The ruling came from Cardinal Robert Sarah of the Vatican’s Congregation for Divine Worship and the Discipline of the Sacraments.
Sarah stated that the issue was a simple matter of wheat:
“The bread used in the celebration of the Most Holy Eucharistic Sacrifice must be unleavened, purely of wheat, and recently made so that there is no danger of decomposition. It follows therefore that bread made from another substance, even if it is grain, or if it is mixed with another substance different from wheat to such an extent that it would not commonly be considered wheat bread, does not constitute valid matter for confecting the Sacrifice and the Eucharistic Sacrament.”
The communion wafer used in the holy rite of transubstantiation must remain primarily wheat. So to is the purity of the wine which must be “natural, from the fruit of the grape, pure and incorrupt, not mixed with other substances.” ,
Here is the letter: Communion Letter