Using residues left behind on prehistoric pots discovered in China, researchers have discovered and recreated what is likely one of the oldest beer recipes in recorded history: a 5,000 year old brew made from broomcorn millet, barley, Job's tears, and tubers.

Writing in the latest edition of the Proceedings of the National Academy of Sciences (PNAS), a team of researchers from Stanford University and Brigham Young University, along with a team of colleagues from China, reported that this is believed to be the earliest direct evidence of in situ beer production in that country, and the first to report the use of barley in such activity. Scientists were surprised to learn that the recipe included barley, making it the "secret ingredient" that scientists didn't expect to find.

Lead author Jiajing Wang, a Ph. D. student at Stanford, and her co-authors explained that they used a recently-developed method based on phytolith morphometrics to identify the presence of the barley. This discovery predates macrobotanical remains of barley by about 1,000 years, even though the brew itself is some 4,000 years younger than the oldest beer ever discovered: a 9,000 year old concoction, also from China, made of rice, honey, and fruit, according to CNET.

The pottery containing the beer residue was discovered, along with other tools required to brew beer, in dwellings at the Mijiaya archaeological site in northern China, the authors explained. It indicates that the residents there has established advanced beer-brewing techniques around 5,000 years ago, and suggest that early brewing may have been the motivation for the initial importing of barley from Western Eurasia long before it became a cultivated food crop.

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Upon discovering pottery coated with a yellowing substance at the Mijiaya site, the researchers analyzed it and found that it was an alcoholic beverage made from broomcorn millet (Panicum miliaceum), barley (Hordeum vulgare), an Asian wild grain known as Job's tears (Coix lacryma-jobi), and tubers from plant roots, which were combined and fermented together.

The archaeologists also found traces of oxalate, a byproduct that forms a scale called beerstone in brewing equipment, according to Live Science. Items such as ceramic pots, funnels and stoves that would have been used in the preparation of the beverage were also discovered, and Wang's team dated them back to the late Stone Age Yangshao period (3400 BC to 2900 BC).