water, as you know, is made of hydrogen and oxygen. As food sits and ages the water molecules inside the food breakdown into hydrogen and oxygen and continue to react with other elements in the food. This does two things, one, it encourages bacterial decomposition and two, it oxidizes nutrients in food.
The purpose of dehydrating the food is to remove the water thus removing the oxygen. This stops the oxidizing processes and helps preserve nutrients including enzymes.
by the way enzymes do not actually die, they are turned off or turned on. Enzymes have an ACTIVE SITE which is a specific shape and fits the molecules it works with.
When exposed to extreme heat, this changes the protein structure and the shape of the active site becomes different.
The molecules the enzyme works with can no longer fit in the active site of the enzyme and it becomes DENATURED.
so heat is an enemy of enzymes, but so is pH levels. A large swing in the pH level, either up or down, will also denature enzymes.