The following supports the thesis that the bread and wine offered by Melchizedek in Gen 14 was not simply a meal but a sacrificial offering:
Appearance
of a Gentile Priest
Gen 14:18-20 || Exod 18)
Genesis 14 and Exodus 18 are
remarkably similar, both in terms of shared language as well as in terms of
common themes. In both stories, a Gentile priest appears (gen 14:18; Exod 18:1)
after a successful military campaign (Gen 14:14-16; Exod 17;13) and prior to
the making of a very significant covenant (Genesis 15; Exodus 19-24).
Melchizedek’s appearance bears all the syntactical marks of an
“ungrammaticality.” If one were to remove verses 18-20 there would be no break
between the arrival of the king of Sodom and what he says to Abram. This
interruption of the flow of the “king of Sodom” narrative gives evidence that
the author has strategically placed it here following the successful military
campaign. In both stories, the Gentile priest offers and/or eats bread (לָחָם)
with the victorious party (Gen 14:18; Exod 18:12). In both stories, the Gentile
priest blesses God for the divine protection afforded the victorious party (וּבָרוּךְ֙
אֵ֣ל עֶלְי֔וֹן, Gen 14:20; בָּר֣וּךְ יְהוָ֔ה, Exod 18:10). In both stories,
the Gentile priest makes an offering in honor of the divine victory (Gen 14:18;
Exod 18;12), bread and wine in the former case, a burnt offering and sacrifice
in the letter case. And in both stories, the Gentile priest surprisingly is
granted a place of greater authority over the victorious party. Abram deferentially
offers a tithe to Melchizedek (Gen 14:20). Moses deferentially submits to
Jethro’s counsel (Exod 18:24). (Seth D. Postell, “Abram as Israel, Israel as
Abram: Literary Analogy as Macro-Structural Strategy in the Torah,” in Text
and Canon: Essays in Honor of John H. Sailhamer, ed. Robert L. Cole and
Paul J. Kissling [Eugene, Oreg.: Pickwick Publications, 2017], 23-24, emphasis added)