Objection 1. It seems that wheaten bread is not requisite for the matter of this sacrament, because this sacrament is a reminder of our Lord's Passion. But barley bread seems to be more in keeping with the Passion than wheaten bread, as being more bitter, and because Christ used it to feed the multitudes upon the mountain, as narrated in John 6. Therefore wheaten bread is not the proper matter of this sacrament.
Objection 2. Further, in natural things the shape is a sign of species. But some cereals resemble wheat, such as spelt and maize, from which in some localities bread is made for the use of this sacrament. Therefore wheaten bread is not the proper matter of this sacrament.
Objection 3. Further, mixing dissolves species. But wheaten flour is hardly to be found unmixed with some other species of grain, except in the instance of specially selected grain. Therefore it does not seem that wheaten bread is the proper matter for this sacrament.
Objection 4. Further, what is corrupted appears to be of another species. But some make the sacrament from bread which is corrupted, and which no longer seems to be wheaten bread. Therefore, it seems that such bread is not the proper matter of this sacrament.
On the contrary, Christ is contained in this sacrament, and He compares Himself to a grain of wheat, saying (John 12:24): "Unless the grain of wheat falling into the ground die, itself remaineth alone." Therefore bread from corn, i.e. wheaten bread, is the matter of this sacrament.
I answer that, As stated above (Article 1), for the use of the sacraments such matter is adopted as is commonly made use of among men. Now among other breads wheaten bread is more commonly used by men; since other breads seem to be employed when this fails. And consequently Christ is believed to have instituted this sacrament under this species of bread. Moreover this bread strengthens man, and so it denotes more suitably the effect of this sacrament. Consequently, the proper matter for this sacrament is wheaten bread.
Reply to Objection 1. Barley bread serves to denote the hardness of the Old Law; both on account of the hardness of the bread, and because, as Augustine says (83): "The flour within the barley, wrapped up as it is within a most tenacious fibre, denotes either the Law itself, which was given in such manner as to be vested in bodily sacraments; or else it denotes the people themselves, who were not yet despoiled of carnal desires, which clung to their hearts like fibre." But this sacrament belongs to Christ's "sweet yoke," and to the truth already manifested, and to a spiritual people. Consequently barley bread would not be a suitable matter for this sacrament.
Reply to Objection 2. A begetter begets a thing like to itself in species. yet there is some unlikeness as to the accidents, owing either to the matter, or to weakness within the generative power. And therefore, if there be any cereals which can be grown from the seed of the wheat (as wild wheat from wheat seed grown in bad ground), the bread made from such grain can be the matter of this sacrament: and this does not obtain either in barley, or in spelt, or even in maize, which is of all grains the one most resembling the wheat grain. But the resemblance as to shape in such seems to denote closeness of species rather than identity; just as the resemblance in shape between the dog and the wolf goes to show that they are allied but not of the same species. Hence from such grains, which cannot in any way be generated from wheat grain, bread cannot be made such as to be the proper matter of this sacrament.
Reply to Objection 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 to Objection 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.
Source: Summa Theologica (New Advent)