The principal ingredient was malted grain, either emmer wheat or barley or both together. It is often said that yeast was added by lightly baking bread and using crumbled bread to start the fermentation. This, however, is not supported by archaeological finds, which suggest instead that cooked grain and malted grain were combined, producing a mixture that contained sufficient sugar for fermentation.
A very different recipe is mentioned in the third tractate of the Babylonian Talmud (42b). According to Rav Yosef b. Hiyya, it contains 1⁄3 barley, 1⁄3 safflower seed and 1⁄3 salt. Rav Papa substituted wheat for barley. The ingredients were steeped, roasted and ground. This would not have produced real beer.
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Apart from recreational drinking, zythum was used as an ancient Egyptian medicine. It was said to work as both a laxative and antidiarrhoeal. Its use was thought dangerous for sick people and pregnant women.