I. General Principles

      Practical questions of heating food on Shabbat must deal with two potential problems:
               1. Actual cooking – a Biblical prohibition
               2. Rabbinic restrictions of warming food

      1. Actual cooking.
      Cooking is one of the 39 prohibited melakhot of Shabbat. It is loosely defined as
      affecting a significant positive change in a food through the use of heat.
      Not every change is considered significant. While cooking raw foods is prohibited on Shabbat,
      reheating certain foods would not violate the prohibition.
      In theory, heating the following things would not be problematic:
              a) Fully cooked dry foods (even cold), or
              b) Fully cooked liquids which have not cooled completely
      Practically speaking, the reheating must be done in a way that does not appear like cooking.

      2. Rabbinic restrictions on warming food.
      Chazal understood that even reheating cooked food might lead people to violate
      Torah prohibitions.
              a) One may give the appearance of cooking.
              b) One might adjust the level of the fire in order to cook a food faster.
      Additionally, Chazal prohibited the insulation of foods under certain conditions, reasoning
      that one who does so might determine that the food was not kept warm enough and
      will inadvertently place the food on an exposed flame on Shabbat.

  II. Speeding Up the Cooking Process

      Not only may one not cook or warm up uncooked food on Shabbat, but one may not
      accelerate the cooking process. Examples of this principle are listed below:
      Moving the food to the flame area: If uncooked food is being cooked on the blech,
      one may not move it directly over the flame on Shabbat because
      this will speed up the cooking process.
      Reducing volume: One may not release some unheated water in an urn into a glass
      so that the remaining water will cook more quickly.
      Stirring and scooping:
              • Non-fully cooked foods and liquids: Stirring food that is in the process of
              being cooked accelerates the cooking process because it causes the food to
              cook more evenly by bringing food that is farther away closer to flame.
              However, stirring liquid that is in the process of being cooked does not
              accelerate the cooking process because there is already an even
              distribution of heat prior to the stirring. Accordingly, one may not stir or
              scoop uncooked foods (e.g., chulent that is being cooked), but one may stir
              uncooked liquids (e.g., chicken soup on a blech that is being cooked).
              However, as a practical matter, one should not stir uncooked soup in a pot
              because this will require covering the soup pot, as will be discussed below.
              • Fully cooked foods: Some halakhic authorities rule that stirring or
              scooping fully cooked foods is prohibited while the foods are on a flame
              and that stirring fully cooked foods should be avoided in any case.
              Therefore, one should not stir or scoop even fully cooked foods while the
              foods remain on the flame area. Additionally, some authorities are
              stringent and rule that one ideally should not stir fully cooked foods even
              when they are removed from the flame area, but one may scoop fully
              cooked foods after they have been removed from the flame.
     Covering the pot with a lid: Placing the lid on a pot of food cooking on the flame
     accelerates the cooking process. As such, one should not cover a pot of non-fully
     cooked foods or liquids. Additionally, it is preferable not to cover the lid on a pot
     of fully-cooked food or liquid while the food or liquid, as the case may be, is on
     the flame area.

  III. Ain Bishul Ahar Bishul

      The principle of “ain bishul ahar bishul,” literally meaning that something cannot
      be cooked twice, is that once a food is fully cooked, it cannot be cooked again.
      Therefore, one may reheat a food that was fully cooked once before and merely
      cooled off. This principle only applies to dry foods and not liquids. Once a liquid
      has cooled down to such an extent that it is no longer desirable to the average
      person as a hot drink, then one would violate the Biblical prohibition of cooking if
      he would reheat the liquid.
      Therefore, one may reheat dry fully cooked foods assuming that they are not
      reheated on a heat source. (We will discuss how to reheat foods on a heat source
      in Section VI.) For example, one may place matza balls or noodles in a pot of
      soup that was removed from the blech. However, if soup had cooled down in a
      bowl so much so that the average person would not enjoy eating it anymore, it
      would be forbidden to pour it back into a pot of soup, even if the pot of soup was
      removed from the blech.

