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Introduction to Shabbat: Cooking
Bishul B'Shabbat/Cooking on Shabbat
The Torah forbids cooking on Shabbat.   “Cooking” means making food edible by heating it to above 120° F (49° C).
Cooking includes:
  • You may not make a soft food hard (such as cooking an egg).
  • You may not make a hard food soft (such as cooking meat).
  • You may not, in any manner, heat (to 120° F or above ) liquids that you will drink or foods with liquids—such as sauces and gravies--whether fully cooked or not. 
Timing of Cooking
According to the Torah, you may eat food on Shabbat that had been placed on the heat source Friday afternoon but was not completely cooked by sunset.
ReasonNo action is being taken and the cooking will be completed by itself.
 
Kli Rishon and Kli Sheini
According to Torah law, food is only considered to be cooked if it has been directly heated from the heat source, such as a kettle on a fire or a pot on a flame (even if that utensil has been removed from its heat source). This is called a kli rishon.
Once you pour water from a kli rishon into a glass, the glass is a kli sheni. Some foods, such as an egg or tea, get cooked in a kli sheni (kalei bishul--easily cooked). These foods are forbidden by Torah law to be put into a hot kli sheni on Shabbat.
Exception Halacha allows spices and water to be “cooked” in a kli sheni.
NoteIf the water is less than 120° F, nothing gets halachically cooked in any kli, even in a kli rishon.
Reheating/Replacing to Heat Source
   1.    Do Not Reheat Food Unless It Is Halachically Dry.
This includes heating on a hotplate, stove, or oven and applies to even fully cooked food. Halachically dry means the food is solid at either the beginning or the end of the cooking, or both. To determine whether a food is liquid (and therefore may not be reheated on Shabbat), shake the container. If the food does not move around, it is considered to be solid. (For when solid food may be reheated, please see Shabbat: Reheating.)
   2.    To Replace Heated Food onto/into Its Heat Source (hachzara):
  • You must have taken it off with the intention of replacing it, and
  • You may not put the utensil down onto a surface; you must continue to hold the food (or the utensil) in your hand.
  • The heat source must be covered.
Reason Chazal were concerned that someone might see you put food on the heat, mistake it for actual cooking, and erroneously think that cooking is permitted on Shabbat.
Note You may not replace incompletely cooked food to a heat source.
Reason To do so would facilitate the cooking.
   3.    Do Not Put Food on a Heat Source that has Adjustable Controls.
This applies even to fully cooked food.
Reason You might adjust the heat and thereby violate a Torah law due to shehiya (stoking the fire or turning up the heat).
Note Shehiya is simple to avoid; just cover the flame (or electric heating element) and any temperature controls before Shabbat, as when using a blech (a metal sheet that covers the flames and controls). Then, on Shabbat, you may put fully cooked solid (but not liquid) food on top of other food (or utensils containing food) that were already on a blech from before sunset on Friday.
Reason Doing so does not look like you are cooking and the blech prevents you from adjusting the heat.
   4.    Do Not Insulate Food to which Heat Is Added.
You may not add insulation (which will help keep in the heat) during Shabbat to foods that are on a heat source, even to fully cooked foods, if they are “wet.” By rabbinic law, you may not apply heat to an insulated utensil—or apply any insulation that adds heat--even before Shabbat began and let it remain that way during Shabbat--even if the food was completely cooked before sunset on Friday.
Note Regarding food on a heat source, you may add insulation if there is at least one uncovered area at least the size of a quarter. Adding insulation on Shabbat is only a problem if the insulation completely surrounds the food or utensil on all surfaces and the top.
Introduction to Food Nullification: Utensils (Kashering)
Food Nullification in Utensils: Torah-Law and Rabbinic Decree
By Torah law (d'oraita), any clean utensil, countertop, etc., automatically reverts to neutral/pareve and kosher after not being heated to more than 120° F (49° C) for 24 hours.
But by rabbinic decree, utensils do not automatically become neutral/pareve even after 24 hours and must be kashered by heat (libun—direct heat; hag'ala—boiling in a pot; or eruy rotchim—pouring boiling water over item) or, if some types of glass, by soaking in water (meluy v'eruy ).
 
