I wrote about the Vatican directives for the bread and wine in the Eucharist for Religion Dispatches:
The genius of the eucharist, in strictly human terms, is in its simplicity: bread and wine, the everyday foods of Mediterranean antiquity, enter the ritual process to become body and blood. The elements are plain and common, so humble as to be almost embarrassing. Yet they represent the nearly miraculous transformation of nature’s abundance by human ingenuity and labor. Body and blood, the ritual products, are scarcely less so. Bread becomes body all the time, in a sense, wherever it is eaten, imparting its peculiar alimentary virtues to a small but complex animal that then, for example, sits and types words onto a screen.
Whether as a holy sacrament or a randomly-generated meme, the practice is perfect. Bread, body, nature, culture, always at hand and always primed for a miraculous transformation of the humble.
So it should come as no surprise that Christian churches sometimes go to considerable lengths to protect and preserve the practice of the eucharist from variations or influences that could distort or obscure its compelling core.