And they set on for him by himself, and for them by themselves, and for the Egyptians, which did eat with him, by themselves. "Joseph eats apart from his brethren, keeping strictly to the Egyptian mode; and the history does not omit to remark that in this point he adhered to the custom of the country" (Havernick, 21).
Because the Egyptians might not eat bread with the Hebrews. Herodotus (2.41) affirms that the Egyptians would neither use the knife, spit, or basin of a Grecian, nor taste the flesh of a clean cow if it happened to be cut with a Grecian knife.
For that is an abomination unto the Egyptians. The reason for this separation from foreigners being that they dreaded being polluted by such as killed and ate cows, which animals were held in high veneration in Egypt.