When does transubstantiation actually place during consecration?
To answer this question, I will switch words to say by the consecration, transubstantiation takes place. At the Last Supper, the Lord Jesus, on the night he was handed over, took bread, and after he had given thanks, broke it and said, "This is my body that is for you. Do this in remembrance of me." In the same way also the cup, after supper, saying, "This cup is the new covenant in my blood. Do this, as often as you drink it, in remembrance of me." For as often as you eat this bread and drink the cup, you proclaim the death of the Lord until he comes. (1 Cor 11:23-26) During the Eucharistic Prayer at the consecration, the priest proclaims Jesus' words at the Last Supper over the bread and wine. "The power of the words and the actions of the species of bread and wine, Christ's Body and Blood, his sacrifice offered on the cross for all" (CCC, no. 1353). By the consecration, the transubstantiation of the bread and wine into the Body and Blood of Christ is brought about. Under the consecrated species of bread and wine, Christ himself, living and glorious, is present in a true, real, and substantial manner: His Body and Blood, with his soul and divinity. (CCC< no. 1413; Council of Trant: DS 1640, 1651)
Transubstantiation means that the substance of the bread and wine is changed into the substance of the Body and Blood of Christ. The appearances of bread and wine remain (color, shape, weight, chemical composition), but the underlying reality - that is, the substance - is now the Body and Blood of Christ.