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):
The Parable of the Leavened Meal
— There are two interpretations of this parable. The first and the most popular is that which treats leaven as the type of the Kingdom. The other claims that the whole picture is required to set forth what the King intended to teach concerning the Kingdom. That is to say, one method of interpretation lays emphasis upon the fact that the Lord said "The Kingdom of Heaven is like unto leaven." The other interpretation insists that to stop there is to miss the Master's meaning, and that it is necessary to read 'The Kingdom of Heaven is like unto leaven, which a woman took, and hid in three measures of meal." Either leaven alone is the type of the Kingdom, or all the facts of the picture - the meal, the woman, the leaven, the hiding, and the issue - are required in order to understand what the King intended to teach.
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 »»»
It is a picture of the age which ends with the advent. "When the Son of Man Cometh, shall He find faith on the earth?" The popular answer of theology is, Yes. Christ's answer is, No; and it is infinitely better in order to do our work as it ought to be done, that we should accept His estimate of our age. It may be objected that this outlook is pessimistic in the extreme. It would be, indeed, if this age were the final one; but it is not so, it is only initial. Beyond the flaming of His advent feet will come the Kingdom administration of the King's own presence. For that the world is waiting, and that we, by consecration, are attempting to hasten.
 "leaven is good" interpretation supports universal salvation
 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
 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.
 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.
 oh, btw
: "Evil" has gotten distinctly worse over the millenia. Originally it seems to have signified nothing more sinister than "uppity"