The Halacha of "Hatmana" (insulating) forbids insulating a food before or during Shabbat to maintain or raise its level of heat. In light of this Halacha, the question arises as to whether one may place a baby’s milk bottle in hot water to warm it on Shabbat. For example, a woman may wish to pour some hot water from an urn on Shabbat into a bowl, and then place the milk bottle into the bowl to warm the milk for the infant. Seemingly, this should be forbidden on Shabbat due to the prohibition of "Hatmana," which forbids maintaining or increasing a food’s heat by insulating it. In this case, the cold milk is "insulated" by the hot water, which seemingly violates the prohibition of "Hatmana."
Hacham Ben Sion Abba Shaul (Israel, 1923-1998), however, in his work Or Le’sion (vol. 2), rules that one may heat a milk bottle in this fashion – in a "Keli Sheni" (a utensil into which water had been poured from an urn, kettle or pot) – provided that it is not entirely submerged in the water. As long as part of the bottle remains out of the water, it is permissible to place the bottle in a bowl of hot water. It is forbidden, however, to entirely submerge a milk bottle in a bowl of hot water.
Many people have the practice of placing a bag of raw rice in their pot of Hamin ("cholent") before Shabbat, so that the rice will cook inside the pot and be ready for Shabbat lunch. Similarly, some people wrap a piece of raw "kishkeh" in aluminum foil and place it inside their pot of Hamin to cook. Is this a permissible practice, or does it violate the prohibition of "Hatmana," insofar as it entails "insulating" a food to increase its heat?
Some authorities, including the Shebet Halevi (Rav Shemuel Wosner, Bnei Brak, contemporary), indeed forbid placing raw food in a Hamin pot in this fashion, viewing this practice as a violation of "Hatmana." Others, however, including Hacham Ben Sion Abba Shaul, rule leniently in this regard. Hacham Ben Sion argues that one does not intend to insulate the rice in the Hamin, but rather to cook it in the pot together with the Hamin. The bag is used simply to prevent the rice from mixing with the Hamin. Therefore, since the intention is to cook the rice in the pot and not to insulate it in the Hamin, this practice does not violate the prohibition of "Hatmana." This is also the position of Rav Shlomo Zalman Auerbach (Jerusalem, 1910-1995). Therefore, those who are accustomed to cooking rice, "kishkeh" or other foods in this fashion certainly have authorities on whom to rely.
Summary: It is permissible to warm a baby’s milk bottle in a bowl of hot water on Shabbat provided that part of the bottle remains outside the water; it is forbidden to submerge the entire bottle in hot water. There is a dispute among the Halachic authorities as to whether one may place raw food – like rice or "kishkeh" – in a bag or aluminum foil inside a pot of Hamin before Shabbat for it to cook by Shabbat morning. Those who wish to be lenient may do so, as they have authorities on whom to rely.