by the Committee on Doctrine of the USCCB 

Does the bread cease to be bread and the wine cease to be wine? 
Yes. In order for the whole Christ to be present—body, blood, soul, and divinity—the bread and wine cannot remain, but must give way so that His glorified Body and Blood may be present. Thus, in the Eucharist the bread ceases to be bread in substance, and becomes the Body of Christ, while the wine ceases to be wine in substance, and becomes the Blood of Christ. As St. Thomas Aquinas observed, Christ is not quoted as saying, “This bread is my body,” but He said, ” This is my body” (Summa Theologiae, III q. 78, a. 5). 

Is it fitting that Christ’s Body and Blood become present in the Eucharist under the appearances of bread and wine?
Yes, for this way of being present corresponds perfectly to the sacramental celebration of the Eucharist. Jesus Christ gives himself to us in a form that employs the symbolism inherent in eating bread and drinking wine. Furthermore, being present under the appearances of bread and wine, Christ gives himself to us in a form that is appropriate for human eating and drinking. Also, this kind of presence corresponds to the virtue of faith, for the presence of the Body and Blood of Christ cannot be detected or discerned by any way other than faith. That is why St. Bonaventure affirmed: “There is no difficulty over Christ’s being present in the sacrament as in a sign; the great difficulty is in the fact that He is really in the sacrament, as He is in heaven. And so, believing this is especially meritorious” (In IV Sent., dist. X, P. I, art. un., qu. I). On the authority of God who reveals himself to us, by faith we believe that which cannot be grasped by our human faculties (cf. Catechism, no. 1381). 

Do the consecrated bread and wine cease to be the Body and Blood of Christ when the Mass is over?
No. During the celebration of the Eucharist, the bread and wine become the Body and Blood of Christ, and this they remain. They cannot turn back into bread and wine, for they are no longer bread and wine at all. There is thus no reason for them to change back to their “normal” state after the special circumstances of the Mass are past. Once the substance has really changed, the presence of the Body and Blood of Christ “endures as long as the Eucharistic species subsist” ( Catechism, no. 1377). Against those who maintained that the bread that is consecrated during the Eucharist has no sanctifying power if it is left over until the next day, St. Cyril of Alexandria replied, “Christ is not altered, nor is his holy body changed, but the power of the consecration and his life-giving grace is perpetual in it” ( Letter 83, to Calosyrius, Bishop of Arsinoe [ PG 76, 1076]). The Church teaches that Christ remains present under the appearances of bread and wine as long as the appearances of bread and wine remain (cf. Catechism, no. 1377).