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).

• 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

       

• 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.)

   

2. Hag'ala (Boiling)
    

• 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.

       

• 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.

     
 

3. Eruy Rotchim (Hot-Water Pour)

         

 
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!

Jewish Festivals are listed in the Torah and are of two types: 

1. Three pilgrimage festivals (shalosh regalim):

• Passover,
• Shavuot, and
• Sukkot (including Shimini Atzeret).

These festivals were celebrated in ancient times by "appearing before God"--by bringing offerings to the Tabernacle or Temple.

2. High Holidays: 

• The Jewish New Year (Rosh Hashana—“Yom Teru'a” in the Torah), and the
• Day of Atonement (Yom Kippur).

 
Duration
Jewish holidays that were originally one day are now observed as follows:

• Rosh Hashana--2 days.
• Yom Kippur--1 day.
• Shavuot--1 day in Eretz Yisrael or 2 days elsewhere.
• Passover has festival days at the beginning and end of the holiday and intermediary days of lesser holiness, which are called chol ha'moed. In Eretz Yisrael, Passover lasts for 7 days in total and the first and last days are festival days. Outside of Eretz Yisrael, Passover lasts 8 days and has two festival days at the beginning and two at the end.
• Sukkot has festival days at the beginning and at the end and intermediary days of lesser holiness, which are called chol ha'moed. In Eretz Yisrael, Sukkot lasts for 8 days and the first and last days are festival days (the last day is Shimini Atzeret). Outside of Eretz Yisrael, Sukkot lasts for 9 days and the first two days and last days are festival days (the 8th day is Shimini Atzeret and the 9th day is Simchat Torah).

Character
Each holiday contributes its own character to Jewish life (Passover--the theme of freedom; Yom Kippur brings atonement, etc.). 

Celebrating
How to celebrate these holidays is detailed in our Oral Law and halacha books. Jewish festivals as practiced today are similar in holiness to Shabbat. As with Shabbat, the Jewish festival has candle lighting, kiddush at two meals, and havdala.   We eat our best food and wear our best clothing on Jewish festivals (we eat our next-best food and wear our next-best clothing on Shabbat!).
The main idea behind eating meals on Jewish festivals is joy (simcha), so you should drink wine and eat meat (only if you enjoy wine and meat).  There is no third meal on Jewish festivals since people used to eat two meals each day (adding a third meal on Shabbat was for enjoyment/oneg).
 
Melachot
Any activities or actions permitted on Shabbat are also permitted on the Jewish festivals. Actions that are forbidden on Shabbat are generally also forbidden on Jewish festivals, but there are some leniencies (only if the actions are needed for that festival day).
Examples

• Lighting from an existing flame,
• Cooking and baking for the Jewish festival day, and
• Carrying outside the eruv (hotza'a--transferring objects between domains).

 
Grama
Grama (indirectly causing an action) is permitted on Jewish festivals (but not on Shabbat). For example, you may advance or delay a timer that will make a light go on or off in the future (the timer must already be plugged in and operating from before sunset of the festival day).
Note For an action to be considered indirect based on time, there must be at least 2.5 seconds after the first action is done before the resulting action begins to happen.

Psik Reisha Dla Neicha Lei
Psik reisha dla neicha lei is forbidden on Jewish festivals, just as it is on Shabbat.

D'oraita Restrictions
D'oraita restrictions apply world-wide to:

• First and seventh days of Passover,
• First and eighth days of Sukkot,
• First day of Shavuot,
• Yom Kippur,
• First day of Rosh Hashana.

Note The same restrictions apply to all other Jewish festival days but are rabbinical.
 
Women
In general, women are not required to perform the positive, time-dependent commandments. Women and girls are not required to eat any Jewish festival meals except the Passover seder meal (but they are not allowed to fast on those days).

Categories of Batel/Nullification
Categories of nullification of non-kosher ingredients:

• Never batel.
• Batel b'shishim when the non-kosher substance is less than 1/60th of the total volume of the food.
• Batel barov when the non-kosher substance is less than 1/2 of the total volume of the food.

