Harvest

Beyond picking at the right maturity, the next most important part of picking grapes is the minimization of unwanted microbial growth or oxidation caused by bruising or rupturing. This can be controlled by:

• ​Temperature or Spoilage Control During Harvest
• Harvest Technique

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Inevitably, some maceration occurs during transportation to the winery. However, its impact can be mitigated by:

• Harvesting at night or storing the grapes at a cool temperature to minimize microbial growth.
• Sulfites can be used instead of temperature control to mitigate microbial growth; however, its usage may hinder desired microbial growth during fermentation.

• ​Manual harvest is gentler, but it can take longer and is more expensive.
• Mechanical harvest is typically less gentle than manual harvesting of grapes.  In mechanical harvesting, individual grapes are removed rather than whole bunches.

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Grapes Processing

• Material Other than Grapes (MOG)
• Stems vs Whole Cluster

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Cleaning and Sorting the Grapes: MOG (Material Other than Grapes)
Removal MOG refers to non-grape material, like leaves, petioles, canes, sticks, stones, and insects that make their way into the bins of harvest grapes.  MOG, classified as a per-centage of the overall weight of the grapes and MOG, is ideally less than 1%.  It’s ac-ceptable at 1-2%, but  starts to have an im-pact at 3% with anything above that being unacceptable.
FOR MORE ON HARVESTING GRAPES READ OUR VITICULTURE ARTICLE AT hawaiibevguide.com/viticulture

Process Options

• Mechanical removal: Various mechanical removal technologies include vibration removal, air purging and optical sorting.
• Hand removal: Hand sorting through grapes to both clean the grapes and eliminate anything unwanted.
• Washing in a citric acid solution: Grapes are not typically washed as some believe that it may compromise sophistication or adulteration of wines with water addition; however, it eliminates many contaminants like chemicals, microorganisms and soil on the musts.2  Cavazza et al (2007) and Checchia et al (2021) found that there are benefits to washing includ-ing a significant reduction in pesticides and the wild yeast, and desired yeast growth during fermentation was faster. [3]

Aroma Impact
The impact on wine aroma depends on the material.  Processing methods vary and include hand removal and machines that can use air purging. 

• Leaves and stems
  Grape leaves and stalks, in the amount present in machine-harvested fruit, do not contribute to the green charac-teristics (methoxypyrazines) of wine compared to that of stems. [4]
• Insects
  ​Excessive amounts of millipedes cre-ate an unpleasant herbal character; ladybirds give a green, capsicum, as-paragus, herbaceous character from 2-methoxy 3-isopropylpyrazine; cica-das create a savory/cheesy smell. [5]​

Stem Removal versus "Whole Cluster Fermentation"

Stems, which make up 2%-5% of the overall weight of a grape bunch, may or may not be removed from the grape bunches.  Inclusion increases the phenolic content of the wine depending on the wine’s style. This maceration can occur at different times and temperatures which also depends on the wine’s style. 

Process: 

• Stems are typically removed mechanically before the grapes are crushed or pressed.
• Whole cluster fermentation: Grapes used to make red wine are NOT destemmed and instead undergo alcoholic fermentation while still attached to the stems before being pressed after alcoholic fermentation concludes.

​Aroma Impact:

• Stem phenols are intermediate in as-tringency and bitterness relative to the less strident tastes of skin tannins and the more assertive seed tannins (Wine Science).
• Stems, when included, provide a great majority of the extractable catechins, oligomeric and also polymeric proanthocyanidins released into wine. [6]
• Stem ripeness refers to the lignification of stems as they turn from being photosynthetic green into brown and woody.  The less ripe the stem, the higher the concentration of me-thoxypyrazines. [7]  Quantifying each compound showed that (Z)-1,5-octadi-en-3-one was the main green odorant compound from stems (Blackford et al 2021). However, stems do not fully lignify and can vary from bunch to bunch.
• The specific impact of stem inclusion varies by culviar, for example:
  • In Cabernet Sauvignon and Chardonnay green aromas from pyrazines, hexanal, (E,Z)-2,6-nonadi-enal, and dodecanol were the most significant. [8]
  • In Syrah production furaneol (caramel/ red fruit jam), eugenol (clove), sotolon (curry), vanillin, γoctalactone (coconut/almond) are also increased by stem usage. [9]
  • Stems may reduce acidity due to their potassium content which binds with ​the tartaric acid and precipitates out of the wine. [10]

For a great review on the impact of stems in winemaking read: 
Blackford M, Comby M, Zeng L, Dienes-Nagy Á, Bourdin G, Lorenzini F, Bach B. A Review on Stems Composition and Their Impact on Wine Quality. Molecules. 2021; 26(5):1240. https://doi.org/10.3390/mol-ecules26051240 

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Crushing

​Grapes are first crushed to rupture the skins of the berries.  This is separate from press-ing, which is the process used to extract the juice.  

