Reply Obj. 3: A moderate mixing does not alter the species, because that little is as it were absorbed by the greater. Consequently, then, if a small quantity of another grain be mixed with a much greater quantity of wheat, bread may be made therefrom so as to be the proper matter of this sacrament; but if the mixing be notable, for instance, half and half; or nearly so, then such mixing alters the species; consequently, bread made therefrom will not be the proper matter of this sacrament.
Reply Obj. 4: Sometimes there is such corruption of the bread that the species of bread is lost, as when the continuity of its parts is destroyed, and the taste, color, and other accidents are changed; hence the body of Christ may not be made from such matter. But sometimes there is not such corruption as to alter the species, but merely disposition towards corruption, which a slight change in the savor betrays, and from such bread the body of Christ may be made: but he who does so, sins from irreverence towards the sacrament. And because starch comes of corrupted wheat, it does not seem as if the body of Christ could be made of the bread made therefrom, although some hold the contrary. _______________________
FOURTH ARTICLE [III, Q. 74, Art. 4]
Whether This Sacrament Ought to Be Made of Unleavened Bread?
Objection 1: It seems that this sacrament ought not to be made of unleavened bread. Because in this sacrament we ought to imitate Christ's institution. But Christ appears to have instituted this sacrament in fermented bread, because, as we have read in Ex. 12, the Jews, according to the Law, began to use unleavened bread on the day of the Passover which is celebrated on the fourteenth day of the moon; and Christ instituted this sacrament at the supper which He celebrated "before the festival day of the Pasch" (John 13:1, 4). Therefore we ought likewise to celebrate this sacrament with fermented bread.
Obj. 2: Further, legal observances ought not to be continued in the time of grace. But the use of unleavened bread was a ceremony of the Law, as is clear from Ex. 12. Therefore we ought not to use unfermented bread in this sacrament of grace.
Obj. 3: Further, as stated above (Q. 65, A. 1; Q. 73, A. 3), the Eucharist is the sacrament of charity just as Baptism is the sacrament of faith. But the fervor of charity is signified by fermented bread, as is declared by the gloss on Matt. 13:33: "The kingdom of heaven is like unto leaven," etc. Therefore this sacrament ought to be made of leavened bread.
Obj. 4: Further, leavened or unleavened are mere accidents of bread, which do not vary the species. But in the matter for the sacrament of Baptism no difference is observed regarding the variation of the accidents, as to whether it be salt or fresh, warm or cold water. Therefore neither ought any distinction to be observed, as to whether the bread be unleavened or leavened.
On the contrary, According to the Decretals (Extra, De Celebr. Miss.), a priest is punished "for presuming to celebrate, using fermented bread and a wooden cup."
I answer that, Two things may be considered touching the matter of this sacrament, namely, what is necessary, and what is suitable. It is necessary that the bread be wheaten, without which the sacrament is not valid, as stated above (A. 3). It is not, however, necessary for the sacrament that the bread be unleavened or leavened, since it can be celebrated in either.