Could you explain why yeast was not to be used is some sacrifices, while being used in others? I am aware of the symbolism in the New Testament, but am a little confused with that in the Old. -- Max Maxwell
You are right. While we usually remember the use of unleavened challah in the OT sacrifices, some sacrifices did in fact allow leavened bread (Leviticus 23:16-17; Numbers 28:17). Yeast seems to be a reminder of the old. In ancient times yeast (the animal that feeds on the sugars in bread, releasing carbon dioxide bubbles which lead to the rising of the bread), was normally added through a piece of dough, left to spoil, from a previous meal. Interestingly, leavened bread was the norm in the Western church until the 10th century, and is still used in the (Eastern) Orthodox church.
As for the (a) presence or (b) absence of yeast, depending on the specific type of sacrifice, the words of John E. Hartley (The Word Biblical Commentary: Leviticus, lxviii and 379, respectively) are apropos.
(a) "The permission of yeast witnesses to the exuberant joy the community felt at harvest time [the Feast of Weeks]."
(b) "[The Feast of Unleavened Bread]... was a feast for the grateful celebration of the barley harvest and the dedication of the first sheaves to God. By the people's giving God the firstfruits of the new harvest, the rest of the harvest was released to their use. To symbolize this break in the annual cycle, fresh kernels of grain were roasted and baked into loaves, but nothing from the past, such as yeast, could be used (Porter, 182; Kraus, Worship, 48). Kraus (Worship, 48-49) claims that when Israel took over the Feast of Unleavened Bread from her Canaanite neighbors, she modified the feast in three ways: (1) She attributed the feast to Yahweh (Exodus 23:15; Leviticus 23:6). This means that she did not recite any myths of fertility such as the victory of Baal [the weather and fertility god] over Mot [the god of death]. (2) The festival was for the entire nation assembled at a central shrine (Exodus 23:17; 34:24). (3) The festival was tied to Israel's redemptive history..."
Yours is a challenging question, though a welcome one. Moreover, the cultural gap between us and ancient Israel is considerable. Some queries can only be answered through the information provided by solid biblical scholarship. I hope this information is helpful and that your question has been answered.