The method for making beer has changed over time. Hops(啤酒花), for example, which give many a modern beer its bitter flavor, are a 26 recent addition to the beverage. This was first mentioned in reference to brewing in the ninth century. Now, researchers have found a 27 ingredient in residue(残留物) from 5,000-year-old beer brewing equipment. While digging two pits at a site in the central plains of China, scientists discovered fragments from pots and vessels. The ditferent shapes of the containers 28 they were used to brew, filter, and store beer. They may be ancient "beer-making tools," and the earliest 29 evidence of beer brewing in China, the researchers reported in the Proceedings of the National Academy of Sciences. To 30 that theory, the team examined the yellowish, dried 31 inside the vessels. The majority of the grains, about 80%, were from cereal crops like barley(大麦), and about 10% were bits of roots, 32 lily, which would have made the beer sweeter, the scientists say. Barley was an unexpected find: the crop was domesticated in Western Eurasia and didn't become a 33 food in central China until about 2,000 years ago, according to the researchers. Based on that timing, they indicate barley may have 34 in the region not as food, but as 35 material for beer brewing.