Coffee gets studied from the inside out and sold from the outside in, and this week's news covered both ends. One research team tracked what a daily habit does to the gut. A giant long-running health study asked what it might do for the brain. Producers keep stretching how long they let beans ferment before they ever reach a roaster. And a drive-thru chain is finding out what happens when a coffee shop simply stays open later. Four stories, one cup.
The common thread is that the cup is rarely as simple as it looks. Each of these stories comes with caveats about what the data can and cannot prove, and those caveats are the useful part.
A Danish Capsule Study Maps How Coffee Drinkers' Guts Differ
Researchers at the University of Copenhagen set out to answer a question most coffee drinkers have asked after a second cup: why does it make the gut move? Their small study, published September 15 in the journal Gut Microbiology, followed 61 healthy Danish adults for nine days and compared regular coffee drinkers with people who drank it less often.
The team tracked what participants ate and drank and analyzed urine for caffeine and its breakdown products. Fifty of the 61 also swallowed a wireless capsule that measured acidity, pressure and movement as it traveled through the digestive tract. Frequent drinkers were defined as people who had coffee on more than three of the eight recorded days, at between one and five cups a day. About 48 of the 61 met that bar.
What showed up in the stool and in the gut
Frequent drinkers had greater microbial richness in their stool samples than people who rarely drank coffee. Three groups of bacteria, including Lawsonibacter and Slackia, were more abundant among them. The Lawsonibacter result echoes earlier work: a much larger 2024 analysis of more than 20,000 metagenomic samples had already tied coffee intake to that microbe and reported a median abundance 4.5 to 8 times higher in coffee drinkers than in non-drinkers, with the link holding for decaf as well.
The Copenhagen capsule data added something those earlier studies could not. Higher levels of caffeine byproducts in urine were associated with more contractions in the colon, and the small intestine was less acidic in frequent drinkers than in rare ones. "But perhaps more importantly, frequent coffee consumption appears to affect the physiology of the gut itself, in other words, how the gut functions," said first author Francesca Bucci, a Ph.D. student in the university's Department of Nutrition, Exercise and Sports.
Why the researchers are cautious about the pH finding
Contractions are the easier result to interpret. "Contractions in the gut can support healthy digestion, help prevent constipation and keep the intestinal muscles in good condition," Bucci said. The lower pH is murkier. The researchers say they cannot yet determine whether it is beneficial or harmful, though acidity in the small intestine matters for how digestive enzymes work and how food gets broken down.
They also stress that a study like this cannot establish that coffee alone is responsible for the differences. Senior author Henrik Munch Roager, an associate professor, still thinks coffee is probably doing real work. "We already know from other studies that coffee can influence a range of processes in the digestive system, including acid secretion in the stomach," he said. "Anecdotally, many people also report that coffee affects their digestion."
The natural next step, Roager said, would be an intervention study in which researchers control how much coffee participants drink and compare gut physiology during high-coffee periods with periods without it. The group hopes the work could eventually help people who struggle with constipation by understanding how to stimulate gut motility naturally. For now it is a snapshot of 61 people, and a reminder that the effects of a daily habit are measurable well beyond the bloodstream.
Two to Three Cups and Dementia: What 43 Years of Data Show
The dementia story is not brand new. The JAMA paper behind it was published in February, led by investigators at Mass General Brigham, the Harvard T.H. Chan School of Public Health and the Broad Institute of MIT and Harvard. It resurfaced this week in new coverage, and it remains one of the larger looks at coffee and cognition.
The researchers drew on two long-running U.S. cohorts, the Nurses' Health Study and the Health Professionals Follow-Up Study. Of 131,821 participants followed for up to 43 years, 11,033 developed dementia. Diet questionnaires, completed every two to four years, captured caffeinated coffee, decaffeinated coffee and tea, and the team also assessed subjective cognitive decline and, in a subset, objective cognitive performance.
The numbers behind the 18 percent
Participants with the highest caffeinated coffee intake had an 18 percent lower risk of dementia than those who reported little or none. After adjustment, dementia incidence was 141 cases per 100,000 person-years in the highest-intake group against 330 in the lowest. Subjective cognitive decline was reported by 7.8 percent of the top group and 9.5 percent of the bottom group.
The relationship was not a straight line. The strongest associations showed up at roughly two to three cups of caffeinated coffee a day, or one to two cups of tea, and drinking more did not appear to add benefit. Decaf told a different story. Decaffeinated coffee was not significantly associated with lower dementia risk or better cognitive performance, and the associations did not differ by APOE4 genotype or polygenic risk for Alzheimer's disease.
How the authors frame it
Lead author Yu Zhang called the work "the best evidence we have so far on the association of caffeine and cognitive health," and the decaf result surprised her. "We didn't observe any association between decaf coffee and dementia," she said, adding that caffeine "seems to be the primary neuroprotective compound" but "we cannot say definitely." Coffee and tea also contain polyphenols, which scientists think may contribute as well.
Senior author Daniel Wang was blunt about scale. "While our results are encouraging, it's important to remember that the effect size is small and there are lots of important ways to protect cognitive function as we age," he said. The study is observational, so it cannot show that caffeine prevents dementia, and lifestyle differences such as diet and exercise could explain part of the gap. It is a finding about patterns across a large population, not a prescription for any individual.
