A sales lady drives an electric Yakult cart to sell yogurts and other beverages for Korea Yakult Co. (Photos: Korea Yakult)

A fleet of bathtub-size four-wheelers started hitting the streets in major cities in South Korea in 2015 to better transport the little bottles of the yellow-colored yogurt drinks South Koreans have long consumed to aid their health.

Yakuruto, or yakult, is a popular after-meal, probiotic milk-like product which is sweet in taste and enjoyed by Koreans at restaurants or at home. It is made by fermenting a mixture of skimmed milk with a special strain of the bacteria which helps in food digestion.

It was originally created by a Japanese scientist and marketed in different sizes worldwide to nearly 30 nations.

In the Americas (including Mexico, one of yakult's largest selling markets), South Korea, Japan, Philippines, and Thailand, 80 ml bottles are available. It is also available in Singapore, Hong Kong, Taiwan and mainland China where it comes in 100 ml bottles.

Yakuruto is made by Yakult Corporation which is one of the largest food companies in South Korea and produces beverages, ramen noodles, and dairy products.

Korea Yakult Co., Ltd is a food company based in Seocho-gu, Seoul; The Korean joint venture between Yoon Deok-byeong, a Korean investor and Japanese Yakult Co. established in 1969.

At the helms are thousands of delivery ladies, as they are known, who for decades pulled carts, hauling heavy ice boxes filled with the drinks to bring them to customers’ doorsteps.

For these women, the unfamiliar vehicles are a technological revolution so bold as to be awkward at times.

“I felt like a monkey at first because people gave me odd looks,” said Lee Chang-sim, an 11-year yogurt-delivery veteran.

Whipping up a yogurtmobile wasn’t a smooth process, even in a place where innovation is part of the culture. Manufacturers were baffled. Officials couldn’t figure out how to classify them. Some of the delivery women didn’t know how to drive.

“It seemed like such a preposterous request at first,” said Lee Young-tae, an executive at DaeChang Motors Co., one of four companies that signed up to develop and manufacture the vehicles.

The best-known seller of yogurt drinks, Korea Yakult Co., started door-to-door delivery in the early 1970s, transporting a successful business model from Japan.

The company now sells around two million bottles of its mini yogurts each day. Its army of 13,000 delivery ladies - there are no men - reaches almost every corner of the country, including remote islands. Customers typically take deliveries once or twice a week for about 15 cents a bottle.

Facing competition from online retailers with delivery trucks selling rival products, Korea Yakult earmarked $67 million for 10,000 electric vehicles to be introduced through 2017. The new wheels are being accompanied by a redesign of delivery ladies’ uniforms with brighter colors and pink helmets.

Many Koreans gulp down yogurt drinks each day in hopes of maintaining healthy digestive systems. The special ingredient is a strain of the Lactobacillus bacteria discovered by a Japanese scientist nearly a century ago that yogurt sellers say complements the production of digestive enzymes.

A recent survey by the Korean Society of Coloproctology showed fermented-milk products were the most favored remedy for constipation. The vehicles are a world’s first according to Korea Yakult.

Delivery women must have drivers’ licenses and stick to one side of the road. Driving is prohibited on sidewalks.

The rollout, of 4,000 carts so far, has had a few hiccups. The vehicles emit a loud beeping noise when reversing for safety reasons, resulting in a bout of complaints in a neighborhood where Korea Yakult operates about 38 carts.“If they all move out together, it can get pretty loud,” said Mr. Kim.