"St. Thomas. . . . observes that in the Eucharistic Prayer Christ is quoted not as saying, 'This bread is my body,' but 'This is my body.' If Christ had said 'this bread' was his body, then the thing referred to would still be bread, but the simple demonstrative pronoun 'this' without a noun implies that it is not bread any longer."
Robert Sokolowski, "The Eucharist and Transubstantiation" (1997), in Christian faith and human understanding: studies on the Eucharist, Trinity, and the human person (Washington, D.C.: The Catholic University of American Press, 2006), 105-106 (95-112), citing Summa theologiae III.78.5.Resp. (Latin here), as well as 75.3 and 75.8.