      • The definition of a dry food, as opposed to a liquid, is that there is no
      significant accumulation of gravy or other liquid. Therefore, moist roast
      chicken is considered “dry” for these purposes.
      • “Dryness” is determined by the current state of the food. As such,
      congealed fat that is on a piece of chicken is still considered “dry” for
      these purposes. Note that it is forbidden to melt congealed fat into a liquid
      (i.e., where it is not absorbed into chicken) because of molid (changing the
      status of a food from a solid to a liquid).
      • Even thick, pourable liquids, such as ketchup and heavy sauces are
      considered liquids for these purposes. Therefore, ketchup may not be
      poured onto chulent that is in a chulent pot that was removed from the
      flame because the ketchup would be “recooked.”
      • Some authorities are stringent and rule that any food that is dissolvable in
      liquid is considered a liquid for these purposes. Therefore, it is advisable
      not to add salt to a pot of soup or a pot of chulent even after the pot is
      removed from its heat source.

      Dry heat vs. wet heat:
      • A food is cooked through wet heat (liquid) and is baked or roasted through
      dry heat. The principle of “ain bishul ahar bishul” only applies if the
      second “cooking” is similar to the first cooking, i.e., if a cooked food was
      reheated in a liquid, such as reheating cooked noodles in a pot of soup.
      However, challah or roasted chicken may not be placed in a pot of soup
      because these foods were originally “cooked” with dry heat so they may
      not be reheated through liquid.
      • Many halakhic authorities are lenient and rule that only very intense dry
      heat of a direct flame (e.g., roasting on a spit over a fire, barbeque on
      coals, etc.) can effect halakhic “roasting.” Therefore, one may place
      boiled chicken or any other dry cooked food on top of a pot that is over a
      heat source (such as an urn or a chulent pot) or on top of a single
      temperature hot plate on Shabbat.
      • Deep-fried foods, such as french fries, doughnuts, soup nuts and chow
      mein noodles, are considered cooked foods for these purposes.
      • The status of pan-fried foods, such as fried onions, potato latkes and
      pancakes is unclear. Therefore, one should be stringent and not heat them
      through liquid or with intense dry heat. However, one may place them on
      top of a pot that is over a heat source (such as an urn or a chulent pot) or
      on top of a single temperature hot plate on Shabbat.

 IV. Secondary Heat – Kli Rishon, Kli Sheni and Kli Shlishi

      Halakhic cooking can occur not only from a heat source, such as an oven, but it
      can also occur from a material that absorbed heat from a flame, such as hot water
      that was poured from a kettle to a glass. There are different forms of “secondary” heat.

      Kli Rishon (first vessel): A pot of soup that is on a burner is considered a “kli rishon”
      and can effectuate halakhic cooking even after it is removed from its burner.
      Therefore, it is forbidden to add cold water to a chulent pot even after the
      chulent pot is removed from the heating element because the chulent pot is a “kli
      rishon” and will cook the cold water.

      Irui Kli Rishon (pouring from a kli rishon onto a plate of food): We assume that
      pouring hot food or liquids from a “kli rishon” will “cook” the outer shell of a
      solid food but it will not “cook” a liquid unless the liquid becomes hot (more than
      110 degrees). Therefore, it is forbidden to pour hot water from a kettle into a
      glass containing a slice of lemon and it is forbidden to pour hot gravy from a pan
      onto fresh vegetables. However, it is permitted to pour hot water from an urn into
      a container of cold water as long as the cold water doesn’t become hot (more than
      110 degrees). Additionally, one may pour hot water directly onto a baby bottle
      from a kettle or urn because the outer layer that is touched by the hot water is only
      the bottle and not its contents.