Changing Gender of Utensil
You may kasher a pot or cooking/eating utensil from:
  • Non-kosher to kosher, or
  • Year-round use (chametz) to kosher for Passover.
 
You may not intentionally kasher a utensil in order to change it from dairy to meat or meat to dairy; you must first kasher it from accidentally (or intentionally) non-kosher to kosher/pareve, or from non-Passover to Passover/pareve. You may then use it for either dairy or meat.

Once you have used it for that gender, the item retains that gender (unless you re-kasher it for Passover or you make it non-kosher first, then kasher it to neutral/pareve).
But if you accidentally heat meat with a dairy utensil or vice versa, you may kasher it back to its original gender by any one of the kashering methods, depending on how it became non-kosher.
 
Items/Materials that Can Be Kashered
The following materials can be kashered:
  • Glass, including Corelle, if not used directly on the stove or oven. Glass does not change gender or other kosher status unless heated on a flame or in the oven. Unless it is heated in this way, glass does not ever need to be kashered (except for Passover) (see Meluy v'Eruy, below).Glass, including Corelle, if not used directly on the stove or oven. Glass does not change gender or other kosher status unless heated on a flame or in the oven. Unless it is heated in this way, glass does not ever need to be kashered (except for Passover) (see Meluy v'Eruy, below).Glass, including Corelle, if not used directly on the stove or oven. Glass does not change gender or other kosher status unless heated on a flame or in the oven. Unless it is heated in this way, glass does not ever need to be kashered (except for Passover) (see Meluy v'Eruy, below).
   NOTE  Glass used directly on fire or in the oven (kli rishon) cannot
  be kashered except by heating in a kiln.
  • Granite (not granite composite)
  • Marble
  • Wood, if smooth (see notes on Eruy Rotchim, below)
  • Metal, including stainless steel, cast iron, and aluminum.
Note While metal can be kashered if thoroughly cleaned, welded handles and other difficult-to-clean parts may render a metal utensil not kasherable. You might be able to use libun kal on the problematic area and still use hag'ala for the remainder of the utensil.
 
Items/Materials that Cannot Be Kashered
  • China
  • Corian
  • Corningware
  • Crockpot
  • Formica
  • Glass that has been used directly (kli rishon) on a stove or in an oven; however it can be kashered in a kiln
  • Granite (composite)
  • Knives with Plastic Handles (knives with wooden handles may be kashered if there are no cracks in the wood and if the rivets do not have spaces that catch food and prevent you from cleaning it completely)
  • Mixer-there might be exceptions. Consult a rabbi.
  • Plastic
  • Porcelain (Enamel)
  • Pyrex (if used directly on stove or in oven--kli rishon)
  • Rubber (synthetic)
  • Silestone
  • Silverstone
  • Stoneware
  • Teflon
  • Toaster/Toaster Oven
  • Waffle Iron.
 
Pot Lid Handle
Kashering
The handle on a pot lid does not need to be kashered for normal use during the year. 
Reason It does not normally get hot.
Cleaning
However, the pot lid handle must be removed and the lid cleaned where the handle attaches, if possible.
Note If the gap between the handle and lid cannot be completely cleaned, you may not use that lid for Passover and you normally may not kasher it if it becomes non-kosher. If the lid handle cannot be removed, consult a rabbi.

Pot or Pan Handle
A plastic handle that gets hot, especially if it is over a flame on a burner, may not be kashered. If the handle becomes non-kosher, it must be replaced. If a plastic handle connects directly to the metal of the utensil, consult a rabbi about what to do.
 
