VEN124 - Introduction to Wine Production
Author: Linda F. Bisson, UC Davis
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Section 2 - Grape and Must Processing
Lecture 7: Overview of White Wine Processing

In this lecture we will discuss white wine production, with a focus on pre-fermentation operations. There are some concerns that are similar for red and white wine production, but others that are specific for white and blush wines.

The Basic Steps of White Wine Production:

  1. Crushing, Destemming
  2. Pressing
  3. Cold Settling / Racking
  4. Fermentation
  5. Racking
  6. Finish of Fermentation

In contrast to red wines, white wines are pressed immediately upon crushing or following a brief period of skin contact. Excessive skin contact is not recommended in white wines as this leads to undue bitterness and astringency. White grapes are frequently destemmed during crushing again to avoid extraction of the phenolic compounds. If minimizing skin contact is desired, the crushing step may be omitted and the fruit taken directly to the press. White must may be subjected to de-juicing to obtain the free-run, which generally yields the highest quality wine. Press fractions may also be fermented separately. The juice is transferred to a tank for cold settling overnight, and then racked off of the grape lees prior to initiation of fermentation. The juice is usually warmed to the temperature of fermentation, typically lower than that for red wines (12-18°C versus 25-30°C). Sulfur dioxide is typically added early, at the crusher or at the point of cold settling.

  Principal White Wine Grape Varietals in California (% of white wine production, 2006 crush report):
    •  Chardonnay 36.4%
    •  French Colombard 19%
    •  Thompson Seedless 16.4%
    •  Sauvignon blanc 7.3%
    •  Pinot Gris 5.1%
    •  Chenin blanc 5.0%
    •  Burger 3.0%
    •  Viognier 0.83%
    •  White Riesling 0.74%
    •  Gewurztraminer 0.6%
    •  Other: 4.5% (each less than 7,000 tons crushed statewide)

A total of just over 1.5 million tons of white grapes were crushed in 2006 for white wine production.

The white grape variety with the largest production in California is Chardonnay. French Colombard, Pinot Gris, Chenin blanc and Sauvignon blanc round out the list of the top five wine varietals. Thompson seedless is a table grape variety that produces a neutral juice. It is used extensively in blending in white, red and rose wine production. Several other varietals of white grapes are also grown in California, but the production levels are relatively minute.


I. Options for White Wine Production:

  Style:
    •  Varietal or Blend
    •  Premium or Picnic
    •  Price/Volume

As with red wines, the first decision to be made is the type of wine to be produced, as this will dictate quality parameters and cost effectiveness. For example, is the wine to be marketed as a varietal wine or as a blend? If it is a blend, is it a well-known blend style or is it going to be a proprietary blend? Is the goal to produce a premium, ultra-premium or artisan style or will it be a picnic wine? To be economically feasible, the price obtainable per bottle will obviously need to cover the costs of production. The profit margin is not high for wine, just as for other commodities, so there is little room for error or poor judgment.

  Premium wine varieties
    •  Chardonnay
    •  Sauvignon blanc
  Dessert wine varieties
    •  White Riesling
    •  Muscat
    •  Gewürztraminer
  Picnic (Jug) wine varieties:
    •  French Colombard
    •  Chenin blanc
    •  Sultanina (Thompson seedless)
  Late Harvest "Botrytized" styles Grapes infected with mold
    •  Mold growth impacts composition
    •  Mold impacts yeast fermentation

Other styles of white wine include dessert wines, which are usually high in sugar as well as ethanol, and specialized styles such as Botrytized wines. Grapes harvested at a very high Brix value (28 Brix or higher) will naturally arrest fermentation. This high sugar level leads to high ethanol levels, which may be high enough to be inhibitory to the yeast present, depending upon other juice and strain factors. Many yeast strains can tolerate ethanol concentrations as high as 17%, but if the nutritional and physical parameters of the fermentation are not optimum, yeast ethanol tolerance is reduced. In this case, fermentations with 15% or higher ethanol may arrest. As a rough rule of thumb, the final ethanol concentration can be estimated as 0.55 of the initial Brix value (this is a rough estimate, as ethanol yield is affected by factors such as temperature of fermentation and the type of fermentation vessel). A juice with a Brix value of 30 or more will likely arrest naturally. This is the basis of one type of dessert wine production - harvest of the fruit at very high sugar levels. Other ways are to arrest fermentation by artificial means, the addition of distillate for example, or a temperature shock. Botrytized wines are made from fruit that is infected with the mold Botrytis. Botrytis is able to penetrate the berry surface and impact the composition of the fruit. The yeast have difficulty completing the fermentation of the juice produced from the mold-infected fruit and characteristic flavors are present in the wines.