Crushing Techniques

• ​Foot stomping  
• ​Pressing against a perforated wall
• ​Rollers
• ​Centrifugal Crushers
• Freeze Extraction

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​Grapes are stepped on inside of a barrel or other fermentation vessel 

​The berries are broken open and the juice, pulp, seeds, and skins pass through openings to be collected and pumped to a retaining tank or vat. 

​Berries are crushed between a pair of rollers turning in opposite directions.

​The fruit is spun against the sides of the crusher. Centrifugal crushers are generally undesirable because they tend to turn the fruit into a pulpy slurry (Wine Science).  This process also results in the clarification of the juice being difficult, and the rupturing of seeds.

Extraction by freezing
Freezing then thawing grapes causes cell walls to rupture and the grape skin to split.  This can result in reduced acidity, increased alcoholic strength and increase skin aroma compounds present in wine. [23]

Techniques include:

• ​Supraextraction: Freezing the grape, defrosting complete defrosting then pressing.
• Cryoextraction: Freezing the grape with only partial defrosting prior to pressing.

For more on freeze extraction read:
Ruiz-Rodríguez A, Durán-Guerrero E, Natera R, Palma M, Barroso CG. Influence of Two Different Cryoextraction Procedures on the Quality of Wine Produced from Muscat Grapes. Foods. 2020; 9(11):1529. https://doi.org/10.3390/foods9111529​

Crushing challenge: Microbial Spoilage (and sulfur dioxide usage during crushing)
Ideally, crushing occurs as quickly as possible after harvest to minimize any microbial spoilage.  If there is a large number of dam-aged or diseased berries or an extended period of time between harvest and crushing, sulfur dioxide may be used because it limits the action of polyphenol oxidases and minimizes any microbial growth.  However, (according to Wine Science) sulfur dioxide usage at this point in wine production may: 

• Cause premature phenolic oxidation or cause its removal
• Minimize the production of acetaldehyde during fermentation
• Enhance phenolic extraction
• Hinder malolactic fermentation

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Maceration (Skin Contact)

Maceration aids the release of the pomace (seeds, skins and pulp) constituents following crushing by liberating and activating the hydrolytic enzymes from crushed cells. In this process, the winemaker uses time and temperature to influence wine’s interaction with the grape’s skin and pulp and extract its components. The wine-maker’s challenge is that differences in cultivar, vineyard, and even vintage create changes in how time and temperature impact the wine due to differences in phenolic content.  An alternative method of adding in skin aromatics is the utilization of press fractions as it provides easier control then maceration (Jackson, Wine Science).   

Maceration Manipulation:  ​Duration, Temperature, Oxygen and Agitation

• Duration
• Temperature
• Oxygen
• Agitation

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Shorter Maceration

• When combined with cool temperature typically creates fresh, fruity wine
• Reduces the uptake of heat-unstable proteins and decreases the need for protein stabilization products (Wine Science).

Longer Maceration

• Increases the quantity of extracted anthocyanins and many aroma compounds.
• Extracted compounds may decrease in concentration due to precipitation or degradation.  For example, extended maceration results in a decline in free anthocyanin content but enhances color stability by encouraging their early polymerization with procyanidins (Wine Science).

Cool Temperatures:

• Reduce the rate of extraction compared to warmer temperatures.
• Minimize unwanted microorganism development during maceration.
• When combined with short duration, it minimizes flavonoid uptake which limits the wine’s potential bitterness and astringency. [13]

​Warmer maceration temperatures

• Most alcohol (except methanol) synthesis is reduced by maceration at warmer temperatures because methanol content increases as the action of grape pectinases release methyl groups from pectins (Wine Science).
• Ramey et al., (1986) compared Chardonnay grapes from a single vineyard block were held after crushing for 20 to 25 hours at four different temperatures between 9°C and 30°C, pressed, and vinified separately but identically and found higher temperatures greatly increased phenolic extraction rates particularly of the flavonoid fraction, deeper color, increased oxidative sensitivity, and corser character which matured more rapidly during barrel aging whereas wine held below 15°C minimized wine protein levels and bentonite requirements.​

Oxygen Exposure
(Hyperoxidation) Oxygen exposure according to Jackson in Wine Science can:

• Impact the sensory influence of mac-eration. This can come from oxygen absorbed during crushing or via intentional exposure.
• Reduce white wine’s susceptibility to in bottle oxidation and lower bitterness by promoting the enzymatic oxidation of the primary phenolics of nonflavonoid odiphenols including caftaric acid because the polymers typically precipitate during fermentation.
• Hyperoxidation may degrade volatile thiols which produce varietal character for cultivars like Sauvignon Blanc.

Cap management refers to agitating the wine in a way that exposes more wine to the floating “cap” of pomace that occurs during maceration. This process is done to increase extraction from the grape pomace, and the more agitation the more
extraction.

Technique

• Punchdowns (pigeage in French): The cap is pushed into the liquid manually using poles or paddles, or mechanically using a hydraulic press.
• Pumpovers (remontage in French): Juice is pumped from the bottom of the tank over the top of the cap.
  

