The principles of Hatmana can be applied to numerous practical issues. One question that arises concerns wrapped foods submersed in a hot liquid. For example, is it permitted to place a kugel wrapped in foil or rice in a cooking bag inside the Hamin before Shabbat? Is the wrapping considered a type of insulation?
One could argue that on Erev Shabbat, it is permitted to insulate with foil and cooking bags, as they are not Mosif Hevel; they don’t generate their own heat. Nevertheless, Shulhan Aruch holds that even non-heat generating insulants when placed on a surface which is Mosif Hevel, namely, the pot that's cooking the hamin, the Hatmana is considered Mosif Hevel. Thus, the question remains as to whether it is permitted to submerse wrapped foods in the hamin on Erev Shabbat.
Hacham Ovadia, in Hazon Ovadia (Section 1, p. 61) rules that it is permitted. He outlines four reasons to be lenient. First, the Shulhan Aruch (318:4) holds that placing a food within a food is not considered Hatmana. Secondly, there are opinions that there is no issue of Hatmana if one is not planning to eat the food until the morning. In our case the hamin is meant for the morning. Although, we do not hold like that opinion, it can be relied on in conjunction with other factors. Third, the Rama (Rabbi Moshe Isserles, Cracow, 1520-1572) brings down an opinion that if the food is fully cooked, there's no problem of Hatmana. In our case the kugel or rice is fully cooked. Again, we would not rely on this opinion alone; but, it can be used along with other justifications.
Finally, the main reason for being lenient is that the definition of Hatmana is determined by the intent of the person. That is, for what purpose is he covering and insulating this item? In our case, his intent is clearly not to retain the heat of the food. Even without the wrapping, the kugel or rice would stay hot. Obviously, he wrapped it so that the kugel or rice won’t intermix with the other foods in the pot; he wants it to remain separate. Therefore, since his intent is not l'shem hatmana, for the sake of insulating heat, rather, it's for separation, it is permitted.
Hacham Ben Sion Abba Shaul (Israel, 1923-1998) also rules, that it's permissible to put food in a bag or aluminum foil and submerse it in the hamin(Or L'Sion, Hilchot Shabbat). On the other hand, the Menuhat Ahaba (Rabbi Moshe Halevi Israel, 1961-2001), and the Shebet Ha’levi (Rav Shemuel Wosner, 1913-2015) disagreed and held that it is prohibited. However, we rely on Hacham Ovadia and Hacham Ben Sion.
Another question that arises is regarding the custom to put an egg in the hamin before Shabbat. There are those who argue that the shell of the egg serves to insulate the egg and would pose a problem of Hatmana. Hacham Ovadia rejects this view, citing the Rashba who points out that an egg shell cannot be considered an insulating element because it is porous. This can be demonstrated by conducting an experiment in which the egg is immersed in colored water; the result, of course, is that the egg itself becomes colored. Therefore, the egg is not considered a separate entity from the rest of the hamin. Just as there is no Hatmana when one cooks rice and potatoes together, so too, the egg is considered part of the mixture. Thus, it is permissible on Erev Shabbat to put an egg in a pot of hamin over the Shabbat.
1. It is permitted on Erev Shabbat to put a kugel wrapped in foil or rice enclosed in a cooking bag into the hamin.
2. It is permitted to put an egg into the hamin before Shabbat.