As nitrogen is the limiting element in the growth of yeast cells it is important to measure the concentration of Yeast Assimilable Nitrogen (YAN). Limitation of YAN has been identified as the main cause of ‘stuck’ fermentation, while high levels in the presence of ethanol can lead to formation of potential carcinogens especially where levels of L-arginine are present. The addition of nutrient supplements to grape juice prior to / during fermentation can now be quantified accurately in terms of YAN by using both Randox Food Diagnostics Primary Amino Nitrogen (NOPA) and ammonia kit combined.