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​Factors influencing cap exposure

• Frequency of agitation can vary from constant to a few times a day.
• Force of agitation can range from violent to gentle agitation.
• Vessel shape influences the amount of cap exposed to the wine with more horizontal surface area providing less cap exposure.
• Oxygen exposure amounts can vary
• Pumpovers may help reduce the formation of temperature gradients in large non-temperature controlled fermentation tanks. [14]

Red and Orange Wine Maceration Techniques 

• Cold Maceration/Cold Soak
• Short duration high temperatures maceration
• Carbonic Maceration

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Cold Maceration/Cold Soak [15]
Cold soak conditions are similar to the natural cooling that occurs in Bourgogne’s
small, unheated, wine cellars in the fall.

Usage

• Cold soaking is used to delay alcoholic fermentation as anthocyanins extract faster before alcohol is formed. It is primarily in red wine production but may also be used in the production of orange wine.
• Cold soaking also increases extraction of aroma and flavour compounds.
• While useful regardless of the grapes growing conditions, the impact of cold soaking is different between warm and cool climate growing conditions of the same cultivar with hot climate being the most responsive to cold soaking.​

Process:

• Time: As little as 5 to 10 hours to 10 days
• Temperatures: Typically 5 to 10 ºC.
• Temperature reduction may be done by heat exchanger or with a cryogen of liquid carbon dioxide, solid carbon dioxide (dry ice), or liquid nitrogen, being poured or placed onto crushed grapes. Cryogen usage minimizes oxygen exposure during cold soaking and creates cryoextraction in the places where the grapes are in contact with cryogen. Read the section of Freeze Extraction to learn more about the process.
• Sulfur Dioxide is commonly used as an antimicrobial and as a solvent for increased extraction of total phenols and total anthocyanins.

​Risks
Spontaneous yeast growth while possible is unlikely due to the low temperatures, and may be further mitigated with the use of carbon dioxide.

Short duration high temperatures maceration:

• Time: ~15 minutes
• Temperature ~70 ºC
• Increases the release of volatile compounds and the concentration of most but not all monoterpenes as the concentration of geraniol decreases (Wine Science)

Carbonic Maceration and Semi-Carbonic Maceration [18]
Carbon dioxide is used to create an anaerobic (no oxygen) environment. This
changes the still living grape cells from using aerobic respiration to intracellular
fermentation as the grape's energy source and by doing so sugars and malic acid are
metabolized into pyruvate, acetaldehyde and then ethanol, succinic acid and aminobutyric acid without producing lactic acid. As the grape cells have minimal alcohol tolerance, they eventually die and release their contents.

Usage
Can be used to reduce acidity and add aroma to red wines with low varietal or fruit
character. 

Aroma compounds

• Creates .05-2.2% alcohol before the grape cells die and burst.
• Glycerol and shikimic acid are produced, with shikimic acid accumulating within the berry, and then degrading to cinnamic acids and further into: benzaldehyde (cherry/kirsch/almond), vinylbenzene (styrene, plastic), ethyl cinnamate (Cinnamon/strawberry/honey)
• Enzyme produced aroma compounds associated with carbonic maceration include: ethyl and methyl vanillate (Vanilla), ethyl 9-decenoate (sweet/fruity/ quince), and 1-octanol (almond/buttery).
• The decreased varietal characteristics may be produced by lower wine ester like helylacetate which forms only in oxygen rich environments during crushing.
• Phenolic extraction: Well in both methods ethanol access a solvent for the extraction of phenolic, in standard form intentions, a greater portion of the skin and Seed contact Time occurs at higher alcohol concentrations than with Carbonic maceration, resulting in Greater extraction anthocyanins. Extraction of any tannin into the pulp at low alcohol would also more likely be skin tannin rather than seed tannin, which could lead to a perception of softer tannin in wine.

Process
Carbonic maceration occurs on a scale from 100% oxygen free to that of mainly carbon dioxide with some oxygen exposure.  In the process:

• Gas can be pumped in before or after the grapes are placed into the tank
• Grapes are added as whole clusters to keep the grapes metabolically active. If yeast are used to generate carbon dioxide in a partial carbonic maceration, the tank may also contain:
  • Partially crushed whole cluster grapes at the bottom of the tank
  • Destemmed grapes
  • Pied de cuve (yeast start from fermenting wine) which is pumped into the tank.  This yeast fermentation creates heat and carbon dioxide which helps the intracellular fermentation.
• After the tank is sealed:
  • Temperature: 30-35 °C
  • Duration: 5-8 Days
  • Longer durations are used for grapes that are less ripe because of the malic acid metabolism.
• Once carbonic maceration concludes, normal yeast fermentation begins.

Styles of Carbonic Maceration
Semi-carbonic maceration: Maceration traditionelle
• Associated with Gamay from Beaujolais
• Grapes are put into sealed cement or steel tanks, where some of the berries are inevitably crushed by the pressure.  This starts alcoholic fermentation caused by yeast, and as CO2 is heavier than O2 it fills up the tank and pushes out the O2.  This gradually causes grapes to die and release their juices.
• The higher pH that results from the intracellular degradation of malic acid means that malolactic fermentation can begin more easily after alcoholic fermentation finishes.

