Nutrient needs during wine fermentation
During wine fermentation, yeast require sufficient nutrients to complete the alcoholic fermentation without the production of off-aromas. Often the grape berry contains enough nutrients for a successful fermentation but there are situations where supplementation of nutrients at the winery is required. Deficiencies in the grape nutrients are caused by a number of factors, such as lack of water and nutrient availability during the growing season, and can be particularly common during hot, dry growing seasons. These conditions often result in fruit with higher Brix content resulting in yeast needing to metabolize a greater amount of sugar with a lower amount of nutrients in a high alcohol environment. The end result is often a very slow/sluggish fermentation or fermentations that do not complete fermentation but rather stall out with a few Brix still remaining (termed "stuck fermentation"). While there are many potential causes of stuck and sluggish fermentations, including improper yeast hydration, temperature management, microbial competition, residual pesticides, low nutrients and high alcohol content are two of the most common.
Yeast assimilable nitrogen (YAN)
The major yeast nutrient we are concerned with in the grape is yeast assimilable nitrogen (YAN). YAN is composed of inorganic nitrogen (ammonia) and organic nitrogen (primary amino acids). It is important to know what the YAN level in your fruit is prior t o fermentation so that you can make appropriate additions. Often, by the time you notice a problem fermentation it is already too late to make any nutrient additions. This is because amino acid uptake is hindered late in fermentation due to membrane disruption from increasing ethanol concentrations. You also do not want to add excessive nutrients at the beginning of fermentation as this can also cause problems. Large additions of nutrients early in the ferment may lead to over vigorous fermentations and alter the aroma compounds produced by the yeast. In addition, residual nutrients in the wine may contribute to microbial spoilage during aging. So how much YAN do you need? Well, it depends. The general recommendation is between 120-220 mg/L for a 21 Brix must. If you have higher Brix must or are using a high nutrient demand yeast strain then you may want to consider higher YAN levels. These are not hard and fast rules as many people may have no problems fermenting juice with much lower YAN levels then these, but these YAN levels have been found by researchers to result in fermentations with good kinetics.
Other nutrient needs
Aside from nitrogen, the other nutrients that are essential factors for yeast growth are the micronutrients such as the vitamins biotin, pantothenic acid, and thiamin. A simple method for analyzing these compounds does not exist so the general rule is that if your grapes are low in nitrogen they are probably also low in micronutrients. If you just want to increase YAN then diammonium phosphate (DAP) is an efficient way to do this. However, DAP does not contain any micronutrients so in addition to DAP you also should use a complex yeast nutrient that contains a blend of organic nitrogen (amino acids, peptides) and micronutrients. A balanced approach of both DAP and complex nutrients works best if you need to significantly increase your YAN levels. If only a small adjustment is needed then an addition of a complex yeast nutrient will usually suffice. Nutrient additions should be carefully monitored and recorded as there are legal limits to the concentrations that can be added. For example there are limits to the amount of DAP (0.96 g/L), thiamin (0.60 mg/L), and pantothenic acid (0.048 mg/L) that can be added. For complex yeast nutrients, read the manufacturer’s instructions carefully to determine the maximum concentration of the product that can be added.
How to deal with stuck fermentations
While preventative strategies such as nutrient additions are often the best way to prevent stuck fermentations what can you do if you have a problematic fermentation that refuse to finish those last few Brix? Just as there are a number of causes for stuck fermentations there are also a few approaches to restart them. In general, these strategies entail building up a healthy population of a rescue yeast (typically a vigorous fermenting yeast) and slowly acclimatizing the yeast population to the stuck wine. If the specific cause of the stuck fermentation is known then specific strategies can be taken. For example, if the stuck ferment was caused by a high population of bacteria (Lactobacillus for example) then an addition of lysozyme may be necessary. Often an addition of yeast hulls is also recommended as this may reduce inhibitory substances. Specific procedures for re-starting stuck fermentations can be found at the following links:
- Winery protocol for restarting a stuck fermentation (by Enartis)
- Good practices for restarting a stuck fermentation (by Lallemand)
- Recommended method to restart stuck fermentations (by Scott Lab)
- Video on how to restart a stuck ferment
The procedures described follow the same general methods but recommend different commercial products to achieve similar goals. Carefully follow the manufacturers recommended procedures depending on what yeast and nutrient products you use.
- Warmer growing seasons can result in grapes with high Brix and low YAN that potentially could result in stuck or sluggish fermentations
- Assessment of YAN is crucial to determine appropriate nutrient additions
- Balance of DAP and complex yeast nutrients recommended to provide YAN and micronutrients
- Nutrients added late in fermentation unlikely to be utilized by yeast - perform additions early and at 1/3 fermentation
- Excessive use of nutrients can cause over vigorous fermentations and change aroma profile - legal limits for some nutrient additives
- Re-starting stuck fermentations involves treating wine with SO2/lysozome/yeast hulls if necessary followed by preparation of a healthy rescue yeast population
- Slow addition of stuck wine to yeast preparation in step-wise manner