      Kli Sheni (second vessel): Since the walls of a “kli sheni” are cold and constantly
      losing heat, it doesn’t have the same capability to “cook” as a “kli rishon.” In
      practice, water and olive oil may be poured into a kli sheni. For example, a cold
      bowl of soup may be poured back into a hot soup tureen. Moreover, one may
      place an ice cube into a bowl of hot soup to cool it down. Additionally,
      previously cooked liquids, even when they are cold, may be poured into a “kli
      sheni.” For example, cold milk and sugar may be added to coffee in a “kli sheni.”
      As mentioned above, sugar or commercially produced table salt are treated by
      some authorities as liquids since they are dissolvable into liquid, but since they
      are processed, they may be poured into a “kli sheni.” Additionally, soup mixes
      and soup bouillon cubes may be added to a “kli sheni.” Tradition soups may be
      poured into a “kli sheni” or one may pour hot water from a “kli sheni” into
      Tradition soups; provided, however, that one must ascertain that every food item
      in the soup is precooked or preprocessed; otherwise, one may not use a “kli sheni”
      to permit one to heat the soup. Similarly, one may add hot water from a “kli
      sheni” to warm up cold tea or coffee.

      Ladle: A ladle is similar to a “kli sheni” because it is not the original container in
      which the food was cooked. However, it is similar to a “kli rishon” because it is
      first immersed in the hot contents of a “kli rishon,” thereby absorbing much of the
      heat of a “kli rishon.” The custom is to treat a ladle like a “kli rishon.” However,
      the ladle is considered a “kli sheni” with respect to a previously baked item that is
      cooked on Shabbat. Therefore, it is permitted to dip challah (a previously baked
      item) into a soup bowl on Shabbat. The pot of soup is considered a “kli rishon,”
      the ladle is considered a “kli sheni” and the soup bowl is considered a “kli shlishi”
      and the challah, even though it was only previously baked, cannot be
      “halakhically cooked” in a “kli shlishi.” However, one may not ladle hot soup
      directly from a “kli rishon” into a bowl containing spices, vegetables, bread
      crumbs, matzah, or any other uncooked foods or liquids.

      Davar Gush (solid hot foods): According to some halakhic authorities, solid hot
      food, such as hot chicken, meat or potatoes, retains its status even after transferred
      to a “kli sheni” or a “kli shlishi,” so one should not sprinkle uncooked spices,
      such as diced onion, garlic, celery, oil, mayonnaise or dressing onto hot solid food
      (more than 110 degrees), just as one may not put them in a “kli rishon.”
      However, one may pour ketchup on hot chicken because ketchup is cooked during
      its processing. Additionally, one may eat hot chicken together with a cold side
      dish on the side plate based on a number leniencies. Furthermore, a “davar gush”
      that is immersed or partially submerged in a liquid has the status of liquid in this
      regard, such as soupy chulent, a bowl of meatballs in sauce, a bowl of chicken
      soup with pieces of chicken.

      Kli shlishi (third vessel): Rav Moshe Feinstein is of the view that “kli shlishi”
      cannot effectuate any form of halakhic cooking. According to this view, a tea bag
      may be introduced only into a “kli shlishi.” A “kli shlishi” is the third receptacle
      for the liquid. The urn would be the “kli rishon” and the cup into which the water
      was originally poured would be a “kli sheni.” The water must be transferred to
      another cup before the tea bag can be introduced into the water. Some authorities
      are stringent and do not permit one to place a tea bag in a “kli shlishi.” According
      to this view, if “tea essence” (concentrate) is made before Shabbat, it may be
      poured even into a “kli sheni,” but one should not pour hot water directly from the
      kettle or urn onto the concentrate.
      Summary of rules of “Cooking” in foods/liquids that are more than 110 degrees &
      not on a flame

