Vietnamese police have arrested five people suspected of using battery chemicals to dye waste coffee beans, the Ministry of Public Security's online newspaper, Cong An Nhan Dan, said on Tuesday, apparently passing the mixture off as black pepper.

Vietnam is the world's largest exporter of black pepper, accounting for half of global trade. The country is also the world's largest producer of Robusta coffee, which has a bitter taste and is used mainly in instant coffee.

The five, led by Nguyen Thi Thanh Loan, 43, were arrested on suspicion of violating food safety regulations after they were caught mixing coffee waste with a black, tar-like liquid made from manganese dioxide found in the batteries, the report said.

Police seized 40 litres of the liquid, along with more than 21 metric tonnes of dyed coffee waste and over 200 kg of used battery parts from Loan's home in the Central Highlands province of Dak Nong, the report said.

Calls to police in the province went unanswered on Tuesday.

The Central Highlands is Vietnam's largest coffee-producing region. It also produces much of the country's black pepper.

Loan sold three metric tonnes of the mixture to a black pepper trading firm in the neighbouring province of Binh Phuoc, the Ministry of Public Security newspaper said.

Nguyen Thi Thanh Loan, owner of a facility caught using dirty and harmful materials in food production in Dak Nong Province in the Central Highlands region of Vietnam. Photo: Tuoi Tre

Police seized nine tonnes of pepper at the firm which had been blended with the battery and coffee mixture, Tuoi Tre newspaper reported. The firm had been selling pepper mixed with the battery-laced coffee granules for years, the paper said.

"Whatever purpose they may have, what they are doing is posing threat to people's health. If they mix that substance with coffee or pepper, it would be awful and unacceptable," said Nguyen Nam Hai, chairman of the Vietnam Pepper Association.

"I don't understand why they do it. Vietnam accounts for half of the world's black pepper output and 60-65 percent of global black pepper trade and prices are falling," said Hai.

"It's just a small firm and I don't think this dirty product could end up being exported."

If found guilty, the five face up to 20 years in prison.