Pesachim 42 - Unwanted substances

The following must be removed (from the world) on Passover. Babylonian sour dip, because it had sour milk and moldy breadcrumbs - ingredients about which people tended to forget; beer from Media - because it had some barley added to it, while most beer brands at the time were made from dates; and wine vinegar from Edom - because it, too, had barley.

Rav Nachman explained: originally wine from Judea would not sour, due to the merit of its being used in the Temple, and to convert it to vinegar one had to add barley. The standard "vinegar" of this time was the one made from Judean wine. However, now it is the wine of Edom that does not sour, so they have to add barley to it, and it is called "Idumean vinegar."

Art: Man Holding a Pipe and a Glass of Beer by David The Younger Teniers