Hilchos Shemittah v’Yoveil 5:3

One may not change the nature of shemittah produce just like we don’t change the nature of terumah or second tithe. For example, something normally eaten raw is not to be eaten cooked, and something normally eaten cooked is not to be eaten raw. Accordingly, animal feed may not be cooked, nor should a person force himself to eat food that has spoiled or bread that has gotten moldy, the same as one doesn’t eat such things from terumah or second tithe.

Hilchos Shemittah v’Yoveil 5:4

One may not cook shemittah vegetables in terumah oil out of concern that the latter (if rendered ritually unclean) may invalidate the former (which would require the shemittah produce to be destroyed). If one cooked only a little and ate it right away, it is permitted (even at the outset) because he didn’t leave it out where it could become invalidated.