DuBois left Coca-Cola in 2011, after two decades of working for the company’s global research and development team. “I was feeding my ideas into this supersecret group responsible for flavor chemistry,” he said when we met for breakfast in the suburbs of Atlanta. Now 67, DuBois runs a sweetener consultancy from his Georgia home, and he seemed most at ease while making presentations. The one he’d prepped for me described how hard it is to find a sweetener that works and why the best natural product that we have is a compromise.

A new product should be considered for development, he explained, only if it meets nine specific requirements. It’s not enough to find a chemical that’s sweet; it also has to sweeten foods in the ways that matter most, to the right degree, without an aftertaste, without a linger and without diminishing effect. And even one that has the taste of granulated sugar would be deficient if it weren’t also stable in solution, resilient to hot and cold, safe to drink, cheap to make and amenable to patenting. An “all-natural” sweetener must meet these nine metrics for success, and then a 10th: It must be taken from a living thing.

That last requirement left DuBois with little room to work. In all, there are about a hundred natural, noncaloric chemicals that, by pure coincidence or some unlikely quirk of evolution, have found a way to hijack the mouth’s machinery for sensing sugar. One category comes from animals. Lysozyme, a chemical found in tears and spit, and also in the whites of eggs, can be very sweet. (In hens and turkeys, it’s sugary but pungent; in soft-shelled turtle eggs, it tastes more like licorice.) The rest derive from vegetation, including: one compound drawn from crushed hydrangea leaves, used in Japan for sweet tea; another from a Malaysian plant called lemba, with tiny yellow flowers and fruits that look like cloves of garlic; the seeds of a swollen caper berry in Yunnan, China, which locals chew as candy; and monk fruit, a cousin to the cucumber and the bitter melon, which grows in Guangxi.

Most of these, DuBois knew, would never stand a chance. He pushed aside his knife and fork and tilted the laptop’s screen to show me why. For the natural-sweetener project, he started with the first and most important of his tests, shown on his display as “Taste Quality Metric 1: Maximal Response.” Were any of the natural compounds strong enough to sweeten Diet Coke? At first this seemed an easy obstacle to overcome: Lysozymes are at least 10 times as sweet as sugar; monk-fruit extract is 20 times as strong as lysozyme; and the sweetener drawn from the lemba plant is 10 times more intense than monk fruit. But those numbers correspond only to tiny doses of the chemicals, enough to match the strength of, say, a teaspoon of sugar mixed into a cup of water — a 2 percent solution. To make a cup of Coca-Cola, you’d need much more: about six teaspoons’ worth of sugar, for a 10.4 percent solution. (Pepsi is a little sweeter, at 11 percent. Root beers and some fruit-flavored sodas can be 12 percent or more.) That’s where many substitutes fall short.

DuBois had a set of graphs tracking how the power of a sweetener changes with its concentration. He included curves for six different compounds, from saccharin to stevia, but they all looked very much the same. Each curve rose steeply, gaining sweetness with every increment in milligrams per liter, then appeared to hit a ceiling, a point at which the sweetness flattened out. Once you reach that threshold concentration, a compound loses its effect: No matter how much more of it you pump into a beverage, you’ll never get a sweeter taste. The ceilings for some chemicals are high enough to flavor carbonated drinks. Aspartame, for example, can match the taste of sugar in a 16 percent solution. But others reach their limit much too soon. That’s why today you’ll never find a Diet Coke that’s made with saccharin. At best it would match the sweetness of a sugar drink at 10.1 percent.

He gestured at the curve for stevia, which didn’t seem much better than the one for saccharin. “That looks like a death sentence,” he said. But when I asked him to name his favorite noncaloric sweetener, DuBois demurred and cleared his throat. “None of them,” he said, “tastes like sugar.”

We clicked ahead to Metric 2 — “Flavor Profile.” Most products have at least a hint of bitterness or licorice; some have a metallic note or even menthol cooling. Metric 3 considers how the compound’s taste develops on the tongue. Sip a soda made with sugar, and the taste should reach its peak of sweetness in four seconds, then fall off 10 seconds later. Zero-calorie substitutes tend to lag behind: They come on too slowly, and then — much worse — they stay too long, clinging to your mouth in a disconcerting glaze. Even aspartame takes an extra second to hit its sweetness high and hangs around an extra four before it goes away.