Genesis 39:6

And he left all that he had in Joseph's hand, and took no knowledge of anything of his, except his wife with whom he lay. And Joseph was of goodly form and beautiful aspect.

Except for the bread he ate. He would not allow Yoseif to touch his bread because he was a Hebrew; see 43:32.

SAVE THE BREAD WHICH HE DID EAT. In the words of our Rabbis, this is a refined expression which refers to his wife.
Rabbi Abraham ibn Ezra said in interpretation of the verse that whatever Potiphar possessed was left in the hands of Joseph excepting the bread which he ate. This he did not even permit him to touch since he was a Hebrew. It was the customary behavior of the Egyptians towards the Hebrews that they not permit the Hebrews to touch their food, because that is abhorrent to the Egyptians.
Possibly this is so. Perhaps the interpretation of the verse is that his lord did not know of Joseph taking anything from him save only the bread which Joseph ate, but no other pleasures as young people are wont to do. Nor did he gather wealth and property, just as it is said of David, And I have found no fault in him since he fell unto me unto this day. Now the verse, Having me, he knoweth not what is in the house, expresses another matter, namely, that he [Joseph’s lord] did not trouble himself to know about anything inside the house. But the present verse, Having him, he knew not aught, is an expression of negation; he knew that nothing in the house is [taken by Joseph except the bread which he eats].

AND JOSEPH WAS HANDSOME AND GOOD-LOOKING. The verse mentions this here in order to indicate that it was on account of his good looks that his master’s wife cast her eyes upon him. And Rashi wrote that because he saw that he was ruler of the house, he began to eat and drink, and curl his hair, etc.

ולא ידע אתו מאומה, in the house

כי אם הלחם אשר הוא אוכל, he did not oversee any of the affairs of his household, not caring where everything originated until the food came on his table, when he proceeded to eat it. The word לחם is an inclusive term applying to any manner of food. It is used in the same sense in Daniel 5,1 עבד לחם רב, “he made a great banquet.” Also in Job 3,24 כי לפני לחמי אנחתי, “my groaning serves me as my food.”

ויעזוב כל אשר לו, he left everything in Joseph’s hands without demanding an accounting from him.

ויהי יוסף יפה תואר ויפה מראה, after Potiphar had entrusted him with all these important tasks Joseph found time to make himself look handsome having no longer to perform demeaning physical labour assigned to most slaves. Psalms 81,7 extols the relief felt when one is freed from such burdensome tasks.

ולא ידע אתו מאומה, he did not involve himself in anything concerning the affairs of his household. Everything rested on Joseph’s shoulders. Everything that needed to be done in the household other than matters relating to the food served in the house. The reason why Joseph was not charged with this relatively minor task was the fact that he was from a different people. We know already from 43,32 that the family of Yaakov, loosely known as העברים, “the Hebrews,” (from the sticks, i.e. a culture beyond the Euphrates river) had such different eating habits that the Egyptians could not eat at the same table with them. Joseph had not changed his eating habits. Other commentators interpret the line כי אם הלחם אשר הוא אוכל, as a euphemism applying to Potiphar’s wife.

ויהי יוסף יפה תואר, the reason why the Torah mentions that Joseph was handsome is only to help us understand why a highly placed person such as Mrs Potiphar would try to get involved with a foreign-born slave. Joseph was so handsome in appearance. We already explained the nuances of the word יפה תואר and יפה מראה respectively in connection with Rachel (29,17).

כי אם הלחם אשר הוא אוכל, (literally) “except for the food that he ate.” We find two more references in the Torah describing that the Egyptians and Hebrews had a mutual distaste for each other’s eating habits and menus. (Compare Genesis 43,32, and verse 9 in this chapter, where Potiphar’s wife is described as out of bounds to Joseph [similar to his bread. Joseph was not afraid to make this comparison. Ed.]