Many people prepare cholent (or "Hamin") for Shabbat by placing meat, potatoes and other foods and flavoring into the crockpot before Shabbat, and letting it cook until Shabbat morning. Sometimes, one may wish to cook another food, such as rice, by placing it in a cooking bag inside the cholent. The rice remains separate from the cholent, as it is held in the bag, and cooks from the heat of the cholent cooking in the crockpot.
At first glance, we might forbid this practice on the grounds of "Hatmana," or "insulating." Halacha forbids insulating food in a heat source even before Shabbat, if the insulation increases the foods temperature. In the case described, one insulates the rice inside the hot cholent, which obviously adds heat to the rice and enables it to cook, which would seemingly constitute a violation of "Hatmana." Indeed, Rabbi Moshe Halevi (Israel, 1961-2001), in his work Menuhat Ahaba, forbade putting food in a cooking bag inside the cholent pot before Shabbat.
Other authorities, however, including Hacham Ovadia Yosef, Hacham Ben Sion Abba Shaul (Israel, 1923-1998) and the Hazon Ish (Rabbi Avaham Yeshaya Karelitz, 1879-1954), ruled leniently on this issue. The prohibition of "Hatmana" does not apply to insulating food in other food. The classic case of "Hatmana" is wrapping a pot with cloths and fabrics such as towels, and enclosing food with other food does not violate this prohibition. As such, one may insulate the rice inside the cholent before Shabbat.
One might have argued that "Hatmana" should apply in this case, since the rice is not insulated directly in the cholent, but rather in a bag. Insulating with a bag indeed constitutes "Hatmana," and thus seemingly this should be forbidden. This argument, however, fails to take into account the fact that the bag is not intended as an insulating material. The person placed the rice in the bag not for the bag to add heat, but simply to keep the rice separate from the cholent. It is the cholent, not the rice, which serves the purpose of insulation, and thus since "Hatmana" does not apply when insulating with food, this arrangement would not violate "Hatmana."
Summary: It is permissible to place a cooking bag with raw food inside ones cholent in the crockpot before Shabbat to cook and be ready for the Shabbat morning meal.
