeast will rapidly divide once exposed to water and sugar in ideal temperatures. In the right temperature, yeast cells will change the sugar into glucose by using the water plus as an enzyme catalyst (invertase). Once the yeast has converted the sugar to glucose fermentation can then occur to produce carbon dioxide and ethanol as per the equation below;

Glucose ⟶ Ethanol + Carbon dioxide

which can be written as…

C 6 H 12 O 6(aq)   ⟶ 2C 2 H 5 OH (aq)  + 2CO 2(g)

In your experiment, you were trapping the carbon dioxide released during the fermentation process. The more active the yeast, the more carbon dioxide the yeast produced! In your experiment, the different water temperatures will have produced different results as bottles may have been too hot for the yeast to survive whereas the other bottles may have been too cold.   By introducing a variable to test in your experiment, you’re doing real science!  The following list of temperatures is worth keeping in mind when assessing your results:

• 55° C – 60° C Yeast cells die (also known as the thermal death point).
• 41° C – 46° C Ideal temperature of water for dry yeast being reconstituted with water and sugar.
• 4° C The temperature of a fridge – yeast will be too cold to work properly.

Yeast is used to make bread rise and to ferment beer. There are many different species of yeast, but the one most commonly used in cooking and baking is called  Saccharomyces cerevisiae , which is also known as brewer’s yeast.

Yeast can break down many types of simple carbohydrates (monosaccharides) however they cannot break down complex carbohydrates such as starch. This means that extra enzymes are needed to break down starch into sugars that the yeast can use, for example during beer production we use enzymes from germinating barley to do this.

Variables to test

More on variables here

• Try different concentrations of vinegar as the growth medium. Can the yeast handle some acidity?
• Vary the amount of sugar used.
• Does the volume of water make a difference?

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4 thoughts on “ Grow yeast experiment ”

What is the amount of water you would like me to put?

Hi! Here’s the detail’s that you need;

> Cup 1 – 200mL of chilled water > Cup 2 – 150mL of chilled water > Cup 3 – 100mL of chilled water > Cup 4 – 50mL of chilled water

With an adult, boil a jug of water and then top up cups 2, 3 and 4 so that they too have 200mL of water as per cup 1. You will be testing the effect of temperature on the growth of yeast by measuring how much gas is released by the yeast under 4 different temperature conditions.

Would this experiment still work if instead i tested how different types of sugars affect the amount of fermentation by yeast. Would i still get different sized balloons in my result.

We’d love it if you try this and let us know! With any experiment you just have to change one thing and then measure the result. So, changing the types of sugars is a completely valid investigation. Good luck!

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3.1.3 Yeast experiment explained

You’ve seen the results of the yeast experiment, but what do these results mean?

Yeasts are microscopic, single-celled organisms, and are a type of fungus that is found all around us, in water, soil, on plants, on animals and in the air. Like all organisms, when yeasts are put in the right type of environment they will thrive; growing and reproducing.

Your experiments were designed to help you identify which environment promotes the most yeast growth. The first three glasses in your experiment contained different temperature environments (cold water, hot water and body temperature water). At very low temperatures the yeast simply does not grow but it is still alive – if the environment were to warm up a bit, it would gradually begin to grow. At very high temperatures the cells within the yeast become damaged beyond repair and even if the temperature of that environment cooled, the yeast would still be unable to grow. At optimum temperatures the yeast thrives.

Your third and fourth glasses both contained environments at optimum temperature (body temperature) for yeast growth, the difference being, the fourth glass was sealed. The variable between these two experiments was the amount of available oxygen. You may have been surprised by your results here, thinking that a living organism in an environment without oxygen cannot survive? However, you should have found that yeast grew pretty well in both experiments.

To understand why yeast was able to thrive in both conditions we need to understand the chemical process occurring in each glass during the experiment. In the three open glasses, oxygen is readily available, and from the moment you added the yeast to the sugar solution it began to chemically convert the sugar in the water and the oxygen in the air into energy, water, and carbon dioxide in a process called aerobic respiration.

Yeast is a slightly unusual organism – it is a ‘facultative anaerobe’. This means that in oxygen-free environments they can still survive. The yeast simply switches from aerobic respiration (requiring oxygen) to anaerobic respiration (not requiring oxygen) and converts its food without oxygen in a process known as fermentation. Due to the absence of oxygen, the waste products of this chemical reaction are different and this fermentation process results in carbon dioxide and ethanol.

Depending on how long you monitored your experiment for and how much space your yeast had to grow you may have noticed that, with time, the experiment sealed with cling film slowed down. This is for two reasons; firstly because less energy is produced by anaerobic respiration than by aerobic respiration and, secondly, because the ethanol produced is actually toxic to the yeast. As the ethanol concentration in the environment increases, the yeast cells begin to get damaged, slowing their growth.

