Sadly, there are some priests who feel compelled to tinker with the rubrics of the Mass–rubrics which the Church in its wisdom set forth for good reasons. Your question comes up frequently from folks who have witnessed priests consecrate (more properly, attempt to consecrate) something other than bread at Mass. Some commit this grave abuse due to poor or heterodox theological training in the seminary, others because they know better but just want to be avant garde, thinking such innovations will endear them to the congregation and make the Mass more “relevant.” Either way, this indicates a lack of belief in the Real Presence of Jesus in the Eucharist and an obstinacy against obedience to the Church.
A noted liturgical expert Nicholas Halligan, O.P., explains the rules regarding the bread to be used for the Eucharist:
The requisite material for the celebration of the Eucharist and the confection of the sacrament is only wheaten bread, recently made whereby the danger of corruption is avoided . . . Unleavened bread alone is to be used in the Latin Rite (canons 924, 926). The bread must be made from wheat, mixed with natural water, baked by the application of fire heat (including electric cooking) and substantially uncorrupted.
The variety of wheat or the region of its origin does not affect its validity, but bread made from any other grain [e.g. barley, corn, rice, peanuts] is invalid material. Bread made with milk, wine, oil, etc., either entirely or in a notable part, is invalid material. Any natural water suffices for validity, (e.g. even mineral water or sea water). The addition of any condiment, such as salt or sugar, is unlawful but valid, unless added in a notable quantity. Unbaked dough or dough fried in butter or cooked in water is invalid matter. . . .
Therefore, the valid material of this sacrament must be made in the common estimation of reasonable men bread made from wheat and not mixed notably with something else that is no longer wheat. Those who make altar breads must be satisfied that they have purchased genuine and pure wheat flour. The bread must be of wheat flour and only in case of necessity a white material thrashed or crushed from wheat. It must be free from mixture with any other substance besides wheat flour and water. It is gravely unlawful to consecrate with doubtful material. (The Sacraments and Their Celebration [Staten Island: Alba House, 1986] 65-66)