This is what the October-November issue of the Food Allergy News says about
nutritional yeast:

"...Nutritional yeast is made from a yeast known as saccharomyoes cerevisiae.
 There are various strains of this yeast which are used to manufacture a
variety of yeast products.  The yeast is grown in molasses obtained from the
processing of either sugar cane or sugar beets.  It absorbs various nutrients
from the molasses, especially B vitamins.

"Some manufacturers will add additional B vitamins and minerals to the
molasses during the growing process, which is referred to as the fermentation
phase.  The yeast is "cold washed" to stop the growing process and
pasteurized to make it safe to eat.

"After pasteurization, more B vitamins (thiamin, riboflavin, niacin, B^ and
B12) may be added before the yeast is dried and packaged.  ...  Nutritional
yeast is also a good source of protein containing approximately 8 grams of
protein per serving."

I hope this helps to answer your question.  I had not heard of nutritional
yeast before I read about it in this newsletter, either.

Sandy