The "true" bread

In the Eastern Church the breads were made by consecrated virgins; in the Western Church, by priests and clerics (Benedict XIV, De Sacrif. Missae, I, section 36).

For valid consecration the hosts must be:
• made of wheaten flour,
• mixed with pure natural water,
• baked in an oven, or between two heated iron moulds, and
• they must not be corrupted (Miss. Rom., De Defectibus, III, 1).

If the host is not made of wheaten flour, or is mixed with flour of another kind in such quantity that it cannot be called wheat bread, it may not be used (ibid.). If not natural but distilled water is used, the consecration becomes of doubtful validity (ibid., 2). If the host begins to be corrupt, it would be a grievous offence to use it, but it is considered valid matter (ibid., 3.) For licit consecration:

• the bread must be, at present unleavened in the Western Church, but leavened bread in the Eastern Church, except among the Maronites, the Armenians, and in the Churches of Jerusalem and Alexandria, where it is unleavened.

http://www.newadvent.org/cathen/01349d.htm