When Can a Non-Kosher Substance Be Nullified in a Mixture?
Whether a non-kosher substance can be nullified in a mixture depends on 3 factors:

• Whether the owner is Jewish;
• Whether the intended eaters are Jewish; and
• Whether the non-kosher substance was added intentionally as non-kosher.

If the answers to all three cases is yes, the food is never batel.

Food “Nullified in 60 Parts”:
Accidentally Adding Non-Kosher to Kosher Food 

• Less than 1/60 of the volume of the whole.
• Mixed in and not lying on the surface.
• Not intentionally added by a Jew.
• Not listed in “Foods that Never Become Nullified” (below).

Min b'Mino
Substances are only batel when they are similar (“min b'mino”). The substances must be the same type, have the same taste, and have the same appearance (the eater cannot identify them as being different).
Note In such situations, it would be batel barov from Torah (d'oraita) but batel b'shishim (1/60^th) by rabbinical order (d'rabanan).
Example A piece of non-kosher meat is mixed in with kosher meat of more than 60 times the volume of the non-kosher piece. The non-kosher meat is batel b'shishim.
Note As a practical matter, this can only apply to ground meat.
Counter Example Non-kosher chocolate syrup or a non-kosher flavored extract mixed into milk or other liquid or onto a solid would NOT be min b'mino even though both are liquids, since their appearances, flavors, and substance are different.

Too Thin To Make Non-Kosher

• Yayin Nesech
  Wine that has been offered to a pagan god or used for idolatrous purposes (yayin nesech) is forbidden in any amount!
• Mixtures of Milk and Meat
  Mixtures of milk and meat are not ever batel if they were cooked together.
  ExceptionBatel in 1/60^th if:
  • You cannot identify either substance AND
  • The mixture is liquid in liquid or solid mixed with solid.

Examples: Milk from a pig mixed with milk from a cow; ground kosher meat mixed in with ground non-kosher meat.

• Chametz

• Jew Intentionally Adding Non-Kosher Item

• Unflavored or Flavored Non-Kosher Ingredient
  
  Non-Jew Adds Unflavored Non-Kosher Ingredient
  

  Situation A non-Jew adds a non-kosher ingredient that has no flavor. 
  Status The non-kosher ingredient is nullified if less than 1/2 of the total (it does not need to be less than 1/60th--batel ba'shishim).

  
  Non-Jew Adds Flavored Non-Kosher Ingredient

  Situation A non-Jew adds a flavored non-kosher ingredient even if to impart flavor.
  Status The non-kosher ingredient is nullified in 60 parts (batel ba'shishim).
  

  Note If a Jew had told the non-Jew to add the ingredient, the mixture is non-kosher, just as if a Jew had added it. 
• Stam Yeinam Added to Water
  Situation A non-Jew adds—to water--stam yeinam (uncooked/non-mevushal) wine that has been handled while open by anyone other than a shomer-Shabbat Jew.  
  Status As long as the wine is less than 1/7th of the final volume, the mixture is kosher. 

  Note For mixtures with liquids other than water, consult a rabbi. 

• Essential Additives

• Food Bought by the Piece

• Important Food

• Permissible in Future (Davar SheYesh Lo Matirin)

• An egg laid on Shabbat will not be nullified by being mixed with eggs laid before Shabbat.
• Matza made of chadash flour will not be nullified by being mixed with matza made from yashan flour.

• Whole Insects

• Frozen or raw chopped or ground vegetables or spices may be considered kosher even without supervision.
  Reason We assume that any bugs in the food would have gotten partly chopped or disintegrated and therefore nullified.
• If a recipe calls for chopping or grinding herbs or vegetables, you may do so without first checking them for bugs.
  Note However, if you know there are bugs, you may not chop the food for the purpose of making the bugs nullified:  You must still check for insects before cooking or eating the food and if you see any bugs, you must remove them.