If the yeast lacks nutrition during fermentation, off flavours and aromas will be formed by the yeast and reduce the overall quality of the wine. In extreme cases of nutritional deficiency fermentation will stick. 

Each Vintner’s Harvest wine yeast sachet contains all thirteen  essential trace vitamins, minerals and co-factors to ensure that yeast remain at optimum health during the fermentation process. However You may still need to add DAP (diammonium phosphate)  in situations where there is insufficient nitrogen or phosphorous provided by the fruit ingredients. The quantity of DAP to use will vary depending on the quantity of fruit used, but will generally be between 7-15 grams per 23 litre fermentation. For ultimate quality delay DAP addition until day 2 to ensure yeast is forced to assimilate available amino acids from the fruit.