By the OU Kosher Halacha Yomis Team

Can I make oatmeal on Shabbat by putting oats into a bowl of hot water?

One might think that retail oats are considered mevushal (cooked) because they are steamed before they are sold. As such, it should be permissible to put oats in hot water because of the principle ein bishul achar bishul (it is permissible to reheat cooked food on Shabbos). However, this is not the case. This heat treatment is done to neutralize an enzyme in the oats which will make the oats turn rancid if left unchecked. This steaming process does not affect the taste of the oats or their edibility, and therefore, halachically, the oats are treated as a raw food.

Can oats be placed in a kli shelishi? Many poskim allow putting raw food in a kli shelishi (third vessel) on Shabbat because the food will not be cooked. Still, Rav Belsky zt”l thought this is not permissible. The main difference between instant oats and regular oats is how thinly the oats are sliced. The thinner the oat slices, the faster they will cook. Rav Belsky said that since raw oats are partially cooked by steam, and we see that they can cook very easily when sliced thin, oats have the status of kalei ha’bishul (foods that cook very easily). Therefore, one may not prepare oatmeal even in a kli shlishi, since even this minimal amount of cooking would constitute bishul on Shabbat.