Chapter 6

Wine Fermentation

Nutrient Management during Wine Fermentation

Yeast nutrition is an essential factor in the overall health and success of fermentation. Managing nutrient requirements not only allow for regular and complete fermentations but enhances sensory quality. In many viticultural regions the natural nutrient composition of grape juice is considered sub-optimal, and this may contribute to a variety of problems such as sluggish or stuck fermentations and the formation of undesirable off-flavors (Section 6.12).

What Forms of Nitrogen Can Yeast Use

Nitrogen plays a critical role in yeast metabolism. Nitrogen that is available for yeast to utilize is called yeast assimilable nitrogen (YAN). Yeast assimilable nitrogen comes in two main forms, amino acids and ammonia. These are referred to as organic and inorganic nitrogen, respectively. In winemaking, organic nitrogen is supplied as amino acids and some peptides. Common winemaking sources are nutrients derived from autolyzed yeast. In winemaking, inorganic nitrogen is supplied as ammonia (NH3). Common winemaking sources are diammonium phosphate (DAP) and other ammonia salts.

Determining the Nitrogen Content

Yeast assimilable nitrogen (YAN) should be measured in juices or musts immediately prior to inoculation after all juice or must conditioning treatments. Fining agents such as bentonite and colloidal silicone dioxide are known to have a strong affinity for amino nitrogen, hence reducing its concentration in the juice. Similarly, pre-fermentation treatments such as cold soaks, delayed sulfur dioxide additions, poor mixing of sulfur dioxide and/or insufficient sulfur dioxide may allow for indigenous microbes such as Kloeckera apiculata to assimilate YAN and other nutrients (Butzke, 2010a).

Methods for Measuring YAN

Since the level of nitrogen can greatly influence the quality of the resulting wine, it is important to monitor levels throughout the fermentation process and adjust them as required. Favored methods of measurement of YAN are: (1) enzymatic assay kits, (2) a method known as the formol titration, which consists of neutralizing a juice sample with a base, then adding an excess of neutralized formaldehyde, and re-titrating the resulting solution to an endpoint; and (3) use of expensive equipment such as a spectrophotometer.

YAN Application Rates

When estimating YAN application rates, one must also consider that yeast strains have varying nutritional demands and are classified in general terms as low, medium, or high nitrogen requiring strains. Further to this point, as the sugar level in any must increases, the nitrogen requirement of the chosen yeast will also rise. Thus, when assessing the nitrogen requirement of any fermentation one must consider the general nitrogen requirement of the inoculating yeast and the specific sugar level present in the must.

Recommended Rates

The published, and generally accepted minimum level of YAN required to prevent stuck or sluggish fermentations, is considered to be 140 to 150mg/L for a 21 °Brix clarified must (O'Kennedy et al., 2008a). A low YAN is usually also an indicator of low vitamin and mineral content. The suggested range recommended for fermentation varies from 200 to 400 mg/L of YAN nitrogen per liter. To determine YAN requirements, analyses of °Brix (Section 6.9) and YAN (already discussed) in must are necessary.

Insufficient Ammonia Levels

Nitrogenous compounds such as ammonia are essential to the winemaking process. A sufficient concentration of nitrogen must be present in the grape juice for healthy yeast metabolism and an efficient fermentation. Low nitrogen levels can result in slow or incomplete fermentations.

Excessive Ammonia Levels

The addition of too much ammonia during fermentation is also not advisable. As mentioned earlier, the yeast will assimilate amino acids early during the fermentation and store them for use at a later stage, when ethanol concentrations inhibit amino acid uptake. The addition of ammonium ions early during the fermentation will act to partly inhibit amino acid uptake leading to a loss of volatile aroma compounds, as the ammonium ion is a preferred nitrogen source.

Timing of Nutrient Additions

The timing of nitrogen additions to fermenting must is very important. There are two optimum times to consider adding YAN to the fermenting must. The first time for YAN addition is during the beginning of the yeast stationary phase (after 30-40% of fermentation) and combine this addition with oxygenation. A second time for a YAN addition is at the end of yeast cell growth, which helps to maintain the yeast's vitality to ensure a satisfactory fermentation finish.

Commercial Wine Yeast Nutrients

There is a very wide variety of wine yeast nutrients currently available under many different brand names. The choice presented to winemakers is staggering and overwhelming, and can be quite confusing. Yeast nutrients fall into seven categories: (1) inorganic nitrogen/diammonium phosphate (DAP), (2) complex nutrients, (3) rehydration nutrients, (4) rehydration protectants, (5) yeast hulls, (6) glutathione enriched inactivated yeast, and (7) vitamin mixes.

Diammonium Phosphate

Diammonium phosphate is the most common source of YAN supplement at present, possibly because it is inexpensive and easy to use. Diammonium phosphate is a white crystalline powder which when dissolved in aqueous solution is 21.5 percent (w/v) nitrogen in its ammonium form and 23 percent (w/v) as the inorganic phosphate. For convenience we can consider 100 mg DAP to contain 20 mg YAN.

Complex Nutrients

Complex nutrients are a blend of various components such as ammonium salts, thiamine, vitamins, minerals, sterols, lipids, and specific inactivated yeast, which are a natural source of amino acids. Low YAN musts require the use of complex yeast nutrients since a low YAN is usually also an indication of low vitamin and mineral content.

Rehydration Nutrients

Yeast rehydration nutrients contain inactivated yeasts which in turn deliver a mixture of vitamins, minerals, amino acids, lipids, and sterols. These are all critical in their own way to yeast fermentation performance. The inactivated yeast-based supplements not only provide organic nitrogen, but they are also composed of yeast cell walls that are beneficial to the yeast during fermentation.

Rehydration Protectants

A rehydration protectant is a partially autolyzed inactivated yeast. The partial autolyzation exposes the sterols and lipids in the cell membrane so that they become more readily available to be incorporated into the cell membranes of the fermenting yeast. It is used during rehydration so that the sterols and lipids are only available to the inoculated yeast and not the wild yeasts present in the must.

Yeast Hulls

Yeast hulls are essentially the remains of dead yeast cells (e.g., cell walls and membranes). Yeast hulls can have very good adsorbing capacities, depending on how they were produced. Yeast hulls are mostly used for sluggish or stuck fermentations. Their main role during fermentation is to bind to toxic medium chain fatty acids secreted by the fermenting yeasts, thereby detoxifying the environment and allowing the fermenting yeast to ferment to dryness, thereby making the environment more fermentation friendly.

Glutathione Enriched Inactivated Yeast

Although the addition of pure glutathione is not allowed under current regulations, the concentration of this compound can be increased in wine through the addition of glutathione-enriched dry yeast preparations. These preparations have been observed to have antioxidant properties and can thus influence wine aroma and sensory characteristics.

Pure Vitamin Mixes

The use of pure vitamin mixes is not permitted in all countries. These mixes normally contain the most important vitamins needed during fermentation. The addition of vitamins to a fermenting must is recommended for very low, as well as very high, YAN musts.

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