Di-ammonium phosphate with some zinc. It is multivitamins for yeast to create strong cultures, and can be used as yeast energizers as well during stuck fermentation. It is good to stop yeast from mutiliating in a stressed environment high sugar, low pH, low temperature, zero oxygen etc.
Recommended dosage 0.5gm per liter of fruit juice (preferably added in 2-3 steps during the open fermentation stage)
For grain mash 0.25gm per liter added along with yeast starter gives best results.
Also recommended to be used along with stir plates if you are washing/propogating yeasts and other wild cultures. Agar & petri dish can be optained from the medical labs.
TTB (American Alcohol & Tobacco Tax & Trade Bureau) limits yeast nutrients to 0.968 g/l, which is enough to give the YAN (yeast assimilable nitrogen) to give 15% v/v strength of ethanol.