When a cold item is placed onto a hot surface, Halacha tells us to view the cold item as though it were hot, even though the item remains cold. Conversely, if a hot item is placed on a cold surface, we view the hot item as becoming cooled down. However, in this case we say that until it cools down, there is a kdei klipa transfer of ta’am. This concept is brought in the Gemara Pesachim (76a) and referred to as ta’tah gavar (the bottom surface overpowers).
The Shach (92:36) brings two criteria for deciding which surface is considered the “ta’tah”.
- The bottom surface because heat rises or because the top item weighs down upon the bottom.
- The stationary surface because the item that remains in its place is considered dominant.
Hot cans exit a retort and are dropped into a cooling bath. Because the cans are heated in a retort they have the status of a kli rishon. Although, the cans become submerged in the bath, the water is considered the ta’tah because the can is considered transitory and the bath the stationary surface. Therefore, so long as the bathwater remains below yad soledes, we say that only a kdei klipa of water is heated by each can. Even if tens of thousands of non-kosher cans pass through this bath, it will not make the water non-kosher, since even thousands of kdei klipa are still a small amount, and are batel b’shishim in the bath. If the water gets above yad soledes, we should view the water as non-kosher.
Hot cans exit a retort and are cooled down by cascading cold water. In this case the cans would be the ta’tah and the water would become non-kosher. The water would need to be replaced for the kosher production, even if it never gets to be yad soledes.
When discussing ta’tah gavar we are discussing only kli rishon heat. If a cold item falls on a kli sheini we do not say ta’tah gavar and there is no transfer of ta’am until the above item becomes hot.
Bottles and cans which are hot-filled, but are not retorted, could be considered like a kli rishon or a kli sheini depending on the temperature. If the temperature of the container is above yad nichves bo (180 F), we consider this to be a kli rishon. If it cools to below yad nichves bo, it is a kli sheini. If kli sheini bottles are cooled with cascading water, even though the bottles are hot (i.e. above yad soledes) the water will not become non-kosher.
Poskim1 say that if there is a heat source that prevents the upper item from cooling down, even if the ta’tah is cold, it does not overpower, but rather the hot item will make the ta’tah surface as though it is also hot.
In a steam jacketed kettle, the hot water is considered the ta’tah even though the kettle remains in place and the hot water circulates. Since the steam has a heat source (i.e. the boiler) it is always considered dominant. Therefore, the kettle will be boleya from the steam even if the kettle remains cold. However, the Chavas Daas 91:5 says that the product inside the kettle will not be boleya from the kettle so long as the inside wall that touches the food remains below yad soledes. Ta’tah gavar says that we should view the kettle as being hot only regarding being boleya from below. There will not be a transfer of ta’am with the product above, until the kettle actually becomes hot.
This is important in whey production. Although whey is a byproduct of gevinas akum, it remains kosher so long as the whey is not cooked together with the cheese.2 Often companies are interested in heating slightly their cheese to help extract the whey. Kettles that contain both whey and cheese are heated to 115°F (slightly below yad soledes). Although these kettles are steam jacketed and the inside jacket of the kettle gets hot, we do not say ta’tah gavar and consider everything in the kettle as being cooked, so long as the inside wall that touches the whey remains below yad soledes. These kettles have agitators that keep the temperature uniform throughout the kettle. Therefore, we can be confidant that so long as the product temperature remains below 115°F, the inside wall will remain below 120°F.3
- Yam Shel Shlomo (Chulin 7:38) brought by Shach 92:33 and Taz 92:25
- Igros Moshe Y.D. III:17 is maikel even if the whey is cooked together with the cheese, but the OU has adopted the position to be machmir not to accept whey that is cooked together with gevinas akum.
- I spoke with a respected engineer who told me that the closest approximation for the temperature of the inside wall of the kettle is the temperature of the product right next to it. So long as the agitation produces uniform temperature, we can expect the inside wall to remain within a degree or two of the product.