This is a case where I am fairly certain that I know the practical halacha, but don't understand the issues underlying the practice.
Let's say you chop up some apples and nuts before Pesach on a chametz cuttingboard (that hasn't been used for several days prior), with a chametz knife (same), and store it in a new, never-used glass jar in your fridge. Can the apples and nuts be used on Pesach, for example as charoset?
My (limited) understanding is that the ta'am of a food is only transmitted from a vessel or utensil into the food item if heat is applied, or if the food has an intrinsic "sharpness" (like onions or radishes). And in any case the ta'am of the chametz would be stale at the time the food is prepared. Furthermore, we know that before Pesach chametz can be batel. So it would seem that there is really no reason one could not use such food on Pesach. And yet, I am sure that everyone would agree it may not be done. What am I overlooking?
A follow-up question: What if instead of charoset, you are making home-made sauerkraut? Again there is no heat, and no intrinsic sharpness in the cabbage. The only difference here is that the chopped cabbage is pounded to soften it and bring out the brine, before being allowed to ferment. Does either the act of pounding, or the natural fermentation process, behave analogously to heat insofar as the transmission of ta'am is concerned?