Dear Muffled,
Actually I baked some leaven bread the day before yesterday, and the process under ideal conditions, took less than 4 hours. Although if I had been on the road, I would have not have been able to replicate the procedure, but I used leaven in a jar, and the Israelites probably used a starter. I would think the problem would be one of timing and the method of baking and not the leaven itself.
Bread - Wikipedia, the free encyclopedia
There were multiple sources of leavening available for early bread. Airborne yeasts could be harnessed by leaving uncooked dough exposed to air for some time before cooking. Pliny the Elder reported that the Gauls and Iberians used the foam skimmed from beer to produce "a lighter kind of bread than other peoples." Parts of the ancient world that drank wine instead of beer used a paste composed of grape juice and flour that was allowed to begin fermenting, or wheat bran steeped in wine, as a source for yeast. The most common source of leavening was to retain a piece of dough from the previous day to use as a form of sourdough starter.[7]
The Israelis probably had wine and grape juice, so that could also be a good source of new leaven. The yeast grows on the skin of a grape.
The use of leaven was similar to that of the use of parables, in which is explained by
Isaiah 6:10,"You will keep on seeing, but will not perceive;" The feast days are lessons based on past events, put focused on the future of Israel, such as nations in the millenium, will keep the feast of booths. (Ze 14:18) The judges of Israel (traditions of men) will be replaced with a judge who judges not by what he sees or hears, but by righteousness. (Isaiah 11:4)