If a liquid remains in a container for 24 consecutive hours it becomes kavush k’mivushal, and we are choshesh that there is a transfer of ta’am between the liquid and the entire container.1 The transfer of ta’am takes place only after 24 hours. Before 24 hours there is no transfer of ta’am. Therefore, bidieved if one did leave cold kosher liquid in a non-kosher container for 24 hours the product remains kosher, since the bliyos in the walls of the container after 24 hours are nosain ta’am lifgam.2
For example: if bidieved pareve soy milk was kavush in a dairy holding tank for 24 hours, the soy milk would remain pareve.
If a non-kosher liquid (with the exception of non-kosher grape juice and wine) became kavush in a container for 24 hours the container needs hagalah.3 If this tank has no heating capability, the proper way to kasher is with an extended irui of roschim. Spray balls connected to a CIP system can be run for several minutes until the walls become saturated with heat and the exiting water is roseyach. Alternatively, a steam jenny can be used to spray steam and hot water against every section of the tank. Some tanks cannot withstand roschim temperatures, and because the bliyos of kavush are weaker bliyos we allow kashering with lower temperatures. Lechatchila the exiting temperature should be close to roschim, but in cases of need, above yad soledes (175 F) is acceptable.4
If stam yeinam or kosher dairy (even chalav stam) was kavush in a tank, it can be kashered with milui v’irui gimmel yamim or irui kli rishon.5 Irui kli rishon means that water, near roschim and certainly not less than yad soledes (175 F), should be sprayed over every area of the tank. It is not necessary for the walls to become saturated with heat. It is often more practical to kasher with the extended irui method as outlined above.
If a non-kosher liquid is kavush in a tank, it will only assur the tank up to the level of the liquid. However if the issur is a davar shamain (greasy) it will assur the entire tank. Although ordinarily we consider ourselves uncertain as to which items are considered shamain, regarding kavush we are maikel to only assur in cases where it is clearly shamain.6 Therefore, if milk was kavush in 5 ft of a 10 ft tall tank, only the bottom 5 ft of the tank needs to be kashered. There is no concern of nitzotzos. If non-kosher oil was kavush in the bottom of the tank, the entire tank should be kashered.
A davar charif has the ability to be m’chalya lei lishvach ta’am pagum. Therefore, if a davar charif is kavush in a non-kosher kli for 24 hours the liquid can become assur. Examples of a davar charif include alcohols, vinegars, peppers (e.g. hot sauce) and citric acid. Products that contain enough of these ingredients can also be consider charif. Salt brine in addition to being charif also has a second chumra that it becomes kavush in the amount of time of כדי שיתן על האור ויתחיל להרתיח (between 6-18 minutes).
Example: A company receives ethyl alcohol in previously used 55 gallon drums. Because ethyl alcohol is a davar charif we cannot view the drum as an aino ben yomo. However, since empty steel drums (not including the lid) typically weigh about 32 pounds, we can compute that the volume of an empty steel drum is approximately .46 gallons. A full steel drum holds approximately 120 times the volume of its walls and any ta’am would be batel. However, this is only true for steel drums, plastic 55 gallon drum do not hold 60 times their volume.
Example: Soy sauce contains very high levels of salt and should be considered like tzir (brine). Even if cold soy sauce is left in a tank for only a few minutes, the soy sauce will be considered kavush. If the tank was not kosher, the soy sauce will become not kosher. Extreme care should be given to brines and very salty sauces since it is very easy for them to become kavush.
1 Shach Y.D. 98:13 says that kavush in a kli requires being mivatel the entire thickness of the kli. Not like Taz 105:1 that says that we only need to be mivatel the kdei klipa of the kli.
2 Taz Y.D. 105:1
3 Shach Y.D. 135:33
4 Pri Migadim says that one can kasher a kli that was kavush in a kli rishon that was removed from the fire. Presumably this means that the water in the pot is no longer roschim. Since roschim is not necessary, the only other temperature that we deal with is yad soledes. Another way to look at this is that kavush is not more chamur than bishul at yad soledes. Since bidieved we can kasher the bishul of yad soledes with a hagalah at yad soledes because k’bolo kach polto, surely we can kasher kavush with hagalah at yad soledes. However, lichatchila we try to always kasher with roschim.
5 Shach Y.D. 135:33
6 Shach Y.D. 105:1