In 2014, Minnesota was one of the few markets where Aldi Inc. tested acceptance of credit cards. Last week, it started taking Visa, MasterCard, Discover and American Express cards nationwide.

Supermarket analysts say the move will help Aldi attract a broader customer base and appeal to consumers' changing preferences. Previously, the German-based, no-frills supermarket chain only accepted cash, select debit cards, SNAP and EBT cards.

"Customers were extremely thrilled with the new payment option," Matt Lilla, division vice president of Aldi in Faribault, said of the test program in Minnesota. "Every month since we instituted the program, credit card usage has increased."

Supermarket analyst David Livingston said he was surprised by Aldi's decision. "Credit cards are pretty costly," he said. "There's usually a 50-cent swipe fee per transaction and a percentage." Even Costco, which operates on slim profit margins as Aldi does, accepts only American Express. Livingston assumes that Aldi may have struck some sort of deal, just as Costco did with American Express (and Citibank Visa, which Costco will switch to later this year).

Analysts worry that Aldi's prices may become less competitive with credit card fees. The company stated that the acceptance of credit cards will have no impact on the price customers pay. "Our customers will continue to save money on the high-quality groceries they buy the most," Jason Hart, chief executive of Aldi's U.S. operation, said in a statement.

Lilla said Aldi negotiated for the lowest prices it could, having an advantage with stores all over the world. Stores in England, Wales and Scotland now accept credit cards.

In other examples of how Aldi is trying to broaden its appeal, it has removed monosodium glutamate, or MSG, synthetic colors and partly hydrogenated oils from Aldi-brand foods, which account for about 90 percent of its goods. The organics selection continues to grow, with more than a dozen items in produce as well as cheeses and grass-fed ground beef. Organic chicken is being tested in other states.

Store design has also been refreshed. New stores in Blaine, Medina, Eden Prairie and Shakopee sport higher ceilings, LED lighting, more windows, streamlined displays and more square footage devoted to fresh produce and meat.

Aldi opened its first U.S. store in 1976 and its first Twin Cities stores in 2004. It now operates 1,500 locations in 32 states. A new store in Cottage Grove will open at the end of April, with five to seven more Twin Cities locations under development, Lilla said. It plans to have 40 stores open in California by the end of the year and expects to have 2,000 stores by 2020.

Its small stores stock only about 1,400 items compared to 40,000 at larger stores. Its prices are 15 to 20 percent lower than Wal-Mart with minimal customer service and amenities. Aldi's customers pay a refundable quarter to "rent" a cart, pay extra for bags to carry their purchases, and often wait in line because of fewer cashiers.

John Ewoldt • 612-673-7633