Getting Ready for Harvest – Part 1: Yeast Nutrition
By: Denise M. Gardner
The “Getting Ready for Harvest” seminar featuring José Santos from Enartis Vinquiry (California) included a detailed discussion about the technical aspects of several key processes that are currently of importance to Pennsylvania winemaking. Some of these topics included:
- White wine oxidation protection
- Yeast nutrition and its importance during fermentation
- Reducing “green” aromas and flavors in red wines
- Color stability in red wines
In this four part series, I will summarize some of the discussions led by José during the 2014 “Getting Ready for Harvest” workshop.
The Components of Nutrients used during Fermentation
There are many components that are required for adequate yeast nutrition. Essentially, in winemaking, yeast need a sugar source to convert into ethanol. However, yeast have additional needs for vitamins, minerals, proteins, and fats. Comparable to human nutrition, the yeast have nutritional requirements essential to their growth and reproduction which allows them to carry out the utilization of sugars.
Thiamine is a vitamin that yeast need in order to multiply or reproduce. In general, thiamine is correlated with yeast assimilable nitrogen (YAN); the higher the YAN, the higher the thiamine concentration. Enologically, thiamine contributes to yeast growth stimulation, improves the fermentation speed, and reduces the production of SO2-binding compounds (e.g. acetaldehyde, pyruvate).
Thiamine is especially important at the start of fermentation, as it will contribute to yeast reproduction and is therefore added at inoculation. Essentially, this vitamin will be used to get yeast out of the initial lag phase and into a logarithmic (growth) phase. Without adequate thiamine reserves, winemakers run a higher risk of stuck fermentations, reduction of yeast growth during fermentation, and a higher accumulation of pyruvate and/or acetaldehyde (which bind to SO2). Several factors may decrease the concentration of thiamine including:
- the consumption of thiamine by wild yeast strains,
- application of long pre-fermentation stages like transportation of grapes/juice or cold soak
- the presence of Botrytis
Nitrogen is often the component that most winemakers are concerned about, and is reflected in juice YAN values. The primary use of nitrogen is for the synthesis of amino acids and proteins. Proteins contribute to structural yeast cell components, enzymes within the yeast cell that are essential for growth, and contribute to transporters on the yeast cell membrane which allows for chemical exchanges between the juice/wine and the internal yeast structure. Without protein development, the yeast cell cannot live.
There are two forms of nitrogen that are available to yeast during fermentation: inorganic (ammonium ions, NH4+), which winemakers can add directly as diammonium phosphate (DAP), and organic (amino acids), which are added at yeast hydration. Amino acids are brought into the cell through transport proteins located on the cell membrane. Two things inhibit the transfer of amino acids from the juice into the yeast cell: the presence of alcohol and the presence of ammonium (NH4+). Therefore, the addition of amino acids to the juice usually takes place at yeast hydration and inoculation, before alcohol is present, and before DAP additions to the wine. This is why winemakers are encouraged NOT to add DAP at inoculation. Figure 1 illustrates this point in which amino acids are added before 3-4% alcohol is produced during fermentation. After this point, the yeast can only take up the inorganic form, or NH4+, which is usually added within a complex nutrient or as DAP.
For more information on YAN, DAP, and complex nutrient additions, please visit this previous blog post.
Fats and Minerals
Fatty acids make up an essential part of the yeast cell: the yeast cell membrane. Fatty acids and sterols actually help to maintain membrane fluidity, which is an important property as alcohol concentrations increase outside of the cell. If the cell membrane becomes too rigid, transport protein functions are inhibited, which prevents the yeast from being able to take up sugar. This can lead to stuck fermentations and an increase in volatile acidity (VA) development. The rise in VA is attributed to the stuck fermentation, which minimizes CO2 output and allows competing, spoilage microorganisms (like acetic acid bacteria or wild yeast) to proliferate without protection from SO2 and availability of oxygen.
To increase the development of fatty acids and sterols during fermentation, José encouraged the use of micro-oxygenation (10 – 15 mg/L O2) at 1/3 of the way through fermentation. If micro-oxygenation cannot be utilized, winemakers were encouraged to incorporate air (i.e. oxygen) in some way: splashing, pumping over, etc. He emphasized that this step is critical in white wine production (especially for fermentations in reductive environments), and for high alcohol wines (in particular, reds).
Minerals, like magnesium (Mg) and zinc (Zn) are also essential during fermentation, as they contribute to yeast cell membranes as well. They actually act as co-factors for many enzymes that are essential during fermentation. Winemakers do have sources of Mg and Zn through many yeast hull derivative products.
Practicality in Production
Where does this information become useful for winemakers?
After decades of research, winemakers have evidence regarding the importance of YAN and nutrient management during fermentation. If you’d like more information on YAN, please visit the follow sites.
- Variation in YAN data across PA and NY
- Cornell’s Management Options for YAN
- Cornell’s Publication Tracking YAN through Harvest
If you are not measuring YAN at harvest, there are several considerations for implementing this essential step:
- Ensure fermentation completion
- Minimize stuck and sluggish fermentations
- Minimize the prevalence of hydrogen sulfide (H2S) produced during fermentation
- Maintain varietal aromas/flavors that were developed during grape ripening
- Enhance aromatics and flavor components of many wines
While budgetary issues may arise, wineries should take the time to consider outsourcing wine ISO-accredited labs to measure YAN in juice or run YANs in house. Cornell has previously written a small “Cellar Dweller” newsletter regarding the equipment needed to run YANs.
It may end up being more economically feasible for wineries to overnight juice samples to a lab and get the result the following day.
Additionally, wineries should consider where money is spent for winemaking products. Some yeast strains can be costly, and while they seem like a worthwhile investment, it may be more practical to purchase a lower-cost yeast that ensures fermentation completion. Meanwhile, wineries can use savings to purchase products that protect the juice that they worked so hard to get over the growing season. This can include antioxidant products used in white juice processing or a more thorough yeast nutrient regime as discussed in this post.