Maceration Carbonique
Grapes are added to a tank that is filled with carbon dioxide.  The grapes then have minimal yeast driven alcoholic fermentation. 

Cold soak vs carbonic maceration

• Cold soak is cold whereas a little warmth helps carbonic maceration
• Carbonic maceration uses whole clusters whereas cold soak may not.

Rosé Wine Maceration

Process

• Maceration occurs after grapes are gently crushed
• Duration: Lasting up to 24 hours to minimize anthocyanin extraction.
• Temperature: Maceration at ~20 ºC to minimize microbial action.
• Maceration ideally occurs under an-aerobic conditions to limit oxidation of important volatile thiols, and protect anthocyanins from oxidative discoloration.

Aroma Compounds:
Jackson in Wine Science references Mu-rat (2005) [21] who found that the extraction of S-3-hexan-1-ol-Lcysteine, from the grape skin is metabolically converted into  3-mercaptohexan-1-ol (grapefruit, passion fruit, and boxwood nuances) during fermentation at about 20 ºC. 

Challenges with rosé wine
Color stability issues can arise because of the minimal extraction of color stabilizing tannins.  These issues can be minimized by enzyme treatment as a Salinas et al. (2003) found enzyme treatments of endozym contact pellicure (ECP), applied during maceration and an endozym cultivar (EC) applied after fermentation improved color stability and released aroma.

 White Wine Maceration

Process: Duration and Temperature:

• Most white wine is not macerated.
• Orange wine/ Romato (Italy)/Amber Wine (from Republic of Georgia): White wine with extended skin contact that becomes darker and almost orange in the bottle.

Extraction Compounds [20]
According to Jackson in Wine Science: In white wine, the increased phenolic content from extended skin contact occurs without the degree of astringency typically found in red wine because in white wines most tannins precipitate out during fermentation whereas in red wine the solubility of tannins is increased by the anthocyanins which bind with catechins and flavonoid tannins keeping them in suspension (retaining their bitter and astringent properties).  The increased phenolic extraction favors subsequent in-bottle browning (which turns the wine orange/amber). 

Cultivar impact on maceration
Varieties differ considerably in the amount of phenolics released during crushing or extracted during maceration (skin contact). Jackson in Wine Science provides the following example:

• ​Fewer must flavanoids:  Palomino and Sauvignon blanc
• Moderate must flavoanoids: Riesling, Sémillon,’ and ‘Chardonnay;’
• Ample must flavanoids: Muscat Gor-do, Colombard, Trebbiano, and Pedro Ximénez.

Other Prefermentation Extraction Techniques

T​he addition of supplementary skins, or seeds to red must during fermentation increases the levels of total phenolics, catechins and dimeric procyanidins, and may stabilize wine color. [24]  This process is similar to Spanish doble pasta, a technique used in Jumilla, Yecla, Utiel-Requena, Manchuela, and Alicante where a portion of the fermented must is run off after two days and more crushed grapes are added to the vat thereby concentrating the ratio of skin to pulp. [25]

Dejuicing

• Dejuicers are contraptions used to remove the freerun juice.
• Juice may be used for Saignée method rose wines.  Saignée means to bleed.
• Dejuicing helps to maximize press capacity​.​

Pressing

Pressing can be done in a batch process or continuous process. In the continuous process, a “continuous screw press” produces a juice that tends to be homogenous, whereas the batch process results in juice with different physicochemical properties depending on the amount of pressure applied. The differences in juice from the different pressures applied, called press fractions, allow winemakers to manipulate the character of the wine. The benefit of a continuous press is that it does not take 1 to 2 hours to refill like in a batch process (Jackson, Wine Science).

Pressing squeezes the juice out of grapes.

Styles of wine that are pressed without crushing

• Sparkling Wine: Pressing without crushing minimizes phenolic extraction.
• Botrytized Grapes: The gentle separation of the juice minimizes the liberation of fungal dextran (β-glucans) polymers into the juice as excessive amounts can plug filters used in the clarification process. In the case of Tokaji Eszencia, for example, only free-run juice is used.

Pressing Process and Press Fractions

• Free-Run Fractions
• Press-Run Fractions/Press Cuts

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Pressure: None. Juice runs out of the press as the press is being filled.

Juice characteristics: Lower levels of suspended solids, phenolic contents, and flavorants are principally derived from the skins.

