Factors affecting amino acid accumulation in berries are broad; cultivar, rootstock, soil management (in particular pH), climate, disease pressure, mineral nutrition and ripeness are all contributing factors. It can be said that the yield this year is big so dilution of YAN (Yeast Assimilable Nitrogen) might be why there is a lower than normal nitrogen content in the berries available as a starting platform for wine makers. Quality and source of supplementary nutrition is critical to the fermentation for both the yeast and the Malo lactic fermentation. Outlined below is some key pointers to help decide on what nutrient sources to use;
Key nitrogen source
The nitrogen sources that can be used by Saccharomyces cerevisiae are ammonium (NH4 +) and amino acids (organic nitrogen). They both represent assimilable nitrogen and are present in must at varying concentrations, sometimes not in sufficient quantities to meet the requirements of the yeast (figure 1). The three following factors must be taken into consideration:
υ Below 150 mg N/L, must is deficient. It is therefore important to supplement it with nitrogen elements.
υ Yeast nitrogen requirements depend on sugar concentration. The higher this concentration, the greater the amount of yeast biomass needed to successfully achieve a thorough breakdown of the sugars during alcoholic fermentation. Although, the yeast biomass must not be too excessive to avoid an induced nitrogen deficiency.
υ The nitrogen initially present in must is rapidly assimilated during the first third of the alcoholic fermentation (d-30), at the point when the biomass is at its highest density. Consequently, irrespective of the initial nitrogen content, its addition during alcoholic fermentation (d-30) allows to preserve the biomass formed, which is dependent on the yeast strain and proportional to the initial nitrogen concentration.
Sulphate reductase pathway
The key enzyme in the production of H2S is sulphate reductase. When the H2S and amino acids pathways meet the sulphur amino acids (cysteine and methionine) are produced. Where there is an imbalance between these two pathways and a nitrogen deficiency, the precursors of these sulphur amino acids are limiting, leading to an accumulation of H2S (Figure 2).
Organic nitrogen is supplied by adding yeast derivatives (usually autolysed yeast). In addition to amino acids, these yeast derivatives include lipids, vitamins and minerals which also contribute to the efficient performance of the yeast. Yeast has the ability to simultaneously assimilate organic nitrogen and mineral nitrogen from the beginning of the alcoholic fermentation. Organic nitrogen must be present in order to:
- Limit the production of SO2 and sulphur compounds (H2S and mercaptans).
- Produce healthy, but not excessive, biomass.
- Limit the risk of stuck or sluggish fermentation
of organic nutrition
Numerous experiments show that improved outcomes of alcoholic fermentation can be achieved with the use of organic nitrogen. Even in the case of wines considered dry (glucose + fructose < 2 g/L), small amounts of fermentable sugars can be used by degrading microorganisms and can have an adverse effect on the quality of the wines.
Besides its effects on fermentation kinetics, the addition of organic nitrogen can increase the fruitiness of wines and limit the aromatic mask linked to the production of sulphur compounds during the alcoholic fermentation. Except for the source of the nitrogen added, a comparison of wines produced under the same conditions reveals significant preferences for wines derived from musts supplemented with Nutristart® Org. The wines are considered fruitier, fresher, less vegetal and subject to less reduction than those supplemented with minerals.