    (1) Definition - (2) Rule - (3) Example

      Kli Rishon - 1-Container in which a food or liquid was cooked or heated.
                           2-One may not heat up anything in a kli rishon except precooked
                           solid foods and precooked warm liquids.
                           3-Yes: Add cooked noodles to a pot of soup.
                              No: Add salt to a pot of soup (absent pressing issue);
                                    add cold water to a pot of chulent.
      Irui Kli Rishon - 1- Pouring from the original container in which a food or liquid
                                       was cooked or heated onto liquids or solids
                                  2- One may pour onto cold liquids if the resulting mixture is
                                      less than 110 degrees.  One may not pour onto uncooked
                                      solid foods because the shell will become “cooked.”
                                  3-Yes: Pour hot water from a kettle into a cold liquid as long as
                                      resulting liquid is not more than 110 degrees; pour hot water onto baby bottle.
                                      No: Pour hot water from a kettle into a glass with lemon;
                                            pour hot gravy from a pan onto fresh vegetables.
      Kli Sheni -1- Container into which hot food or liquid from a kli rishon was directly poured.
                        2- One may pour certain “spices” (which either dissolve in liquid or
                            are not eaten with the liquid) or cold liquids into a kli sheni.
                            One may place processed solids in kli sheni if they will dissolve in the kli sheni.
                            One may not place non-precooked solids into a kli sheni.
                        3-Yes: Pour salt or pepper in a bowl of hot soup; place an ice cube, cold water,
                           sugar or lemon in a glass of hot tea or a bowl of hot soup, as the case may be
                           (but don’t squeeze lemon); add instant coffee, tea or cocoa to a glass of hot water
                           to make coffee; add tea essence to a glass of hot water to make tea;
                           pour Tradition soup mix into a kli sheni or from kli sheni into a soup cup only if
                           all contents are pre-cooked.
                           No: Dunk a doughnut in a glass of hot coffee that is in a kli sheni.
                           (Making oatmeal on Shabbat is prohibited for “non-bishul” reasons.)
      Kli Shlishi -1- Container into which hot food or liquid from a kli sheni was poured.
                          2- One may place roasted or baked solids in a kli shlishi. There is a disagreement
                              as to whether one may place nonprecooked solids into a klishlishi.
                          3- Yes: Dunk a doughnut in a kli shlishi; place bag of roasted coffee beans
                              in a kli shlishi.
                         Subject to dispute: Placing a tea bag in a kli shlishi.
      Ladle -1- Dispute as to whether considered kli rishon or kli sheni
                  2- One should consider ladle as a kli rishon absent other lenient considerations.
                  3- When one serves soup on Friday night, one should remove soup fromthe stovetop,
                      then ladle soup into a bowl and then one may dip challah into the bowl
                      (which is considered kli shlishi).
      Davar Gush -1- Hot solid food.
                             2- One should consider davar gush as a kli rishon absent other
                                 lenient considerations.
                             3- Yes: Margarine on hot potato (margarine is precooked); ketchup on hot chicken;
                                 eat hot foods with cold foods on same plate.
                                 No: Mayonnaise, dressing, oil or raw vegetables onto hot solid food.

  V. Sh’hiya (Leaving Foods on a Flame Before Shabbat)

      Stovetops: According to Torah law, one may place a pot of raw food over an
      uncovered flame just prior to Shabbat because, in this case, no act of cooking (i.e.,
      placing the food on a flame) is being done on Shabbat. Nevertheless, our Sages
      were concerned that one may inadvertently adjust the flame on Shabbat in this
      instance. To solve this problem, most halakhic authorities rule that all raw foods
      that are not half-cooked at the onset of Shabbat should not be placed directly on a
      flame but they should be placed on a blech (minimally a layer of aluminum foil)
      on top of a flame. Some authorities advise that one should cover the range top
      with a blech before Shabbat in all instances, even if the food is fully cooked. The
      blech serves as a reminder that the flame must not be raised. Some have the
      custom of covering the knobs, as well.
      If one inadvertently left raw food on a range without a blech that was less than
      1/3 cooked at the onset of Shabbat, then one may not eat that food on Shabbat.
      To determine “1/3 cooked food,” one should calculate how much time it takes to
      cook that food and, for example, if it takes an hour to cook the food, then if the
      food was cooked for at least twenty minutes before the onset of Shabbat, one may
      eat the food on Shabbat.

      Ovens: Many halakhic authorities permit one to leave food cooking in the oven
      before Shabbat provided that the food is half cooked before the onset of Shabbat.
      On Shabbat, one may open an oven door to remove the food as long as this action
      doesn’t definitely cause either an immediate change in the oven temperature or
      any lights to turn on.

