Wikipedia recognizes two basic interpretations for Parable of the Leaven: evil and good (Matthew 13:33 and Luke 13:20–21) ~ "Although leaven symbolises evil influences elsewhere in the New Testament (as in Luke 12:1), it is not generally interpreted that way in this parable. However, a few commentators do see the leaven as reflecting future corrupting influences in the Church." G. Campbell Morgan's Parables of the Kingdom discusses both interpretations (Ray Stedman's The Case of the Sneaky Housewife is somewhat similar):
If the first interpretation of the parable, that leaven is the symbol of the Kingdom, be the correct one, we are necessarily driven to the conclusion that in this instance leaven must be the type of good, and that as a result of its working all things will be finally brought into subjection to the King. That is the view which seems to be held to-day by the great majority of expositors.
According to the second view leaven is not a type of good but of evil, as it is in every other case in Scripture. It is thus the type of a principle which affects for evil the Kingdom testimony of this particular age. The ultimate issue, therefore, described is not the conquest of the age by the principles of the Kingdom, but rather the intermixture with the Kingdom testimony of forces which enfeeble it and render it comparatively inoperative.
If a view is not to be accepted because of its popularity, neither ought it to be rejected on that account. There are, however, other reasons which compel me to accept the second theory as the true one. I do so in the first place because the former view is out of harmony with the symbolic use of the Bible in other places. Those who hold the first view admit frankly that this is the only case in which leaven is used as a type of good. Uniformly, from its first mention to its last, with this one exception - if it be an exception - leaven is a type of evil. more »»»
Morgan concludes:
[1] "leaven is good" interpretation supports universal salvation.
[2] meaning of "hiding". The Thermochemical Joy of Cooking. chemical reactions in cooking may have looked like a mystery back then, but brewing has a long history. Godisgood!
[3] Morgan makes a "passing proposition" that "the use of leaven is injurious physically". contrast: ergotism caused by bad rye bread as possible theory for "bewitchment" in Salem.
[4] these aren't about leaven, but thought they were worth mentioning: Elijah and the Widow at Zarephath & the unending supply of flour and oil; Parable of the empty jar and the missing flour.
[5] oh, btw: "Evil" has gotten distinctly worse over the millenia. Originally it seems to have signified nothing more sinister than "uppity".