Pressure: Varies. The designation of fractions is set by the winemaker and made using sensory clues, including acidity and sugar level for white juice and tannin perception in red wine. This occurs because the science of press fractions is general in its understanding of what compounds are increasing and decreasing as more pressure is applied. However, for a specific pressure in PSI, results will vary and depend on the cultivar, growing location and vintage variation.
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Juice characteristics: Increasing amounts of suspended solids, anthocyanins, tannins and skin flavorants (Wine Science).
According to Dwayne Bershaw in Winemaker Magazine [26]:

• Brix decreases slightly (1 °Brix)
• pH increases (becomes less acidic): 0.2 to 0.4 pH units
• Potassium concentration increases due to skin maceration, which can cause reduced acidity due to its reaction with Tartaric Acid
• Phenolic content increases, which may double over the course of pressing
• Grape solids increase, which are more likely to cause oxidative browning

Usage of press fractions:

• Wines typically blend free and press-run fractions
• For premium wine, fractions derived from higher pressure pressing may be sold as bulk wine. [27]
• White grapes use earlier press fractions than red grapes.

For a winemaker’s perspective on press fractions read:
winemakermag.com/article/making-the-cut

For an interesting paper on press run fractions in Sauvignon Blanc read: Parish-Virtue K, Herbst-Johnstone M, Bouda F, Fedrizzi B, Deed RC, Kilmartin PA. Aroma and Sensory Profiles of Sauvignon Blanc Wines from Commercially Produced Free Run and Pressed Juices. Beverages. 2021; 7(2):29. https://doi.org/10.3390/beverages7020029
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Pressing Byproducts: Pomace Cake/ Press Cake

​The residual skins and other solids from pressing, known as pomace, can be used as fertilizer, fermented and distilled into a pomace brandy like grappa, and can even be dehydrated and powdered into a “flour.” [28]

Types of Presses

• Horizonal (Moving Head) Presses
• ​Pneumatic Press (Bladder /Membrane)
• ​Vacuum pressing
• ​Continuous Screw Press
• Vertical (Basket) Presses

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Usages:  Pressing  crushed and uncrushed grapes, as well as fermented juices
Design:

• Pressing time: 100-120 minutes 
• Loading occurs through an opening in the upper, raised portion of the press
• Pressure is applied by hydraulically forcing end-plate(s) inward from one or both ends. The resulting juice escapes through slats in the pressing cylinder. However, horizontal presses experience a progressive reduction in drainage surface during operation thereby requiring additional force to maintain juice extraction. This correspondingly increases the extraction of suspended solids and tannins. The last fraction of juice may be discarded due to excessive suspended solids and tannins. To mitigate this problem, stems may be used to create channels for easier juice escape.
• First, pomace is broken by chains or rotation of the press between successive pressings. Then, retraction of the endplate(s) and inversion permits dumping.

Pressing crushed or uncrushed grapes, as well as fermented must using gentle pressure

​Design:

• Pressing time: 50-90 minutes
• Size: 5–22 hl versions are commercially available
• The press is filled through an elongated opening in the top.
• Pressure is applied by: 
  • A plastic bladder filled with compressed gas which presses the grapes against a solid outer cylinder wall and compresses the grape mass against perforated plates that project along the central cavity
  • A central or side-positioned bladder that extracts juice at lower pressures by pressing the grapes more uniformly against the larger surface area of the perforated outer cylinder wall, which reduces the release of suspended solids and phenolics (Found in more recent and efficient models).
• Crumbling pomace cake between successive presses by rotating the pressing cylinder before being released through a filling trap between presses

​Negative pressure instead of positive pressure is used to crush the grapes.

Usages: Uses a high volume press as it can run continuously thereby minimizing the time and labor costs associated with filling and emptying the other types of presses

Disadvantages

• Does not function well with uncrushed grapes
• Juice or wine quality may not be as good as that yielded from other press types as the fractions are more difficult to separate

Design:

• Crushed grapes, as well as fermenting or fermented must, are pumped into the press via a hopper at one end of the press.
• Pressure is applied using a fixed helical screw that forces the material into a pressing chamber. The perforated walls allow the juice or wine to escape.
• Pomace is discharged through an exit portal after accumulating at the end of the pressing cylinder.

Fractions:

• The first fractions (closest to the intake) possess characteristics similar to free-run fractions.
• Fractions obtained near the end of the pressing cylinder progressively resemble the first, second, and third pressings of conventional presses.
• Slower pressing can decrease the incorporation of suspended solids that diminish juice or wine quality, but also reduces one of the principal advantages of continuous-type presses, namely speed.

Jackson, in Wine Science, noted some of the following differences between presses:

Usage:

• Pressing small quantities of fruit
• Pressing frozen grapes for the production of ice wine
• Pressing sparkling wine grapes using a vertical press (wider than standard) to maximizes the surface area to volume ratio applying minimal pressing pressure

Design:

• Pressing time: 100 to 120 minutes
• Generally consists of a series of concentrically arranged slats between which the juice can escape
• Pressure is applied hydraulically from above using a plate.
• The plate that presses the grapes is usually retracted before the sides are removed to extract or crumble the press cake. Alternatively, the bottom may be lowered.

Pre-Fermentation Adjustments to Must

Adjustments to Sugar

• Chaptalization ​
• Reverse Osmosis
• Cryoextraction

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Chaptalization (the addition of sugar)
This process, typically only legal in cool climate regions where grapes regularly have difficulty reaching the necessary Brix for adequate ethanol creation, is named after Dr Chaptal who advocated for its usage in 1801.

