Commentary for Avodah Zarah 137:11

The Talmud asks why in the first case the food is assumed to be pure and the second it is assumed to be impure. R. Yitzchak explains that in the first case the Jew made sure that his workers were pure. We should not that this is clearly not the simple reading of the baraita.
Clearly the difference is between telling them that he was going away and simply going away. The Talmud will get to such an explanation below.