Chemical detectives have traced deposits of fat in Scottish peat bogs to foodstuffs buried by people hundreds of years ago. The 'bog butter' is the remains of both dairy products and meat encased in the peat, say Richard Evershed of the University of Bristol and colleagues.

Those who live in the countryside of Ireland and Scotland and dig up chunks of peat for fuel have long been familiar with bog butter. While gathering the compressed plant matter, which can be burned in fires, diggers occasionally slice into a white substance with the appearance and texture of paraffin wax.

This is thought to be the remains of food once buried in the bog to preserve it. Waterlogged peat is cool and contains very little oxygen, so it can be used as a primitive kind of fridge.

The question is what type of food was buried in the peat. Local lore sometimes says that the waxy stuff is literally the remains of butter. For example, the seventeenth-century English writer Samuel Butler remarked in one of his famous poems that butter in Ireland "was seven years buried in a bog".

But there could be an alternative source for the waxy material: dead animals. In the eighteenth century, French chemists discovered that human corpses often contain adipocere, a substance also known as 'grave-wax'. So bog butter could be the remains of carcasses rather than dairy products.

To find out, Evershed and his colleagues took a close look at the fatty acids in bog butter. The chains of hydrocarbons in these molecules differ between those derived from dairy and those from meat. The chains in dairy products tend to be shorter than those in animal fat. And there are also differences in the relative amounts of normal and 'heavy' carbon they contain. Most of the carbon in organic material is carbon-12, but about one percent consists of the heavier isotope carbon-13. The exact amount of carbon-13 depends in part on whether the fat came from meat or dairy products.

The team verified some of these differences by analysing artificial bog butters, which were made in the 1970s from mutton fat and butter mixed with soil and water. They then looked at nine samples of bog butter provided by the National Museum of Scotland, some of which are 2000 years old. Six of the bog butter samples come from dairy products, and three are from animal fat, they report in The Analyst¹. So ancient Scots clearly used the peat to store both types of food, they say.

But there remains some mystery: researchers still do not know for sure if the food was buried solely to preserve it. Perhaps chemical reactions in the soil helped to transform the foods to more palatable products in a kind of primitive food processing, says Evershed. He plans to bury some modern fatty foods in peat to find out if anything interesting happens to them.

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