Coffee made espresso makers contain more toxic, carcinogenic furans than regular drip machine brews or even instant coffee, according to research conducted at the University of Barcelona. However, all are still within safe levels.

Scientists led by Javier Santos, a professor at the Department of Analytical Chemistry at the University of Barcelona said in the journal Food Chemistry that higher concentrations of furans are found in espresso (43‐146 nanograms per mililitre) than in coffee made in drip coffee makers, with normal coffee getting higher readings (20‐78 ng/ml) than decaffeinated coffee (14‐65 ng/ml). Instant coffee registers even lower at 12-35 ng/ml but the big gorilla of the group are the coffee capsule products sold by several name brands, one of which read 117 - 244 ng/ml.

According to Santos, "The reason for these higher levels is due to the fact that hermetically-sealed capsules prevent furan, which is highly volatile, from being released, and the coffee makers used to brew this kind coffee use hot water at higher pressures, which leads to the compound being extracted into the drink." The longer that coffee is exposed to the air in cups or jugs, meanwhile, the more the furan evaporates. Fans of tepid coffee can rejoice.

However, even for the brands that produce the highest values, furan ingestion is still expected to be within safe levels. In order for furan levels to exceed the maximum acceptable values, a person would have to drink at least 20 cups of capsule coffee or 30 espressos per day, or 200 instant coffees.

Furans, are a group of carcinogenic substances that can form when foods and drinks are subject to heat treatment. So roasting coffee at a lower temperature for a longer period produces less furans than the traditional methods that take only 10 minutes, Santos said.