​General Fermentation Techniques

  • Batch Fermentation
  • Continuous Fermentation
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Batch Fermentation
Batch fermentation is the primary approach to wine, beer and spirits production and consists of the liquid being fermented in a vessel with yeast for a duration of time. To make more of the fermented liquid, the vessel is then emptied and refilled and the process is repeated. Since this is almost exclusively the fermentation technique used for wine, it will be the focus of this article.
Continuous Fermentation
In many industrial fermentations, continuous fermentation is employed. This is not a common practice in wine production, even in inexpensive wines, because grapes are produced seasonally and require expensive storage under sterile, nonoxidizing conditions, which negate any cost savings from the technique. Immobilization of the yeasts may make the technique more accepted as a means of keeping costs down in the production of low priced wines.1

Technique:
  • Fermentable liquid (substrate) is added at a relatively constant rate or at frequent intervals. An equivalent volume of the ferment is removed to maintain a constant volume.
  • For the industrial production of single metabolic products, synthesized primarily during a particular phase of colony growth.

Benefit:
  • Lower overall cost for high-volume output produced on a production line operating 24 hours a day, 365 days a year than batch fermentation, due to less labor. However, there are high upfront costs, due to the expensive computerized equipment necessary.
  • Ideal for fermentation of year-round high-volume distilled alcohol made from grain or molasses, products which can be cheaply stored in bulk.

​General Wine Fermentation Styles

Red and white wine fermentations differ due to the skin contact practices, phenolic content, and source of aromatic compounds. White wine aroma is primarily developed from the juice and yeast metabolites, whereas red wine flavor is primarily derived from the grape’s skin and seeds
  • White and Rose Wine
  • Red Wine Fermentation
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White and Rose Wine Fermentation
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Vessel type: Typically temperature-controlled tanks. Barrels may be used in colder climates.
Temperature of the fermentation is maintained within a narrow range throughout the duration of the process, with the specific range depending on the general style.
  • 10-15 °C to encourage the production and retention of fruit esters.
  • 15-20 °C to encourage the development of the varietal fragrance in certain cultivar.s

pH: 3.1-3.42

Process Notes:
  • The juice is typically chilled to fermentation temperature using cooling coils, food-grade dry ice, or liquid nitrogen before inoculation.
  • Skins are not typically included in the fermentation vessel.
Red Wine Fermentation

​Vessel type: Historically, red wine was fermented in 50-100 hl wooden open-top vats that allowed sufficient passive cooling and easy cap management. The higher phenolic content of red wine provided adequate oxygen protection (Wine Science).
Currently fermentation vessels are highly variable, and dependent on the winemaker’s style.

​Temperature of fermentation is utilized to manipulate the rate of skin phenolic extraction, with higher temperatures occurring at faster rates.
  • 20-30 °C ( 68-86 °F) is typical.
  • pH: 3.4-3.5

Process Notes:
  • Skins are typically included in the fermentation vessel.
  • Cap punch-downs or pump-overs during fermentation are used to increase skin contact. The frequency and force of punch-downs and pump-overs during primary fermentation are dependent on the level of extraction desired. According to Jackson in Wine Science, punch-downs and pump-overs also:
  • Help limit the growth of potential unwanted microorganisms by aerating the ferment, cooling the fermenting must, exposing potential spoilage organisms in the cap to the toxic action of ethanol, and facilitating the release and dispersion of potassium throughout the must (limiting an undesirable pH rise in the cap).
  • Promote the rapid extraction of anthocyanins while limiting excessive tannin uptake.

Physical Conditions: The Fermentation Vessel

​Vessel Shape

  • Fermentation Vessel Materials
  • Vessel Size
  • Vessel Shape
  • Vessel Orientation
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Fermentation Vessel Materials [3]

Wood (typically oak)
  • Inert: No
  • Microbial Spoilage Potential: High but depends on sanitization practices
  • Oxygen Permeability: Some
  • Maintenance: High
  • Other Features: For more on oak barrel variations our guide to oak aging, visit hawaiibevguide.com/oak

Cement/Clay
  • Inert: No (if uncoated), yes, if coated with epoxy. Gil i Cortiellaet al. 2021 noted: Clay jarsfavor the development of more chemically distinctive wines than wine making using other kinds of vessels. Wine aged in concrete showed the lowest titratable acidity and the highest pH among all treatments, followed by clay, whereas polyethylene and stainless steel had higher and similar TA and pH. This may be caused by a release of inorganic elements, like silicon, sodium, magnesium, iron, and manganese, which cause the formation of inorganic salts, that, in turn, alter the ionic strength and the equilibrium between tartaric acid and hydrogen tartrate forms in dissolution, leading to the changes observed in the pH and TA.
  • Microbial Spoilage Potential: Low
  • Oxygen Permeability: Some
  • Maintenance: Medium
  • Other Features:
    • Cement and clay are difficult to surface-sterilize. This can be mitigated by epoxy coating the interior of the cement tank, or by using ceramic tiles.
    • Cement tanks are generally cheaper to construct than stainless steel tanks of equal size.
  • Note: There are differences in porosity and chemical composition for both ceramics and cement tanks.

