On a recent afternoon Dave Miller was literally minding his own business at his Lincoln Square restaurant, Baker Miller, when a customer implied his coffee was dirty.

It wasn't that she'd seen something floating in her cup. Instead, she'd recently heard about a new place called Limitless Coffee that sourced "washed" beans. She suggested that Miller switch to it because it was supposed to be "cleaner and healthier," he recalled her saying.

As a veteran coffee roaster and buyer, Miller says the claim struck him as "absolutely ridiculous." He knew that "washed coffee" (more than half the coffee on the market) simply refers to the wet-processing (as opposed to dry/natural-processing) method used to prepare coffee beans for separation from the fruit. It didn't mean that one coffee was clean and the other was dirty.

So Miller looked into Limitless Coffee, which started selling retail beans and cold brew last year and opened a pop-up cafe in Logan Square (2355 N. Milwaukee Ave.) in January. Its website says, "We exist to produce the purest coffee and tea on earth," and it throws plenty of shade on those who don't use the same methods.

It's one thing, Miller says, for a new roaster to talk about the quality of its own coffee, "but when you start comparing it to other people and their coffee and making health claims about things that don't add up, that concerns me."

So why would a new Chicago roaster base its marketing on implications that its competitors' naturally processed coffee is dirty?

"But it is," says Matt Matros, co-founder and brand director of Limitless Coffee. "I've been to the farms. I saw it."

Matros explains that in 2015, while he was on a yoga retreat in Bali, he took a tour of a coffee farm.

"And I'm thinking, 'Wow that is filthy.' I couldn't believe how dirty the coffee (harvesting) process is," he says.

Specifically, Matros was horrified by the "natural" processing, which allows coffee cherries to dry under the sun for weeks.

"They're fermenting and decomposing and dying and attracting bugs, birds and wild animals," he says. "I saw this with my own eyes. And then it might rain, and then the sun comes out, and you get mold."

Ting Shen / Chicago Tribune Dave Miller, co-owner of Baker Miller, 4610 N. Western Ave., says a customer suggested he use washed beans for his coffee, which caused him to look into the claims. Dave Miller, co-owner of Baker Miller, 4610 N. Western Ave., says a customer suggested he use washed beans for his coffee, which caused him to look into the claims. (Ting Shen / Chicago Tribune)

When Matros — founder of the Protein Bar restaurant chain — returned to the United States, he learned about wet-processed or "washed" coffee, and was impressed. It's a faster method — often used in areas with more access to water — that soaks the cherries before separating the bean from the pulp, and then air dries them for a shorter time. Depending on conditions, this can potentially reduce mold formation and resultant mycotoxins on the beans.

Fear of these mycotoxins — warranted or not — has freaked out some coffee drinkers in recent years and driven sales of the Upgraded coffee brand sold by Bulletproof coffee founder Dave Asprey. And that fear motivated Matros.

So at Limitless Coffee, whose roastery is at 316 N. Elizabeth St., the entrepreneur says he sources only wet-processed, "clean" beans.

"With that clean coffee, we have low toxicity, and I think it's that low toxicity that really drives performance and productivity," says Matros. "We want to help people own the day and really win at productivity."

Limitless Coffee also air roasts its beans, which Matros claims reduces "carcinogenic chemicals" that he believes occur with traditional drum roasting.

Antonio Perez / Chicago Tribune Matt Matros, co-founder and brand director of Limitless Coffee, says he sources only wet-processed, “clean” beans for the new company's roastery at 316 N. Elizabeth in Chicago. Matt Matros, co-founder and brand director of Limitless Coffee, says he sources only wet-processed, “clean” beans for the new company's roastery at 316 N. Elizabeth in Chicago. (Antonio Perez / Chicago Tribune)

But do these processes really create a "cleaner, healthier" coffee?

"I think a lot of this sounds like hyperbole and someone who is a little confused about the science," says Spencer Turer, vice president of Coffee Analysts, an independent coffee testing and consulting firm based in Vermont.

Dan Cox, president of Coffee Analysts, went further.

"I was offended by what I saw" on the Limitless Coffee site, says Cox, a certified coffee grader who's been in business for 35 years. "I don't think he knows what the (heck) he's talking about."

Specifically, Cox calls out Matros' blanket statements about the cleanliness of washed beans versus natural processed beans. When it comes to coffee quality, a number of factors come into play.

"You can have washed coffee, and it can be filthy," Cox says. "Or it can be very clean."

Cox further challenges Matros' assertion that drum-roasted coffee is full of charred contaminants like burning chaff.

"Almost all (unroasted) coffee is going to have silver skin or chaff," Cox says. "And during the roasting process, whether you're using a drum or air roaster, all roasters collect the chaff."

One of the main proponents of the theory that drum roasting creates carcinogens is the late inventor of the Sivetz air roaster Michael Sivetz.

As for mycotoxins, Cox notes that they can be present in coffee and in many plant-based foods. These include most grains, nuts, wine and beer.

But coffee is actually not a significant contributor, says Spanish toxicology researcher Ana Garcia Moraleja. In 2015, while at the University of Valencia, she co-authored a risk assessment of mycotoxin exposure from coffee in adults and adolescents.

"We found levels of exposure are significantly lower than toxicological (limits)" set by international health agencies, she wrote in an email interview. She further found that mycotoxin input from coffee was tiny compared with overall dietary consumption — which still wasn't very high.

For its part, the U.S. Food and Drug Administration says it routinely tests imported coffee for mycotoxins and can take enforcement action if any elevated levels are found.

When presented with some of this evidence, Matros says the real proof is in how his coffee makes people feel.

"At the end of the day, if you had two cups of coffee in front of you, would you want the one with toxins in it or not?" he asks. "Maybe some people don't care. I know some people who eat at McDonald's, but I don't want to."

Despite this comparison, it's worth noting that in 2012 the New England Journal of Medicine published a large, 13-year study showing that coffee drinking was associated with reduced mortality from a number of causes — and it didn't differentiate among people who drank wet-processed, natural-processed, drum-roasted or air-roasted beans.