Usage:

• Generates the desired alcohol content from low brix grapes caused by immaturity or rain-swelling. The added sugar is completely fermented leaving no residual sweetness
• Possibly increases glycerol, succinic acid, 2,3-butanediol and the synthesis of some aromatically important esters while decreasing others (Wine Science)
• Common technique in France and Germany and is permissible in Oregon, New York, Canada, New Zealand and the United Kingdom; however, it is not permissible in California, Argentina, Australia, Italy, Spain, Portugal, and South Africa

Process:
Sugar is slowly dissolved into grape must then is added to the main fermentation near the end of the yeast growth phase (about 2–4 days after the commencement of fermentation [29) in order to avoid shocking the yeast, which can result in a stuck fermentation. [30]

Reverse Osmosis (the removal of water)
Usage: Reverse osmosis reduces the water content of grape must (often caused by pre-harvest rains) thereby concentrating the grape sugars.

Disadvantages:

• Causes an increase in concentration of malic acid, and important water soluble aroma compounds of esters and aldehydes are lost (Wine Science)
• Additional steps are needed to mitigate the effects of reverse osmosis - untreated juice can be added to the concentrated juice, or the volatiles that were removed with the water can be concentrated and added back into the treated juice

Process:
Juice is concentrated by removing a portion of the water via a semipermeable membrane.

To learn more about the differences between chaptalization and reverse osmosis read:
Mietton-Peuchot, M., Milisic, V., & Noilet, P. (2002). Grape must concentration by using reverse osmosis. Comparison with chaptalization. Desalination, 148(1-3), 125-129. source: www.academia.edu/27128208/Grape_must_concentration_by_using_reverse_osmosis_Comparison_with_chaptalization?sm=b

Cryoextraction is the process of freezing the grapes and removing the ice (frozen water) to reduce the overall water content.

Pre-fermentation Color Adjustments

• Hyperoxidation
• Heat Treatment/Thermovinification

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Reducing white wine browning potential
Phenolic oxidation does not typically occur, as white wine is typically not maerated (the main source of oxidative browning) and any oxidized phenols typically precipitate out during fermentation (Jackson, Wine Science). However, to further reduce the potential for oxidative browning:

Gentle pressing, short maceration, phenolic removal with fining agents, acid addition to musts of high pH, and the addition of sulfur dioxide before bottling are practiced.

​Hyperoxidation, typically performed immediately after crushing and before clarification and sulfur dioxide addition, removes readily oxidized phenolics before fermentation by allowing the yeast cells (that precipitate post-fermentation) to bind to the already oxidized molecules.
This process entails:

• Air exposure during crushing
• Activating oxidation by bubbling air through the must
• Removing the precipitated phenolics through hyperoxidation clarification to prevent their resolubilization

Flavor modifications:

• Hyperoxidation reduces the bitterness and astringency phenolics. [31]
• The impact on wine aroma depends on the variety employed, the potential for oxidative browning, and the type of clarification used.

For more on must hyperoxidation read:
Schneider, Volker. (1998). Must Hyperoxidation: A Review. American Journal of Enology and Viticulture. 49. Retrieved from: www.researchgate.net/publication/277791257_Must_Hyperoxidation_A_Review

Color Enhancement of Red Wine Using Heat Treatments [32]
When maceration, pressing technique, enzyme addition, pumping over, yeast strain, or the addition of enological tannins are insufficient, red cultivars like Pinot Noir or diseased grapes contaminated with laccase, may require increased anthocyanin extraction using heat. This kills the grape skin cells and releases anthocyanins. Generally, the higher the temperature, the shorter the duration of maceration time.​

Types of heat treatment:

• Thermovinification (Hot pressing): Heating the grapes up to 70ºC for less than one hour followed by fermentation without skins
• Pre-fermentation hot maceration (MPC): Heating the grapes up to 70ºC for up to 15 hours followed by fermentation with or without skins
• Short time high temperature treatment with warm maceration (KZHE): Heating the grapes up to 85 ºC for approximately 2 minutes, cooling to 45ºC by prea-heating incoming grapes and holding them for 6-10 hours before fermentation without skins
• Flash detente: Heating up the grapes to 85 ºC for a short time then exposing them to a vacuum that vaporizes a portion of them, cools the remainder and weakens their cell walls, followed by solid and/or liquid phase fermentation
• Thermo dente: Heating the grapes up to 75 ºC before pressurization using compressed gas and release, and then pre-fermentation maceration followed by fermentation with or without skins

Aroma Impact: Heat treatment may cause aroma degradation. This can be negative or positive.

Heat treatment drawbacks:

• Bluish colors and cooked flavors may occur. This can be avoided by strict exclusion of oxygen and keeping the duration of heating as short as possible.
• The denaturation of grape pectinases can cause clarification and filtration issues. This can be corrected by the addition of pectinase.

Heat treatment advantages: Astringency is reduced as well as the undesired vegetative, grassy aromas, allowing fruity aromas to be better expressed, especially in red French-American hybrids (Jackson, Wine Science).

