vayikach chem'ah vechalav uven-habakar - he took butter, milk, and the calf
No mention is made of Avraham serving the bread. Our Sages (Bava Metzia 87) use this to surmise that at that time Sarah became a menstruant, (although she had long ceased to be), and that since Avraham was in the habit of eating even ordinary food only if it could have been eaten by a Kohen in a state of ritual purity, he did not want to offer such food to his guests either.
Some people ask that seeing there was no halachic reason that Avraham should impose such restrictions on people other than himself, it seems unreasonable for him to not serve the very thing he had first said he would offer. These scholars suggest a different reason for Avraham not serving the bread. They claim it was the date on which Pesach would occur in the future and that this was the reason he spoke of the bead in terms of ugot (cakes), the Torah completes the expression by adding the word matzot (unleavened bread), when the same men arrive at Lot's home in Sedom the same evening (19:3) and the Torah reports that this is what Lot served these men. It is typical of the Torah to reveal hints of something here and there. Something not made clear in one part of the Torah can often be understood properly only when comparing two different sections of the Torah.