      Crock pots, hot plates, and warming drawers: One may place even raw food on a
      single-temperature electrical cooking appliance (such as a non-adjustable hot
      plate or crock pot) without a blech before Shabbat. Crock pots, hot plates and
      warming drawers with temperature controls are treated as stovetops for purposes
      of sh’hiya. See Section VII, however, regarding hatmana (insulating) issues
      relating to the use of crock pots with metal cylinder enclosures.

  VI. Chazara (Returning Food to a “Covered Flame” on Shabbat)

      Subject to the exceptions listed below, our Sages prohibited one from placing any
      food, even fully cooked food, directly onto the blech during the course of
      Shabbat. The Sages reasoned that placing the food onto a stovetop or a blech or
      returning a crock pot into its cylinder gives the outward appearance of cooking.

      Exceptions to the prohibition: The only foods or liquids that may ever be placed
      directly on the blech are those which were removed from a blech. There are four
      conditions under which a cooked food may be returned to the blech:
              1) The food must be fully cooked.
              2) When originally removing the food from the blech, the person who
              does so must intend to return the food to the blech.
              3) The food must be held the entire time until returned. (The food may
              rest on a surface, e.g., a counter, table, etc., as long as it is being held
              by the handle.
              4) The food must remain warm.
      Note that under no circumstances may one return even fully cooked food
      on Shabbat to an oven which is turned on. Additionally, some authorities rule that
      one may not return even fully cooked food in a crock pot to its metal cylinder
      enclosure even if the conditions above are satisfied and the walls of the cylinder
      are lined with aluminum foil, reasoning that this situation is similar to placing
      food in an oven. (This stringency would not apply to crock pots that are merely
      heated bases with no enclosures.) Other authorities rule that one may return the
      fully cooked food in this crock pot as long as the walls of the cylinder are lined
      with aluminum foil.
      If, inadvertently, conditions two and/or three are omitted, then, as long as
      the food is fully cooked & returned to a blech on Shabbat, the food is generally
      permitted on Shabbat. For example, if one mistakenly removed the wrong pot
      from the blech, expecting not to return it afterward, and then he releases his hold
      of the pot, he may return the pot to the stovetop if the pot contains the only warm
      food or the main part of the Shabbat meal.
      Additionally, only the food that was removed from the blech may be
      returned to the blech if the conditions above are followed. Therefore, one may
      not remove a pot of soup from the blech, take some cooked noodles from the
      refrigerator and place them in the pot of soup, and return the pot to the blech,
      because the noodles were not removed from the blech.

      Other permissible methods: There are several acceptable ways of reheating food
      on Shabbat morning. Reheating food is permissible if there is “no appearance of
      cooking.” In the following cases, halakhic authorities have determined that
      reheating fully cooked dry foods and fully cooked liquids which have not cooled
      completely (and are still suitable as a warm drink) in the following manner
      contain “no appearance of cooking” and therefore, one may reheat these type of
      foods by placing them:
              a) On top of a pot of food or water which is sitting on the range,
              b) On top of an urn,
              c) On top of a chulent pot,
              d) Near a flame, in a place where the food cannot reach 110 degrees – no
              matter how long one would leave it there. Such as:
                      • The side of the range
                      • The metal between burners of a range (using the heat of the oven or
                      the pilot light)
                     • Inside an oven which is turned off but still has a pilot light burning
              e) On a single temperature hot plate or in a single-temperature warming
              f) On a hot plate and a warming drawer with temperature controls, provided
              that, in the case of the hot plate, the knobs and surface of the hot plate are
              lined with aluminum foil and, in the case of the warming drawer, the
              knobs and surface of the warming drawer are lined with aluminum foil and
              the lining extends up the wall of the drawer and juts out of the drawer such
              that it is visible when the warming drawer is closed.
              g) According to some opinions, on a special “kedeira-blech” – a large
              covered pan that looks like a blech but actually holds water,
              h) According to some opinions, the food may be placed on even an empty
              (overturned) pot or pan over the fire. Many disagree. Still, this is much
              better than placing food directly onto the blech or into the oven.

