I would use pure wheat bread, preferably organic, because GMO bread would be dangerous and not entirely within the spirit of the canons. Wouldn't people also be able to commune during the Saturday Liturgy of St. John Chrysostom during Lent?
What does "pure" mean for you? If you think that pure is the white part of wheat, you are wrong.
God made all the things good. Until ~ 200 years ago, there was no white bread because the corresponding mills were invented after not long ago. Therefore Christ and His Apostles communed from an integral bread.
It is scientifically proved that the best part of the wheat is the crust, the bran. Remove it and you'll loose 90% of grain's benefits for health and, in parallel, you will get in time diseases due to the big concentration of sugar and other unbalanced things.
Theologically speaking, the bread is used to become the Body of Christ - which is actually The_Church = All_of_Us -, so God uses it to gather us in one big harmonious unit. The white flour is only part of the perfect designed wheat, therefore it's inappropriate to use it for such an important purpose. We know who is called The Splitter, the one that from the beginning came into the world with lies, separating the humans. He also convinced us that it's good to separate the kernel from the peel, obtaining an inferior substance that the priest offers to God as being the Holy Gift in the end... As far as I know, we have to offer the best products for sacrifices, not the worst.
So which canons specified this practice and when do they date (originate)?