A piece of dough that is set aside for a day or longer and allowed to sour or ferment. The Hebrew term seʼorʹ denotes such sourdough and means “fermented or leavened mass.” Sourdough readily leavens new mixtures to which it is added.

The Israelites used sourdough in making leavened bread. The lump of dough preserved from a former baking was generally dissolved in water in the kneading trough prior to the adding of the flour, or it might be put in the flour and then kneaded along with it. The latter seems to be the method referred to by Jesus Christ when he said: “The kingdom of the heavens is like leaven, which a woman took and hid in three large measures of flour, until the whole mass was fermented.” (Mt 13:33; Lu 13:20, 21) Though there is no direct evidence, it has been suggested that the Jews also used wine dregs as yeast.

Israel’s grain offerings presented by fire to Jehovah were not to be made with sourdough. (Le 2:11) Also, the Israelites were expressly commanded not to have sourdough (here an apparent symbol of corruption and sin) in their homes or within the boundaries of their territory during the seven-day Festival of Unleavened Bread. (Ex 12:15; 13:7; De 16:4) Anyone eating something leavened during that time was to be “cut off from the assembly of Israel.”​—Ex 12:19.

In ancient Egypt it was also customary, when baking, to set aside some dough, to be used for leavening fresh dough. Even today, when the kneading of dough has been completed, some people of Cyprus, for instance, put aside a piece of dough in a warm place. After 36 to 48 hours it can be used to ferment an entire lump of new dough.

Paul may have had sourdough in mind when he urged the Corinthians: “Clear away the old leaven [Gr., zyʹmen], that you may be a new lump, according as you are free from ferment.”​—1Co 5:7; see LEAVEN.