By: Halacha Yomit team of OU KOSHER

I’m preparing a picnic. Can I buy sliced watermelon from the supermarket?

The Shach (YD 96:3) cites a concern that a knife often contains a fatty residue even after it has been washed or wiped with a rag. Therefore, if a non-kosher knife was used to cut kosher food, some of the residue on the knife would transfer to the food. Rema (96:1, 4) writes that with regards to fruit, we can assume that the manufacturer or processor has dedicated utensils. Even if the knife is not dedicated to cutting fruit, if large quantities of fruit are being cut or sliced, we can assume that whatever non-kosher residue was on the knife was removed when cutting the first few fruits, which are batel (nullified) against the majority of other fruit.

One may, therefore, purchase cut watermelon in a supermarket or in a fruit store. The market would likely have dedicated utensils for cutting fruit, and in any event is preparing large quantities of fruit. In a non-kosher restaurant or catered event, however, the fruit would not be permitted because the knives very likely are not dedicated and food preparation switches from one product to the next.