Food Nullification: Heat-Kashering
Three Methods of Heat-Kashering
Heat-Kashering is of three types:  Libun, Hag'ala, and Eruy Rotchim.
  1. Libun (Direct Heat)
    How It Works  Burns up any residual food taste
 
 What It Works On
Complete Burning (Libun gamur --heating metal red-hot).  Stoves, ovens, grills, grates, baking pans, roasting pans, etc., that were ever used with direct heat MUST be kashered by heating to red-hot (libun gamur). Libun gamur works on anything except pottery (this is a rabbinic injunction since you might not do a good job).
 
Light Burning (Libun kal--heating metal hot enough to burn paper on the side opposite the one being heated).  You may use this method whenever there is a question of whether an item needs libun. For example, food may have overflowed onto gas-stove grates. Due to safek, we use libun kal-- gas-stove grates do not need libun gamur.
 
 Process 
Libun Gamur The entire metal substance of a utensil, oven, or other cooking surface becomes red hot, but the item does not need to be red hot all at the same time: it may be heated sequentially as long as the entire surface gets red hot at some time. Libun gamur can be done by blowtorch or by placing the item in a kiln. 
 
Libun Kal
  • Direct a flame, such as a blowtorch, onto the inside of a pot. Pot is hot enough when a piece of paper that touches the outside of the utensil burns (it does not need to burst into flame, just to smolder), or
  • Put the pot into the oven at 500 ° F for 40 minutes. (First, remove any non-metal handles; they will need to be kashered separately or not used.)
 
   Waiting Time  You do not need to wait at all before kashering by libun--and certainly not the 24 hours needed before kashering by hag'ala.
 
  1. Hag'ala (Boiling)
     
How It Works
Any non-kosher or meat or milk taste is removed from the walls of the utensil during boiling (hag'ala). You may kasher a pot or utensil by either:
  • Boil Method Boiling water within the pot to be kashered, and making the boiling water overflow, or
  • Dip Method Dipping a smaller pot or utensil to be kashered into a larger pot of boiling water.
What It Works On   
Pots and utensils that are used with liquids (meaning, liquid all the time) can be kashered by being immersed in boiling water (hag'ala). The utensil being kashered by hag'ala must be made of a material that can release flavor, such as metal or wood. Materials that cannot be kashered (except in a glazing furnace!) are pottery--and, by extension--china, enamel, and similar materials. 
 
Note  The Boil Method only helps if the utensil became non-kosher due to food inside the utensil. If the non-kosher food was on the outside of the utensil, you may only kasher it by the Dip Method or by libun kal.
 
Note The boiling water must reach at least the same temperature during kashering as when the utensil became non-kosher.
 
Note Once the Passover holiday has begun, chametz cannot be nullified with hot water/hag'ala (only libun can kasher something during Passover). You may only kasher during chol hamoed, not during the first and last (festival) days.
 
Note Whenever hag'ala is effective, you may instead use libun kal, since libun kal is a stronger form of kashering. Sometimes you may find it more convenient to use libun kal to kasher an item that needs only hag'ala.
Situation A metal pot of the opposite gender went through a dishwasher cleaning.
What To Do Even though the pot only needs hag'ala, you may instead kasher it by libun kal by putting it in an oven at 500° F (for this application).

Process
The Boil Method can be used as:
  • Batel BaShishim ("nullifying in 60 times" the volume), or
  • Batel BaRov ("nullifying in a majority"--that is, boiling the item in water that is more than twice the volume but less than 60 times the volume of the non-kosher element).
NoteIf a pot is hot (over 120° F, or 49° C) when only part of the pot becomes non-kosher, the entire pot is non-kosher and its volume is figured into the volume of water needed for boiling.
NoteFor whether the lid becomes non-kosher, consult a rabbi.

In Batel BaShishim, by the actual halacha, you do not need to wait at all before kashering. But the custom is to wait 24 hours--except in extreme circumstances--because it is too hard to figure out 1/60th. In Batel BaRov, you must wait 24 hours.
 
