The Gemarah Chulin 111b tells us that כבוש הרי הוא כמבושל. The Chochmos Adam (58:4) explains that this is a הלכה למשה מסיני, and therefore in cases of safek one must be machmir, ספק דאורייתא לחומרא. Therefore great care should be taken when dealing with even possible situation of kavush, and in any situation of doubt shailos should be asked.
Kavush only applies to liquids:
Kevisha not only applies to loose liquids such as water and wine, but also applies to thicker liquids such as honey and cream1. The Teshuvos HaRosh 20:1 sets forth the general rule in determining what a liquid is for these purposes as follows: If an object placed inside the liquid can wobble around, it is still considered liquid. Once the liquid congeals and holds the object in place, there is no longer kevisha.
Although liquids that are frozen solid do not become kavush, one should not assume that everything placed into a freezer is not subject to kevisha. Aside from the fact that it can take many hours for a drum of liquid to solidify, additionally the drum might contain alcohols or other anti-freezing agents that might prevent it from freezing. For example though pure glycerin freezes at 62° F and water freezes at 32° F, a 2/3 to 1/3 mixture of glycerin and water will only freeze at -51.7° F (much colder than your average freezer!). Just knowing that a liquid was wheeled into the freezer is not enough to remove the chashash of kavush.
If a tank containing non-charif liquids is emptied before 24 hours then there is no concern of kavush. One should not leave the emptying to the last minute since it can often take quite a while to pump all the liquid out of a tank. One must verify that the tank has been emptied before 24 hours. The mashgiach can either visit the plant, or rely on charts that would indicate that the product was removed. One such method would be to have the plant run a CIP on the tank immediately after it is emptied. The time of the CIP will be indicated on the Taylor chart, and prove that the tank was certainly emptied beforehand.
In some situations there is no available tank in which to move the product. Simply agitating the liquid is not sufficient to stop kevisha. Can one pump the product through pipes and back into the tank to avoid kevisha, or do we view the pumping as merely another form of agitation? Rav Belsky questioned whether pumping is adequate but has agreed that regarding chalav stam/dairy/pareve one can be maikel. Rav Schachter said that if the pumping is done properly it is adequate. Therefore in situations where no other solution exists, we can be mitzaref other mitigating factors to be maikel2. In cases where one may pump around the liquid, in order to make sure that rov of the liquid was not in the tank for 24 hours; one must cycle at least 70% of the liquid3.
For example, a tank containing 500 gallons of milk requires that more than 350 gallons of milk to be pumped around and back into the tank every 24 hours to avoid kevisha.
Kli shemachniso l’kiyum
Storage of wine and grape juice has an additional chumra. If a tank is designated for holding liquids for more than 72 hours4, it is considered a kli shemachniso l’kiyum. Rav Belsky said that even if the tank ordinarily holds apple juice, if non-kosher grape juice or wine is placed into such a tank even momentarily, the tank will need to be kashered. Until it is kashered, one may not put kosher wine or grape juice into that tank even momentarily.
If a kosher liquid became kavush in a non-kosher tank, because kevisha takes 24 hours, the tank will always be aino ben yomo. Still the kosher liquid should be emptied from that tank as soon as possible, since it improper to allow for the continued use of a treif kli.
However if an alcohol (davar charif) became kavush in a non-kosher tank, since a davar charif is michalya lishvach aino ben yomo ta’am, the alcohol becomes non-kosher, unless the tank contains enough liquid to be mivatel the volume of the walls b’shishim. Most tankers trucks that are at least 60% full will meet this criterion. There is a dispute between Achronim5 as to whether the alcohol will become instantly assur if it falls below shishim. In a case that arose, Rav Belsky was maikel bidieved, b’tziruf other sfeikos (was the cleaning done on the tanker an adequate kashering; is alcohol michalya li’shvach or only a chiltis6; is there kavush in a metal kli7).
1 Chochmos Adam 58:1
2 Issur V’heter (brought by Taz 105:1) holds that there is no kevisha in a metal kli. Chavas Daas and Chut Hashani 63 are maikel like Issur V’heter, and Pri Migadim 105:1 leaves it as a safek “ומתכות יש “מחלוקת. Additionally, kavush kailim are usually aino ben yomo.
3 A mathematical formula can show that it can take until 70% of the liquid is cycled, in order to guarantee that less than 50% of the original liquid remains. This is because the cycled liquid will be mixed back into the tank, together with the original liquid. Some of the liquid will be cycled twice before other liquid is cycled even once.
4 See Rama 135:9
5 Chamudei Daniel (Hilchos Ta’aruvos 36) is machmir; Mahari Haleivi 13 is maikel; and Chesed L’Avrohom 31 sets it up as a machlokes Magen Avrohom (maikel) and Chavas Ya’ir (machmir).
6 See Taz 96:8
7 Issur V’heter (brought by Taz 105:1) holds that there is no kevisha in a metal kli.