  Residual Sugar:
    •  Dry (<0.2% RS)
    •  Semisweet (0.5-2% RS)
    •  Sweet (5-10% RS)

Another decision that needs to be made is the level of desired sugar at the end of fermentation. In dry table wines, the goal is a sugar content of less than 0.2%. As we will see in the lectures on fermentation, the residual sugar is generally fructose rather than glucose. Semisweet wines range from 0.5 to 2% residual sugar and wines containing greater than 5% sugar are classified as sweet .

  How to achieve desired residual sugar:
    •  Arrest fermentation
    •  Temperature
    •  Ethanol addition: fortification
    •  Add juice concentrate
    •  Late harvest: natural arrest of fermentation
    •  Add sweet reserve

Other than the various methods of arrest of fermentation, there are several techniques for adjusting the level of sugar post-fermentation. Juice or juice concentrate may be added or a sweet reserve (sugar plus ethanol); however if ethanol is added the wine will need to be labeled as fortified. If the ethanol content is below that which is inhibitory to continued yeast fermentation, the wines may undergo a spontane

  Carbonation:
    •  None - still wine
    •  Lightly carbonated
    •  Heavily carbonated (Sparkling wines)

The level of carbonation may also vary in the finished wines. The amount of CO 2 varies with the style of wine produced. Carbon dioxide enhances flavor perception; so many winemaking regions produce a lightly carbonated style. This style is more popular outside of the United States . Sparkling wines are heavily carbonated. We will not have time in this course to cover the complex process of Champagne and Sparkling wine production, but there are several methods that are used to generate carbon dioxide in the bottle. The carbon dioxide can be produced naturally by having a secondary fermentation occur in the bottle or can be added artificially at the time of bottling.

II. White Wine Production: The Goals

The goals for white wines differ from red wine production in several respects. Generally little to no skin contact is desired. This is because the principle flavor and aroma compounds are located in the pulp of the grape with the skin providing little other than bitterness and astringency. Many white wine styles are designed to be consumed relatively young (less than five years of age), which is insufficient time to allow polymerization and softening of the phenolic content. In addition to bitterness, phenolic compounds lead to off-color production under oxidizing conditions. This color change is generally undesirable in white wines. Given the more delicate flavors of white wines, other spoilage characters are not well masked, such as aldehydes, higher alcohols and acetic acid. It is therefore more important to protect white wines from oxidative damage than red wines.

In addition to limiting both skin contact and oxidation , white wines are usually protected against excessive loss of volatile components by fermentation in enclosed vessels generally with refrigeration. If fermented in barrels, care is taken to minimize loss of volume due to evaporation, especially post-active phase of fermentation when a protective carbon dioxide blanket is no longer present.

III. White Wine Processing: Processing Options

There are several production techniques that can be varied in white wine production that will affect the composition of the final wine.

  White Wine Processing: Variables
    •  Solids Content
    •  Skin Contact
    •  Pressing Conditions
    •  Temperature
    •  Oxygen Exposure
    •  Lees Contact
    •  Oak

 

  Solids Content:
    •  High solids
      Better yeast and ML fermentation
      Decrease fruity content due to esterases of solids
      Increase phenolics/astringency
    •  Control of solid content
      Take free run
      Settle/Rack at low temperature
      Centrifugation

The solids content of the must can be varied, using techniques described in an earlier lecture. The higher the solids the less fruity the wine will be because the solid material contains esterases which break down aromatic compounds. If varietal character is to be minimized this is an advantage, if not, then it should be discouraged. The higher the solids, the higher the phenolics and the astringency, and the increase in the astringency might not be desirable. Settling and racking, and using a settling aid if necessary can control solids level. Centrifugation and filtration will also reduce solids but may lead to oxygen exposure unless performed under a modified atmosphere. High solids are stimulatory to the growth of both yeast and bacteria; over-clarified juices tend to ferment poorly. Another option we have discussed previously is skin contact. The more contact with the skin, the higher the phenolic content.

  Skin Contact:
    •  Increase phenolics/astringency
    •  Impact yeast fermentation products
    •  Impacts microbial flora

Skin contact refers to the length of time the juice is left in contact with the skins and seeds. The longer the time of contact the greater the extraction of the components of the skins into the juice. In contrast to red wine production, the majority of the important sensory components of white grapes are in the pulp not in the skins. Since the microbial flora of the grapes is located on the skins, skin contact also increases the contact of these organisms with the juice. If the skins are separated from the juice quickly, the microbes are also separated, minimizing their numbers in the primary fermentation. If the presence of the wild flora is desired, then some skin contact is beneficial. Since the skins are a source of phenolic compounds and, as we will see in later lectures, microbes can convert phenolic compounds to aroma compounds, the amount of skin contact will affect the aroma composition of the wine if those microbes are present. Pressing conditions can also be used to modify the solids content. The more pressure applied, the higher the solids content and the more damage to the skins and seeds due to shear forces. One of your assigned readings is a seminal paper that covers the changes to must and juice composition with varying pressing regimes.

