Heads up to anyone who has attended the Kentucky Derby in the last 18 years. Guess what? That Mint Julep you've likely guzzled, it wasn't a real Mint Julep.

Tomorrow marks an important break from tradition for the Derby. After nearly two decades, Old Forester bourbon will replace Early Times Kentucky whisky as the event’s official spirit. Meaning, for the first time in a long while Derby spectators will sip Mint Juleps made with actual bourbon, as called for in the classic cocktail's traditional recipe.

... legend has it that track founder Meriwether Lewis Clark, Jr., grew fresh mint for Juleps outside the clubhouse as early as 1875 ...

Early Times isn’t officially bourbon. One of the mandates for bourbon to be classified as bourbon is that the whiskey must be aged in new charred oak barrels. Early Times rests in used barrels and is therefore labeled a Kentucky whiskey.

Both Early Times and Old Forester are produced by Kentucky's Brown-Forman Distillery, and before the liquor-maker's involvement in the race, Derby Julep's were made to order and guests could specify their choice booze in the drink.

So what made Brown-Forman switch from a longstanding tradition? After all, Old Forester has been around since before the Derby. It was actually created by Brown-Forman founder George Garvin Brown in 1870, five years before the first Kentucky Derby, making it America’s first bottled bourbon.

Turns out this is the year the company has decided to focus its efforts and $65 million on establishing the whiskey as Louisville’s hometown bourbon. So, in addition to building a new Old Forester distillery in downtown Louisville’s Whiskey Row, which opens in the fall of 2016, Brown-Forman decided to align the spirit with the state's biggest annual event.

It took the Mint Julep, a staple of the South since its creation in the 18th century, a long while to settle in as the official Derby drink. Even though legend has it that track founder Meriwether Lewis Clark, Jr. grew fresh mint for Juleps outside the clubhouse as early as 1875, the Mint Julep wasn't christened the official race cocktail until 1939—64 years after the first Derby race.

The Julep became synonymous with the Derby thanks to Matt Winn, who was part of Churchill Downs' leadership group from 1902 to 1949 and a savvy marketer. "His team was responsible for creating and promoting the Kentucky Derby Gold Cup, the Garland of Roses, the 'My Old Kentucky Home' song and even the Mint Julep glasses," explains Derby Museum curator Chris Goodlett.

It’s estimated that 120,000 juleps are served during the two-day event ...

The Kentucky Derby Mint Julep not only looks different from the traditional cocktail (in 1939 Derby organizers decided to serve Juleps in commemorative cups to market the drink), but the careful ritual of muddled sugar cubes and bitters, bruised mint, crushed ice, and bourbon is streamlined considerably for the race. Organizers estimated that 120,000 juleps are served during the two-day event which attracts about 160,000 spectators. Like last year and the 17 years before, Derby bartenders will build drinks from a pre-made mix, fresh mint and crushed ice, but this year's "mix" involves Kentucky straight bourbon in place of Kentucky whiskey, plus a different species of mint and an amended sugar ratio.

Will regular Derby goers notice a difference between the Old Forester Mint Julep, which sells for $11, and the previous Early Times version? Yes, according to Early Times and Old Forester brand manager Therese McGuire. "Early Times is a heavy mint candy, sweeter finish. You have some light chocolate flavors coming out in the Early Times. Old Forester is like fresh spearmint with muddled mint leaves on top of bourbon. It certainly has a distinct bourbon taste."