Nutritional yeast flakes are deactivated yeast (single-cell fungi) that are first dried and then processed into small flakes. They are low in sodium and used in cooking as a seasoning; a binder for soups, salads, and sauces; and a topping for spicy dishes.
From Wikipedia: “Nutritional yeast is a deactivated yeast, often a strain of Saccharomyces cerevisiae, which is sold commercially as a food product. It is sold in the form of flakes or as a yellow powder and can be found in the bulk aisle of most natural food stores. It is popular with vegans and vegetarians and may be used as an ingredient in recipes or as a condiment.”
“Nutritional yeast is produced by culturing a yeast in a nutrient medium for several days. The primary ingredient in the "growing" medium is glucose, often from either sugarcane or beet molasses. When the yeast is ready, it is killed (deactivated) with heat and then harvested, washed, dried and packaged. The species of yeast used is often a strain of Saccharomyces cerevisiae. The strains are cultured and selected for desirable characteristics and often exhibit a different phenotype from strains of S. cerevisiae used in baking and brewing.”
“Nutritional values for nutritional yeast vary from one manufacturer to another. On average, two tablespoons provides 60 calories with 5 g of carbohydrates (four grams of which is fiber). A serving also provides 9 g of protein and is a complete protein, providing all nine amino acids the human body cannot produce. While fortified and unfortified nutritional yeast both provide iron, the fortified yeast provides 20 percent of the recommended daily value, while unfortified yeast provides only 5 percent. Unfortified nutritional yeast provides from 35 to 100 percent of vitamins B1 and B2.
Because nutritional yeast is often used by vegans, who need to supplement their diets with vitamin B12, there has been confusion about the source of the B12 in nutritional yeast. Yeast cannot produce B12, which is only naturally produced by bacteria. Some brands of nutritional yeast, though not all, are fortified with vitamin B12. When fortified, the vitamin B12 is produced separately (commonly cyanocobalamin) and then added to the yeast.
Although some species of bacteria that can produce B12 could potentially grow along with S. cerevisiae in the wild, commercially produced nutritional yeast is grown in controlled conditions that would normally not allow those bacteria to grow. Therefore, nutritional yeast is not a reliable source of B12 unless it is fortified.”
“Nutritional yeast products do not have any added monosodium glutamate; however, all inactive yeast contains a certain amount of free glutamic acid because when the yeast cells are killed, the proteins that compose the cell walls begins to degrade, breaking down into the amino acids that originally formed it. Glutamic acid is a naturally occurring amino acid in all yeast cells, as well as in vegetables, fungi, and animals.”