Stainless Steel
  • Inert: Yes
  • Microbial Spoilage Potential: Low
  • Oxygen permeability: Some
  • Maintenance: Low
  • Other Features: Rapid heat transfer used to facilitate cooling of the fermenting juice.

Polyethylene (Plastic)
  • Inert: Yes
  • Microbial Spoilage Potential: Low
  • Oxygen Permeability: None (unless specially designed to be semi-permeable)
  • Maintenance: Low
  • Other features: Lightweight, low cost


​Fiberglass (No longer common)
  • Inert: Yes
  • Microbial Spoilage Potential: Low
  • Oxygen Permeability: None
  • Maintenance: Low
Vessel Size
Fermentation vessel size is mainly influenced by:
  • The volume of grape must to be fermented in a single batch. Excess space in the fermentor (headspace) needs to be filled with costly inert gas to minimize the oxygen in the tank.
  • Surface area-to-volume ratio, because it influences passive radiation (passive cooling) of heat caused by fermentation’s exothermic reaction, through the vessel’s walls. Smaller vessels typically have higher ratios.

General Wine Tank Sizes
According to Jackson in Wine Science, tank size is also influenced by the type of wine being fermented:
  • Premium wines typically use a 50 to 100 hl vessel to balance economical/ ease of operation with maintaining wine individuality.
  • Standard quality wines: 200 hl to more than 2000 hl ( 50,000 gal), because economics of size favors fewer but larger fermentors.
  • Computers using biosensors are used to monitor temperature regulation, since the large volumes of must can produce more heat than can be dissipated by the tank, and increase the likelihood of excessive foaming and wine loss and the need for defoamers ( mixture of mono- and diglycerides of oleic acid and polydimethylsiloxane).
Generalizations on how size impacts fermentation

Small Fermentors

Benefits:
  • Smaller fermentation batches mean that vineyards can be broken down by specific lot, rather than having to ferment all the grapes together.
  • Small fermentors may allow wine to be aged on the lees longer than in larger fermentors.
Drawbacks: Small fermentors require the more labor-intensive tasks of topping, racking, cleaning, sterilizing, and maintenance.


Large Fermentors:
Benefits: Despite the high up-front cost, as they are typically outfitted with a multitude of sensors, mechanized processes, and computerized controls, this type may provide substantial savings in the long run, due to economies of scale.

Drawbacks:
  • Requires active temperature control solutions, because heat does not sufficiently dissipate via the surface.
  • Increases the likelihood of excessive foaming and wine loss (and the need for defoamers).
  • Increases the likelihood of sedimentation of grape solids and yeasts, which can negatively impact fermentation, necessitating agitators or pump-overs.
  • Larger equipment can be more difficult to work with than smaller equipment, as manual tasks require automation.

For an insightful discussion on the impacts of fermentor size, read: Patterson, T. (2010, February). Wines & Vines - With Fermenters, Does Size Matter? Wines Vines Analytics. Retrieved April 15, 2022, from https://winesvinesanalytics.com/sections/printout_article.cfm?content=70980&article=column
Cylindrical Shaped Tanks
The cylindrical shape distributes the tank’s pressure more evenly compared to a rectangular
tank, which concentrates the pressure on the corners as the pressure applied
to the tank walls pushes the two sides apart.

Oblong (Egg Shaped) and Conical Fermentors
Benefit: Turbidity within the fermentor is passively increased by the enhancement of convection currents (Gil i Cortiella et al. 2021).

Drawback: These vessels are more difficult to produce due to their shape. The multitude of conical fermentors originating from different countries include:
  • Amphora:
    • Country of Origin: Greece
    • Shape: Tapered, rounded body shaped like a vase.
  • Tinaja:
    • Country of Origin: Spain
    • •Shape: Tapered at the top and the bottom and large enough to ferment in.
  • Dolium:
    • Country of Origin: Ancient Rome
    • Shape: Wide top, tapering towards the flat bottom.
  •  Qvevri (pronounced kway-vree):
    • Country of Origin: Georgia
    • Shape: Inver se teardrop.

For additional insight into commercially available concrete egg-shaped wine tanks:
Vertical Oriented Tanks
  • Enables easier cap management by providing a smaller cap surface area for pump overs and punch-downs.
  • Take up a minimal footprint in the wine cellar. Horizontal Oriented Tanks
  • Increase the surface area of the wine’s cap, thereby increasing extraction.
  • Aids the flocculation (settling) of lees to the bottom of the tank by reducing the distance in which it travels to the bottom.
  • Drawbacks: Has a larger wine cellar footprint, compared to vertical tanks.

