As brewmaster at Anheuser-Busch InBev's pilot brewery in St. Louis, the 29-year-old chemical engineer experiments with new beers in a scaled-down replica of the main brewery next door. Each batch of beer is just 10 barrels, barely a drop in the bucket for a company that ships 100 million barrels in the U.S. each year. Almost all of the 500 recipes she and her team brew each year never make it out of the building.

But Ms. Reid's efforts are increasingly critical for Anheuser-Busch, which has watched a growing number of Americans drop its giant domestic brands such as Budweiser, Bud Light and Michelob in favor of small "craft'' beers sprouting across the country. In recent years, other types of alcohol such as liquor and cider also have swiped restless U.S. beer drinkers. Anheuser-Busch's beer shipments in the U.S. rose 0.6% last year but its share of the U.S. beer market dipped to 46.3% from 46.8%, estimates Beer Marketer's Insights, a trade publication and data provider

Ms. Reid recently helped reverse Bud Light's slide by creating the recipe for Bud Light Platinum after the company ordered up a sweeter variation of America's top-selling beer that's higher in alcohol but still goes down smoothly and isn't heavy in calories. Such a request wasn't straightforward: More alcohol typically produces a harsher taste and more calories.

The Indiana native began playing with the formula in late 2010, experimenting with several batches over a year and tinkering with brewing times, temperatures and ingredients. Making beer can take weeks as barley and sometimes other grains are converted into malt, sugar and alcohol through heating, cooling and filtering tanks while being combined with water, hops and yeast.

For Platinum, she overhauled the mix of grains, settling on a combination that lent a caramel-like note. She also altered the time the malt sat in the mash cooker; the longer it sits, the lighter and smoother the beer becomes. And she went low on the hops, which produce bitterness, to bring out more sweetness, among other things.

The new beer was rolled out nationally in early 2012 and quickly captured more than 1% market share, making it one of the most successful launches by a big brewer in years.

Ms. Reid plays with the mix of grains, such as malted barley, for each batch of beer, and keeps ingredients like rice, rye, juniper, nutmeg, ginger and orange peel on hand to fine-tune the flavor.

Ms. Reid is trying to stretch the boundaries of what typically ends up in a beer. Recently she mixed hibiscus flowers with wheat and lemon peels to approximate the taste of a "strawberry lemonade'' beer. That idea came from a baking recipe for chocolate chili-powder cupcakes with hibiscus frosting. She said the beer batch tasted pretty good, but not good enough, and she plans to keep tinkering.

Other botanicals that she wants to experiment with in the coming months include thyme and rosemary. "If they go well in food, they should be good in beer,'' she says.

Men are still the biggest beer drinkers, and women brewers remain rare. Teri Fahrendorf, a pioneer female brewmaster in the 1990s and founder of the Pink Boots Society for women in the beer industry, estimates 99% of brewmasters are men. The Siebel Institute, a brewing academy, says only about 5% of its students are women. As an African-American, Ms. Reid is even more atypical.

Unlike Pete Kraemer, a fifth-generation brewmaster who heads Anheuser-Busch's North American brewing operations, Ms. Reid says there was very little beer in her house growing up. But her science background is common at big breweries, where many brewmasters have degrees in chemistry or biology.

She spent two summers at Eastman Chemical, working on chemicals used in plastics and gas-collection systems, before graduating from Purdue University in 2005 and joining Anheuser-Busch. "I always liked beer, but I honestly didn't know a lot about it when I got here," she says. She learned quickly: She began as a production supervisor at the main brewery before switching to the innovation group in 2010, then becoming head of the pilot brewery last year.

Last year Anheuser-Busch began selling gluten-free Michelob Ultra Light Cider, which Ms. Reid helped develop in the pilot brewery. Because cider is made from fermented apples instead of grains it is not technically beer, but brewers are increasingly plunging in because cider sales are growing rapidly.

Ms. Reid and her dozen assistants also recently experimented with several Christmas beers, brewing a peppermint stout as well as gingerbread, raisin and apple-pie ales that might one day make their way to store shelves.