Leaven or Unleavened?

From a discussion in CDF:
Sandra has challenged Dana to document that the Latin
Church has always used unleavened bread. That too has
been (and remains) my belief as well. Does this mean
the Latin Church ALWAYS used unleavened bread? No!
There may have been times when leavened bread was used,
but the NORM of the Latin Church has ALWAYS been to use
unleavened bread.

The controversy really arose at the time of the Great
Schism between the Oriental and Occidental (Eastern
and Western or Latin) churches. Prior to the schism
though the Latin Church almost, if not exclusively
used unleavened bread - the Greek (Eastern) Church
almost exclusively used leavened bread.

Why the difference? Does the use of one or the other
invalidate the consecration of the Eucharist? Well,
answering the latter question first - it depends on
whom you ask! Eastern Orthodox Christians may say
the use of unleavened bread invalidates the
consecration. Eastern Orthodoxy has long been very
anti-Jewish. Just about anything Jewish or of Jewish
tradition is rejected by Eastern Orthodoxy. The
"turbulent Michael Cerularius Patriarch of
Constantinople in 1043 in order to make the rupture
between the two churches as great as possible so
far as to assert that consecration in any other
bread leavened was invalid and that hence the whole
Latin Church was heretical because it used unleavened
But the theologians never adopted this teaching nor
is it to day although with the exception of the
Armenians (and) Maronites all the Oriental churches
follow the Greek in the use of leavened bread."
http://books.google.com/books
Another text only view of O'Brien's book:
http://www.archive.org/stream/historyofmass00obriuoft/historyofmass00obriuoft_djvu.txt
Or in PDF:
http://www.cimmay.us/oncall/pc_obrian.pdf
[A History of the Mass and its Ceremonies in the
Eastern and Western Church, pg 154, By John O'Brien]

So we can see from John O'Brien's research that the
Latin Church used unleavened bread and by the decree
of the Patriarch of Constantinople in 1043, he was
declaring the whole of the Latin Church to be in
heresy. In short, it was a political move of the
Eastern Patriarch which was not and is not held by
theologians - even though they do exclusively use
leavened bread - with the exception of the Maronites
and the Armenians.

Why do the Greeks use leavened bread?
It is really a matter of symbolism, with leaven
being, in their view, more symbolic of the body
of Christ. Leaven is life whereas unleavened is
a bread of affliction. The Orthodox see Passover
as a feast of hardship and affliction and since
leaven is life - it is a feast of thanksgiving
and "Eucharist" is defined as "thanksgiving."
http://www.prosphora.org/page27.html

Why do the Latins use unleavened bread?
Latins use unleavened bread because Jesus was
celebrating the Day of Unleavened Bread, or the
Pasch. During this Passover time no leaven is
permitted in the homes of any faithful Jews,
(Exodus 12:15), since Jesus would not have
sinned in breaking that law, He could not have
had leavened bread during the Pasch. Latins
take the command to "Do this in remembrance
of Me" (Luke 22:19) quite literally, they do as
Jesus Christ did.

Does using leavened v. unleavened invalidate?
No. Whether the bread is leavened or unleavened
is a matter of discipline, not dogma. It would be,
however, illicit for a Latin Rite priest to use leavened
bread or an Eastern Rite priest to use unleavened.
Each church has their own set of disciplines but is
is not dogma for either of them, however not just
any flour will do!
Can. 924 §1 The most holy Sacrifice of the Eucharist
must be celebrated in bread, and in wine to which a
small quantity of water is to be added.
§2 The bread must be wheaten only, and recently made,
so that there is no danger of corruption.
The above is from Canon Law in the Latin Church, but
for Eastern tradition, though they use leaven (yeast)
the requirements of purity are still there.
Click herefor an Eastern Orthodox presentation of Prosphora.

Conclusion
In short, exactly what the earliest of the Early Church did
is in debate. The fact of the matter is that the use of
leaven or unleavened is a matter of discipline - not dogma.
A Latin priest could use leavened bread, but that would be
illicit but still valid (consecration does take place, it is the
Eucharist). Both sides in this debate have valid points.