Jerusalem Will Be Riddled With Famine and Its Inhabitants Will Dwell Among the Nations in Uncleanness.
“Also take to yourself wheat, and barley, and beans, and lentils, and millet, and spelt, and put them in one vessel, and make of it bread for yourself. According to the number of days that you will lie in your side, even three hundred and ninety days, you will eat of it.”
The purpose of these and the following instructions was to indicate siege rations (Ezekiel 4:16). This is confirmed by the quantity of the rations (Ezekiel 4:10), and the fact that it was purportedly to be baked on human dung (Ezekiel 4:12; compare Deuteronomy 23:13-14) rather than cow dung, because they were shut up in the city. It also indicated that the children of Israel, once taken captive, would eat their food ‘unclean' among the nations (Ezekiel 4:13; compare Hosea 9:3. See also Daniel 1:8). In other words from the beginning of the siege onwards into captivity they would experience poor food, short rations, and ritual uncleanness. There was nothing ritually unclean about the food itself as far as we are aware from Leviticus and Deuteronomy (and the Mishnah - the later Jewish oral law). Among other things it would be the way such foods came in contact with uncleanness and unclean things, and the way that they might be grown (e.g. Leviticus 19:19) or stored, that would render them unclean. With regard to meat, its source, and whether it had been killed correctly, would often not be known. Foreigners could not be depended on to maintain ritual cleanness and to kill meat in the right way.
We should note, in fact, that on his protesting in horror (Ezekiel 4:14) God graciously allowed Ezekiel to use cow dung instead of human dung (Ezekiel 4:15). This was in order to maintain his own ceremonial cleanness. The use of cow dung for baking on was a recognised method of baking.
The various items were all to be baked together in some form of bread. When they were under siege people would put together whatever they had, mixing it together, in order to prepare food. In Ezekiel's case this was then to form his means of sustenance for the 390 days, which was possibly intended to represent roughly the prospective length of the siege of Jerusalem (i.e ‘a year').