Yeast Selection and Hydrogen Sulfide: A Summary of the American Society of Enology & Viticulture (ASEV) and ASEV-Eastern Section Conferences: PART 2
By: Denise M. Gardner
This post will feature summaries from the enology industry sessions during the ASEV Conference. These topics are of relevance to the entire Mid-Atlantic and Midwestern wine community. For previous information regarding the Texas Hill Country and the presence of Penn State at the conferences, please click here.
Yeast Selection: Malic Acid Degradation and Yeast Lees
(Based on presentations by Chandra Richter of E&J Gallo and Katie Cook from the University of Minnesota)
The research literature is often split between the importance of yeast selection when it comes to the effects on the aroma and flavor profile of the wine 6 months post-fermentation. Immediately post-fermentation, the flavor differences associated with yeast strains seem dramatic. However, some note that these differences in the flavor profile are depleted over time.
However, there are several other factors that should be considered by winemakers when selecting yeasts for their fermentations.
- Ability to complete fermentation efficiently and completely.
- Long-term mouthfeel effects.
- Secondary byproduct production (aroma and flavor compounds).
- Extent of yeast strain malic acid degradation.
- Yeast strain compatability with malolactic bacteria strains.
- Post-fermentation properties (use of yeast lees).
A recent paper by researchers from E&J Gallo titled “Comparative metabolic footprinting of a large number of commercial wine yeast strains in Chardonnay fermentations” (by C.L. Richter, B. Dunn, G. Sherlock, and T. Pugh in 2013 FEMS Yeast Res 13:394-410) reviewed the ability of 69 Saccharomyces cerevisiae commercial strains to deplete malic acid through primary fermentation. Some commercial yeast strains could deplete the malic acid content by more than 20%, which could have implication for regions that suffer from high malic acid concentrations at harvest.
De-malification properties of yeasts could be important for high acid varieties produced in cool-climate regions (i.e. Pennsylvania, Mid-Atlantic), or for varieties that retain high concentrations of malic acid (> 5 g/L malic acid). Use of yeast strains that degrade the malic acid concentration during primary fermentation can have significant sensory implications: help soften the mouthfeel of high acid varieties and minimize the sharp bite in the finish that is often caused by malic acid. With sugar additions, the loss of malic acid can minimize the production of “sweet-tart” wines (wines that often taste like the candy Sweet Tarts – sweet then sour). To retain the malic acid degradation property of a selected yeast strain, aeration of the must is usually recommended. This is especially important when malolactic bacteria is not desired for a specific wine variety, but minimizing the malic acid content is preferred by the winemaker.
Additionally, wineries may opt to incorporate the use of Schizosaccharomyces strains, including encapsulated Schizosaccharomyces yeast strains, which have higher malic acid degradation properties. Although appealing for wineries that deal with high malic acid grape juice (musts), there are two primary concerns associated with Schizosaccharomyces yeast strains: the high production of acetic acid (volatile acidity) through primary fermentation, and with encapsulated yeast products, the associated higher cost.
Winemakers should also consider the use of yeast lees when making yeast selections. While removal of the gross lees is exceptionally important to wine quality to ensure freshness and minimize the influence of potential off-flavors, use of the fine lees (or lees that remain after 24 hours) may be an important component used by the winemaker to enhance wine quality.
Yeast autolysis (or death) is a slower process in low pH wines (<3.5) compared to higher pH wines. The optimal parameters for yeast autolysis occur at a pH of 5.0 and at a temperature of 45°C (113°F). These conditions are obviously not conducive for quality wine production. Lees aging for many wines may take over a year in order to obtain the full extent of their benefits. However, the timing parameters should be based on wine style and lees quality.
Over time, yeast autolysis may contribute several components to the wine. After 3 – 9 months of aging, yeasts lees may contribute polysaccharides (long-chained sugars) that begin to break down into mannose and glucose. After 9 – 12 months of aging with lees, the yeasts’ cell walls begin to degrade, releasing proteins and peptides into the wine. Proteins and peptides may contribute to a “salivating bitterness” or sweetness, dependent upon the chemical composition of those proteins. Additionally, ribonucleotides are also released, which act as flavor enhancers (similar to the flavor enhancer MSG).
Katie Cook, from the University of Minnesota, presented several general advantages and disadvantages to lees aging in winemaking:
- Remove ochratoxin A
- Remove diacetyl (the buttery flavor developed through MLF)
- Remove fungicides
- Remove copper residues
- Remove sulfur-containing aroma compounds
- Increase the reduction potential
- Increase biogenic amines (which may have effects for individuals with allergies)
- Increase the fatty acid concentration
- Increase the concentration of higher alcohols (may have positive or negative flavor impacts)
- Increase sulfur-containing compounds
- Increase the microbial population (if not properly managed with SO2 and if MLF gets stuck)
- Remove esters (fruity aroma compounds)
- Remove oak-based flavor compounds
It is important for a winemaker to consider all options before deciding on whether or not to use yeast lees for a specific wine. Yeast selection can contribute to the quality of lees at the end of primary fermentation.
