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Spoiled Chametz:^[1]

To point not edible to humans:[2] Chametz that was fit for eating from the time of its leavening [and baking], such as Chametz bread or yeast, and Chametz wheat kernels [and the like of Biblical Chametz], remains Biblically forbidden, even if the Chametz has spoiled and rotted to the point that it is no longer fit for human consumption, so long as it still remains edible for dogs, as explained next.[3] Such spoiled Chametz is Biblically obligated to be destroyed before Pesach.

To point not edible even to dogs-before 6^th hour Erev Pesach:[4] If the Chametz became inedible for even dogs prior to the 6^th hour on Erev Pesach, then one is not obligated to destroy it, and it is permitted for one to own on Pesach.[5] One may likewise benefit from it on Pesach.[6] Thus, if prior to the 6^th hour of Erev Pesach one burnt the Chametz until the point it is inedible even for dogs, he may now own it and benefit from it over Pesach.[7] It is, however, Rabbinically forbidden to be intentionally eaten.[8]

To point not edible even to dogs-after 6^th hour Erev Pesach:[9] If the Chametz became inedible even for dogs only after the 6^th hour on Erev Pesach, then one is Biblically obligated to destroy it completely from the world.[10] This applies whether the Chametz was created before the 6^th hour on Erev Pesach, and whether it was created after the 6^th hour on Erev Pesach.[11] Furthermore, Chametz that is burnt after the 6^th hour of Erev Pesach, remains forbidden in benefit even after it is destroyed and has turned into ash.[12]

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^[1] Admur 422:21; Michaber 442:2; Pesachim 45b

Sour Chametz: If the Chametz did not spoil but is too sour to be edible for a dog, since it can still be used to leaven bread, therefore it is still considered Chametz. [Biur Halacha 442]

Yeast: Some Poskim rule that spoiled yeast is not Chametz if it is no longer edible to a dog. [Rambam; Kesef Mishneh; Maggid Mishan; Magan Avraham] Other Poskim rule that even if yeast spoiled to point it is not edible for a dog, it remains forbidden as Chametz as it can still give fermentation to other yeasts. [Ravaad; Pri Chadash]  See Biur Halacha 442

[2] Admur ibid; M”A 442:1

[3] The reason its forbidden even though it’s not edible to humans: Although in general we rule that any food that is not edible for humans does not contain a Biblical Kashrus prohibition [see Rama Y.D. 155:1] nonetheless, regarding Chametz, it must reach a state that it is inedible even for animals, as the Torah prohibits even yeast, which is inedible for humans in its natural state. Thus, any Chametz that still contains the ability to ferment dough, like yeast, contains the prohibition. It is only after Chametz reaches a state that it is not fit even for a dog, which it loses all its leavening powers, and is thus no longer prohibited. [M”A 442:14; Ran; See P”M 442 A”A 2 and 14 for a lengthy discussion on this matter; M”B 442:9; Biur Halacha 442]

The reason it is not considered Chametz Nuksha: The above spoiled Chametz is not considered under the category of Chametz Nuksha, being that it was originally fit to be eaten. Chametz Nuksha only refers to Chametz that from the time of its initial leavening it was not fit for eating unless in a pressing situation. Such Chametz was never fit for regular eating in its entire leavened lifespan. However, Chametz that was fit for eating even for moment during its leavened lifespan, remains forbidden even after spoilage for human consumption. [Admur ibid]

[4] Admur 442:21 and 32-34; 433:25 and 445:4 and 11 and 466:3; Michaber 442:9 and 445:2; Pesachim 21b; Rif Pesachim 13b; See P”M 442 A”A 14

[5] The reason: As if the Chametz is inedible even to dogs by the time the obligation to destroy Chametz arrives, then this obligation does not befall onto this form of Chametz, as it does not have a status of food and is considered like mere dust. [Admur ibid]

[6] Admur 445:11; 442:24 “Any item that is permitted to be owned on Pesach may likewise be benefited from”; Michaber 445:2; Pesachim 21b

[7] Admur 442:32 and 445:11; Michaber 442:9 and 445:2; Pesachim ibid

[8] Admur 442:32 and 22; Implication of Michaber 442:9; Taz 442:8; Bach 442; Tur 442 in name of Rosh and Riy Abartzelona; Rabbeinu Yeurcham 5:13; Terumos Hadeshen 129; Erech Hashulchan 442:5; Kaf Hachaim 442:99

The reason: Although if the Chametz became inedible even for dogs before the 6^th hour on Erev Pesach it is permitted to own on Pesach, nevertheless, it is forbidden to be eaten, as although it’s not [Biblically] considered food at all, nevertheless, since the person intends to eat it, he gives it a status of food, and it is thus Rabbinically considered for him like complete food due to the thoughts that he has of eating from it. [Admur ibid; Taz ibid; Terumos Hadeshen 129; see Rama Y.D. 155:1]

Other opinions: Some Poskim rule the Chametz is permitted to even be eaten. [Peri Chadash and Rashbatz, brought in Kaf Hachaim ibid]

[9] Admur 442:21 and 34; Yerushalmi 2:6 as explains Ran and Maggid Mishneh; See P”M 442 A”A 14

[10] The reason: As since it retained a Chametz status for even one moment after [the prohibition of Chametz began and] the time of its destruction arrived, this Chametz status remains forever intact until one destroys the Chametz completely from the world in the same method required to be done to all other Chametz [as explained in 445:3-7 that it must be destroyed from the world and spoiling it is not enough.]. [Admur ibid]

[11] Admur 442:21

[12] Admur 445:8

^[13] Piskeiy Teshuvah 442:3