By: Brent Nakano
A Guide to: Wine Alcoholic Fermentation Part 1
During the alcoholic fermentation of wine, beyond the production of ethanol, the chemistry of grape must is changed and aroma compounds are generated. This occurs in a multitude of ways including changes in compound solubility due to additional ethanol and the yeast’s enzymatic and metabolic interaction with the grapes’ chemistry. To learn more about the alcoholic fermentation of wine, we consulted our favorite book on wine: Wine Science by Ronald Jackson. We highly recommend purchasing the book; it can be found here in both print and digital versions at: www.elsevier.com/books/wine-science/jackson/978-0-12-816118-0
Primairy Functions of Alcoholic Fermentation in Wine
Modification to Sugar Content
Winegrapes are predominantly composed of fructose and glucose. These sugars are converted into alcohol and, even if fermentation reaches completion, small quantities of sugars without sensory significance will remain. According to Jackson in Wine Science:
Winegrape’s sugar concentration at maturity: 19-25° Brix
Wine’s sugar concentration after vinification:
Higher quantities of residual sugar may be retained by prematurely stopping fermentation through chilling, centrifugation, distilled alcohol addition or filtration. Common examples include:
Winegrapes are predominantly composed of fructose and glucose. These sugars are converted into alcohol and, even if fermentation reaches completion, small quantities of sugars without sensory significance will remain. According to Jackson in Wine Science:
Winegrape’s sugar concentration at maturity: 19-25° Brix
Wine’s sugar concentration after vinification:
- Fermentable sugars: preferably 1g/liter (< 0° Brix)
- Non-fermentable sugars including arabinose, rhamnose, and xylose: (0.2 g/liter).
Higher quantities of residual sugar may be retained by prematurely stopping fermentation through chilling, centrifugation, distilled alcohol addition or filtration. Common examples include:
- Sparkling wine, where sugar is reserved for secondary fermentation.
- Dessert wines like port are made by the addition of distilled alcohol.
Decrease in acidity
Titratable acidity decreases and pH increases. Vilela (2017) in a literature review of deacidification by microbes noted:
For more on microbial deacidification read:
Titratable acidity decreases and pH increases. Vilela (2017) in a literature review of deacidification by microbes noted:
- Saccharomyces Cerevisiae metabolizes acetic acid during the fermentation process.
- Other Saccharomyces strains like Schizosaccharomyces pombe and Saccharomyces paradoxus can deacidify wine through the metabolism of malic acid. However, the high fermentation temperatures of Schizosaccharomyces yeasts (relative to Saccharomyces cerevisiae) can adversely affect wine quality.
- Malolactic Fermentation by selected strains of Oenococcus oeni or Lactobacillus plantarum can be used to lower acidity. Malolactic Fermentation will be discussed in depth in another issue.
For more on microbial deacidification read:
- Vilela, Alice. 2017. "Biological Demalication and Deacetification of Musts and Wines: Can Wine Yeasts Make the Wine Taste Better?" Fermentation 3, no. 4: 51. https://doi.org/10.3390/fermentation3040051
- Vilela, Alice. 2019. "Use of Nonconventional Yeasts for Modulating Wine Acidity" Fermentation 5, no. 1: 27. https://doi.org/10.3390/fermentation5010027
Additional Phenolic Extraction from Grape Solids
The extraction of color and aroma from grape phenolics occurs when grape skins, seeds and other solids are left in the fermentation vessel. And, as discussed in Hawaii Beverage Guide’s February issue, phenolic extraction can occur before, during and/or after fermentation.
The extraction of color and aroma from grape phenolics occurs when grape skins, seeds and other solids are left in the fermentation vessel. And, as discussed in Hawaii Beverage Guide’s February issue, phenolic extraction can occur before, during and/or after fermentation.