Hydrogen Sulfide and its Development in Wine
(Based on presentations by Denise Gardner from Penn State Extension, Benedicte Rhyne from Wine Country Consulting, and Karl Antink from J. Lohr Vineyards & Wines)
Hydrogen sulfide (H2S) is an universal winemaking problem. Most researchers agree that H2S is developed through the sulfate reduction pathway.
While this image looks complicated, there are only a few key things winemakers need to be aware of:
- When sulfate (SO42-) is brought into the cell, it undergoes a series of transformations before it is made into sulfide (S2-) and H2S.
- H2S is used to develop the two sulfur-containing amino acids, methionine and cysteine.
- In order for cysteine and methionine production to continue, an equal amount of nitrogen (from the nitrogen compound pool) needs to be available to match with the H2S concentration.
- If there is not enough nitrogen available to make the amino acids, H2S concentrates within the yeast cell. As H2S is toxic to the yeast cell, it is diffused into the surrounding media: wine.
- Sulfite (SO2), added to the must as potassium metabisulfite (KMBS), bypasses the regulatory sulfate reduction pathway by diffusing through the cell membrane and immediately reorienting in hydrogen sulfide. Large additions of KMBS to the must (>80 ppm) have been shown to increase H2S concentrations in wine.
- H2S can also be produced as methionine and cysteine degradation, which may be a reflection of yeast stress or autolysis.
(For a more detailed explanation of the sulfate reduction pathway, please visit Virginia Tech’s Enology Grape Chemistry Group website here.)
What can winemakers do to prevent the production of H2S?
The current understanding of H2S development is related to nitrogen management during primary fermentation. Yeast assimilable nitrogen (YAN) can be measured in incoming fruit or in the must prior to primary fermentation to ensure that adequate nitrogen is available through the duration of fermentation. YAN can be measured in the winery, but many wine analysis labs also measure YAN for wineries throughout harvest. A past report posted by Penn State Extension Enology indicated the annual variability of YAN within specific plots across Pennsylvania and New York. For this reason, measuring YAN on an annual basis should be a part of a winery’s quality control practices.
The selection and addition of nutrients is also important. The rate and timing of addition should be based off of supplier recommendation.
Diammonium phosphate, or DAP, should never be added with yeast hydration, as it is toxic to the yeast and can inhibit primary fermentation from starting properly. Yeast hydration nutrients (like GoFerm or Nutriferm) should be used for each inoculation. The use of complex nutrients (like Fermaid K or complimentary products) should be used prior to the use of DAP after primary fermentation has started. DAP is recommended to obtain that minimum YAN value when the yeast hydration nutrient and complex nutrient are not enough to reach the critical minimum. Although DAP is less expensive than many complex nutrients, it only contains the inorganic form of nitrogen, ammonium. Recent research at Cornell University has found that overuse of DAP simplifies the aroma and flavor composition of wines, and may contribute to increased biomass of yeast cells, which may contribute to more H2S production as a function of yeast stress.
Research on YAN and its relationship with hybrid or native varieties is ongoing, especially in the Mid-Atlantic. And following adequate nutrient strategies is not a 100% guarantee against hydrogen sulfide production. However, measuring YAN, understanding nutrient additions, and adequately feeding the yeast during fermentation can minimize H2S production in the winery. While this task may seem overwhelming at first, especially during the harvest season, many wineries (including those in PA) have found taking this preventative step to be less time consuming and better for wine quality than dealing with copper additions, copper trials, and potentially not being able to adequately fix reductive wines.
Wineries can also take the initiative to adequately train cellar personnel on identifying H2S and other sulfur-containing flavors or aromas. (A previous blog post discussed the value of training individuals associated with the production.) Many defect kits exist on the market: Le Nez Du Vin, Wine Awakenings, Enartis Vinquiry. Additionally, if you have not already received one, you may email Denise Gardner (firstname.lastname@example.org) to receive a free Penn State Extension Digital Defects Kit, which can be used to train winery and tasting room personnel on wine defects with the use of common household items.
Additionally, yeast strain selection can be important when dealing with hydrogen sulfide issues. It is imperative for wineries to read the technical data related to yeast strains that they purchase. Some strains have higher nutrient demands than others. Adequately treating the must with the proper nutrient regime is essential. Wineries should not hesitate to contact yeast suppliers and talk to a technical representative if they have questions regarding a yeast strain.