Objection 1. It seems that the sacramental species cannot be corrupted, because corruption comes of the separation of the form from the matter. But the matter of the bread does not remain in this sacrament, as is clear from what was said above (Question 75, Article 2). Therefore these species cannot be corrupted.
Objection 2. Further, no form is corrupted except accidentally, that is, when its subject is corrupted; hence self-subsisting forms are incorruptible, as is seen in spiritual substances. But the sacramental species are forms without a subject. Therefore they cannot be corrupted.
Objection 3. Further, if they be corrupted, it will either be naturally or miraculously. But they cannot be corrupted naturally, because no subject of corruption can be assigned as remaining after the corruption has taken place. Neither can they be corrupted miraculously, because the miracles which occur in this sacrament take place in virtue of the consecration, whereby the sacramental species are preserved: and the same thing is not the cause of preservation and of corruption. Therefore, in no way can the sacramental species be corrupted.
On the contrary, We perceive by our senses that the consecrated hosts become putrefied and corrupted.
I answer that, Corruption is "movement from being into non-being" (Aristotle, Phys. v). Now it has been stated (3) that the sacramental species retain the same being as they had before when the substance of the bread was present. Consequently, as the being of those accidents could be corrupted while the substance of the bread and wine was present, so likewise they can be corrupted now that the substance has passed away.
But such accidents could have been previously corrupted in two ways: in one way, of themselves; in another way, accidentally. They could be corrupted of themselves, as by alteration of the qualities, and increase or decrease of the quantity, not in the way in which increase or decrease is found only in animated bodies, such as the substances of the bread and wine are not, but by addition or division; for, as is said in Metaph. iii, one dimension is dissolved by division, and two dimensions result; while on the contrary, by addition, two dimensions become one. And in this way such accidents can be corrupted manifestly after consecration, because the dimensive quantity which remains can receive division and addition; and since it is the subject of sensible qualities, as stated above (Article 1), it can likewise be the subject of their alteration, for instance, if the color or the savor of the bread or wine be altered.
An accident can be corrupted in another way, through the corruption of its subject, and in this way also they can be corrupted after consecration; for although the subject does not remain, still the being which they had in the subject does remain, which being is proper, and suited to the subject. And therefore such being can be corrupted by a contrary agent, as the substance of the bread or wine was subject to corruption, and, moreover, was not corrupted except by a preceding alteration regarding the accidents.
Nevertheless, a distinction must be made between each of the aforesaid corruptions; because, when the body and the blood of Christ succeed in this sacrament to the substance of the bread and wine, if there be such change on the part of the accidents as would not have sufficed for the corruption of the bread and wine, then the body and blood of Christ do not cease to be under this sacrament on account of such change, whether the change be on the part of the quality, as for instance, when the color or the savor of the bread or wine is slightly modified; or on the part of the quantity, as when the bread or the wine is divided into such parts as to keep in them the nature of bread or of wine. But if the change be so great that the substance of the bread or wine would have been corrupted, then Christ's body and blood do not remain under this sacrament; and this either on the part of the qualities, as when the color, savor, and other qualities of the bread and wine are so altered as to be incompatible with the nature of bread or of wine; or else on the part of the quantity, as, for instance, if the bread be reduced to fine particles, or the wine divided into such tiny drops that the species of bread or wine no longer remain.
Reply to Objection 1. Since it belongs essentially to corruption to take away the being of a thing, in so far as the being of some form is in matter, it results that by corruption the form is separated from the matter. But if such being were not in matter, yet like such being as is in matter, it could be taken away by corruption, even where there is no matter; as takes place in this sacrament, as is evident from what was said above.
Reply to Objection 2. Although the sacramental species are forms not in matter, yet they have the being which they had in matter.
Reply to Objection 3. This corruption of species is not miraculous, but natural; nevertheless, it presupposes the miracle which is wrought in the consecration, namely, that those sacramental species retain without a subject, the same being as they had in a subject; just as a blind man, to whom sight is given miraculously, sees naturally.
Source: Summa Theologica (New Advent)