Yeast and Hydrogen Peroxide: Foam as a Measure of Enzyme Speed
Yeast makes catalase, the same enzyme your liver uses to break down hydrogen peroxide. A spoonful of yeast in 3 percent peroxide produces a column of foam that lets you compare enzyme activity at three different temperatures without any laboratory equipment.
Materials
| Item | Amount | If you do not have it |
|---|---|---|
| Hydrogen peroxide, 3% | 150 ml | The 3% first-aid solution; never use 30% or higher |
| Dried active yeast | 7 g, one sachet | Fresh baker's yeast, 15 g crumbled |
| Warm water at 40 C | 50 ml | Tap water at blood temperature, tested on your wrist |
| Narrow-necked bottle | 1, 500 ml | A clean plastic drinks bottle, which is safer than glass |
| Washing-up liquid | 5 ml | Any surfactant; it slows the foam collapsing so you can measure it |
| Thermometer and ruler | 1 each | A digital probe thermometer changes the result far less than guessing |
Method
Stir the yeast into 50 ml of 40 C water and leave it for five minutes until it froths slightly.
Watch for: The mixture smells strongly of bread and turns a pale tan; if the water was over 60 C it will smell flat and inactive.
Pour 50 ml of peroxide into the bottle, add the yeast mixture and 5 ml of washing-up liquid, and swirl once.
Watch for: Foam rises from the bottom within three seconds and reaches the neck in about eight seconds.
Measure the peak foam height against a ruler taped to the bottle, then note the time to peak.
Watch for: At 40 C the peak arrives in 12 to 18 seconds and the foam is warm to the touch.
Repeat with 10 C water and with 60 C water, using fresh peroxide and a clean bottle each time.
Watch for: The cold run peaks at roughly half the height and takes three times as long; the 60 C run barely foams at all.
Touch the bottle wall during the warm run and then again after the reaction dies down.
Watch for: The bottle is noticeably warm during the reaction and returns to room temperature within four minutes.
Why it works
Catalase is a large four-part enzyme that accelerates the breakdown of hydrogen peroxide into water and oxygen by roughly a factor of a million compared with the uncatalysed reaction. It works by holding the peroxide molecule against an iron-containing haem group, which lowers the activation energy of the decomposition. The enzyme is a protein, so it has a narrow working temperature range: at 10 C the molecules move too slowly to meet the substrate often, and above about 55 C the protein unfolds and the active site is destroyed permanently. The foam is oxygen gas trapped by the surfactant, and it is a rough but honest measure of reaction rate, because faster decomposition releases gas faster and builds a taller column before the surfactant drains.
Think of it like: The enzyme is a workbench that holds two parts together at the right angle. Chill the workshop and the workers move sluggishly; overheat it and the bench itself warps so nothing fits any more. Neither cold nor heat changes the parts, only the bench.
Clean-up and disposal
Dilute everything with plenty of cold water and pour it away; the residue is water, oxygen and dead yeast. Rinse the bottle while the foam is still wet, because dried yeast foam sets into a crust. Throw away any leftover peroxide solution that has been sitting open, and never return it to the original bottle.
Bench notes
Temperature is the dial
This card is really a temperature experiment with a foam readout. Hold the yeast amount and the peroxide volume fixed, and the only thing that changes between runs is the folding of the enzyme.
Observed / 实测Foam height at 10 C, 40 C and 60 C runs roughly 4 cm, 11 cm and 1 cm in a 500 ml bottle.
Reading rate without a stopwatch
The time from mixing to peak is almost as informative as the height. Cold slows both the reaction and the foam drainage, so the peak is late and low; hot denatures the enzyme so there is no peak at all.
Observed / 实测The 40 C run warms the bottle by about 6 C above room temperature, which is direct evidence that the decomposition is exothermic.
Peroxide discipline
Variations worth running
- Test yeast, mashed potato, raw liver and a slice of apple separately and rank them by catalase activity.
- Run the warm test with half a sachet and then with two sachets to show that the rate depends on enzyme concentration as well as temperature.
- Add the yeast in one go versus in three portions thirty seconds apart and compare the total foam height, which should be nearly identical.
Questions that come up on the bench
- Why did the 60 C run produce almost no foam?
- The catalase protein was denatured. Above roughly 55 C the folded structure of the enzyme unrolls, and the shape of the active site is what does the chemistry. This change is permanent, which is why you cannot rescue it by cooling the mixture down again.
- Is the foam itself the oxygen?
- The bubbles are oxygen, but the foam is mostly liquid. Oxygen makes up only a small fraction of the volume you see; the rest is the thin films of water and the surfactant that hold the bubbles in place.
- Can I use a higher concentration of peroxide to get more foam?
- No. Three percent is the safe working limit for a kitchen bench, and 30 percent peroxide causes instant chemical burns to skin and eyes. Concentrating the reaction is not worth the risk when temperature and enzyme amount already give a clear signal.