"... The parable of the leaven (13:33) confirms the view that Jesus chose startling images to convey the humble and mysterious nature of the kingdom. Leaven was a symbol of corruption in the Hebrew Scriptures (Exodus 12:17-20; 23:18; 34:18; Leviticus 2:11, 6:17; Hos 7:4), in Matthew (16:6, 11), and elsewhere in the New Testament (1 Corinthians 5:6-13; Galatians 5:9; see also Mishna 'Orla 2:8-12; Menachot 5:1-2; Genesis Rabbah 34:10; Babylonian Talmud Berakot 17a; Plutarch, Roman Questions 289F: “Yeast [leaven] is itself also the product of corruption, and produces corruption in the dough with which it is mixed; ... and altogether the process of leavening seems to be one of putrefaction”). What leaven meant to the first-century Jew has been obscured by its translation as “yeast,” as if yeast and leaven were one and the same thing. Leaven was produced by keeping back a piece of the previous week’s dough, storing it in suitable conditions, and adding juices to promote the process of fermentation. After several days the old dough was sufficiently fermented to be used in a large mass of dough to give it lightness. This homemade rising agent, however, was fraught with health hazards. If it became tainted, it would spread poison to the rest of the dough in baking, and that batch of dough could infect the next batch and so on (see Pliny, Natural History 18:26). For this reason, leaven became a symbol for the infectious power of evil ..."