Hack said these should be peeled, indicating berry used in health food (4)
I believe the answer is:
acai
I'm afraid I can't even guess how this answer relates to the clue.
Can you help me to learn more?
(Other definitions for acai that I've seen before include "fruit which is nutritious" , "something nutritional to eat" , "South American food product" , "South American palm tree" .)