HOHHOT (China) - CHINA'S dairy heartland promised it has banished a toxic chemical from its milk and launched a media campaign to restore its reputation among consumers wary after thousands of children were poisoned.

There are 2.5 million cows in Inner Mongolia, which are a lifeline for tens of thousands of poor farmers and supply two of China's biggest dairy firms, Inner Mongolia Yili Industrial Group and Mengniu Dairy .

But they were forced to literally pour away their income after the discovery of the industrial compound melamine in baby formula that made tens of thousands of children sick and caused frightened consumers to shun Chinese dairy products globally.

'Of course this has influenced the reputation of our area, and the two companies, but now we are trying to think of ways to turn the negative positive, and win a wider amount of customers' trust,' said Inner Mongolia's deputy governor Ren Yaping.

'The slogan we are using now is 'trustworthy milk',' he told foreign reporters during a government-organised visit to regional capital Hohhot.

Yili and Mengniu were named in a list of 22 companies found to have sold melamine-tainted dairy products when the scandal broke last month.

Officials say they have pinned down the source of the poisoning - middlemen who sell the big companies much of the milk they process, because their massive plants are not yet matched by massive industrial farms.

Instead, much of the milk they sell is from small farms or even one or two cows raised by local farmers, who take their milk to collecting centres where it is mixed together and then passed on.

The government says it is at these stations that the compound was mixed in to help cheat nutrition tests and six people are now under arrest, three for adding the melamine and three for selling it.

'There was a small portion of immoral people who at the collecting stations added melamine, but this had nothing to do with the production, management, equipment or oversight at the two companies,' said Hohhot's mayor Tang Aijun.

There is now tighter control of the process of collecting and selling the milk, and more quality checks along the way, including for melamine, officials said.

But ultimately the best solution may be eliminating the middlemen who can blame quality on farmers and are keen to squeeze the most possible out of the factories.

'It is the source of the milk that is a problem, so it's important for the next step to tackle the origin of the milk. First we have to scale up. Without that it is harder to enforce checks,' Mr Ren said.