"Some have held that the substance of the bread and wine remains in this sacrament after the consecration. But this opinion cannot stand: first of all, because by such an opinion the truth of this sacrament is destroyed, to which it belongs that Christ's true body exists in this sacrament; which indeed was not there [...]
"At the heart of the Eucharistic celebration are the bread and wine that, by the words of Christ and the invocation of the Holy Spirit, become Christ's Body and Blood. Faithful to the Lord's command the Church continues to do, in his memory and until his glorious return, what he did on the eve of [...]
"There is no quantity, however small, of the bread and wine which cannot be consecrated. Therefore, neither is there any quantity, however great, which cannot be consecrated." Source: St. Thomas Aquinas, Summa Theologica