The bread to be consecrated must be wheat bread. If it is not wheat, or not bread, it is not valid matter. The wheat must have some gluten in it, because gluten is necessary for it to be wheat.
Not Valid Matter:
* Anything that is not wheat: tubers – such as potato, sweet potato, yuca.
* Grains that are not wheat: rye, triticale (a rye-wheat cross), oats, etc.
* Pseudo-cereals: amaranth, quinoa, buckwheat, hemp.
* Totally-gluten-free wheat is not wheat, and so is not valid.
* Some wheat products are not bread: semolina, freekeh, bulgar, farro; those are all made from wheat, but in a way that makes it not bread.
* whole, refined, or enriched wheat.
* GMO wheat.
* wheat with a rye gene added.
* varieties of wheat: kamut, emmer, durum, spelt, einkorn, wheat-wheat crosses.
* modern wheat is a cross of three ancient varieties of wheat.
* very low-gluten wheat, even if it is so low in gluten (<20 ppm) as to legally qualify as "gluten free".
“But the Food and Drug Administration’s guidelines for a gluten-free designation is that the food must contain less than 20 parts per million gluten. The U.S.C.C.B.’s approved providers of low-gluten Communion hosts have less than 20 parts per million of gluten. So while the church considers “low-gluten” hosts to have enough gluten to be “valid matter,” the F.D.A. says the hosts are low enough in gluten to be considered gluten-free.” [Explainer]
In the 1940’s, a plant disease called “rust” threatened to devastate the wheat crop. The solution was to cross wheat with rye, and then continue crossing the resultant plant with wheat, again and again, until the end result was wheat, but with a rye gene resistant to rust retained. That is the type of wheat used for decades in the Eucharist in most places.
Is leavened bread valid matter?
Yes, it is valid, but illicit. The reason it is valid is that wheat grains in the field have yeast on them. It is still possible with whole-grain wheat flour to leaven the bread without adding wheat. You just take advantage of the yeast already in the flour. So unleavened bread has usually had some leaven (yeast) added to it. It is not allowed to rise. But the yeast may be somewhat active, causing a minute amount of rising. This is analogous to the minute amount of gluten that still makes the wheat valid matter. So the minute amount of rising makes most bread, especially before modern flour processing (which kills the yeast), still somewhat leaven.
Can. 926 According to the ancient tradition of the Latin Church, the priest is to use unleavened bread in the eucharistic celebration whenever he offers it.
It is an ancient tradition to use unleavened bread, but it is not absolutely required for validity.
For the wine to be consecrated:
It must be grape wine with at least a little alcohol. Mustum is wine with very little alcohol as the fermentation process is stopped shortly after it begins. But there must be some fermentation and some alcohol for valid matter.
The wine used should be red wine, as it resembles blood, and so it more fitting for the symbolic aspects of the Sacrament. All Sacraments have a symbolic aspect, even the Eucharist, which is literally Christ himself under appearances of bread and wine.
White wine is permissible, as are other types of grape wine.
Not valid matter: non-grape wine; wine that has added alcohol (fortified wine); wine that has any non-wine liquid added.
Valid matter: wine with sulfites added; wine from GMO grapes (does this exist?); any wine made from grapes of various varieties or grape-grape crosses.
A recent discussion about permitting a tuber called yuca, to be used in the Amazon is not an open question. Valid matter must use wheat only. Yuca is a tuber, not wheat and not even a grain.