Read page 217-218 and 238 of The Church’s Liturgy By Michael Kunzler. This is the first volume published in English of a series of International Handbooks of Catholic Theology under the general editorship of Cardinal Christoph Schnborn, Archbishop of Vienna.
The Church’s Liturgy By Michael Kunzler (page 217-218)
The main information you want is in section 3.6.2. The “Fruits of the Earth and Work of Human Hands”, at the bottom of page 217, but there is also relevant information just prior to that
The Church’s Liturgy By Michael Kunzler (page 238)
Read section 3.8.3. The Fraction, Agnus Dei and Commingling.
It speaks of the West’s former traditional use of leavened bread for the eucharist.
Primary Readings on the Eucharist
This goes into the reason why the West introduced unleavened bread for the eucharist around the 800’s
Fr. Joseph Jungman – in his book The Mass of the Roman Rite – states that:
"In the West, various ordinances appeared from the ninth century on, all demanding the exclusive use of unleavened bread for the Eucharist. A growing solicitude for the Blessed Sacrament and a desire to employ only the best and whitest bread, along with various scriptural considerations – all favored this development.
Code:
"Still, the new custom did not come into exclusive vogue until the middle of the eleventh century. Particularly in Rome it was not universally accepted till after the general infiltration of various usages from the North" [Joseph Jungman, The Mass of the Roman Rite, volume II, pages 33-34]
Fr. Jungman goes on to say that,
“. . . the opinion put forward by J. Mabillon, Dissertatio de pane eucharistia, in his answer to the Jesuit J. Sirmond, Disquisitio de azymo, namely, that in the West it was always the practice to use only unleavened bread, is no longer tenable” [Jungman, The Mass of the Roman Rite, volume II, page 33]
Now, the fact that the West changed its practice and began using unleavened bread in the 8th and 9th century – instead of the traditional leavened bread – is confirmed by the research of Fr. William O’Shea, who noted that along with various other innovative practices from Northern Europe, the use of unleavened bread began to infiltrate into the Roman liturgy at the end of the first millennium, because as he put it,
“Another change introduced into the Roman Rite in France and Germany at the time * was the use of unleavened bread and of thin white wafers or hosts instead of the loaves of leavened bread used hitherto” [Fr. William O’Shea, The Worship of the Church, page 128].*
Moreover, this change in Western liturgical practice was also noted by Dr. Johannes H. Emminghaus in his book, The Eucharist: Essence, Form, Celebration, because as he said:
“The Eucharistic bread has been unleavened in the Latin rite since the 8th century – that is, it is prepared simply from flour and water, without the addition of leaven or yeast. . . . in the first millennium of the Church’s history, both in East and West, the bread normally used for the Eucharist was ordinary ‘daily bread,’ that is, leavened bread, and the Eastern Church uses it still today; for the most part, they strictly forbid the use of unleavened bread. The Latin Church, by contrast, has not considered this question very important.” [Dr. Johannes H. Emminghaus, The Eucharist: Essence, Form, Celebration, page 162]
In reading some of these quotes, I don’t see how they are inconsistent with is what is in the Catholic Encyclopedia. Since the West acknowledges both leavened and unleavened historically, it doesn’t make any difference if in the 9th century there was call for exclusively unleavened bread. That wouldn’t make it a novelty. No? In some of my research from memory, part of the fear of using unleavened bread was the appearance of appearing too “Jewish,” because that’s what was used for Passover. But, just as with the issue of iconography, which had to lose the stigma in the early Church of being idolatrous, once the Church established distance from the stigma, it became more widely accepted. Even some of the quotes you cite above had to concede that leavened bread was “for the most part” used in whatever historical sample they examined. Bottom line, it seems there is evidence and historians willing to admit that unleavened bread was indeed used in authentic Christian Churches even early on.