After more than a year in beta, Amazon opened their cashier-less grocery store to the public

More than 20 percent of adults who use their smartphones to shop will use an app to order groceries by 2019, according to a forecast.

Apps that deliver perishable food items — including meal kits — are rapidly increasing in popularity. The eMarketer forecast predicts that the number of U.S. adults who order groceries at least once a month with an app will increase by 49.6 percent to 18 million adults this year compared to last year.

Amazon's acquisition of Whole Foods has pushed other grocery chains to step up their grocery delivery efforts. Walmart is expanding its grocery delivery to 100 cities by the end of 2018. Regional chain Kroger is investing in initiatives to make delivery more efficient.

Retailers are also trying to make customers more comfortable with ordering produce and meat online. Apps that let customers order online and then pick up their groceries in the store have helped build shoppers' trust, according to eMarketer senior analyst Patricia Orsini.

"Shoppers are becoming more comfortable with ordering online in general, and grocery is a part of that," Orsini said in the forecast's release. "A key hurdle, traditionally, for ordering fresh produce and other perishable items online has been delivery time, and the desire to hand-select produce and meat."

Even at its current growth rate, food and beverage retail sales will make up only 2.8 percent of all U.S. e-commerce sales in 2018 at $14.94 billion.