So according to Catholicism, the wafer/wine becomes Jesus' body/blood instantly when the priest says so, but when does it stop being Jesus' body/blood?
When it is chewed up, or when it is in the throat, or when it reaches the stomach, or when it passes through the intestines, or when it is evacuated out of the body? Or does it never stop being Jesus' body?
To complement Belinda’s answer, although the Church does not descend in so much detail, St. Thomas Aquinas in his Summa theologiae makes the following commentary in III Pars, q. 77, a. 4, responsum.
But if the change be so great that the substance of the bread or wine would have been corrupted, then Christ's body and blood do not remain under this sacrament; and this either on the part of the qualities, as when the color, savor, and other qualities of the bread and wine are so altered as to be incompatible with the nature of bread or of wine; or else on the part of the quantity, as, for instance, if the bread be reduced to fine particles, or the wine divided into such tiny drops that the species of bread or wine no longer remain.
In other words, once the Eucharistic species suffer a change sufficiently great that they no longer have the properties of bread or wine, then the sacramental presence ceases.
Hence, in essence, for the host, the Presence remains until it has been dissolved into a paste or divided into minuscule particles; for the wine, the Presence remains until it is diluted or divided into minuscule droplets.
(That is why, for example, the priest, deacon, or acolyte uses water to purify the vessels after Communion: this action dissolves any remaining particles of the host and dilutes any remaining drops of the Precious Blood. Although minuscule particles of the Host technically do not retain the Presence, out of reverence for the Presence that was once there, care should be taken lest these fall to the floor. Thus, for example, any breaking of the host should always be done over the corporal—the white cloth on which the species are consecrated—and when Communion is given out, someone should generally hold a paten or other receptacle underneath to catch any falling hosts or particles.)
It is hard to pinpoint the exact moment after receiving Communion that the Presence ceases, but these reflections should give the general idea.
The Eucharistic presence of Christ begins at the moment of the consecration and endures as long as the Eucharistic species subsist. Christ is present whole and entire in each of the species and whole and entire in each of their parts, in such a way that the breaking of the bread does not divide Christ
The Eucharist remains Christ's body until it is broken down in our digestive systems.
There have been DNA studies that refute the claim of turning the bread into Christ’s body. The Eucharist was obtained during mass and placed in plastic bags instead of ingesting it. This was done by ex-Catholics who had left the church because they felt violated by the church’s doctrines.
The DNA showed no change had occurred.
However, I believe the faith of the person receiving the host plays a major part of the transformation. Once the Eucharist leaves the priests hands and given to another of faith it would retain the transformation to the body of Christ. If given to a faithless person, the miracle ceases to be.