Horizontal Oriented Tanks
  • Increase the surface area of the wine’s cap, thereby increasing extraction.
  • Aids the flocculation (settling) of lees to the bottom of the tank by reducing the distance in which it travels to the bottom.
  • Drawbacks: Has a larger wine cellar footprint, compared to vertical tanks.

Common Fermentation Vessel Types

Barrels
  • Design: Cylindrical
  • Material: Wood or plastic
  • Oxygen Exposure: Semi-permeable
  • External Microbial Exposure: None if plastic, some if wood. The extent is dependent on barrel material and sanitization practices.
  • Size: Small
  • Temperature Control: None
  • Disadvantages: More effort is involved in topping, racking, cleaning, sterilizing, and maintaining small wood fermentors.

Vats
  • Design: A container with an open top.
  • Oxygen Exposure: Yes
  • External Microbial Exposure: Yes
  • Size: Varies
  • Temperature Control: None.
Tanks
  • Design: A completely enclosed container. A sloped bottom with a trap door or removable tank bottom eases pomace discharge.
  • Material: Typically stainless steel, cement, or fiberglass; formerly made of wood.
  • Oxygen Exposure: None, unless purposefully added.
  • External Microbial Exposure: No
  • Additional feature: Can act as a longterm storage container.
  • Disadvantages: High purchase cost;however the shorter holding periods reduce the number of fermentors needed to process the same volume of must.

​Mechanized Fermentor Modifications

Fermentation vessels have been outfitted to control two mechanisms in fermentation:
  • Temperature Controlled Tanks
  • Automated Cap Management
  • Rotary (roto) Fermentors
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Temperature Controlled Tanks
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Problem it solves: Fermentation is exothermic, meaning it gives off heat. This can cause the contents to become hotter than ideal for the yeast. Adequate temperature control usually
avoids the need for adding defoaming agents, as foam causes wine loss associated with discharge through overflow valves (Jackson, Wine Science).

Design: Water jacket, cooling coils, or plates placed at various points in the fermentation vessel
Automated Pump-overs

Problem it solves: Automates the labor intensive process of manual pump-overs.

Design:
  • Automated periodic or continuous pumping of the juice over the cap is done via computer managed program. This may be combined with temperature control by passing the wine through cooling coils.
  • To limit oxygen uptake during pumping over, the head space is often filled with inert gas (N2 or CO2). This is most important at the beginning and end of fermentation, when flushing of the headspace by carbon dioxide, released during fermentation, is limited.

Pileage Fermentor (Automated CapPunch-down)

Problem it solves: Automates the labor-intensive process of manual punch-downs.

Design: Automated cap punchers.

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Ganimede Fermentors (Automated punch-down/pump-over/Délestage) [4]
Problem it solves: This patented design passively automates délestage fermentation and punching down or pump-overs.

Délestage (meaning “lightening”) fermentation5
A technique for making red wine that uses the separation of juice and grape solids to lower tannin concentration, while increasing esters and other fruit aromas. It also enhances the early formation of polymeric pigments, especially with incompletely ripened grapes. [5]

Manual Process: A “rack-and-return” process (racking is the transfer of wine from one container to another), where:
  1. Racking: As the juice is drained away from the grape solids, the cap settles to the bottom of the fermentation vessel. This process may involve seed deportation, a process in which a portion of the grape seeds, particularly the immature ones which possess a high proportion of extractable phenolics, are removed. The grape solids are kept separate from the fermenting wine for a few hours before reintegration. This allows the solids to settle.
  2. This softens the tannins and stabilizes the wine’s color by oxidation. It differs from maceration, which is typically done with minimal oxygen exposure. It differs from pump-overs by separating the juice from the solids.
  3. Return: The juice is returned to the fermentation vessel to soak again in the solids.
  4. This process is repeated several times, or even daily for the entire duration of fermentation.

Benefits
  • Generally this method results in a higher percentage of color derived from large polymeric pigments than manual cap-punched wines.
  • It favors juice extraction from grape solids and increases free-run yield, therefore requiring less pressing of the solids at the end of fermentation. [6]

Drawbacks: Délestage-fermented wines may not age as well as more tannin-rich wines.

Ganimede Fermentors Process
Utilizes CO2 generated from fermentation instead of a motor to power a gentle cap mixing action.

Design
  •  An internal cone traps volitized CO2 below it during fermentation to pressurize the system.
  • Once the system is pressurized, large bubbles are released. As they rise to the surface through the neck of the cone, they agitate the cap and keep all the skins wet and evenly dispersed.
  • When the bypass valves are opened, the CO2 trapped below the cone is released, causing a vigorous mixing action to take place.
  • The mixing action caused by both opening the bypass valves and agitating the cap with CO2 bubbles causes much of the seeds to fall to the bottom of the tank. The seeds can be removed from the conical bottom via a discharge valve.
  • Nitrogen or oxygen may be manually injected to eliminate reductive aromas that can be caused during fermentation, without pumping over or punching down in the tank.

