Nitrogen's critical role in yeast fermentation and wine quality

Winemaking, a practice that has been carried out for centuries, involves a series of complex biochemical processes that are anything but simple. Though the act of fermentation may seem spontaneous and unproblematic, maintaining the vitality of yeast is a significant challenge, particularly when striving to produce high-quality, flawless wine. It turns out that yeast, in addition to sugar, requires nitrogen as an essential nutrient for growth and development. Moreover, grape musts are often deficient in one or several of the nutrients necessary for yeast, further complicating the process.

Nutritional deficiencies can be responsible for causing delays or even cessation in fermentation, or even the production of hydrogen sulfide (H2S). If this happens, unwanted volatile sulfur compounds are generated in the wine long after fermentation is complete, resulting in flawed, off-tasting wine. Thankfully, modern winemaking is no longer a craft of yore and is now supported by oenologists, a group of professionals specializing in the chemical sciences. As such, contemporary winemaking practices take these deficiencies into account, employing the addition of nitrogen-based nutrients during fermentations.

However, an excessive abundance of nitrogen in the must can also bring about negative consequences. Fermentation could be overly swift in the initial phases, reaching temperatures that could stress the yeast, and potentially kill them. An excess of nitrogen could also lend a salty taste to the wine due to the production of sulfides and ethyl carbamate, the latter being a carcinogenic compound.

Therefore, striking a balance in the levels of nitrogen is critical, not only for producing excellent wine, but also for our health. It is vital to measure the quantity of nitrogen present in the must and add the remainder until desired levels are reached, and all in precise measure.

The concept of Yeast Assimilable Nitrogen (YAN) is generally considered the most efficient method to determine if nitrogen supplementation is needed. The YAN value measures the total nitrogen in the must that is available for yeast, that is, alpha amino-nitrogen (amino-N), which is measured with the NOPA test, and ammoniacal nitrogen (ammoniacal-N). Simply put, Yeast Assimilable Nitrogen would be YAN = ammoniacal-N + NOPA.

To measure the levels of amino-N nitrogen, it's also possible to use another test called the FAN. The NOPA and FAN tests measure only nitrogen complexes based on amino, not measuring the ammoniacal nitrogen. The addition of nutrients from the results of NOPA and FAN will be excessive, as they are based on a lower nitrogen level (only amino-N) than the actual nitrogen level (amino-N + ammoniacal-N). The nitrogen results, both ammoniacal-N and amino-N, are only valid if they are determined before any yeast growth, whether inoculated or native.

So, how much nitrogen do yeasts need? Yeasts generally require between 200mg/l - 400mg/l of nitrogen to complete fermentation without issues. This range varies depending on the sugar content. Musts with high sugar levels require a higher amount of nitrogen. If the must presents a higher risk of nitrogen deficiency or the possibility of a halt in fermentation, an additional 50 mg/l to 100mg/l of nitrogen should be added for each additional degree Brix of sugar (approximately). There are tables or algorithms that allow precise calculation.

The musts at high risk of nitrogen deficiencies and stops in fermentation are those with elevated sugar levels, those originating from vines under water stress, from vines with mineral deficiencies such as nitrogen, from low vigor vines or with diseases, and vineyards with a history of slow fermentations, stops, or with the formation of H2S. Also, high risk are musts that are highly clarified, filtered, or refined, those prone to spontaneous fermentation without inoculating yeasts, and those undergoing malolactic fermentation during alcoholic fermentation. Certain varieties such as Cabernet Sauvignon, Cabernet Franc, Merlot, Zinfandel, and sometimes Chardonnay, Syrah, and Riesling, are also prone to exhibit these problems in their musts.

For these types of musts, an additional amount of nitrogen to that recommended for normal musts should be used, which can be obtained through the use of oenology tables or calculators.

The addition of nitrogen in the form of ammonia and amino acids is essential. Diammonium phosphate (DAP) and ammonia solutions are sources of ammoniacal nitrogen. However, for effective fermentation, yeasts also require a proper balance in amino nitrogen levels. Therefore, in addition to using DAP or ammonia, it is recommended to use a mixture of a nitrogen complex containing amino acids. Remember, YAN is the sum of two types of nitrogen: ammoniacal and amino.

It is important to understand that in grape juice, the amount of amino nitrogen is greater than that of ammoniacal nitrogen. If only DAP or ammonia is added to the must, the natural balance required for normal ammonia metabolism is altered. This alteration is exaggerated when large amounts of DAP/ammonia are added in cases where the must presents a large nitrogen deficiency. This situation can lead to the production of H2S, even when sufficient nitrogen is available (in ammoniacal form), especially if methionine (an amino acid) and pantothenic acid (a vitamin) are in low concentrations. Therefore, to achieve the greatest benefits, it is advisable to use DAP/ammonia in conjunction with a source of amino acids, complemented with vitamins and minerals.

Amino acids, in the form of yeast extracts, are a viable source of amino nitrogen that can be added to the must. It is important to note that the quality and source of these extracts can vary greatly, which can have a significant impact on the wine's quality. Yeast extracts derived from previously used yeast do not have the nutrient concentration found in "primary growth" yeast extracts, which are specifically prepared to assist in winemaking. Additionally, previously used yeasts can transfer unwanted odors and flavors to the wine.

Vitamins play a crucial role in yeast nutrition. They can be found in many yeast extract products or can be prepared separately. Among the most important vitamins are pantothenic acid (Vitamin B5), vital for methionine synthesis; thiamine (Vitamin B1), essential for healthy cell growth; and biotin (Vitamin B7), necessary for ensuring efficient nitrogen metabolism by yeasts.

Regarding minerals, it is known that certain elements, such as phosphate (found in DAP), magnesium, manganese, zinc, and copper, are beneficial for the proper progression of fermentation, although further research is still being conducted on this topic.

The importance of yeast hulls should not be overlooked either. While they are not a yeast nutrient per se, yeast hulls provide sterols to active yeast. Sterols are of great importance in protecting active yeast from high levels of alcohol. Moreover, yeast hulls present benefits towards the end of fermentation as they adsorb decanoic acid derived from yeast, which is toxic to fermenting yeasts.

Lastly, although not nutrients for yeast per se, the presence of oxygen and suspended solids greatly enhances fermentation efficiency. Oxygen allows yeasts to produce the sterols required for alcohol tolerance, while suspended solids seem to promote healthy fermentation. However, since clarified musts produce finer and more aromatic wines, striking a balance is necessary.

Nutrient Addition: The Right Timing

In addition to providing yeast with a healthy and balanced diet consisting of sugar, nitrogen, amino acids, vitamins, and minerals, it is essential to do so at the right time. Nutrients for yeast should be added during fermentation at different stages. The first stage is during yeast inoculation for growth. The second stage occurs when fermentation has begun and the sugar level has dropped by 2 to 3 degrees Brix. At this point, most of the ammoniacal nitrogen has been depleted. The third stage is reached when fermentation is halfway or slightly before. At this stage, nitrogen is needed to replenish the existing yeast cell reserves without promoting further cell growth. The alcohol level is still sufficiently low to allow yeasts to take up nitrogen.

As we have seen, the role of nitrogen in winemaking is essential to ensure successful fermentation and the production of high-quality wine. However, its management requires precision and knowledge, from accurately measuring nitrogen levels in the must to selecting and adding the necessary nutrients for yeasts. This delicate balance is a fundamental piece in the fascinating art of winemaking.