Genesis 42:7

‘And Joseph saw his brothers, and he knew them, but he made himself a stranger to them and spoke harshly to them. And he said to them, “From where have you come?” And they said, “From the land of Canaan to buy food.”

When Joseph saw his brothers he knew them immediately, but he did not make himself known. Rather he signalled to his underlings to bring them forward so that he could speak with them. They were probably quite apprehensive at being selected out to speak to this great Egyptian overlord, and were even more so when he addressed them harshly. They must have wondered why they should be picked out. All they could do was answer his questions and hope for the best.