On a Fire
The issur of bishul akum applies to all types of cooking with a fire (רמ''א סי' קי''ב סי''ג ), e.g., cooking [בישול ], which is done via boiling water; baking [אפיה ], which is done via dry heat (מאירי חולין דף ל''ז ע''ב ד''ה אע''פ, רמ''א ס''ב ); “שליקה ,” which means thoroughly cooked or lightly cooked (כלשון הגמ'); roasting [צליה ], which is directly over an actual fire; and frying [טיגון], which is cooking in oil.
Without a Fire
Food prepared for eating without a fire, e.g., via salting, pickling (רמ''א סי''ג ), smoking, sugaring, dehydrating, or the like, is not assur under the gezeira of bishul akum.
Steaming
Steaming food is done by boiling water until steam rises and cooking food with the steam. There is a machlokes whether this is subject to the issur of bishul akum.
Meikilim. Some hold steaming is like smoking and not subject to the issur of bishul akum (זר זהב שער מ''ג סק''ד, שו''ת אבן שתיה יו''ד סי' נ', בין איש חי ש''ב פ' חוקת אות כ''ב ).
Machmirim. However, many poskim hold it is a problem of bishul akum and is not comparable to smoking (שו''ת שם אריה סי' כ''ב, שו''ת אחיעזר ח''ד סי' ל''ו, שו''ת שבט הלוי ח''ט סי' קס''ב ).
Therefore, many poskim do not rely solely on the fact a food is steamed to permit bishul akum. They only use it as a factor to allow the food when there are additional elements of heter (שו''ת שרידי אש יו''ד סי' כ''ב סוף אות י', שו''ת חשב האפוד ח''ג סי' כ''ט, שו''ת מנחת יצחק ח''ג סי' ע''ב ).
Cooked in Water Heated via Steaming
Most poskim hold that cooking something in water that was boiled via steaming is considered regular cooking and subject to the conditions of bishul akum (שו''ת שבט הלוי ח''ו סי' ק''ח אות ו', שו''ת חשב האפוד ח''ג סי' שפ''ב ).
Steam system. One of the methods of cooking used in factories and industrial kitchens today is through a steam system. The way it works is that steam is carried to pots via a boiler from which a piping network runs to the various cooking pots in the kitchen. In practice, the pots are heated by the hot steam inside the pipes. This too is subject to the halachos of bishul akum according to most poskim mentioned above, as it is considered regular cooking.
Canned food. Similarly, factories producing canned foods commonly fill the cans with raw food, seal the cans, then insert the cans into a pressurized chamber and inject very hot steam into the chamber. This cooks the cans’ contents and kills bacteria, keeping the food good for a long time. This is also considered regular cooking for the purposes of bishul akum according to most poskim, as the food is cooked in the water that is in each can, and the can is heated via steaming. Thus, it is considered cooking, as mentioned above (ס' בישולי עכו''ם פ''ב אות ה').
Cooking in the Sun
Cooking in the sun or something heated by the sun is not subject to the issur of bishul akum (פרי מגדים סי' קי''ב שפ''ד סק''ד ).
Microwave
The poskim discuss whether food cooked by a non-Jew in a microwave is subject to bishul akum, as it is not cooking via fire. Instead, waves heat the water molecules in the food.
Meikilim. Some hold since this is not the cooking method using fire that was used in the days of Chazal, it is not a problem of bishul akum (שו''ת להורות נתן ח''ז סי' ס''ד, שו''ת רבבות אפרים ח''ח סי' תקי''א ).
Machmirim. However, most poskim hold there is a problem of bishul akum. This is because everyone views it as a regular cooking method. It is not like salting, pickling, or smoking, which is viewed as a distinctly different method (שו''ת שבט הלוי ח''ח סי' קפ''ה, תשובות והנהגות ח''ה סי' רמ''ט אות ב').
Electric Stove
Heating element. An electric stove that operates with a heating element heated via electricity is considered regular cooking on a fire, as the heating element heats up just like fire, and it is obviously subject to the conditions of bishul akum (שו''ת מנחת יצחק ח''ז סי' ס''ב, שו''ת שבט הלוי ח''ח סי' קפ''ה, תשובות והנהגות ח''א סי' תל''ט ).
Induction. There is a newish type of electric stove called an induction cooktop. It does not operate with a heating element. Instead, under a glass surface, there are copper coils which, using an electric current flowing through them, generate a magnetic field. As long as no pot is on the surface, no heat can be felt on the surface. Only when a metal pan is placed on the surface does the magnetic field cause an electric current in the walls of the pot. This heats the walls of the pot, which then cook the contents.
Here, too, the poskim discuss bishul akum. The characteristics are somewhat similar to a microwave, in that it works with a magnetic force, not a heating element. However, the consensus of the poskim is that since the heating occurs in the pot’s metal walls, unlike a microwave, which directly heats the water particles, it is much more similar to traditional cooking. Thus, even those who are meikel with a microwave would agree that one should be machmir with an induction cooktop (ס' בישולי עכו''ם פ''ב אות י''ב בהערה ).
Cooking After Cooking, Salting, or Smoking
After cooking. A food that was first cooked in a permissible way and then recooked by a non-Jew may be eaten, as the second cooking is insignificant [אין בישול אחר בישול ] (ט''ז סי' קי''ג סק''ב, ש''ך סק''ה ). Even if the food cannot be eaten cold after the initial cooking and the subsequent cooking by the non-Jew was necessary, the food is still not assur since it was initially fully cooked permissibly (מו''ר בשבט הקהתי ח''א סי' רכ''ז ).
After salting or smoking. If food was salted or smoked into an edible state and then cooked regularly by a non-Jew, it does not become assur through the extra cooking. However, if the salting or smoking did not enable the food to be eaten regularly, even if it was able to be eaten with difficulty, the food becomes assur due to bishul akum (רמב''ם פי''ז מאכ''כ הט''ז, כס''מ שם, סתימת השו''ע שם סי''ב ). However, some allow it to be eaten as long as it was able to be eaten, albeit with difficulty (יש מתירים בשו''ע שם ).
