The dairy materials were thought to be from the same batch that should have been destroyed following the 2008 scandal, which resulted in 300,000 cases of illness and six deaths.
Arrests and seizures
Of the six people arrested at the end of last week, three were employees from a factory in the Qinghai Province, in northwest China, according to news reports. They stand accused of shipping contaminated milk powder to the neighbouring Gansu province.
The three others arrested are suspected of involvement in the hiding of tainted milk powder that should have been destroyed in 2008 and then selling it to the Qinghai plant.
State news agency Xinhua said an additional 41 people have been detained as police continue their investigations. Police have also seized suspect dairy materials although reports as to the exact amount vary.
Melamine, which is an industrial chemical used in the manufacture of plastics, was added to thousands of tonnes of watered-down milk products in 2008 to artificially boost protein levels in one of the most high-profile contamination cases in recent years.
Cases of contaminated materials have continued to appear since despite high profile efforts to destroy any melamine tainted stock, combat the melamine threat and tighten up food safety standards generally.
Continued safety scares hamper any potential recovery to the Chinese dairy export industry, which was almost wiped out after the 2008 scandal.
Earlier in the month, NASDAQ-listed Chinese company Synutra had to face down accusations that its milk powder had resulted in premature breast growth among a group of infant girls. The Chinese health ministry investigated the claims and found no link between the milk products and the unusual breast development.