How is coffee decaffeinated, and is it safe to drink?

According to the National Coffee Association, only 10% of adults in the US drink decaffeinated coffee daily. Recent regulation passed by the US Environmental Protection Agency has caused both consumers and advocacy groups to question whether the jitter-free beverage is safe to drink.

The main concern is that one of the primary methods companies use to decaffeinate coffee involves methylene chloride, a solvent that has been linked to an increased risk of cancer and other adverse health effects. These health hazards prompted the EPA to ban most commercial uses of the solvent, which has been used in a variety of products, including adhesives and paint strippers.

But the new EPA rule does not apply to the use of methylene chloride in food, since it’s outside the agency’s purview, so producers can still use the solvent to decaffeinate coffee. And the US Food and Drug Administration, which regulates this use case, says that the solvent is safe to use as long as the amount left in roasted coffee beans does not exceed 10 parts per million.

Organizations such as the Environmental Defense Fund have argued that because methylene chloride is a known carcinogen, no amount of it is safe for human consumption. In collaboration with other environmental and health advocacy groups, the EDF filed a petition with the FDA earlier this year to ban methylene chloride in food.

National Coffee Association CEO William Murray has pushed back against the petition and says in an email, “there is no evidence that drinking decaffeinated coffee causes health problems.”

But how and why is methylene chloride used to remove the caffeine from coffee, and can other solvents accomplish the same task?

The chemistry of decaffeination

All decaffeination methods begin the same way: by steaming the dense, dried green coffee beans or soaking them in hot water. This step prepares the beans for caffeine extraction by opening up their pores and separating the caffeine from the chlorogenic acid it’s affixed to (Crit. Rev. Food Sci. Nutr. 1999, DOI: 10.1080/10408699991279231). The hot temperature also increases caffeine’s solubility.

From there, most producers use an organic solvent that the partially polar caffeine molecules are soluble in.

Some producers opt to soak the waterlogged beans directly in the organic solvent. Often that’s methylene chloride, though ethyl acetate has also been used since the early 1980s.

“There’s less concern about ethyl acetate because it’s a naturally occurring chemical” found in fruits and vegetables, says Tonya Kuhl, a professor of chemical engineering at the University of California, Davis, and codirector of the UC Davis Coffee Center. “Of course, it’s all industrially produced,” she points out. “They’re not stripping it out of fruits for decaffeination.”

For the more hazardous methylene chloride, Kuhl explains that because it isn’t soluble in water, the solvent doesn’t penetrate very far into the waterlogged seed. Instead, caffeine removal happens where the aqueous and organic layer meet, near the surface of the bean.

Any trace amount of either solvent that remains in the coffee then evaporates off as the beans are dried and roasted. Both steps typically occur above 200 °C, far above the boiling point of both methylene chloride or ethyl acetate.

“Of course, it’s hard to remove the last little tiny bits of anything,” Kuhl says.

The Clean Label Project, a nonprofit organization that advocates transparent food and consumer product labeling, tested for methylene chloride in popular coffee brands. The nonprofit’s staff found traces of the solvent in several of their samples, though the amounts present were always below the current FDA limit.

“Even with all this, it’s still a healthy drink for you,” Kuhl says. Many studies have documented positive health effects associated with coffee, including decaffeinated coffee (Crit. Rev. Food Sci. Nutr. 2021, DOI: 10.1080/10408398.2020.1779175).

But not all companies decaffeinate coffee by putting the bean in direct contact with organic solvents. In a process known as the indirect method, some producers remove the beans from their hot water bath and then treat the water—not the bean—with methylene chloride or ethyl acetate.

Because caffeine has a higher solubility in these solvents than in water, the caffeine molecules will move from the water to the organic layer that sits on top. Once enough caffeine has been removed, the organic solvent is separated from the solution and the beans are soaked in the treated water to reintroduce some of the flavor compounds that leached out with the caffeine.

According to the previously mentioned 1999 review of coffee decaffeination methods, “decaffeinated coffee made by the indirect method does not pose the problem of residual solvent in the beans.” But according to a National Coffee Association spokesperson, there are no data to definitively determine how common this practice is compared with the more direct decaffeination method, which is thought to be the most popular.

Alternative methods

In either case, the chemistry seems to suggest that coffee decaffeinated with methylene chloride is safe to drink, as the amount of solvent left in the drink is minimal. But it’s understandable that regular decaf coffee consumers might want to avoid these types of beans until either the FDA reviews the methylene chloride petition or more studies investigate how methylene chloride affects health when it’s ingested through food.

The problem is that it’s hard to know which brand uses which method. US manufacturers are not required to label which decaffeination process they used.

Some manufacturers do label their coffee, but usually with a label such as “solvent-free,” “chemical-free,” or “naturally decaffeinated.” All of these are meant to suggest they don’t use methylene chloride in their decaffeination process.

One exception to this labeling trend is the Swiss water process, a label that coffee drinkers may have noticed pop up at their local grocery stores or coffee shops. The Swiss water process, created in the 1930s in Switzerland, uses only water to decaffeinate coffee beans.

As in methods based on organic solvents, the green beans are soaked in hot water. “When you first start to soak these beans, the concentration of the caffeine in water is less than it is in the coffee bean,” says Michael W. Crowder, a professor of chemistry at Miami University, who recently wrote about the science of decaf coffee for The Conversation. “So the caffeine moves into the water to try to reach equilibrium.”

After some time has passed, the coffee is removed from the water and placed into a second, fresh water bath, which forces even more caffeine out of the bean. This process is repeated up to 10 times. After the last soaking step, all of the now-caffeinated water is passed through an activated carbon filter that’s set up to trap the caffeine and other similarly sized compounds.

As in the indirect method, the water is then sprayed back on the beans to reintroduce any lost flavor compounds.

But because the beans are soaked multiple times, the Swiss water process is more time consuming, and costly, making it less enticing for companies to switch over. According to a National Coffee Association spokesperson, “industry estimates suggest that just 15% of decaf is water processed, made primarily by just one company.”

That cost is then also passed onto the consumer. Coffee that has been decaffeinated via the Swiss water process tends to be more expensive than regular decaf.

Another type of methylene chloride–free decaffeination relies on carbon dioxide, but not in its gas form. “You have to increase the pressure,” Crowder says. “At a certain pressure, carbon dioxide starts to have the properties of a solvent. It’s called supercritical carbon dioxide.”

Supercritical CO₂ is unique in that it can extract the caffeine in the moistened, green coffee beans but leave most of the other flavor compounds untouched. It is also easily removed after the coffee bean has been adequately decaffeinated, since the CO₂ returns to the gas phase and evaporates out of the beans at room temperature and pressure. The gas is subsequently filtered and recaptured to be used in the next batch of coffee.

The downside to this method is that the start-up costs are quite high. Crowder says that is why this method is primarily used by larger manufacturers to decaffeinate large amounts of commercial-grade coffee, which likely helps to ensure the coffee can still be sold at a reasonable price.

The first method for decaffeinating coffee was developed in 1900. Since then, several solvents have been tried and tested, including some toxic ones like benzene, chloroform, carbon tetrachloride, and trichloroethylene.

As new information about the health impacts of these solvents arose, they were eventually replaced, so it’s possible that the same could happen to methylene chloride too.

“It’s not like chemical engineers and chemists can’t figure out how to make better decaffeinated coffee,” Kuhl says. “It’s a question of: Are people going to purchase that coffee at a price point that makes it economically viable?”.