If the appearance, taste, texture and chemical composition of an object remains the same, in what ways does it change?
From Fr. John Hardon, S.J.
Modern Catholic Dictionary:
“TRANSUBSTANTIATION. The complete change of the substance of bread and wine into the substance of Christ’s body and blood by a validly ordained priest during the consecration at Mass, so that only the accidents of bread and wine remain. While the faith behind the term was already believed in apostolic times, the term itself was a later development. With the Eastern Fathers before the sixth century, the favored expression was meta-ousiosis “change of being”; the Latin tradition coined the word transubstantiatio, “change of substance,” which was incorporated into the creed of the Fourth Lateran Council in 1215. The Council of Trent, in defining the “wonderful and singular conversion of the whole substance of the bread into the body, and the whole substance of the wine into the blood” of Christ, added “which conversion the Catholic Church calls transubstantiation” (Denzinger 1652). After transubstantiation, the accidents of bread and wine do not inhere in any subject or substance whatever. Yet they are not make-believe; they are sustained in existence by divine power. (Etym. Latin trans-, so as to change + substantia, substance: transubstantiatio, change of substance.)”
Here is a lengthier article regarding
transubstantiation from the
Catholic Encyclopedia, and a link to a
Called to Communion website regarding the
Church fathers on the change in the bread at the Consecration.
Fr. Antoninus Wall, O.P. asks these questions of those who question, do not grasp, or who deny transubstantiation: "What did you have for breakfast this morning? Toast? What became of that toast when you ate it? Did it not become your living flesh, your body automatically converting it? If your perishable human body can convert bread into living flesh, how can you deny that God did the same, when He specifically says that He did?
Another way of looking at it is to consider the hypostatic union, which all Christians believe. Even though Christ’s human body remained fully human, it was alloyed/fused/united with the Divine, although not changed in
appearance or function (“accidents”) in the slightest. Arius and his spiritual descendants, the Jehovah’s Witnesses denied

that Christ was divine, in part because His appearance was fully and completely human. Yet, we Christians know with certainty that His 100% human Body contained the Divine nature.
Consider also the tomb as Jesus passed through either the earthen walls or the stone: Their appearance (“accidents”) did not change even though their substance contained Christ in
both His human
and Divine nature as He passed through them. The same may be said for the locked doors of the upper room when our Lord appeared to the disciples: Those locked doors did not change in appearance (“accidents”), but the Lord was fully present in them as He passed through them. Jesus passed through the stone and the doors, but chose to remain in the consecrated bread - the only difference. Keep in mind that, as the Passover Lamb, He had to be eaten and so He sacramentally inhabits/possesses the bread, changing its substance so that that we might receive the Bread come down from Heaven, eat it and never die.
As an aside, the empty tomb and locked doors also indicate the rationality of the Catholic doctrine of Mary’s perpetual virginity. Just as Jesus passed through the walls/stone of the tomb and the locked doors without physically altering them; as their accidents remained unchanged, so also He passed from Mary’s womb to earthly life without physically altering Mary’s body. The principle involved is that
Creation yields to Creator.