Over the centuries East and West have argued when, precisely, the body and blood of Christ become present in the Eucharist.
According to Johannes Emminghaus in The Eucharist, a practical question was at the base of the discussion—"What is to be done if, for some reason (for example, the sudden death of the celebrant), the Canon is broken off? When could the bread and wine simply be removed? From what point on is it consecrated?"
The Western Church asserted the Body and Blood are present at the completion of the words of consecration. The Eastern Church supported the view that the real presence takes place through the epiclesis (the prayer for the sending or coming of the Spirit to sanctify the gifts of bread and wine).
According to Richard McBrien in his Encyclopedia of Catholicism, ecumenical theologians avoid attempts to locate a moment of consecration at either the epiclesis or words of institution. They prefer, he says, to consider the entire prayer over the gifts, and not one of its isolated moments, as the consecratory prayer.
Emminghaus observes that the Church has never made a dogmatic pronouncement on the point.
Ludwig Ott, however, in his Fundamentals of Catholic Dogma it as certain that the form of the Eucharist consists of Christ's words of institution uttered at the consecration. And Ott cites the Council of Trent as teaching, according to the standing belief of the Church, "'immediately after the consecration,' that is, after the uttering of the words of institution, the true body and the true blood of the Lord are present."