Ḥametz that was originally fit for eating, but that became moldy or spoiled to the point that it is not fit for human consumption, is still considered ḥametz gamur, since it can still be used as a leavening agent. In this respect, ḥametz differs from all other forbidden foods, which are no longer forbidden once they become unfit for human consumption. Ḥametz, in contrast, remains forbidden even when it becomes unfit for human consumption, since it can still serve as a starter for another dough. Only if the ḥametz became so spoiled that it is unfit for consumption by a dog, it is not considered food at all, it no longer has the status of ḥametz, and it is permissible to keep it during Pesaḥ and to derive benefit from it (SA 442:2; MB 10). By rabbinic decree, however, it is still forbidden to eat it on its own, for by eating it one shows that he considers it food (MB 442:43). The same principle applies to all food prohibitions: Even if it is no longer fit for human consumption, the Sages forbid eating it.
The yardstick of being fit for a dog’s consumption is relevant only for measuring the spoilage of ḥametz or se’or. But if se’or was not spoiled, but only became so sour that it is not fit even for a dog, since it is good se’or (that functions as a leavening agent and is usable like regular yeast), all the laws of ḥametz apply to it, and one is required by Torah law to destroy it (BHL 442:9).
This law – that one does not have to burn ḥametz that was spoiled to the point of not being fit for a dog’s consumption – applies only if it became spoiled before the time that ḥametz becomes prohibited. But if it was fit to be eaten by a dog when the prohibition of ḥametz began, then even if it became spoiled later, to the point of being unfit for a dog’s consumption, one must eliminate it. Since the mitzva of eliminating the ḥametz has taken effect, one has not discharged his obligation until he has destroyed the ḥametz completely (MB 442:9; see below, ch. 5, n. 5).
It is important to note that all of these laws are conditional on the ḥametz having initially been fit for human consumption or for preparing human food, like starter dough. But if it was never fit for human consumption, even if it was fit to be eaten by a dog, the prohibition of ḥametz never applied to it. However, the ḥametz in food prepared for dogs and cats was initially fit for human consumption, and it therefore must be eliminated. Something that was not initially meant for eating but was, in fact, fit for human consumption under pressing circumstances is ḥametz nuksheh, as described above.