They stood in front of him and did not know him. Targum Pseudo-Jonathan on Genesis 42:7 records the moment: Joseph saw his brothers, recognized them, and then "made himself as a stranger in their eyes, and spake hard words to them."
The performance of a stranger
The Aramaic phrasing is careful. Joseph does not become a stranger; he performs strangeness. The Hebrew verb vayitnaker carries the same reflexive sense — he disguised himself, he acted unknown. His voice deepens, his questions turn cold, his Egyptian interpreter (Genesis 42:23) stands between him and the brothers so they never hear the Hebrew of their childhood. Joseph is choreographing this meeting down to the accent.
Why speak harshly?
The rabbinic tradition asks the obvious question. If Joseph recognized them, why not embrace them? The answer running through Bereishit Rabbah 91:6, a commentary on Genesis compiled in the Land of Israel around the fifth century CE, is that forgiveness without testing would have been premature. Joseph needed to know what the brothers had become. Were they still the men who sold him? Or had twenty-two years changed them? The hard words were a diagnostic tool. He had to see how they would act under pressure before he could know what to do with them.
The takeaway
Sometimes silence is the wisest first response to reunion. Joseph withholds himself not from cruelty but from the need to see what his family has become before he tells them who he has become.