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In his PhD dissertation, Dr Patrick Setford developed a simulation of phenolic extraction during red wine fermentation. He outlined the phenolic extraction process as follows:
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Key factors that influence phenolic extraction
(as outlined in Setford et. al 2019): Solvent Composition: Increased concentrations of ethanol and sulfur dioxide (SO2) increases the rate of internal diffusion; however, SO2 at levels normally associated with red wine fermentation appear to have a much lower effect. •Temperature: Extraction increases as temperature increases. This occurs because temperature influences the permeability of the cell membranes in the grape solids, the solubility of phenolic compounds, the rate of ethanol production during fermentation, and the viscosity and density of solvents. •Contact area between the grape liquid and the grape solids can impact the rate of extraction and the amount of extraction. This contact area, which is predominantly dictated by the cap management technique (e.g. manual punch-down, mechanical punch-down, pump-over and fermentation in a spiral rotor tank), influences the rate of anthocyanin extraction, however the quantity of extraction was dependent on the grape variety. Setford also noted that the amount of contact time has not been studied. Reactions of extracted phenolics to the fermentation environment also influence phenolic extraction. The most notable are: •Oxidation, if increased during pre-fermentation, is found to decrease both the anthocyanin and total phenolic evolution. •Copigmentation, the process of anthocyanins being stabilized through non-covalent bonds forming between themselves or colorless cofactors, may account for up to 50% of the color observed in young red wines. Examples of copigmentation include: The flavonol quercetin is the most effective copigment at room temperature. However, if two malvidin-3-glucoside (responsible primarily for the color of red wine) molecules bond together, the self-copigmentation reaction provides more stability than quercetin. A literature review of microorganism influence on wine color by Tofalo et al (2021) noted yeast can: Assist in pyranoanthocyanin development, especially Vitisins type A and B. This occurs through the secretion of pyruvic acid or acetaldehyde which can then bind to anthocyanins. These vitisins contribute 11-14 times more color than unmodified anthocyanins, shift the wine towards an orange-red hue, and affect the intensity and tonality of wine color by the action of β-glycosidase on anthocyanins or anthocyanidase enzymes. •Yeast cell walls can absorb tannins, anthocyanins and other volatile compounds that are released during maceration. The rate of absorption is influenced by temperature and ethanol concentration with higher temperatures increasing anthocyanin adsorption and higher ethanol concentrations having lower absorption. |
Modification of wine aroma compounds through enzymatic and metabolic activity of yeast
The multitude of yeast metabolic pathways that create additional aromatic compounds and what influences their development will be explored in-depth in another issue.
The following summarizes the following article which we highly recommend reading:
Swiegers,Jan & Bartowsky, Eveline & Henschke, P.A. & Pretorius, Isak. (2005). Yeast and bacterial modulation of wine aroma and flavour. Australian Journal of Grape and Wine Research.11. 139-173. Source: www.researchgate.net/publication/280765129
_Yeast_and_bacterial_modulation_of_wine_aroma_and_flavour
The multitude of yeast metabolic pathways that create additional aromatic compounds and what influences their development will be explored in-depth in another issue.
