"In which food can we find the whole grain ?"
Grains (seeds of grasses) include an mechanical covering (chaff),
a chemical coating (bran), an energy store (endosperm), and the
seed itself (germ). Human digestion can't use chaff at all. Bran
provides soluble fiber to our digestion, but also phytates which
impede our absorption of some minerals. Endosperm is stable and
has most of the energy. The germ is highest in nutrients, but
degrades rapidly when exposed to air.
So, what do you want from "whole" grain? Even the strictest
health food stores remove chaff. But, to soak the remainder to
remove bran phytates then dry the grain rapidly enough to prevent
any possibility of moulds (many of which are far more of a
nutrition problem than phytates) is never done before grinding
because it is too expensive, risky (the fungus ergot is widely
believed to be the source of the insane behaviour for a year at
Salem MA), and difficult. So, good health food stores grind
bran+endosperm+germ as is then refrigerate it to prevent
degradation of the germ. Another method (which I used to bring up
my family) is to grind at low speed (little heating) then bake
immediately.
If you buy pot barley (pearled has the bran removed), wheat
berries, or red or black rice from a Chinese grocery, you will
get all of grain that we can use. Brown rice, and rolled oats or
rye from the cooking aisle (as opposed to the Rice Krispies
aisle) are usually close.
The best route I know of to the best nutrition is to not fuss too
much about any one food, but to eat as wide a variety of foods as
possible. Humans didn't eat grains until very recently in our
evolutionary history, but the concentration of population that
characterises civilization was not possible without them.