The creators of Starbucks' famous PSL poured espresso shots over pumpkin pies to nail the flavor

• Peter Dukes, known as the "father of the PSL," spoke to Insider about its origin story.
• Dukes and his Starbucks team poured espresso shots on pumpkin pies to figure out the flavor.

How did Starbucks come up with the flavor for its iconic Pumpkin Spice Latte? With a lot of pumpkin pies and espresso shots.

Starbucks is celebrating the famous fall drink's 20th anniversary, so Insider sat down with Peter Dukes, the "father of the PSL," to learn more about its origin story.

Dukes told Insider that, early in 2003, he and his team got together at the "Liquid Lab" — the former name of Starbucks' research and development space at its Seattle headquarters — and came up with 100 different ideas for the company's first-ever fall drink. One of those ideas was a pumpkin latte.

The new flavor seemed like a big risk, but Starbucks wanted to give it a chance — which kicked off plenty of taste tests.

Pumpkin products weren't common 20 years ago, so Dukes and his team had to get creative.

"The idea of pumpkin in a beverage did not exist," Dukes said. "So we started pouring espresso shots on the pumpkin pies and tasting it, getting a sense of what those flavors tasted like when you combine the pumpkin pie and espresso."

"And we found — wow, that tastes pretty good," he added.

Dukes said some of his coworkers would bring pumpkin pies from their local grocery stores, while others would bake their own pumpkin pies so they could experiment with different flavors.

"Our team would come together multiple times a week, tasting different iterations of the PSL and working to get it optimized to the recipe that exists today," Dukes recalled. "We knew we had something that was unique."

Starbucks' Pumpkin Spice Latte — which is now available hot, iced, or blended — features the coffee giant's signature espresso combined with steamed milk, real pumpkin purée, cinnamon, nutmeg, and clove.

The recipe hasn't changed in more than a decade, and Dukes believes it will stay that way.

"I think we got a good thing," he said. "I think we might have a revolt on our hands if we changed the recipe."

The PSL has since taken on a life of its own and become an integral part of fall for people around the world — and Dukes thinks nostalgia is a big part of it.

"Fall is a unique time period when it comes to the weather changing and those crisp, cool autumn days," Dukes said. "Kids are heading back to school, families are getting back in the routine of things, and there's the holidays and the flavors that come with the holidays."

"Pumpkin spice encapsulates all of that in a cup," he added. "It helps to capture that and evoke those warm, comfortable feelings when you take those first few sips. It's cozying up to a warm fire with your family and friends."