Fermentation Clocks Keep Running Longer on the Farm
Coffee processing has a numbers problem. Fermentation that once meant a night in a tank now stretches to days and sometimes weeks, and a new Perfect Daily Grind feature, titled "96 to 740 hours: The push for longer coffee fermentation," looks at why producers keep pushing the clock. According to summaries of the piece, extended fermentation is becoming more common in specialty coffee, but limited research suggests it does not reliably improve cup scores, and research into fermentation beyond 96 hours remains thin. The same summaries say hour counts are not directly comparable from lot to lot, because headline numbers can hide variables such as temperature control.
Where the 740-hour figure comes from
The top of that range has a documented example. Barista Hustle has described a Colombian coffee fermented for a total of 740 hours, about a month. Per that account, whole cherries were fermented in GrainPro bags for 36 hours, then dried in the sun for four days, then returned to the bags for a final fermentation of 29 days. A drying break in the middle is a reminder that a single hours figure can describe very different processes.
Barista Hustle also quoted fermentation specialist Lucia Solis arguing that duration alone says little. In her view, whether the coffee is pulped or in cherry, hot or cool, oxygen-rich or oxygen-free, submerged or dry, matters more than the clock, and a coffee is not better simply because it fermented longer.
What a Colombian research station found past 96 hours
A September 2025 study from Cenicafe, Colombia's national coffee research center, put numbers on that doubt. Researchers fermented Castillo-variety arabica, either as whole fruit or as pulped beans, for 192 hours at its Naranjal Experimental Station in Caldas, at about 1,381 meters. They compared an open tank with a sealed tank where microbes use up the oxygen themselves, sampled the bioreactors every 24 hours at a controlled 20 degrees Celsius, and tracked chemistry, bean health and sensory quality.
The authors note that most earlier work stopped at about 96 hours, which is where the push described in the Perfect Daily Grind headline begins. Their combined quality index put shorter fermentation times and the sealed-tank condition closest to the best outcome of 1.0. Longer runs in the open tank scored 0.497 for pulped coffee and 0.369 for whole fruit. They also write that overly long fermentation is assumed to bring off-flavors or astringency, particularly if the process is not closely monitored. That is one study of one variety under one set of conditions, and it says nothing about every producer chasing a distinctive profile. It does show why buyers who see a big number on a bag are advised to ask about temperature, oxygen and drying, not hours alone.
For the drinker, the practical takeaway is modest. A long fermentation number on a label signals a deliberate, experimental process, and some of those coffees taste remarkable. It is not a guarantee of quality, and the science on whether longer is better has not caught up with the marketing.
Black Rock Coffee Bar Bets on Staying Open Later
On the retail side, Black Rock Coffee Bar reported third-quarter results on November 10 that put a number on a simple idea: more hours, more customers. Revenue rose 29.7 percent to $66.8 million, helped by 11 new stores. Same-store sales increased 5.8 percent, up from 4.2 percent in the second quarter, and 16.6 percent on a two-year basis, the company's 15th consecutive quarter of positive same-store sales growth.
Traffic comes back, and the hours help
Transactions at comparable stores grew 2.4 percent after declines in the first half of the year, when the company was comparing against a prior-year loyalty program rollout. Chief Financial Officer Rodd Booth said pricing contributed 2.4 percentage points to same-store growth and check size contributed 1 point.
Black Rock began extending hours around the end of June and beginning of July. CEO Mark Davis said about 70 percent of stores now run the longer schedule, and the company has decided to make it permanent. He called the added hours profitable and said they have contributed to comparable sales and transaction growth, though he did not quantify the benefit. He also said later hours attract a younger customer and that food has benefited from the later trading period. New stores are opening with the extended schedule. In the company's words on the call, "These early results reinforce our belief that staying open later can support increasing average unit volumes over time, and driving incremental same store sales."
The cost side of a longer day
Late hours are not free. Labor rose 110 basis points to 22.1 percent of revenue, which Booth attributed partly to new store openings and the longer afternoon and evening hours. Store-level profit rose 26.5 percent to $19.3 million at a 28.9 percent margin, while adjusted EBITDA grew 22.1 percent to about $8.5 million, slower than revenue. Davis cited public-company costs, new store investment and labor for extended hours among the reasons.
The chain ended the quarter with 211 stores, raised its 2026 opening target to 40 from 38, and kept its goal of 1,000 stores by 2035. Food made up 14.2 percent of the third-quarter mix, and the company said this year's store class is costing about $800,000 per store against roughly $650,000 last year. Management expects the fourth quarter to be the strongest of the year for same-store sales and transactions, and it held full-year guidance of $255 million to $257 million in revenue. The coffee itself has not changed. The hours around it have, and so far the customers are showing up.
What These Four Stories Share
Read together, the stories describe coffee as a habit with measurable footprints: in the microbes of 61 Danes, in four decades of questionnaires, in fermentation tanks in Colombia and in the closing time on a drive-thru sign. Two of them are observational findings that stop short of cause and effect. One is a trade debate where the evidence has not settled. One is a company's own account of its results. None of it is a reason to change how much coffee anyone drinks, but all of it is a reason to be a little more curious about what is in the cup and how it got there.