For more on the process, read: www.ganimede.com/en/mg_come_funziona.aspx


Rotary (roto) Fermentors
Problem it solves: Automates cap management and can be used to shorten the maceration time for white wines like Chardonnay, Gewürztraminer, and Riesling, where varietal aromas are derived from the skins.

Design
  • Minimizes oxygen exposure.
  • The fermentor lays horizontally rather than vertically, to increase the surface contact between the juice and the pomace.
  • Internally, there are spiral-shaped paddles that gently and continuously mix the fermenting juice with the seeds and skins. This also limits temperature stratification between the cap and fermenting juice.
  • Permits earlier pressing, allowing the juice to be separated from the pomace before undesirable levels of bitter/astringent polyphenolics are extracted. Occasionally, the early pressing property permits the early transfer of the ferment to barrel for completion of fermentation.
  • Rapidly extracts phenolic compounds thereby creating a red wine that can be consumed early, with less barrel aging.

​Physical Conditions: Temperature, Oxygen, Pressure

Temperature 

Temperature is one of the most significant environmental factors that influence the final wine’s aroma, primarily because of its influence over yeast growth and metabolism, and secondly, because of its influence over the rate of reaction between aroma compounds and enzymes. It is also impacted by the volatility of aroma compounds. According to Jackson in Wine Science, the preferred vinification temperature is typically less than that of optimum ethanol production or yeast growth.
  • Impact on Yeast
  • Temperature is influenced by:
  • Temperature Control
  • Typical Wine Fermentation Temperatures
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Impact on Yeast
Within an operating range which varies by strain and species, yeast typically increases growth and metabolism as temperature increases, and decreases growth and metabolism as temperature decreases.

Temperature impact on yeast growth
Jackson in Wine Science cited Arroyo-López et al 2009, who found [7]:
  • Growth of yeast is impacted by temperature, particularly during the exponential growth phase.
  • Above 20 °C, yeast cells rapidly decline in viability, while at cooler temperatures, cell growth is retarded, but viability is enhanced.
  • Cool temperatures can be used to prolong the lag phase of fermentation, or keep cells dormant, therefore white juice is sometimes warmed to 20 °C before yeast inoculation, then cooled once fermentation has commenced.

The type of yeast that develops is influenced by temperature.
Heard and Fleet, 1988 researched the effects of temperature and pH on the survival and growth of Saccharomyces cerevisiae, Kloeckera apiculata, Candida stellata, Candida krusei, Candida pulcherrima and Hansenula anomala, examined during mixed culture in grape juice.

They found:
  • At 25 °C, pH 3.0 and pH 3.5, Saccharomyces cerevisiae dominated the fermentation, and the other species died off before fermentation was completed.
  • At 20 °C, Saccharomyces cerevisiae also dominated the fermentation, but there was increased growth and survival of the other species.
  • At 10 °C: The fermentation was dominated by the growth of both Saccharomyces cerevisiae and Kloeckera apiculata, and there was extended growth and survival of Candida stellata and Candida krusei.
  • Juices fermented at 10 °C exhibited ethanol concentrations between 7.4 and 13.4% and populations of Kloeckera apiculata, Candida stellata and Candida krusei in the range 106-108 cells/ml. However, these species produced maximum ethanol concentrations in the range of 2.7-6.6% when grown as single cultures in grape juice.
  • For more on the impact of spontaneously occurring yeast species: hawaiibevguide.com/a-guide-to-wine-fermentation-and-yeast

Losses in volatile compounds [8,9]
  • Reduce production of ethanol and higher alcohols.
  • Reduce the release of yeast colloids, thereby facilitating clarification.
  • The rate of ethanol loss during vinification increases with an increase in temperature.,
  • The volatilization of fruity esters and other hydrophobic low-molecular weight compounds increases with temperature and has a greater potential sensory impact than the loss of ethanol (Jackson, Wine Science).
Temperature is Influenced by
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Fermentor surface area to volume ratio; the lower the ratio, the less passive radiation. This can be influenced by size as well as shape, as tall, narrow fermentation tanks have more surface area than shorter, stout ones.

Cool temperatures at the beginning of fermentation can limit the degree of temperature control required. Up to a point, the higher the initial juice temperature, the greater the initial rate of fermentation and heat release, and the sooner a lethal temperature may be reached.

The presence of skins in the fermentation:
  • White and rosé wine, fermented without skins (no cap), releases enough carbon dioxide during fermentation to sufficiently create enough turbulence to maintain a relatively consistent temperature that has a lateral variation seldom more than 1 °C in the tank (Jackson, Wine Science). However, if temperatures are too cool, turbulence may be insufficient.
  • The presence of a cap can cause “hot spots” or stratifications to form as it prevents the circulation of the liquid through the tank, and can create a maximum cap-to-liquid temperature difference of about 10 °C (Jackson, Wine Science). This can be mitigated by cap management techniques like punch-downs, pump-overs, or the use of a rotary fermenter.


