I recently read the OK "Kosher Spirit" magazine, including an article about canned vegetables and kashrut concerns.
The article reviews the steps in the production of canned vegetables and details how cans are filled and sealed, and then cooked in hot water:
The brine or sauce is poured into the cans with the prepared vegetables and sealed. The cans are then sent to be cooked in a retort. (A retort is a large pressure cooker filled with water.) This process kills the bacteria that cause food spoilage, allowing for the canned vegetable to be stored for long periods and remain fresh. At the end of the cooking process, the cans are cooled with cold water, dried, and labeled.
It appears, then, that the food inside the can never comes in contact with the water in the "retort."
Later, the article expresses a concern that
Additionally, there are very large retorts that can process two products simultaneously and a plain vegetable product can be cooked at the same time as a pork and bean product. Even if they are processed in separate retorts, often the water used for heating or cooling the product is recycled between the lines.
But if the cans are sealed, why would that have any impact on the status on the water outside the can? Can water be milchig, fleishig or treif even when it has only been in contact with the outside of a package. Is there some other status (eg. "water that was used in the preparation of treif cannot be used for something kosher even though the water is not trief")? [secondarily, if, hypothetically, some actual treif got into the water, could one then not use that water for boiling sealed cans of kosher food?]
Is there, then, any other limitations on the use of this water for other purposes?
Is this impact on the water because there is only a single layer between the food and the water and not a double layer (I'm thinking of the "double wrapping" that I have heard discussed when cooking kosher food in a non-kosher oven).
Side note -- I sent this question as a letter to the editor of the magazine and have not received any acknowledgment or response.