Is using a flameless heater considered cooking on Shabbos?
New technology has introduced chemical cooking: a combination of minerals (iron, magnesium and salt) is activated simply by adding cold water to a plastic pouch and the flameless heat produced by this chemical reaction can warm up a meal in as little as twelve minutes. Flameless heaters are used extensively in the military and are a convenient option for travelers without a traditional cooking source. Is this process of heating water and cooking food permissible on Shabbos?
Sources as far back as the Gemara discuss the concept of cooking in the sun on Shabbos. The conclusion: It is not considered bishul (cooking with direct heat) since putting food out in sunlight is not derech bishul (the normal manner of cooking). It can be argued that the chemical cooking of a flameless heater is similar to direct solar cooking. It would also not be considered mav’ir (creating a fire), since it does not yield any live fire or red-hot metals.
What about the melachos of molid (creating something new) by producing heat, causing a chemical reaction or dissolving the chemicals, and makeh b’patish (lit. translation: striking a hammer, i.e. the act of completion) when the chemicals combust or the food becomes ready to eat? One of the factors in the above melachos is to create something enduring—but flameless heat immediately dissipates, and therefore, at first glance it would seem that the process is permissible on Shabbos. Hatmanah (wrapping in a heat source) also isn’t an issue since the food gradually cools off even while wrapped. In addition, there is a significant gap between the food container and the outer, heat-producing packaging.
Nevertheless, since the parameters of molid and makeh b’patish are not crystal clear and are the subject of much halachic argument—likewise, what is considered bishul in a technologically advanced era is heavily contested in recent halachic responsa—contemporary poskim recommend not to use these heaters on Shabbos, except in an emergency, such as if food needs to be prepared for a choleh (ill person) and no other warmed food is available.
Mixing the chemicals with water, which is part of the process of flameless cooking, may involve the additional melachah of lishah—kneading, i.e. creating a doughy mixture.
On Yom Tov there is more room for leniency, since many sevaros (arguments) brought by those who oppose the use of flameless heaters aren’t applicable on Yom Tov, when it is permitted to perform certain melachos associated with ochel nefesh (human consumption).
(It should be noted that certain melachos restrict the manner in which food packages can be opened on Shabbos and Yom Tov, which may apply to the different components of the flameless heater and the heated food packets.)
שקו״ט מזה בכ״מ בדור האחרון, ותליא מילתא ביסוד ההיתר בבישול בחמה, שכמה אופנים נאמרו בד״ז.
ונלאיתי נשוא מלהאריך בכ״ז, יען כי כבדה עלי המלאכה לעת כזאת. וכבר האריכו בכ״ז חכמי הזמן הלא היא בספרתם. ומכלל דבריהם דברינו - ראה שש״כ פ״א הע׳ יב. תשו׳ שנד׳ באוצרות השבת הטמנה ובישול בסוה״ס. שו״ת תשוה״נ ח״ג סק״ו. ותח״י גם שו״ת בענין זה מבעל מנחת אשר ווייס. ואריכות גדולה - בקובץ ישורון תשס״ח ותשס״ט (חו׳ כ וכא), עיי״ש באורך. תחומין חו׳ יג. שו״ת במראה הבזק ח״ו סמ״ג. באהלה של תורה סכ״ו. אהלי שם (פרץ) מטמין ומבשל בשבת סי״ג. וראה גם שו״ת מאזני הלכה סי״ז מש״כ באריכות בדברים של טעם. ותו לא מידי.