Coffee and feces share several of the same sulfur-containing aroma compounds, which is why your brain can sometimes blur the line between the two. In most cases, the overlap is subtle enough that you’d never notice. But certain conditions, from a post-viral smell disorder to the way your beans were processed or stored, can tip the balance and make that resemblance impossible to ignore.
Shared Sulfur Compounds in Coffee and Waste
Roasted coffee contains roughly 1,000 volatile compounds, and around 100 of them contribute meaningfully to what you smell when you open a bag of beans. Among the most potent are sulfur-containing molecules: thiophenes, thiazoles, and thiol compounds that carry descriptors like “roasted,” “meaty,” “onion,” and “vegetal.” These are the same chemical families found in decomposing organic matter, including human waste.
One compound in particular, 2-furanmethanethiol, is considered the single most important contributor to the characteristic smell of brewed coffee. It has an extraordinarily low detection threshold, meaning your nose picks it up at vanishingly small concentrations. The catch is that sulfur compounds are inherently unstable. As they break down or recombine, their pleasant “roasty” quality can shift toward something more reminiscent of rotten eggs, cooked cabbage, or sewage. When coffee goes stale, these sulfur molecules degrade first, and the result is often a flat, unpleasant odor that reads as fecal or garbage-like rather than rich and inviting.
Parosmia: When Your Nose Rewires the Signal
If coffee suddenly smells like sewage or feces to you and nothing has changed about your beans, the most likely explanation is parosmia. This is a smell disorder where odors are perceived incorrectly, often as something disgusting, even though the original scent is still present. Before COVID-19, parosmia was relatively rare. It has since become far more common as a post-viral complication, typically appearing weeks or months after an initial loss of smell.
Researchers at the University of Reading identified why coffee is one of the most frequently reported trigger foods for people with parosmia, alongside onion, garlic, meat, and green peppers. From the roughly 100 aroma compounds in brewed coffee, they narrowed the problem down to 15 that parosmic participants consistently described as repulsive. The standout was 2-furanmethanethiol, the same potent sulfur compound responsible for coffee’s normal aroma. In a healthy nose, dozens of receptor types fire simultaneously and send a balanced signal that your brain interprets as “coffee.” In parosmia, damaged olfactory nerves only partially regenerate, so high-potency compounds like 2-furanmethanethiol dominate the signal. Instead of a complex, pleasant aroma, your brain receives a lopsided message it interprets as burnt, chemical, or rotten.
Dr. Jane Parker, who led the research, emphasized that this is not psychological. “This is solid evidence that it’s not all ‘in the head,’ and that the sense of disgust can be related to the compounds in the distorted foods,” she noted. The experience combines what’s happening at the nerve receptor level with how the central nervous system interprets incomplete signals. For most people, parosmia improves over months as olfactory neurons continue to heal, though recovery timelines vary widely.
How Processing and Roasting Affect the Smell
The way coffee beans are handled after harvest plays a major role in their final aroma. In “natural” or dry processing, the coffee cherry’s fruit is left on the bean while it dries in the sun. This triggers wild fermentation by ambient bacteria and yeasts. Done well, it produces fruity, wine-like notes. Done poorly or inconsistently, it creates what coffee professionals politely call “ferment” or “barnyard” flavors, which can land squarely in fecal territory. If your coffee has an animalic funk, the processing method is a likely culprit.
The most extreme example is kopi luwak, one of the world’s most expensive coffees at nearly $1,000 per kilogram. These beans are eaten and excreted by the Asian palm civet, a small mammal native to Southeast Asia. The beans ferment inside the animal’s digestive tract, producing a unique fatty acid profile. Researchers found that kopi luwak contains elevated levels of caprylic and capric acid methyl esters, compounds that impart a pungent, dairy-like flavor. The gut fermentation process fundamentally changes the bean’s chemistry in ways that conventional processing cannot replicate.
Roast level matters too. Lighter roasts preserve more of the bean’s origin character, including any fermentation-related off-notes from processing. Darker roasts generate more sulfur volatiles through the Maillard reaction (the same browning chemistry that gives grilled meat its aroma), which can push the smell toward smoky, ashy, or sulfurous. If you’ve noticed a poop-like smell from a very dark roast, you’re likely picking up on an excess of those same sulfur compounds that normally make coffee smell appealing at lower concentrations.
Stale Coffee and Storage Problems
Freshness is one of the most overlooked factors. Roasted coffee begins losing its volatile aromatics within days of roasting, and the pleasant top notes disappear first. What remains are heavier, less appealing compounds. Stale ground coffee in particular can develop a musty, fecal odor because the increased surface area accelerates oxidation. Beans stored in warm, humid conditions or left in an open bag will degrade faster. If your coffee smells fine when you first open the bag but develops an off-putting quality over a week or two, staleness is the simplest explanation.
Your brewing equipment can compound the problem. Coffee oils left on a French press screen, inside a drip machine’s water reservoir, or caked onto a reusable filter turn rancid over time. These rancid oils produce their own set of unpleasant volatiles that mix with your fresh brew’s aroma. A thorough cleaning cycle often resolves the issue entirely.
Your Gut May Play a Role Too
Coffee speeds up digestion. It stimulates the colon within minutes of drinking, which is why many people associate their morning cup with a trip to the bathroom. This effect isn’t just mechanical. Coffee alters the composition of gut bacteria, increasing populations of beneficial microbes like Bifidobacterium and Faecalibacterium while decreasing others like Prevotella. These bacterial shifts change the profile of short-chain fatty acids in the gut, which can affect both stool consistency and odor. If you notice that your stool smells distinctly of coffee, or that the smell in the bathroom after drinking coffee reminds you of the cup itself, you’re observing the same sulfur compounds making their way through your digestive system and being modified by gut bacteria along the way.
Sorting Out the Cause
If coffee has always had a faint poop-like quality to you, your nose may simply be more sensitive to the sulfur compounds that overlap between the two smells. Some people are genetically better at detecting specific thiols, and for them, the resemblance is obvious where others wouldn’t notice it at all.
If the smell changed suddenly, parosmia is the most likely explanation, especially if it followed a cold, flu, or COVID infection. Other foods will usually taste or smell wrong too, particularly garlic, onions, and cooked meat. If only your current bag of coffee smells off, try a different brand, roast level, or processing method. And if the problem is confined to coffee made in a particular machine, clean it thoroughly before blaming the beans.