The following summarizes the following article which we highly recommend reading:
Swiegers,Jan & Bartowsky, Eveline & Henschke, P.A. & Pretorius, Isak. (2005). Yeast and bacterial modulation of wine aroma and flavour. Australian Journal of Grape and Wine Research.11. 139-173. Source: www.researchgate.net/publication/280765129
_Yeast_and_bacterial_modulation_of_wine_aroma_and_flavour
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Key aroma compounds in wine that are produced by yeast
(as noted in Swiegers et al. 2005): •Higher alcohols of: •Aliphatic alcohols: propanol, isoamyl alcohol, isobutanol and active amyl alcohol •Aromatic alcohols: 2-phenylethyl alcohol and tyrosol •Carbonyl compounds including acetaldehyde •Volatile phenols, produced by the decarboxylation of hydroxycinnamic acids can create barnyard-like off-odors which mask the fresh/floral characteristics of white and rosé wine. •Ethylphenol: 4-ethylguaiacol and 4-ethylphenol •Vinylphenol: 4-vinylguaiacol and 4-vinylphenol Esters: •Significant esters include: Ethyl acetate (fruity, solvent-like), isoamyl acetate (isopentyl acetate, pear-drops aromas), isobutyl acetate (banana aroma), ethyl caproate (ethyl hexanoate, apple aroma) and 2- phenylethyl acetate (honey, fruity, floral aromas) •Production includes both Saccharomyces and non-Saccharomyces strains including Hanseniaspora guilliermondii and Pichia anomala. |
Sulfur Compounds:
•Produced by yeast species Kluyveromyces lactis, Torulaspora delbrueckii and Ambrosiozyma monospora •Found in hops and grapes Other influences on wine aroma by yeast include, according to Jackson in Wine Science:
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Wine Fermentation Microbes
Grape must is typically inoculated with laboratory cultivated yeast to make wine. Unlike the boiling wort in beer production, grape must is not sterilized before inoculation. This means epiphytic yeasts (those living on the grape in the vineyard) are typically present in a wine fermentation. Sulfur dioxide may be used to minimize undesired epiphytic yeast and its impact varies by cultivar, growing location, and winemaking style (including the duration of skin contact), clarification and other processing techniques. It should also be noted that using blends of inoculated strains or yeast species is commonplace.
Grape must is typically inoculated with laboratory cultivated yeast to make wine. Unlike the boiling wort in beer production, grape must is not sterilized before inoculation. This means epiphytic yeasts (those living on the grape in the vineyard) are typically present in a wine fermentation. Sulfur dioxide may be used to minimize undesired epiphytic yeast and its impact varies by cultivar, growing location, and winemaking style (including the duration of skin contact), clarification and other processing techniques. It should also be noted that using blends of inoculated strains or yeast species is commonplace.
General Microbial Development Cycle of
Inoculated Wine Fermentation
(from Demuyter et al 2004)
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Spontaneously Occurring Yeast vs Epiphytic Yeast vs Indigenous Yeast vs Endemic Yeast vs Native Yeast vs Wild Yeast
Though typically used interchangeably, these terms have different technical definitions. To clarify:
For the purpose of this article and for Hawaii Beverage Guide’s future articles, we believe:
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Sources of Wine Yeast
Laboratory cultured yeast are the primary fermenters used by most commercial wineries. A Epiphytic yeast As epiphytic and other spontaneously occurring yeast occur in all wine fermentations, there is a significant amount of academic research on what naturally occurring yeast exist and where they come from. The general answer is: There are a multitude of microbes initially present in fermented grape must, some of which have been identified. Only a few have known enological significance while the others are probably not metabolically active because of the acidic, generally anaerobic, and alcoholic conditions which are generally inhospitable to most yeasts, fungi, and bacteria (Jackson, Wine Science). Some general trends on the sources of yeast that have known significance to winemaking according to Jackson in Wine Science are: •The winery building and winery equipment like crushers and presses are the major sources of yeast for spontaneous fermentation.10 This may be more prevalent in older equipment and buildings as modern wineries strive for conditions that limit unintended microbial development. •Damaged grapes have significantly larger, but still relatively small (compared to other microbes) populations, of epiphytic Saccharomyces cerevisiae yeasts, which may be brought to the berries by insects.11 •Strains used in intentional inoculations of wine may spread over short distances outside the winery predominantly through water runoff, though they can spread further if the pomace is used as vineyard fertilizer.12 •Strains can differ by region13 and vineyard management practices14 making spontaneous fermentation unique to the particular winery. Spontaneous Fermentations Spontaneous fermentations may result in wine that: •Showcases yearly variations in character because of the way the yeast act within the particular grape's chemistry •Showcases the wine’s “unique” terroir. However, given that Saccharomyces cerevisiae is predominantly from winery equipment rather than the vineyard, terroir’s definition needs to include the winery. •Has an extended lag period which, due to the low quantity of starting cells of Saccharomyces cerevisiae relative to that of inoculated fermentation, may be more susceptible to undesirable microbes. To mitigate uncertainty caused by the potential development of off-flavors, cultivated “wild yeast” can be used in inoculations. |
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