In 1745 John Needham boiled broth, sealed it and saw it turn cloudy with tiny creatures. He argued they arose spontaneously. Lazzaro Spallanzani later did many careful experiments with heated broth, and his sealed flasks stayed clear unless opened to air. Both men used sealed flasks of heated broth, yet the two men got different results, so the evidence from the two flasks pointed in different directions. In one flask the broth turned cloudy, and in the other flask it stayed clear unless air got in. Louis Pasteur showed that yeast, a living organism, causes alcoholic fermentation. He also showed that a living organism turns sugar into lactic acid in milk, so souring is caused by life, not just chemistry.
In 1864 Pasteur found that heating wine to 50 to 60 degrees C (122 to 140 degrees F) killed the microorganisms that spoil it. The process, pasteurization, was later used for milk too. Heat is one way to stop spoilers, and cold is another. Cold slows down the tiny living things that spoil food, which is why we keep food in the fridge. Mould had a part in this story in 1928, when Fleming found that a mould culture stopped staphylococci from growing even when it was diluted 800 times. He named the active substance penicillin. Each of these works on the tiny living things in its own way. Heat killed them, cold slowed them down, and penicillin stopped staphylococci from growing.
Now look at what rot does to plant matter. Cellulose is essentially glucose rings attached to one another. Decomposer enzymes break the bond between two glucose molecules, and the freed glucose is taken up as food. Rot also frees nitrogen, phosphorus and about two dozen other nutrients that living things need to grow, not just carbon. In a pile with oxygen, about two-thirds of the carbon is breathed out as carbon dioxide and about one-third is built into the cells of the microbes. In plain terms, the enzymes cut the sugar rings apart, and the pieces become food. Not all of the carbon is lost, because a share of it ends up inside new living cells, which is the one-third.
Nitrogen takes a longer road, and it passes through several forms. In the nitrogen cycle, decomposing bacteria and fungi break down organic nitrogen from dead organisms and release ammonium. Other bacteria then turn ammonium into nitrite and nitrate, which plants can absorb. Still other bacteria turn nitrate back into nitrogen gas, which re-enters the air. This is called denitrification and it finishes the nitrogen cycle. Every step in this cycle is done by living things, and the last step sends nitrogen gas back into the air. The nitrogen leaves a dead organism and goes through these steps, and then it goes back to the air.
Spoiling also happens on a kitchen counter, and it shows up in the national numbers. Ethylene works in a loop in climacteric fruit. A little ethylene makes the fruit produce more, which speeds ripening. This is why a ripe fruit can quickly soften and spoil when it is stored. In 2019 the US produced 66.2 million tons of wasted food in retail, food service and homes, and only 5% was composted, as of November 2026. Food is the most common material sent to US landfills, at 24.1 percent of municipal solid waste. That means most of that wasted food went somewhere other than a compost pile.
A home pile puts these ideas to work. Compost is dark, crumbly and smells like fresh soil, and you can compost fruit and vegetable scraps and dry leaves. In a well-kept pile, compost is ready in about three to five months, and left alone, a pile can take a year or more. A pile can also run too hot. If it goes above about 162 to 172 degrees F, bacterial activity falls and breakdown slows. Backyard piles usually do not get hot enough to fully break down dairy, fats and cooked food, but commercial composters reach higher temperatures and can accept them. Composting is nature's way of recycling.
Cold can hold rot back, and farmers can use rot on purpose. Frozen northern soil called permafrost holds rich deposits of plant carbon because cold slows decay. When the soil warms, the organic matter decays and carbon seeps out as methane and carbon dioxide. In no-till farming, growers leave stalks and other debris on the field to rot. Much of the carbon returns to the air, but some carbon and nitrogen stay in the soil and make it more fertile.
Words to know
- pasteurization: heat treatment
- denitrification: nitrate-to-gas step
- cellulose: plant wall fiber
- permafrost: frozen ground
