Irui can only kasher the outer layer of a kli. Hot water poured against a cold kli is an example of חם לתוך צונן (hot onto cold) of which we say (Pesachim 76a) תתאה גבר ואדמיקר ליה בלע. The bottom surface succeeds in cooling down the water but not before the water succeeds in kashering the topmost layer.
A kli rishon has the ability to be boleya and to be polet throughout its entire thickness. Therefore, one cannot kasher a kli that was used as a kli rishon with irui, even if the irui is performed with water that is significantly hotter than the temperature of the non-kosher product.
The appropriate way to kasher a kli rishon is according to the manner that it was used:
- A spoon placed into a non-kosher kli rishon al ha’aish – must be kashered in a kli rishon that is on the fire.
- A spoon placed into a non-kosher kli rishon that was off the fire – must be kashered in a kli rishon, but the kli rishon need not be on the fire.
In industry today kettles are most often heated by means of circulating steam or hot water. Yet these kettles are considered to be like a kli rishon even though they are heated with an irui of hot water. Rav Belsky explains that an extended irui of hot water can also create a kli rishon. Because circulating hot water/steam heats the walls of the kettle such that there aren’t any דפנות מקררות (quite the contrary the walls of the kettle heat the inside product), this too can be considered a kli rishon and the entire thickness of the walls requires kashering. The source for this idea comes from Tosfos (Shabbos 40b) who explains the distinction between a kli rishon and a kli sheni. The walls of a kli sheni are cold and cause the contents of the kli to immediately cool down. However, a kli rishon has hot walls that retain the heat and cause bishul. Additionally, the Rashba (Shabbos 42a) says that an אמבטי (bathtub) can be mevashel even if it is a kli sheni, because the water in it is much hotter. Rav Belsky explains that because a bathtub contains a great quantity of water and a relatively small amount of surface area it can retain its heat even though it is a kli sheni. Similarly, a continuous irui of large quantities of hot water although technically a kli sheni, can also be viewed as a kli rishon.
While the appropriate method for kashering a jacketed kettle is by turning on the hot water/steam and boiling up the kettle, in cases of need, one can rely on an extended irui on the inside of the kettle, through the use of spray balls. Roschim water should be sprayed until the walls become saturated with heat and the exiting water does not differ in temperature from the entering water. Typically this process takes about 15-20 minutes. This method is employed for kashering tanker trucks.
Other common applications for which we rely on kashering with spray balls are holding tanks that have no independent heat source but were filled with hot non-kosher product or held cold non-kosher product for 24 hours. Lechatchila, these tanks should be sprayed with roschim water for 15-20 minutes. In cases of need there is room to be more lenient and allow for slightly lower temperatures since the tanks have no independent heat source.
Viewing an extended irui as a kli rishon leads to a stringency regarding kashering pipes. When hot non-kosher product flowed through the pipes for an extended period of time, the walls of the pipe became saturated with heat and bliyos were able to be absorbed into the entire thickness of the pipe. It is therefore insufficient to kasher the pipes by merely passing boiling water through them for one minute. Rather, boiling water must be circulated until once again the walls of the pipe become saturated and there cease to be דפנות מקררות. While there is no exact way to tell how long this will take, other than to manually check the outside of the pipe with a thermometer for the point at which the temperature ceases to increase, typically we can assume that this takes about 15 minutes.
Another issue that has recently been raised concerns the use of CIP systems for both kosher and non-kosher lines. Even if a plant has completely separate lines for kosher and non-kosher, if they share a CIP system in which the waters are circulated back to the CIP tank, then the CIP tank can become non-kosher. Future use of the CIP system on the kosher line can potentially compromise its kosher status. Typically a CIP consists of three or four stages; an initial flush which is usually done below yad soledes, a caustic wash and acid wash which are done above yad soledes and a fresh water rinse which can also be done hot. Although the caustic and acid cycles are typically pagum and do not present an issue, if the final rinse is above yad soledes, and is circulated through the CIP tank this can present a problem. Since every CIP system is built and performs differently, it is important for Mashgichim to be aware of how the systems in their plants operate and report to the office any situations of concern.