​​​Temperature Control:
As fermentation releases ~23.5 kcal/mol glucose, [10] a juice of 23 °Brix may cause a temperature rise of ~30 °C without heat dissipation, thereby killing the yeast before completing fermentation. For this reason, heat management techniques have been developed. These include:
  • Selective harvest timing, which has the potential to provide fruit at a desired temperature.
  • Relatively small fermentors of ~225 liters and vinification in cool cellars.
  • Pumping over to provide cooling and temperature equilibrium in red wine vinification.
  • Direct cooling, using a water or a glycol insulating jacket, pumping the fermenting liquid through external heat exchangers. Cooling coils may be inserted directly into the tank as noted in the previous section, “Mechanized Fermentor Modifications.”
  • Fermentors designed to trap carbon dioxide and create a pressure buildup can be used to slow heat accumulation by slowing fermentation.
Typical Wine Fermentation Temperatures
Jackson in Wine Science provides the following insight into wine fermentation temperatures:

White wine fermentation Temperature: 15-20 °C
  • 20-25 °C fermentations are preferred by some traditional European regions.
  • Cooler temperatures can be used to create fresher, more fruity wine by increased synthesis and retention of fruit esters, such as isoamyl, isobutyl, and hexyl acetates. This is common among New World winemakers.
  • At 15 °C: Fatty acid ethyl esters, such as ethyl octanoate and ethyl decanoate, are produced more effectively. Some of these fatty acid esters add a fruity note, as is the case with the apple aromas of ethyl caproate and ethyl caprylate.
  • At 12–16 ºC temperatures limits the activity of acetic acid bacteria.

Red wine fermentation temperature: 24-27 °C
  • This temperature range is ideal for anthocyanin and tannin extraction.
  • Red wine vinification consists of two simultaneous but different phases:
    • Liquid phase, in which the temperature is cooler and readily controlled.
    • High temperature phase, which is largely uncontrolled, rapid, and occurs in the cap. In this phase, the alcohol content may quickly rise to above 10%, which, along with the higher temperature, can increase the speed and efficiency of phenol extraction from the skins trapped in the cap (Jackson, Wine Science).
  • The higher fermentation temperatures may cause:
    • Increased amounts of acetic acid, acetaldehyde, and acetoin; along with lower concentrations of some esters, though these may be less noticeable due to the intense fragrance of red wines.
    • Greater synthesis of glycerol, which can enhance wine mouthfeel.
  • Punching down produces only transitory temperature equilibration between the cap and the juice. High cap temperatures are a common feature of many traditional red wine fermentations.

Oxygen [11]

​The alcoholic fermentation process by definition is anaerobic; however, the trace amounts of oxygen typically dissolved into the liquid during stemming and crushing can indirectly favor fermentation by permitting the biosynthesis of sterols and long-chain unsaturated fatty acids, which are used by the cell membrane (Jackson, Wine Science). Anli and Cavuldak (2012) reviewed micro-oxygenation, and provided a great literature review of the impacts of oxygen on wine. We have highlighted some of the points below:

The two general ends of the spectrum for oxygen usage in winemaking are:
  • Oxidative winemaking allows more oxygen to come into contact with the wine through controlled oxygen exposure, so that the wine can develop some non-primary fruit and textural complexity. Oak barrel-fermented wines are primary examples of this style.
  • Reductive winemaking minimizes, to the greatest extent possible, the amount of oxygen that contacts the wine, in order to protect the volatile fruit aromas. To accomplish this, cooler temperature fermentations with more sulfur dioxide and inert gases are used.​
  • Amount of Oxygen Needed
  • ​Function of Oxygen
  • ​Oxygen Addition Techniques
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Amount of Oxygen Needed

The oxygenation rate and the amount of applied oxygen is dependent on:
  • Phenolic composition and concentration. Wines that lack or have minimal phenolics, in particular white wine, are substrates for polymerization reactions and cannot take much oxygen. If oxygen is applied, it occurs after malolactic fermentation at low rates and amounts.
  • The quantity of volatile sulfides that need to be reduced.
  • The ability of the wine to consume this oxygen.
  • The ideal oxygenation rate should be equal to or lower than the rate at which it is consumed by the wine, in order to avoid oxygen accumulation (as seen by an increase in the level of dissolved oxygen).
  • Ideal oxygen is difficult to calculate therefore tasting is used as an indication of its need.

Excessive oxygen can:
  • Create excessive amounts of phenol oxidation including browning in white wine or ferric and oxidative casse development.
  • Increase the perception of tannin dryness, astringency, loss of wine freshness, oxidized aroma, excessive acetaldehyde aroma, and microbial spoilage by Brettanomyces and ​acetic acid bacteria.

