Fixation. Fetish. Call it what you will, there’s something percolating in the fast-food liquid zeitgeist with McDonald’s Diet Coke.
Since its debut in the early 1980s, Diet Coke has enjoyed a large following, with some fans guzzling gallons each day, for better or for worse. Recently, however, a very specific form of this beloved low-calorie beverage has taken over social media, namely that vended by McDonald’s soda fountains. TikTokers are uniting over its ability to create a uniquely “peaceful” Diet Coke moment, with many claiming Diet Coke is a necessary part of their self-care routines.
For some, this fascination can rapidly devolve into a dangerous obsession.
“It started innocent,” explained Doug E. Fresca, of Dover-Foxcroft, Maine, who notes the irony in his name. “A quick large Diet Coke from McDonald’s on my lunch break. … That golden ratio of syrup to carbonation to slightly-too-much-ice. But then … I spiraled. At my lowest, I pulled up to the window, handed them dirty bills I begged for on the street, [and] ashamed, I whispered: ‘Make it extra cold. You know what I need.’”
Public relations executive Dutch Small of Mexico City recounts similar chronic struggles (victories?): “Let’s just say if McDonald’s ever starts a loyalty program based on Diet Coke consumption, I’ll be their first platinum member. I don’t drink it every day … just every day that ends in ‘y.’ It’s the life-revival infusion you can’t live without, like the deserts miss the rain, especially first thing in the morning with a McMuffin.”
Small waxes poetic even further: “Bottled Diet Coke is like watching fireworks on TV. McDonald’s Diet Coke is the live show.”
Just what makes Mickey D’s Diet Coke so special? Some assert it positively differs from canned or bottled (glass or plastic) variants because it’s “spicy,” while others cite “crispy” as its hallmark virtue. Regardless, the verdict is in: Diet Coke from McDonald’s is the GOAT.
But why? The thirsty hoi polloi have their own theories, some of which are more rigorously researched than others. “I say it’s sorcery. You could hand me a blind taste test with a can, a bottle, and a McDonald’s cup and I’d pick the McDiet Coke every time. It’s like they hacked the soda matrix,” adds superfan Small.
“Bottled Diet Coke is like watching fireworks on TV. McDonald’s Diet Coke is the live show.”
- Dutch Small
Magical origins aside, I wondered if there was more traditional scholarly support for why McDonald’s Diet Coke is objectively better than others? Cursory internet research reveals that several key factors play into how McDonald’s Diet Coke distinguishes itself, including but not limited to the composition, a top-notch filtration system and temperature control.
When it comes quality, the nuances are in the nitty-gritty. To delve into the finer points of the McDonald’s Diet Coke popularity (and the reasons for this phenomenon) I consulted with scientists, engineers, current and former McDonald’s employees, and one of the world’s leading experts on carbonation, Brown University professor Roberto Zenit.
The conclusion? Every step of the way, McDonald’s goes the extra mile to ensure the best Diet Coke experience. Fundamentally, it comes down to a superior solute (syrup) to solvent (water) mixture to yield the solution (har) that is Diet Coke.
For this optimal solution, a premium solvent is key: McDonald’s uses filtered, pre-chilled water. This latter detail is key because, Zenit explains, the behavior of bubbles in carbonated drinks is determined by a physicochemical principle known as Henry’s Law. According to Henry’s Law, the solubility of a gas (in this case, CO2) is directly proportional to its partial pressure and inversely proportional to the temperature of the liquid solvent (the water). Therefore, the colder the water, the more easily the carbon dioxide dissolves in it and maintains its gaseous state.
Having established a long-lasting fizzy solvent, next comes the solute in the form of the diet cola syrup. Unlike most other establishments that get their cola syrup in plastic bags, McDonald’s mandates receipt in stainless steel tanks, which keeps it fresher and better-tasting by shielding it from the elements (sunlight, air, oxygen, etc.). But it’s also the quantity as well as quality of the syrup; McDonald’s deliberately uses a higher syrup-to-carbonated water ratio than standard Diet Coke products to compensate for dilution due to melting ice (which is why ordering your Diet Coke straight will give you the most concentrated flavor).
Finally, there’s the consumption delivery system of this prime solution, aka the “straw.” McDonald’s itself has publicly disclosed (bragged, really) that its straws are wider than the standard version (by an estimated 0.02 inches), and this marginal increase can make a major difference in your perceived satisfaction level while imbibing.
But why is that sizzling Diet Coke so satisfying? “Why do we like bubbly drinks?” Zenit wrote in a paper, “The fluid mechanics of bubbly drinks.” He went on to write, “A scientific answer has proven elusive. Carbonation, it turns out, triggers the same pain receptors in the deep brain that are activated by tasting spicy food.” Perhaps even more fascinating is that while we humans like the “burn,” other creatures great and small do not. “Curiously,” notes Zenit, “when carbonated water is fed to other animals, such as mice, dogs and horses, the animals refuse to drink it. But humans appear to enjoy the mildly irritating effects.”
Not all past and present McDonald’s employees with whom I spoke were familiar with the nuances of their employer’s Diet Coke dispensation system, but virtually all were cognizant of the effects of its absence. When the soda machine at the Dover-Foxcroft, Maine, location was on the fritz, workers were forced to offer customers canned sodas. “People were not lovin’ it,” recounts former employee Shauna Walsh. “They really prefer the fountain.”
Although Diet Coke has been voted to be par excellence by an unofficial jury of pop peers, a recent corporate announcement concerning dispensation may threaten its popularity, since many of its followers say being able to fill (and refill) their own giant tumblers is part of their Diet Coke infatuation.
In 2023, McDonald’s announced it was phasing out its self-serve drink stations in favor of automated “crew pours” for sanitary and hygiene reasons. The transition is set to be complete by 2032 — which means if a large part of your McDonald’s Diet Coke fixation is contingent on serving yourself that crispy, spicy elixir, you have seven years left in soda Shangri-la.