The matter of the material served as Holy Communion is determined by Christ Himself and reaffirmed by the Church. At the Last Supper, Christ used wine and unleavened bread. This is important for many reasons.
First, the Last Supper was believed to be celebrated in the context of the Passover Meal. At Passover Meals, matzo (unleavened bread made from wheat) is used and wine is blessed and taken. Passover marked the liberation of the Jews from the slavery of the Egyptians.
It is in this context that Jesus, Who is the Author of the New Covenant, is the new Passover. By His death and resurrection He liberates us from the slavery of sin. Jesus took bread and wine and consecrated them into His Body and Blood and prefigured His death and resurrection, so that the apostles consumed the Glorified and Risen Savior under the appearances of bread and wine.
In the time of Christ, bread and wine were considered common elements by those who lived in the Mediterranean section of the world. Indeed, wine was drunk more than water, because often water was not clean or pure. When our Lord used these common elements, He was indicating that the Eucharist was for all people, for all times even until the end of time. Bread and wine refer to the universality of God’s redemption and plenteousness of grace that He offers to humanity.
The Church continues the commission of Jesus to “do this in memory of me.” The magisterium of the Church taught in creeds, doctrines, Canon Law, and Ecumenical Council that for the validity of the mass, the matter that is used must be unleavened wheat bread and wine. The Holy Eucharist is referred to as a Holy Banquet. Even in today’s society, the two things commonly on every table at a celebration are bread and wine. Bread conveys nourishment, and wine, health and prosperity. The Eucharist gives us all three; it nourishes our souls and gives us spiritual health, and the grace of the sacrament is our prosperity.
That said, there are certain times the Church allows a slight variation of the matter of bread and wine. For example, if a priest is a recovering alcoholic, he may apply for permission to use mustum, which is grape juice that has no sugar added and is beginning to go under the fermentation process. It is fermentation that makes grape juice into wine, turning the sugar in the juice into alcohol, so mustum contains a very low percentage of alcohol—less than a half percent. Many recovering alcoholics may take a minute sip of this and not fall off their recovery, yet it is still considered wine or wine in the making.
Second, a person who is allergic to wheat (provided he is not a priest) is encouraged to receive from the chalice which contains the Precious Blood. If this is not possible, a special host that is made from very low gluten, again less than one percent, can be used as valid matter which results in incredibly little side effect.
Any other types of matter such as rice, oats, or different kinds of juices or spirits would invalidate the presence of Jesus in the Eucharist.