​Dissolved oxygen rate requires the following parameters to be monitored:
  • Turbidity, as some clarification assists in efficient micro-oxygenation.
  • Temperature
  • Temperature
    • 14-17 °C is ideal for oxygen addition, especially by micro-oxygenation.
    • Excessive temperature results in a reduction in dissolved oxygen and poor oxygen
      solubility.
    • Decreases in temperature can cause accumulation of oxygen in the headspace of the tank, and slow the rate of oxygen consumption by chemical reactions like
      polymerization and condensation.
  • Free Sulfur Dioxide, as a decrease or increase of the free SO2 level is indicative of how the wine is responding to micro-oxygenation.

Oxygen Addition Process
Oxygen can be applied during different stages of the wine-making process. It can be applied:
  • Before and/or after malolactic fermentation. [12]
  • Before oak barrel aging. [13]
  • During storage or the aging of wine in stain- less steel tanks. [14]  This can be in conjunction with toasted oak chips to replicate barrel aging. [15]
  • For an in-depth look into the impact of oxygen’s addition during various parts of the winemaking process, read: Day, M. P., Schmidt, S. A., Smith, P. A., & Wilkes, E. N. (2015). Use and impact of oxygen during winemaking. Australian Journal of Grape and Wine Research, 21, 693-704. Retrievable upon request from: from: https://www.awri. com.au/industry_support/winemaking_resources/aeration-of-ferments/resources- aeration-of-ferments/
​Function of Oxygen

​Influence on Phenols
Polyphenols are the main compounds responsible for oxygen consumption. In other words, phenolic components from the grape are the primary substrates in wine that is oxidized, therefore the more phenolics, the more oxidization the wine can tolerate. For this reason, red wines are typically made with more oxidation than white wines. From a general chemical perspective, the addition of oxygen to wine leads to the polymerization of certain phenolic compounds, especially monomeric anthocyanins and flavanol moieties. This occurs when the formation of H2O2 oxidizes ethanol to yield acetaldehyde, and then forms a bridge between phenolic molecules and leads to polymerization reactions.
Impact on Astringency
Oxygen reduces the astringency in red wines by promoting the polymerization of proanthocyanidins, a class of compounds thought to provide the majority of the astringent sensations in red wine. Additionally, acetaldehyde, which can react with the tannins and form bridges between tannin molecules, can create macromolecular structures and precipitate, thus resulting in a decrease in astringency. Excess bitterness can result from excessive oxygen exposure, however.

Impact on Color
  • Color stability, especially with changes in SO3 and pH, are increased by sufficient oxygen, however, there is a decrease in overall phenolic content. Pyranoanthocyanin-derived pigments and ethyl linked compounds show changes in color characteristics that are partially attributed to an increase in the CDRSO2 parameter, which defines the color owed to pigments resistant to SO2 bleaching.
  • This occurs because of a reaction between anthocyanins and flavan-3-ols mediated by acetaldehyde, which increase the concentration of the higher molecular weight compounds of pyranoanthocyanins and adducts.
  • Micro-oxygenated wines show a higher color intensity than untreated wines

Influence on Volatile aroma Compounds
Volatile aroma compounds can be reduced through oxygen exposure. This includes:
  • Depending on vintage and cultivar, oxygen addition can increase the sensory characteristics of plum/currant, tobacco and nutty notes, as well as toasting, spices and coffee perceptions.
  • Herbaceousness or vegetal characteristics in red wines can be reduced
  • Increases the impact of aromas derived from wood, especially from wood chips, if micro-oxygenation was performed before oak aging.
  • Increases in red fruit character. [16]
  • Reduce hydrogen sulfide (rotten egg) notes by short aeration at the beginning, or a few days after the commencement of fermentation.
  • Reduction of urea in the final wine as, after an initial accumulation caused by yeast’s metabolism of arginine, oxygen exposure through aeration can, depending on timing can [17]:
    • Cause complete degradation which results in minimal concentration in the final wine if continuous aeration is performed.
    • Limited aerobic conditions cause partial urea utilization and elevated levels in the wine. This can be mitigated by aeration during the latter stage of fermentation or supplementing the ferment with excess ammonia (1 g/L).

Impact on white wine fermentation [18]:
If air or oxygen is added during key growth stages of white wine fermentation, it can reduce the duration of a challenging fermentation, or, if added at the onset of the stationary phase, can help avoid stuck fermentations. However, the technique's usage is dependent on the yeast strain (Julien et al., 2000). The aeration of Cuvée wines prior to the second fermentation (in sparkling wine production) may also be of benefit, unless sur lie maturation as sufficient oxygen uptake occurs when the wine is periodically stirred with the lees (bâtonnage).


