Tiny Hot Pile

1. Put equal amounts of compost and coffee grounds in the cooler, filling it about two-thirds full. Add water and mix with the spoon until the mixture is moist to touch, but not soggy enough to leak water when you hold a clump in your hand.
2. Put the thermometer deep into the mixture, and make a note of the starting temperature. (If you have a probe thermometer, you can leave it in for the duration of the experiment.)
3. Put the lid on the cooler, put the cooler on the tray, and leave the setup undisturbed for 24 hours.

After about 24 hours have passed, remove the lid and feel the compost/coffee mixture with your hand. What do you notice? Take the temperature of the mixture. Compare the temperature of the mixture to the ambient (room) temperature, and to the initial temperature you recorded.

Check the temperature of the mixture every day for a week. What happens?

If you notice that the temperature goes down over the course of the week, stir the mixture a bit to aerate it, and replace the lid. Check the temperature again after 24 hours.

Compost is full of organisms (bacteria, fungi, worms, and other invertebrates) that break down, or decompose, the complex carbon and nitrogen compounds in dead plant and animal matter. The process of decomposition releases energy, some of which is used by the decomposers, and some of which takes the form of heat.

Coffee grounds are excellent food for decomposers, so when you mix compost with coffee, you’re providing a meal for the microorganisms living in the compost. Over time, you should notice that the temperature of the compost-and-coffee mixture increases—an increase that you should be able to feel with your hand.

Decomposition in your compost pile requires oxygen, water, and a food source. These are the very same things that you—an animal—require to live. And just like the compost pile, your animal self generates a fair amount of heat.

The metabolic process that produces energy by breaking down the carbohydrates, or sugars, in food is called cellular respiration. Cellular respiration is fundamentally similar among animals, plants, and many microorganisms. However, the cellular respiration happening in your tiny “hot pile” involves the breakdown of cellulose—an additional step that doesn’t happen in animals like yourself.

Cellulose, commonly known as fiber, is one of the major forms of carbohydrates made by plants. Microorganisms in the compost pile (and in the guts of many animals, including cows and goats) have enzymes that can break down cellulose into its component sugars, which can then be used for cellular respiration.

Eventually, your mini compost pile will cool down as the microorganisms run out of food, water, or oxygen. You can try to revive it by adding more coffee for food, moistening it if it’s dry, and aerating it by stirring to add oxygen.

Your tiny compost pile shouldn’t smell bad. If it does start to smell, you’re most likely observing anaerobic respiration, a different type of respiration carried out by microorganisms that don’t require oxygen. These microorganisms are as bad for you as they smell, so you should dispose of a stinky hot pile immediately.

Where compost is concerned, size matters. A large compost pile of the size usually kept outdoors will generate and retain much more heat than your little one. Similarly, a too-tiny compost pile (the result of using a cooler smaller than the recommended size) may not even generate enough heat for you to measure. However, the processes happening in a compost pile are happening in almost every soil, everywhere on the planet—they’re just happening more slowly, and at lower levels.

If you want to experiment further, try setting up one or more coolers with different mixtures to see what affects the temperature of your pile, and therefore the rate of decomposition. Will potting soil work the same way as compost when mixed with coffee grounds? Will compost alone produce heat? What about coffee grounds alone? If you see an increase in temperature in any of these mixtures, what do you think that tells you about the things you’re using?

Are there food sources that will generate more heat than others? Experiment to see what your microbial friends prefer: Try adding shredded paper (you’ll need extra water) or food scraps (be warned: They may smell). Or add half a cup of sugar to your mixture and see what happens.

Try eliminating all microbes by sterilizing your substrate (potting soil, compost, or food sources). To sterilize, moisten your substrate and cook it in a pressure cooker at 12.5 psi for about an hour. How does this affect the decomposition?

Record the temperature at regular intervals and chart the changes. How long does it take your tiny hot pile to warm up? What’s the hottest it gets? How long does it take before the temperature drops, and how fast does it go back up if you add more food or air? What do you think this tells you about the resources needed for respiration and heat generation?

This Science Snack is part of a collection that highlights Black artists, scientists, inventors, and thinkers whose work aids or expands our understanding of the phenomena explored in the Snack.

Dr. Suzanne Pierre (1991–present), pictured above, is a microbial biologist, biogeochemist, and environmental science educator whose research focuses on global environmental change. Formerly part of the science teaching staff at the Exploratorium, Dr. Pierre is particularly interested in the impacts of climatic change on the elemental relationships among plants, microbes, and soils. You can find out more about these relationships by building your own Tiny Hot Pile.

This activity can serve as the basis for a lab investigating cellular respiration; the temperature of the tiny hot pile is an indirect measurement of the rate of respiration by organisms in the compost. The experiments suggested in the Going Further section can help students gather data for the inputs and outputs of respiration—described in the equation below—and find evidence for how each component affects rate of respiration.

$$C_{6}H_{12}O_{6} \text{ (glucose)}+ O_{2} \text{ (oxygen gas)} \rightarrow CO_{2} \text{ (carbon dioxide)}+ H_{2}O \text{ (water)}+ \text{energy (ATP and heat)}$$

Another byproduct of respiration is carbon dioxide, or CO₂. If you have a sensitive CO₂ meter handy you may be able to measure increased levels close to your compost pile (or from your exhaled breath). Carbon dioxide is matter that has mass, and therefore the overall mass of your compost pile decreases as CO₂ is released.

Learn more about composting at a larger scale in this episode of Build Your Own Exploratorium.