Objection 1. It seems that wine of the grape is not the proper matter of this sacrament. Because, as water is the matter of Baptism, so is wine the matter of this sacrament. But Baptism can be conferred with any kind of water. Therefore this sacrament can be celebrated in any kind of wine, such as of pomegranates, or of mulberries; since vines do not grow in some countries.
Objection 2. Further, vinegar is a kind of wine drawn from the grape, as Isidore says (Etym. xx). But this sacrament cannot be celebrated with vinegar. Therefore, it seems that wine from the grape is not the proper matter of this sacrament.
Objection 3. Further, just as the clarified wine is drawn from grapes, so also are the juice of unripe grapes and must. But it does not appear that this sacrament may be made from such, according to what we read in the Sixth Council (Trull., Can. 28): "We have learned that in some churches the priests add grapes to the sacrifice of the oblation; and so they dispense both together to the people. Consequently we give order that no priest shall do this in future." And Pope Julius I rebukes some priests "who offer wine pressed from the grape in the sacrament of the Lord's chalice." Consequently, it seems that wine from the grape is not the proper matter of this sacrament.
On the contrary, As our Lord compared Himself to the grain of wheat, so also He compared Himself to the vine, saying (John 15:1): "I am the true vine." But only bread from wheat is the matter of this sacrament, as stated above (Article 3). Therefore, only wine from the grape is the proper matter of this sacrament.
I answer that, This sacrament can only be performed with wine from the grape. First of all on account of Christ's institution, since He instituted this sacrament in wine from the grape, as is evident from His own words, in instituting this sacrament (Matthew 26:29): "I will not drink from henceforth of this fruit of the vine." Secondly, because, as stated above (Article 3), that is adopted as the matter of the sacraments which is properly and universally considered as such. Now that is properly called wine, which is drawn from the grape, whereas other liquors are called wine from resemblance to the wine of the grape. Thirdly, because the wine from the grape is more in keeping with the effect of this sacrament, which is spiritual; because it is written (Psalm 103:15): "That wine may cheer the heart of man."
Reply to Objection 1. Such liquors are called wine, not properly but only from their resemblance thereto. But genuine wine can be conveyed to such countries wherein the grape-vine does not flourish, in a quantity sufficient for this sacrament.
Reply to Objection 2. Wine becomes vinegar by corruption; hence there is no returning from vinegar to wine, as is said in Metaph. viii. And consequently, just as this sacrament may not be made from bread which is utterly corrupt, so neither can it be made from vinegar. It can, however, be made from wine which is turning sour, just as from bread turning corrupt, although he who does so sins, as stated above (Article 3).
Reply to Objection 3. The juice of unripe grapes is at the stage of incomplete generation, and therefore it has not yet the species of wine: on which account it may not be used for this sacrament. Must, however, has already the species of wine, for its sweetness ["Aut dulcis musti Vulcano decoquit humorem"; Virgil, Georg. i, 295] indicates fermentation which is "the result of its natural heat" (Meteor. iv); consequently this sacrament can be made from must. Nevertheless entire grapes ought not to be mixed with this sacrament, because then there would be something else besides wine. It is furthermore forbidden to offer must in the chalice, as soon as it has been squeezed from the grape, since this is unbecoming owing to the impurity of the must. But in case of necessity it may be done: for it is said by the same Pope Julius, in the passage quoted in the argument: "If necessary, let the grape be pressed into the chalice."
Source: Summa Theologica (New Advent)