Oxygen Addition Techniques
Oxygenation of wine is an authorized oenological practice in the International Code of Oenological Practices of the Organisation Internationale De La Vigne Et Du Vin (International Organization of Vine and Wine). [19]

Open-top fermentors
  • Oxygen is assimilated through the air contact surface.
  • Primarily used for red wine production, and to aid in the accessibility of punch-downs or pump-overs.
  • Punch-downs and Pump-overs
  • Can be done with or without oxygen through the usage of closed fermentors into which an inert gas is pumped.
  • Increases the amount of oxygen in the must through aeration.

Barrel Aging
Oak barrels are semi-permeable to oxygen. For more on oxygen in barrels, 

​​Racking, the process of transferring wine from one container to another, can be used to aerate the wine.

Micro-oxygenation (MOX)
Developed and patented in the early 1990s in France by Patrick Ducournau Laplace, MOX is an evolution of micro-oxygenation via oak barrel aging, and has gained popularity for its ability for more specific control of oxygen additions to wine.

The Process
  • Stainless steel fermentors are outfitted with a monitored oxygen supply, which is applied through a porous diffuser at a rate of a few milliliters of oxygen per liter of wine per month. The fermentor must be at least 2.2 m tall, as the oxygen bubbles must fully dissolve into the wine before reaching the top of the vessel for an accurate amount of oxygenation.
  • The monitoring of acetaldehyde concentration can be used to determine the endpoint of microoxygenation.

Benefits of MOX
  • More precise control.
  • Can accelerate the oxygenation process compared to barrels.
  • Can be more cost effective.

Hyperoxidation:
This process can be used to favor cell growth, promotes fermentation and diminishes oxidative browning, and the reductive influence often associated with sur lie maturation. (Jackson, Wine Science). However, its benefit to wine depends on the yeast strain’s response to oxygen, the degree and timing of oxygen addition. For more on hyperoxidation, read the section on Fermentation Environment: Chemistry

For more insight into oxygen in wine we highly recommend reading
  • The full version of Anli and Cavuldak (2012) and the underlying sources. This can be found at: Anli, R. E., & Cavuldak, Ö. A. (2012). A review of micro-oxygenation application in wine. Journal of the Institute of Brewing, 118(4), 368-385. https://doi.org/10.1002/jib.51
  • Du Toit, W. J., Marais, J., Pretorius, I. S., & Du Toit, M. (2006). Oxygen in must and wine: A review. South African Journal of Enology and Viticulture, 27(1), 76-94. https://doi.org/10.21548/27-1-1610​​
Pressure
(from Jackson, Wine Science)
During fermentation, 260 ml/g glucose of CO2 is generated, which equals over 50 times the volume of the juice fermented. Typically, the built-up gas is dissipated into the air and helps remove ~20% of the heat generated during fermentation, but desired volatile aroma compounds may also be released.

Pressure influence on wine: Aroma compound volatility is increased by lower liquid pressures, and is dictated by fermentor size and shape. Higher surface area/volume ratio and the lower liquid pressure of small fermentors favor volatility.

A convection current created by CO2 can help equilibrate the nutrients and temperature of the fermentation, but can be impeded by a floating or submerged cap.

Pressure influences yeast: [20]
  • Pressure above 1 atm (atmospheric pressure at sea level) slows yeast growth and is further impacted by pH and ethanol concentration.
  • Elevated pressure slows yeast metabolism; above 7 atm and 30 atm will kill yeast. This is especially important for the secondary fermentation of sparkling wine, where the final bottle pressure can reach 7 atm.
  • Some microorganisms are less sensitive to high pressure than Saccharomyces cerevisiae, like the spoilage yeasts of Torulopsis (Candida glabrata) and Kloeckera, which can create an acetic acid vinegar taint and lactic acid bacteria.

Resources and Suggested Reading

1. Verbelen, P. J., De Schutter, D. P., Delvaux, F., Verstrepen, K. J., & Delvaux, F. R. (2006). Immobilized yeast cell systems for continuous fermentation applications. Biotechnology letters, 28(19), 1515-1525. https://www,academia.edu/download/54720461/Verbelen_2006.pdf

​2. Acidity and ph. The Australian Wine Research Institute. (2021, April 14). Retrieved April 20, 2022, from https://www.awri.com.au/industry_support/winemaking_resources/frequently_
asked_questions/acidity_and_ph/

3. Gil i Cortiella M, Ubeda C, Covarrubias JI, Laurie VF, Peña-Neira Á. Chemical and Physical Implications of the Use of Alternative Vessels to Oak Barrels during the Production of White Wines. Molecules. 2021; 26(3):554. https://doi.org/10.3390/molecules26030554

4. Ganimede srl – Metodo Ganimede, IL Fermentatore Innovativo. (n.d.). Retrieved April 20, 2022, from https://www.ganimede.com/en/index.aspx

