Actually, black currant is one of a few berries, which saves all the vitamins and minerals on heat treated. (?)

When heated = при нагревании. Может так?

Может.

https://books.google.com/ngrams/graph?content=one+of+%2A+few&year_start=1800&year_end=2000&corpus=15&smoothing=3&direct_url=t2%3B%2Cone%20of%20%2A%20few%3B%2Cc0%3B%2Cs0%3B%3Bone%20of%20the%20few%3B%2Cc0%3B%3Bone%20of%20those%20few%3B%2Cc0%3B%3Bone%20of%20a%20few%3B%2Cc0%3B%3Bone%20of%20his%20few%3B%2Cc0%3B%3Bone%20of%20very%20few%3B%2Cc0%3B%3Bone%20of%20our%20few%3B%2Cc0%3B%3Bone%20of%20my%20few%3B%2Cc0%3B%3Bone%20of%20these%20few%3B%2Cc0%3B%3Bone%20of%20its%20few%3B%2Cc0%3B%3Bone%20of%20her%20few%3B%2Cc0

Так и запишем - one of the few. Thanks!

As a matter of fact, the blackcurrant is few and far between due to capability of preserving all vitamins and minerals despite going through the heating.(?)

Few and far between - it's interesting.

https://books.google.com/ngrams/graph?content=one+of+*+few&year_start=1800&year_end=2012&corpus=15&smoothing=3