* Caution: Spoilers ahead.

For millions of devoted fans who watched the highly anticipated series finale of "Breaking Bad" on AMC Sunday night, the show offered a satisfying farewell to the story of chemistry-teacher-turned-meth-kingpin Walter White.

Not everything worked out how White originally planned, but he did manage to tie up lots of loose ends. And for one of his biggest enemies, revenge was especially sweet.

Throughout the show, White used his science smarts to solve problems and settle scores. And in one clever scene from Sunday night's finale, he capitalized on a routine-oriented drug smuggler's predictability by replacing stevia with ricin at a diner she frequented. Here's an animated GIF of Lydia Rodarte-Quayle "sweetening" her cup of comeuppance:

Ricin is a powerful poison derived from Castor beans, and it has played a pivotal role in several episodes of "Breaking Bad." But the prominence of stevia in White's switcheroo has left the Internet buzzing about the sweetener. What is stevia, exactly?

It comes from a South American plant called Stevia rebaudiana, whose sweet leaves have been used by humans for hundreds of years. Native Paraguayans introduced stevia to early European settlers via herbal and medicinal teas, and in 1931 a French chemist isolated the eight glycosides that produce its sweet taste. The most prevalent of these compounds, stevioside, can be up to 300 times sweeter than a 0.4 percent table sugar solution, according to the University of Nebraska–Lincoln Extension Service.

Japan began cultivating stevia in the 1970s as an alternative to sugar and artificial sweeteners, eventually paving the way for today's burgeoning global industry. Stevia now represents about 40 percent of the Japanese sweetener market, making Japan its No. 1 consumer, but the substance is increasingly popular around the world — and not just among high-level international drug smugglers.

Consumers wary of artificial foods and dental cavities are increasingly drawn to stevia's natural sweetness, often using lemon or ginger to mask its strong aftertaste. Based on data from the World Health Organization, California-based Stevia First Corp. has projected its namesake could one day command a third of the $58 billion global sweetener market. The U.S. Food and Drug Administration doesn't allow stevia to be sold as a food sweetener because animal studies have linked its overuse to health problems, but since certain extracts are considered safe, they're widely sold as dietary supplements.

Stevia's cameo on Sunday's "Breaking Bad" finale gave its already-rising profile a quick pop-culture boost, but some fans of the sweetener were left wondering whether its association with a fictional revenge murder — as well as a far less innocuous plant-based substance like ricin — would help or hurt its public image:

The acclaimed run of "Breaking Bad" may be over, but at least it didn't leave its fans with a sour taste. And if you're still hungry for more of the sly, sciencey screenwriting that helped make the show a hit — possibly including chemistry-centric cameos like ricin, stevia and Lily of the Valley — watch for AMC's "Better Call Saul," a spinoff series that may sweeten Sunday's ending by letting us linger in the "Breaking Bad" universe a little longer.

Related sweetener stories on MNN:

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.

What is stevia?
The sweetener is enjoying a pop-culture bounce thanks to its role in the finale of 'Breaking Bad,' but what exactly is Stevia? Here's an explainer.