nitrogen
FACTS
nitrogen content decreases 30% during fermentation
large concentration of nitrogen is found in wort amino acids 
yeast assimilates nitrogen from amino acids and small proteins
yeast will manufacture missing amino acids, yielding off-flavors, like phenylethanol (rose)

AMINO ACIDS IN ORDER OF YEAST PREFERENCE
threonine
methionine
isoleucine, leucine
valine
tyrosine
tryptophan
phenylalanine
alanine