The ethanol produced is a type of alcohol, so it is this process that allows us to use it to make beer and wine. When used in bread making, the yeast begins by respiring aerobically, the carbon dioxide from which makes the bread rise. Eventually the available oxygen is used up, and the yeast switches to anaerobic respiration producing alcohol and carbon dioxide instead. Do not worry though; this alcohol evaporates during the baking process, so you won’t get drunk at lunchtime from eating your sandwiches.

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Yeast Fermentation Experiment

Fermentation is a fascinating process that kids can easily explore through a simple experiment using yeast and sugar. This hands-on activity teaches students about fermentation and introduces them to the scientific method, data collection, and analysis.

Investigate how different types of sugar (white, brown, and honey) affect the rate of yeast fermentation by measuring the amount of carbon dioxide (CO₂) produced.

Example Hypothesis: If yeast is added to different types of sugar, then the type of sugar will affect the amount of carbon dioxide produced, with white sugar producing more CO₂ than the others.

💡 Learn more about using the scientific method [here] and choosing variables .

Watch the Video:

• Active dry yeast
• White sugar
• Brown sugar
• Measuring spoons and measuring cups
• Small bottles or test tubes
• Rubber bands
• Ruler or measuring tape
• Notebook and pen for recording data ( grab free journal sheets here )
• Printable Experiment Page (see below)

Instructions:

STEP 1. Prepare a yeast solution by dissolving a packet of active dry yeast in warm water according to the package instructions.

STEP 2. Label 3 bottles and add 1 tablespoon of white sugar to the “White Sugar” bottle. Add 1 tablespoon of brown sugar to the “Brown Sugar” bottle. Measure 1 tablespoon of honey and add it to the “Honey” bottle.

STEP 3. Measure and pour an equal amount of the yeast solution into each bottle, ensuring the yeast is well mixed with the sugar.

STEP 4. Quickly stretch a balloon over the mouth of each bottle. Secure the balloons with rubber bands if needed. Ensure the balloons are sealed tightly to prevent CO₂ from escaping.

STEP 5. Place the bottles in a warm, consistent environment to promote fermentation.

STEP 6. Observe and record the size of the balloons at regular intervals (e.g., every 15 minutes) for 1-2 hours. Use a ruler or measuring tape to measure the circumference of each balloon.

TIP: Note the time it takes for the balloons to start inflating and the differences in balloon size over time for each type of sugar.

STEP 7: Analyze the data by comparing the amount of CO₂ produced (balloon size) for each type of sugar. Create a graph showing the balloon size over time for each sugar type.

STEP 8. Determine which sugar type resulted in the most and least CO₂ production. Discuss possible reasons for the differences, considering what each sugar is made of. Think about whether the results support or disprove the hypothesis. Can you come up with further experiments or variations to explore other factors affecting yeast fermentation?

Free Printable Yeast and Sugar Experiment Project

Grab the free fermentation experiment worksheet here. Join our STEM club for a printable version of the video!

The Science Behind Yeast Fermentation

For Our Younger Scientists: Yeast is a type of fungus that feeds on sugars. When you mix yeast with sugar and water, it starts to eat the sugar and convert it into alcohol and carbon dioxide gas. The gas gets trapped in the balloon, causing it to inflate. This shows that fermentation is happening!

Yeast fermentation is a biological process where yeast converts sugars into alcohol and carbon dioxide (CO₂) in the absence of oxygen. This process is used in baking, brewing, wine making and biofuel production. How much fermentation occurs can vary depending on the type of sugar used.

Yeast contains enzymes that break down sugar molecules through a series of chemical reactions . Here’s how it works:

Enzymes are molecules, usually proteins, that act as catalysts to speed up chemical reactions within living organisms.

First the yeast is mixed with warm water, and it becomes activated. The warm environment “wakes up” the yeast cells, preparing them to consume sugars.

Yeast cells produce enzymes that break down sugar molecules (sucrose, glucose, and fructose) into simpler molecules. This process is called glycolysis. During glycolysis, sugar molecules are converted into pyruvate, releasing a small amount of energy.

In the absence of oxygen (anaerobic conditions), yeast cells convert pyruvate into ethanol (alcohol) and carbon dioxide gas (CO₂). The carbon dioxide produced during fermentation is what inflates the balloons in the experiment.

Different Sugars & Fermentation

Different sugars can affect the rate of fermentation. This is how:

• White Sugar (Sucrose): Composed of glucose and fructose and is easily broken down by yeast, leading to efficient CO₂ production.
• Brown Sugar: Contains sucrose along with molasses, which includes minerals and additional nutrients. May result in a slightly different fermentation rate due to its composition.
• Honey: Contains a mixture of glucose, fructose, and other components. The additional components can influence the fermentation process, potentially leading to different CO₂ production rates compared to pure sucrose.

The amount of CO₂ produced depends on how easily the yeast can break down the sugar molecules and convert them into ethanol and CO₂. Sugars that are more readily broken down by yeast will typically produce more CO₂ faster.

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