Here's what I have learned about nutritional yeast...this is copied and pasted from another message board I belong to. I don't have much info on brewers yeast.
It has been confusing to me in the past, and now I think I understand a lot more about nutritional yeast. This comes from "The Uncheese Cookbook" by Joanne Stepaniak.
Nutritional yeast, saccharomyces cerevisaie, is a natural whole plant grown as a food crop. It is prized for its delicious, cheesy taste and high nutritional content. It is a reliable source of high-quality, easily assimilated protien and is the formost natural shource of B-complex vitamins. It is easy to recognize because of its yellow color which ranges from light tan to bright gold. It is an inactive yeast which means it has no fermenting power as does the live yeast used in leavening and brewing, rendering it more digestible. Nutritional yeast is NOT dried torula, a yeast like organism which is grown on waste products from the wood pulp industry, nor is it brewers yeast or baking yeast. None of these products should ever be substituted in recipes calling for nutritional yeast.
Some brands of nutritional yeast have been combined with WHEY, a by product of cheese processing. Pure nutrional yeast does not contain whey or other dairy products, so read the product labels carefully.
When kept in a cool dry place, the physical characteristics and nutritive values of nutritional yeast remain unchanged for long periods.
Red Star Vegetarian Support Formula is one good brand to look for. It's not available at any of the places that I shop, so I've found a suitable replacement. I used to get mine out of the bulk bins at my local co-op but then I learned that it wasn't supplying nearly the same nutritional value as the kind meant for vegetarian support. I've also run into the kind that is made with whey, so you really do have to be careful.
The kind that I buy is from a company called "KAL" It comes in a big yellow canister, similar to a coffee tin. It was less than $7.00 for 12 oz (by weight). I found it in the supplements section of Whole Foods, near the rest of the vegetarian and B-12 stuff - on the lowest shelf because of the size of the canister I suppose.
It says on the package that it is grown on molassas.
I use mine in tofu scramble, I sprinkle it on veggies, I sprinkle it on toast, popcorn, pasta, rice, etc. The cookbook I mentioned above is really helpful too.
According to the package, a serving is one scoop...the scoop looks like it's about a quarter cup?? I'm not certian though. One scoop provides (here's only a sample of a few nutritional values)
130% of B12
480% of B6
60% of Folic acid
640% of B1
570% of B2
and the list goes on and on!!
Whew! Hopefully I've answered some of your questions!
PS...this company has a website and they sell the Red Star vegetarian support online.http://www.chreese.com/index.php