The Boil Method: Batel BaShishim
Using batel ba'shishim for the Boil Method is not customary.  You may use it for emergencies ONLY; ask a rabbi in this case.
Example To kasher a spoon with the batel ba'shishim type of hag'ala, immerse the spoon in boiling water of a volume at least the volume of 60 spoons. No waiting is needed before kashering with this method.
 
The Boil Method: Batel BaRov
To kasher a pot or utensil by hag'ala using batel ba'rov:
  • Clean the pot or utensil well.
  • Wait 24 hours after the pot or utensil was last heated to more than 120° F, or 49° C (such as when it was cleaned).
Reason Waiting 24 hours allows the taste to become “ruined” and then to be nullified (batel) in a majority (ba'rov) of boiling water.
Note During the 24-hour waiting period, you could still “use” the utensil for watering plants, etc., as long as the water remains under 120° F.
  • Fill the pot to the brim with water.
  • Bring the water in the pot to a boil.
  • Cause the water to overflow the entire rim of the pot by:
    • Plunging something hot into the pot (any item that will not cause the water to stop boiling is OK), or
    • Tilting the pot to slosh water over all of the pot's rim.
  • Cool off the pot by dipping it in cold water or putting it under cold running water.
Note If you did not put the utensil under cold water, it is still kosher b'di'avad.
 
The Dip Method
To kasher a smaller pot or any other kasherable cooking or eating utensil by hag'ala, you may dip the pot or utensil into a large, kashered pot containing boiling water.
  • If the pot in which you are kashering the items had been heated to 120° F (49° C), with food of that gender in the pot, or more within the previous 24 hours, the items you are kashering will assume the gender of the pot.
  • If the pot in which you are kashering the items had NOT been heated to 120° F or more for at least 24 hours, any items that are kashered in it will become kosher and pareve.
Note When kashering a utensil by hagala, you may dip it into boiling water one part at a time; that is, you do not need to immerse the entire utensil under the water all at the same time. This is different from doing tevila since for tevila, the entire utensil must be immersed completely.
 
Calculating 24-Hour Waiting Time
Once a pot has become non-kosher due to any reason, if it gets heated to 120° F (49° C) or more with any food or liquid in it, you must wait another 24 hours from the latest heating before you can kasher it, since everything inside the utensil becomes non-kosher again.
 
Calculating Volume
If only part of a pot becomes non-kosher, as long as the pot was hot (over 120° F, or 49° C), the entire pot becomes non-kosher and its volume gets figured into the volume of water needed for boiling.
 
 
  1. Eruy Rotchim (Hot-Water Pour)
 
Process  Pouring hot water over, for example, a sink to kasher it.
 
Waiting Time You must wait 24 hours before kashering by eruy rotchim.
 
Note Only items that became non-kosher by being poured onto, may be kashered via eruy rotchim.
 
Note Smooth-surfaced wood may be kashered through eruy rotchim (pouring boiling water) but ONLY if it became non-kosher through eruy. If it became non-kosher by being cooked or heated in an oven, it may not be kashered via eruy rotchim.
 
Note A wooden cutting board may be kashered if the board is smooth. If it has cracks and crevices, it can be sanded until smooth and then kashered.

 
Food Nullification: Meluy V'Eruy
Meluy V'Eruy To Kasher Glass
Halachically, “glass” includes Arcoroc, Corelle, crystal, Duralex, and Pyrex.
NOTE In pre-war Europe, where glass was expensive and hard to obtain, it was customary to kasher drinking glasses, especially for Passover, by soaking the glasses for three 24-hour periods (meluy v'eruy), as follows:
Step 1: Submerge glasses in cold water for 24 hours.
Step 2: Empty water, refill, and submerge glasses again.
Step 3: Repeat Step 2.
NOTE If any of these materials were heated directly on a flame or other heat source, they cannot be kashered by meluy v'eruy!