Genesis 42:8

And Joseph knew his brethren, but they knew not him. The lapse of time since the tragedy of Dothan, twenty years before, the high position occupied by Joseph, the Egyptian manners he had by this time assumed, and the strange tongue m which he conversed with them, all conspired to prevent Jacob's sons from recognizing their younger brother; while the facts that Joseph's brethren were all grown men when he had last looked upon them, that he was quite familiar with their appearances, and that he perfectly understood their speech, would account for his almost instantaneous detection of them.