Other heat treatment usages (from Wine Science):

• Can denature glucans in Botrytis-infected red grapes.  This is helpful as glucans are highly colloidal and can clog the membrane filters used for microbial steralization.
• Primarily used for early consumption wine as thermovinification does not facilitate tannin extraction.
• Improves juice fermentability (both alcoholic and malolactic), enabling efficient use of fermentor capacity; however, may correspondingly cause rapid heat production and increase the need for temperature control.

Pre-fermentation Aids

Enzymes may be used to accelerate extraction, or enhance the clarification, filtration, stability of grape must as well as influence microbial activity. For more information about grape enzyme usage: www.apps.fst.vt.edu/extension/enology/downloads/wm_issues/Enzymes%20In%20Winemaking.pdf

Sulfur dioxide is commonly used in winemaking as an antimicrobial and antioxidant and may be added at multiple points during the winemaking process, including after harvest, after crushing, after pressing, or after fermentation. Its usage depends on the grape's health and maturity, the cultivar involved, and the wine style desired.

• Its usage as an antimicrobial can prevent unwanted development of microbes; however, if applied at the wrong time or in the wrong quantity, it can negatively impact desired microbial growth during fermentation.
• Its usage as an antioxidant can prevent the unwanted oxidative compounds from forming, such as the unwanted browning of white grapes; however, some oxidative reactions, such as color stabilization, are desired.
  
  

Nitrogen (typically ammonium salts and vitamins) may be added to improve the fermentability of botrytized and highly clarified white juice.

Flash heating Flash heating (80–90 ºC exposure for a few seconds followed by rapid cooling) may be used to treat juice from moldy grapes to inactivate laccase and denature grape polyphenol oxidases (Wine Science).

​Clarification

​The macro process of separating solids from liquids, and unlike fining or filtration, the removed solids are almost exclusively those that are visible. Clarification typically occurs at two points during winemaking: Before fermentation of white grapes, and after fining of all grapes. The processes can be the same, however. Techniques for clarification include:

• Racking
• Flotation
• Centrifugation

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​Racking
The process of spontaneous precipitation and natural gravity settling before separation by transferring wine from a settling container to a different container with minimal agitation to avoid resuspending particulate matter. It is not used for wines bottled a few weeks or months after fermentation, as the time required is too slow for adequate clarification.

Differences between racking techniques

• Manual draining uses gravity or a hand pump.
• Mechanical pumping allows for more precise aeration and sulfiting but has a higher equipment cost.
• Larger storage vessels require more frequent racking to avoid the development of a thick sediment layer that may cause off-odors in the wine.
• Small cooperage is generally more effective in clarifying wine.

Process
Pre-fermentation settling
(débourbage) and racking
Prior to fermentation of white and rosé wines it reduces solids content to 60-250 NTUs, or 1-2%. Freshly pressed juice contains up to 20% solids consisting of small particles of cellulose, hemicellulose, pectin, mineral salts, lipids, and proteins. This reduction can help to produce fruitier wines, reduce sulfur-like off-odors, oxidative browning potential, vineyard residues, and acetic bacteria.

​The first racking:

• Is often done several weeks after alcoholic fermentation, after malolactic fermentation, or after sur lies maturation, depending on what process is used last.
• Removes most of the yeast, bacterial, and grape-cell fragments that have settled out.

Subsequent racking

• Removes most of the residual microbial population, along with precipitated tannins, pigments, and crystalline material.
• Later rackings remove sediment generated as a consequence of fining.
• The number of rackings also varies considerably from region to region and is often dependent on regional practices (Wine Science).
• Decanting stops when unavoidable turbulence makes the wine cloudy. The residue may be filtered to retrieve wine otherwise lost with the lees.

Benefits beyond clarification

• Enhances microbial stability by removing microbial cells and other nutrients conducive to microbial growth.
• Mixes the wine as it is being transferred between containers. This prevents any stratification or layers that may cause variations in flavors, particularly in large storage tanks.
• Removes the primary sources of reductive-sulfur taints, including hydrogen sulfide and mercaptans, which may form under the low-redox conditions that develop in thick layers of lees.
• Aeration/oxygen exposure can occur if desired. To prevent oxygen exposure, the pumping system can use a blanket of nitrogen.
• Releases any built up CO2 from fermentation.

Flotation/Reverse Settling (Pre-fermentation Clarification) [33].
​
The pre-fermentation clarification process typically uses nitrogen bubbles to float the solids away from the liquids, thereby speeding up the process of traditional settling and racking. In Australia, it is a common practice for wineries producing more than 1,000 tonnes [34].