5. Zoecklein, B. W., Pelanne, L. M., & Birkenmaier, S. S. (2008, June). Effect of délestage with partial seed deportation on ... Retrieved February 20, 2022, from https://www.apps.fst.vt.edu/extension/enology/downloads/DelestageZoeckelin2008.pdf

6. Pambianchi, D. (2020, June 29). Delestage fermentation: Techniques. WineMakerMag.com. Retrieved April 20, 2022, from https://winemakermag.com/technique/237-delestage-fermentation-techniques

7. Arroyo-López, F. N., Orlić, S., Querol, A., & Barrio, E. (2009, February 5). Effects of temperature, ph and sugar concentration on the growth parameters of saccharomyces cerevisiae, S. Kudriavzevii and their interspecific hybrid. International Journal of Food Microbiology. Retrieved February 20, 2022, from http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.597.2698&rep=rep1&type=pdf

8. Williams, L. A., & Boulton, R. (1983). Modeling and prediction of evaporative loss fermentations ... Environmental Protection Agency. Retrieved February 20, 2022, from www3.epa.gov/ttn/chief/old/ap42/ch09/s123/reference/bref06_c09s1203_ch4_1997.pdf

9. Williams, L. A., & Boulton, R. (1983). Modeling and prediction of evaporative loss fermentations ... Environmental Protection Agency . Retrieved February 20, 2022, from www3.epa.gov/ttn/chief/old/ap42/ch09/s123/reference/bref06_c09s1203_ch4_1997.pdf

​10. Williams, L. A. (1982, January 1). Heat release in alcoholic fermentation: A critical reappraisal. American Journal of Enology and Viticulture. Retrieved February 20, 2022, from www.ajevonline.org/content/33/3/149

11. Anli, R. E., & Cavuldak, Ö. A. (2012). A review of microoxygenation application in wine. Journal of the Institute of Brewing, 118(4), 368-385. https://doi.org/10.1002/jib.51
12. Yang, Y., Deed, R. C., Araujo, L. D., Waterhouse, A. L., & Kilmartin, P. A. (2022). Effect of micro-oxygenation on acetaldehyde, yeast and colour before and after malolactic fermentation on Pinot Noir wine. Australian Journal of Grape and Wine Research, 28(1), 50-60.https://doi.org/10.1111/ajgw.12512.
​
13. del Carmen Llaudy, M., Canals, R., González-Manzano, S., Canals, J. M., Santos-Buelga, C., & Zamora, F. (2006). Influence of micro-oxygenation treatment before oak aging on phenolic compounds composition, astringency, and color of red wine. Journal of agricultural and food chemistry, 54(12), 4246-4252. https://doi.org/10.1021/jf052842t

14. Castellari, M., Matricardi, L., Arfelli, G., Galassi, S., & Amati, A. (2000). Level of single bioactive phenolics in red wine as a function of the oxygen supplied during storage. Food Chemistry, 69(1), 61-67.
https://www.academia.edu/6483302/Level_of_single_bioactive_phenolics_in_red_wine_as_a_function_of_the_oxygen_supplied_during_storage

15. McCord, J. (2003). Application of toasted oak and micro-oxygenation to ageing of Cabernet Sauvignon wines. Australian and New Zealand Grapegrower Winemaker, 7, 43-51. Retrieved from: www.stavin.com/stavin_microox_report.pdf

16. Day, M. P., Espinase Nandorfy, D., Bekker, M. Z., Bindon, K. A., Solomon, M., Smith, P. A., & Schmidt, S. A. (2021). Aeration of Vitis vinifera Shiraz fermentation and its effect on wine chemical composition and sensory attributes. Australian Journal of Grape and Wine Research, 27(3), 360-377.https://doi.org/10.1111/ajgw.12490
Retrievable upon request from: https://www.awri.com.au/industry_support/winemaking_resources/aeration-of-ferments/resources-aeration-of-ferments/

17. enschke, P. A., & Ough, C. S. (1991). Urea accumulation in fermenting grape juice. American journal of enology and viticulture, 42(4), 317-321. Retrieved from: https://www.ajevonline.org/content/42/4/317.short

18. Australian Wine Research Institute. (n.d.). Aeration of ferments. The Australian Wine Research Institute. Retrieved April 21, 2022, from www.awri.com.au/industry_support/winemaking_resources/aeration-of-ferments/

19. Organisation Internationale De La Vigne Et Du Vin. (2015, January). International Code of Oenological Practices. International Organisation of Vine and Wine. Retrieved April 21, 2022, from www.oiv.int/en/technical-standards-and-documents/oenological-practices/international-code-of-oenological-practices

20. Kunkee, R. E., & Ough, C. S. (1966). Multiplication and fermentation of Saccharomyces cerevisiae under carbon dioxide pressure in wine. Applied microbiology, 14(4), 643-648. Retrieved from: https://journals.asm.org/doi/epdf/10.1128/am.14.4.643-648.1966