      Note: It is important to remember that even if the Rabbinic prohibition of
      “chazara” does not apply, one should be careful not to transgress the Biblical
      prohibition of cooking. For example, on Shabbat, it is forbidden to warm up a
      cold pot of soup on a non-adjustable hot plate. Even though one would not
      violate the Rabbinic prohibition of chazara, he would violate the Biblical
      prohibition of cooking. Similarly, on Shabbat, even though one may reheat
      meat in the ways mentioned above, one may not do so if the meat is covered
      with a significant accumulation of gravy. (See III, “Ain Bishul Ahar Bishul” above.)

      Transferring from one heat source to another heat source: One may transfer
      hot food from an oven that is hot onto a blech that is on top of a stovetop, but
      one may not transfer hot food from a blech or from a stovetop to an oven
      without an insert. Additionally, one may add hot water from an urn or kettle
      to a pot of fully cooked cholent that has dried up. Ideally, the pot should be
      removed from blech and the water should come directly from the urn or kettle
      into the pot. However, if necessary, one may add water to the pot even from a
      “kli sheni,” e.g., from an urn to a cup to the pot.

  VII. Hatmana - Insulating

      Our Sages prohibited the insulation of foods on Shabbat and before Shabbat,
      subject to the rules below. They reasoned that one who insulates his food does so
      out of a strong concern that the food will be warm enough for Shabbat and
      therefore, if the individual discovers that the food was not kept warm enough, he
      is likely to inadvertently place food on a proper flame on Shabbat.

      Conditions to hatmana: Four conditions must be necessary for hatmana to be
      prohibited on Shabbat if the hatmana retains heat:
              1. The food must completely covered by the insulating material.
                      • Therefore, one may put toweling over an urn as long as a
                      small but significant part of the urn is left exposed.
              2. The purpose of the insulation is to retain or generate heat and not
                  merely to protect the food.
                      • Therefore, one may cover a hot pot of food with the lid
                      because the primary purpose is to protect the food.
                      • Moreover, one may leave cholent kishke inside the pot while
                      wrapped in aluminum foil even if the kishke is completely
                      submerged in the stew because, among other reasons, the
                      main purpose of covering the kishke is to separate it from the
                      cholent and not to insulate and retain heat.
                      • Additionally, one may wrap a hot potato in aluminum foil
                      and place it in the refrigerator on Shabbat.
                      • However, one may not completely wrap a hot potato on
                      Shabbat with aluminum foil to keep the hot potato warm.
              3. The wrapping material must be in direct contact with the food
                   itself or with the outer walls of the pot containing the food.
                      • Therefore, one may place a tray on top of an urn and then
                      drape a towel over it because the insulating material does not
                      touch the walls of the urn.
              4. The food must still be in the container in which it was originally cooked.
                      • Therefore, one may pour soup into a thermos to retain its heat.

      Hatmana before Shabbat: Before Shabbat, one may insulate food with a material
      if there is no active source of heat present (and the purpose of the insulation is
      merely to retain the heat), but one may not insulate food with a material if there is
      an active source of heat present (“davar shemosif hevel”). For example, one may
      not completely wrap a hot water urn even before Shabbat, but one may wrap a hot
      potato in aluminum foil before Shabbat in order to retain its heat. Conditions one
      through three above (but not condition four above) must be satisfied to prohibit
      hatmana if there is an active source of heat present (both before and during Shabbat).

      Crock pots: Some halakhic authorities have ruled that leaving a crock pot with
      cooked food inside its metal cylinder enclosure is considered hatmana with an
      active source of heat present, reasoning that the close fitting enclosure is
      halakhically considered an insulator and there is a heating element. Other
      authorities disagree. In order to satisfy all halakhic opinions, some authorities
      rule that one should place a crushed can underneath the crock pot. Note that
      crock pots that are merely heated bases with no enclosures do not pose a
      hatmana problem.