There are only four ways for Belios to transfer from one
object to another.
Here are the ways:
The amount of heat needed to transfer Belios depends upon
the following two factors:
1) The nature of the hot object: Is it a liquid, a
food-solid, or a utensil?
and 2) How removed that object is from its original source
If you boil water in a kettle, that kettle is called a Kli
Rishon ("original vessel"). If you then pour the boiled water
into, say, a thermos, the thermos is called a Kli Sheni ("second
vessel"). And yes, if you pour the water from the thermos into a
cup, the cup is called a Kli Shlishi ("third vessel").
The intensity of heat necessary to facilitate the
transference of Belios to any given object is inversely
proportionate to the closeness of that object to its source of
heat. Meaning, the closer you get to the fire, the easier it is
to make things Treif.
Case #1: Solid Food in a Kli Rishon on the Fire
The closest any food will ever get to your fire, hopefully,
is when it's cooking in a Kli Rishon. While doing so, Belios can
be absorbed into the outside layer of the food with even a
minimal amount of heat, according to some Poskim (See Shach 105:5, who requires Kelipah, Maharshal quoted
therein is reluctant ("Mistapinah") to be lenient, as it is
likely that a safeguard was made in such a case against usage of
a Kli Rishon. Chochmas Adam (59:1) rules it is better to be
strict regarding Kelipah, even if the pot was not on the fire.
However, the Shach also states that "many are lenient" in this
case. See also Taz 94:1. In order for
Belios penetrate into the food's interior, however, you would
need enough heat to burn a baby's stomach
Rabeinu Yeruchem Shabbos 12, Chochmas Adam 59:1 (Yad Soledes Bo).
This is approximately 110o, R.S.Z. Aurbach, quoted in Shemiras Shabbos Kehilchaso 1:1:3.)
When the outer layer of your food is infested with Belios,
peeling off that outer layer will remove the Belios. A
sufficient thickness of food should be removed so that the
"shell" can be peeled off in one piece; i.e. it should not break
into pieces because of its thinness¼.â ÿ
The removal of Belios from an object by peeling off its
outer layer is called "Netilah K'dei Klipah."
To review: You put a raw meat ball into a Milchig (Ben Yomo)
pot and turn on the fire. After a few minutes you remember that
what you're doing will make both your meatball and your pot
Treif. So you run like the wind to shut the fire off, but by the time you
do so, the meat ball has been heated to 98.6oF. You must shave
the outer layer off your meatball and discard it like cooked
Basar B'Cholov. The pot remains 100% kosher, and the rest of the
meatball may be cooked in another pot, and eaten.
If, however, the meatball was heated to 110o before you
managed to shut off the flame, both your meatball and your pot
are Treif, and you should do Teshuva for transgressing the
prohibitoin of cooking meat and milk together.
Important note: This Halacha, that even a minimal amount of
heat can cause absorption of Belios in the outer layer of food,
is the subject of great debate in the Poskim.
Therefore, even in cases where no great necessity is involved, a Rav should be