When kosher food is opened is it still kosher?

Answer 1

When a container of food that has been certified kosher is opened, the opportunity for deliberate or accidental contamination occurs. Thus, an observant Jew might be hesitant to assume the food in such a container was still kosher unless he, or someone he trusted, had opened it and watched over it. More important, if the food were to be opened and heated up in an oven with non-kosher food, or which had been used for non-kosher food, it would no longer be kosher. This is why (for example) kosher airline meals must be served unopened.