Question: What’s the difference between rinsing off fruit which is forbidden on Shabbos according to the Chazon Ish and rinsing dishes on Shabbos which is permitted on Shabbos according to the Chazon Ish?
A. The Mishna Berura (319:8:28) says that one cannot soak fruits or vegetables in water to get off dirt on shabbos because it is borer (choosing bad from the good, using the water as a utensil).
B. The gemara in Shabbos 118b says you are allowed to rinse your dishes in running water or dip then in and out to clean the dishes because it’s cleaning not borer. Rav Moshe Feinstein, Rav Shlomo Zalman Aurbach and Rav Elyashiv all have a side to say that one can also rinse fruit and vegetables for the same reason as the dishes that it’s just cleaning not borer, if you do it right before you eat it. But Rav Elyashiv says it’s better to rinse fruit and vegetables before shabbos.
C. Hint: Borer is fixing the object being chosen.
Answer: Dirshu footnote 32: Rav Nissim Karelitz explains the Chazon Ish’s view that since borer is fixing the object being chosen, but vessels, since the way of using vessels is to use them, wash them and clean them off after each usage, then rinsing the vessel from its dirt is not considered fixing the vessel. But removing the dirt from fruits and vegetables is fixing the fruit and is considered borer. Rav Shlomo Zalman Aurbach adds that vessels last a while, so even if there is dirt or food stuck to it, it’s not a mixture and removing it isn’t considered washing or borer, however food, since it’s nature is to rot and go bad, therefore if dirt or the like is mixed in with it, then removing it is considered borer.