Sunday, September 21, 2008

Melamine

Perhaps, like me, you have been wondering why some unscrupulous Chinese suppliers would put plastic first into pet food (2007) and then milk (2008). The extent of my knowledge was that melamine is a monomer used in making hard plastic products as diverse as bowls, shoehorns and bench coverings. It seemed to me that such a compound would be more expensive than the product it was substituting and thus not worth the effort. So I did a little searching on the Internet.

There is some confusion in terminology. Melamine is a ring structured compound containing a lot of nitrogen (66% by mass). It is combined with formaldehyde to produce melamine resin, a thermosetting plastic. However, the plastic is also often called simply “Melamine”. There are a number of ways of making the monomer; these days it is usually made from urea, but it can also be made from coal. China is the largest producer of melamine in the world.

The current story probably has its origins in a patent from 1958, which described the use of melamine as a source of non-protein nitrogen in cattle feed. However, a study conducted in 1978 suggested that melamine is digested more slowly and less completely than cottonseed or urea, so its use is probably limited.

Melamine itself has a low toxicity – approximately the same LD50* as sodium chloride. However, the combination of melamine and cyanuric acid in the diet causes acute renal failure in cats. This came to light after the contaminated pet food incident. I think it is unlikely that many humans consume cyanuric acid in their diets, though this compound has been approved by FDA for use in animal feed and is used as a precursor to N-chlorinated cyanurates, which are used to disinfect water e.g. in spa and swimming pools. Chronic ingestion of melamine can lead to formation of kidney and bladder stones and it may be that this is a particular problem in very young children.

Perhaps the reason for the adulteration of the Chinese products was that melamine can increase the apparent protein content of the food. It has been reported that melamine scrap is widely incorporated into livestock and fish feed in parts of China (New York Times, 30th April, 2007). Since “melamine scrap” is an impure and less valuable form of the chemical produced as a by-product, for example in coal gasification, this could explain why it might be economical to replace milk or meat protein with this non-protein nitrogen source if it can be concealed from the authorities and customers. Measurement of protein content by the Kjeldahl or Dumas methods, which actually measure nitrogen content, would not detect the adulteration. Testing for melamine directly is difficult and expensive, though test kits are now available.

As an aside, since melamine is used in food packaging and tableware, small amounts of the compound (parts per million level) may be found in foods and beverages as a result of migration from melamine polymers in contact with the food. These levels are not considered to be a health risk.

* LD50 is the amount of a material, given all at once, which causes the death of 50% (one half) of a group of test animals.

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