Process:

• Grapes are pressed and/or crushed, then pumped into a holding tank, where they are treated with pectic enzymes for 4-6 hours. When grapes are pressed, pectin is released into the must and, if not broken down, can prevent settling.
• The treated must is then transferred to the flotation tank, where flocculation aids like bentonite, gelatin, PVPP, or chitosan are added to assist in aggregating the solids and enhance gas bubble-particle interaction. Then air or nitrogen is pumped from the bottom of the tank before being racked into another tank for fermentation (~16 °C). Nitrogen will be used if reductive winemaking techniques are desired. Air can be used to hyper oxidize the wine, thereby reducing browning potential.
• Microbubbles (air, nitrogen, or oxygen) are injected into the juice. The gas causes suspended solids to rise to the surface where they can be skimmed off. Flocculation aids like bentonite and gelatin can assist in aggregating the solids and enhance gas bubble-particle interaction [35].

Benefits

• Juice quickly clarifies, relative to traditional racking.
• The juice does not need to be chilled for the process to occur, which results in energy savings.
• The clarified juice can be directly inoculated at the post-floating temperature.
• Enhanced control over the degree of clarification.

Drawbacks
Few risks, with the biggest risk being excessive foaming if the flow rate of nitrogen gas is not managed correctly.

For more, read: www.awri.com.au/industry_support/winemaking_resources/winemaking-practices/winemaking-treatment-juice-clarification-by-flotation/

Centrifugation
Wine or juice is rotated at high speed in a centrifuge to speed up settling times.

Process

• Pre-fermentation clarification of white grape juice.
• It may be more ideal for its usage in the clarification of wine than juice as there is a greater density between yeast cells and liquid than there is between grape solids and liquid [36].
• Post-fermentation clarification may also be performed to remove lees.

Benefits

• Exponentially speeds up the settling process.
• Minimizes losses from clarification due to wine binding to the filtration medium.
• Can be cost-effective if used to process a high ​enough volume of wine, due to automation and minimization of wine loss. 
• Has the least impact on the chemical composition of the juice, relative to other clarification techniques.
• Reduces potential off-odors that may develop in racked wine with significant particulate matter.
• More efficient in removing large amounts of particulates than plate filters.
• Avoids potential health problems (dust and worker allergy) associated with the use and disposal of diatomaceous earth and other filter aids.

Drawbacks

• High capital cost.
• Can use a lot of electricity.​​

Sources and Additional Reading

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3.Checchia I, Binati RL, Troiano E, Ugliano M, Felis GE, Torriani S. Unravelling the Impact of Grape Washing, SO2, and Multi-Starter Inoculation in Lab-Scale Vinification Trials of Withered Black Grapes. Fermentation. 2021; 7(1):43. https://doi.org/10.3390/fermentation7010043

4.Allan, W. (2021, February 25). Winemaking treatment – matter other than grapes (MOG). The Australian Wine Research Institute. Retrieved January 26, 2022, from https://www.awri.com.au/industry_support/winemaking_resources/winemaking-practices/winemaking-treatment-matter-other-than-grapes-mog/

5.The Australian Wine Research Institute. (n.d.). How and why identify matter other than grapes. The Australian Wine Research Institute. Retrieved January 26, 2022, from https://www.awri.com.au/wp-content/uploads/2018/08/s1439.pdf

6.Sun, B. S., Pinto, T., Leandro, M. C., Ricardo-Da-Silva, J. M., & Spranger, M. I. (1999, January 1). Transfer of catechins and proanthocyanidins from solid parts of the grape cluster into wine. American Journal of Enology and Viticulture. Retrieved January 26, 2022, from https://www.ajevonline.org/content/50/2/179

7.The Australian Wine Research Institute. (2021, July 27). The Australian Wine Research Institute. Retrieved January 26, 2022, from https://www.awri.com.au/
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19.Flanzy, C., Flanzy, M., Benard, P. (1987). La vinification par macération carbonique. Spain: Institut national de la recherche agronomique. https://www.google.com/books/edition/La_vinification_par_mac%C3%A9ration_carboniq/LC0qHwqKeUwC?hl=en&gbpv=0

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21.Murat, M.-L. (2005). Recent findings on rose wine aromas. Part 1: identifying aromas studying the aromatic potential of grapes and juice. ProQuest. Retrieved
from https://www.proquest.com/docview/216597018?pq-origsite=gscholar&fromopenview=true

22.Salinas, M. R., Garijo, J., Pardo, F., Zalacain, A., & Alonso, G. L. (2003, January 1). Color, polyphenol, and aroma compounds in rosé wines after prefermentative maceration and enzymatic treatments. American Journal of Enology and Viticulture. Retrieved January 23, 2022, from https://www.ajevonline.org/content/54/3/195.short

23.Ruiz-Rodríguez A, Durán-Guerrero E, Natera R, Palma M, Barroso CG. Influence of Two Different Cryoextraction Procedures on the Quality of Wine Produced from Muscat Grapes. Foods. 2020; 9(11):1529. https://doi.org/10.3390/foods9111529


24.Revilla, I., & González-SanJosé, M. L. (1998, November 9). Methanol release during fermentation of red grapes treated with pectolytic enzymes. Food Chemistry. Retrieved January 23, 2022, from https://www.sciencedirect.com/science/article/abs/pii/S0308814698000491


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