Evaluation of Nitrogen Deficiencies in Oregon Grapevines, Musts, and Wines 1997-1998

Over the last several years 'stuck' and 'sluggish' fermenations have become a ma or concern of\ud winemakers in Oregon as well as in Washington and California. During the 1996 vintage we began a\ud preliminary survey of Oregon musts and juices at harvest and showed that many were low or marginal in\ud fermentable nitrogen content. The readily fermentable nitrogen content in juice and musts is composed\ud primarily of ammonia (NH4) and the alpha-amino acids present (particularly arginine, serine, glutarnate,\ud threonine, aspartate, and lysine). An approximation of the total yeast fermentable nitrogen is taken as the\ud sum of the ammonia content plus the alpha-amino acid content. If the levels of fermentable nitrogen\ud present in juice or must at harvest are too low, the total yeast cell biomass produced will be low and\ud fermentations will be slower and may stop or 'stick' before all the fermentable sugar is utilized. Sluggish\ud and stuck fermentations are also sometimes accompanied by production of hydrogen sulfide and other\ud 'reduced' sulfur odors. Some researchers have also reported an increase in fruity aroma intensity and\ud wine quality with moderate nitrogen supplementation (Rapp and Versini, 1995; Vos, 1982). Many of the\ud alpha amino acids are metabolized by yeast to produce important aromatic alcohols and esters. Nitrogen\ud supplementation in the winery and in the vineyard need to be evaluated for their effects on wine aroma,\ud flavor, and wine quality