Ancient denizens of what is now Ireland and Scotland buried stashes of so-called "bog butter" in peat bogs, presumably to stave off spoilage. Thanks to the unique chemistry of those bogs, the stashes have survived for thousands of years. Now, scientists at University College Dublin have conducted chemical analysis and radiocarbon dating of several bog butters recovered from archaeological sites in Ireland. They found that the practice was a remarkably long-lived tradition, spanning at least 3,500 years, according to their new paper in Nature: Scientific Reports.

The researchers also uncovered the first conclusive evidence that Irish bog butters are derived from dairy fat as opposed to being meat-based. According to bioarchaeologist Kristina Killgrove, writing in Forbes, "Previous attempts at analyzing bog butter have come up short, because even though the butter is known to have an animal origin, techniques were unable to distinguish between adipose tissue where lipids or fats are stored and milk fats from ruminants like cows and sheep, particularly on an archaeological time-depth."

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There are some 430 recorded stashes of bog butter, according to Benjamin Reade of the Nordic Food Lab, 274 of which were found in Scotland and Ireland. It's usually found wrapped in some kind of wooden container—buckets, kegs, barrels, etc.—or animal bladders. The bog butter may have been buried as a means of meat preservation, based on a 1995 study demonstrating that meat buried in peat bogs for up to two years had roughly the same levels of bacteria and pathogens as meat stored in a modern freezer. Alternatively, it may have been a kind of primitive food processing.

The Dublin researchers compared their analysis of stable carbon isotopes (targeting fatty acids in the bog butter samples) to a modern global database of animal fats. They found that 26 of their 32 samples were definitely made from dairy, and three more were very likely made from dairy. Their analysis of the last three samples was inconclusive. In addition, radiocarbon dating of the samples showed they dated as far back as the Early Bronze Age (circa 1700 BC).

And yes, there have been modern experiments in making bog butter, most notably samples presented at the 2012 Oxford Symposium on Food and Cookery. Reade headed the project and was pleased to find that his homemade bog butter did not go rancid during its three months underground. (A second stash has been allowed to age for seven years.) However, the result was something of an acquired taste, "causing disgust in some and enjoyment in others," he wrote. "The fat absorbs a considerable amount of flavor from its surroundings, gaining flavor notes which were described primarily as ‘animal,’ or ‘gamey,’ ‘moss,' ‘funky,’ ‘pungent,’ and ‘salami.’"


But Reade thinks that flavor profile would work well in "strong and pungent [contemporary] dishes, in a similar manner to aged ghee." In fact, there's a North African version of bog butter called smen (also similar to ghee, aka clarified butter) that is still used today, considered a delicacy on par with fine cheese.

DOI: Scientific Reports, 2019. 10.1038/s41598-019-40975-y (About DOIs).