  Pressing Conditions:
    •  Higher pressure, higher solids content, harder to settle
    •  Warmer pressing, greater extraction

The warmer the temperature at pressing, the greater the level of extraction of components into the wine. Temperature of both skin contact and fermentation is important. Whites are generally produced between 12 and 16°C. Lower temperatures of fermentation are desired, but the yeast are inhibited below 12°C, and fermentation is exceedingly slow increasing the chances of oxidative damage because a protective carbon dioxide blanket does not form - the rate of fermentation versus the rate of equilibration with the atmosphere is too low.

  Temperature:
    •  12-16°C
    •  Lower temperature
    •  Greater retention

 In general, the lower the temperature the greater the retention of the volatile aroma characters in white wines. One of the major aspirations of commercial yeast companies is the generation of yeast strains that will ferment well at lower temperatures. The level of oxygen exposure will also have a profound affect on the composition and therefore quality of white wines.

  Oxygen Exposure:
    •  Can stimulate yeast
    •  Can lead to off-color (pink, brown) formation
    •  Can lead to development of oxidized characters

Oxygen exposure, as we have mentioned before, stimulates yeast. It also leads to off color production, and it can lead to the development of oxidized characters. So how much oxygen exposure is desired depends upon what the main problems are. If the wine tends toward arrest of fermentation, then oxygenation so that the yeast will be stimulated may be desired. However, oxygenation would not be desired if the wine were at high risk for a pink or a brown character developing, or if other oxidized characters, such as aldehydes, appear in the wine. How can you prevent oxidation of white wine? Chemical antioxidants, which we have already discussed, can be used.

  Preventing Oxidation of White Juices / Wines:
    •  Use of chemical antioxidants: to block/mask oxidation reactions
    •  High Temperature / Short Time (HTST): to inactivate oxidases
    •  Use of low temperature: to inhibit oxidases
    •  Carbon dioxide / Nitrogen blanketing: to eliminate oxygen
    •  Clarification: to remove oxidases

High Temperature/Short Time (HTST) treatments, which expose the juice to a high temperature for a short time, can be employed to inactivate polyphenol oxidase activity. PPO can also be inhibited by sulfur dioxide. Molds, particularly Botrytis , also produce an enzyme capable of oxidizing phenolic compounds called laccase. Laccase is far more resistant to SO2 , and is not inactivated at the levels commonly used in wine production. If laccase is a problem, then HTST is the solution, as this enzyme will be denatured at high temperatures. However, HTST treatments will impact other components of the wine, and can lead to protein denaturation, aggregate formation and cloudiness in the finished wine. The rate of enzymatic reactions is temperature dependent and will be slower the lower the temperature. PPO activity is reduced at low temperatures , so if the temperature is low enough and the yeast hearty enough to compete well for dissolved oxygen, browning can be minimized. Enzymes can also be physically removed, by adsorption to a fining agent binding proteins such as bentonite. However, such fining agents are not specific and other components may be stripped as well. If protection against oxygen is desired in wines before the yeast fermentation becomes active, the juice can be provided with an artificial gas blanket , by addition of carbon dioxide, nitrogen or argon. This can be done to the top of the tank or from the bottom racking valve, allowing the gas to rise to the surface. This latter technique also serves to mix the juice or must. In this case, dissolved oxygen in the juice will also be driven out of the tank. Again, what options are the most appropriate depends upon the nature of the problems with production of wine from a given vineyard.

  Lees Contact:
    •  Increases flavors / complexity
    •  Enhances mouth feel

 One post-fermentation variable used in white wine production is the length of time the wine remains in contact with the yeast sediment or lees. Yeast lees contact influences the aroma profile of the wine. At the end of fermentation, yeast cells lose viability because an energy source is no longer available. Non-viable cells go through a process know as autolysis. During autolysis, the vacuole of the yeast cell deteriorates releasing the hydrolytic enzymes located in this organelle. These enzymes then degrade components of the yeast cell itself and lead to the lysis of the cell and release of cellular contents. This process affects the mouth feel of the wine and results in the production of distinctive characters.

  Oak:
    •  Fermentation in barrel
      New
      Used
    •  Kind of oak
    •  Kind of toasting
    •  Use of alternatives
      Chips
      Staves

A final important processing variable for white (and red) wines is oak contact. Oak imparts distinctive characters to the wine. Oak contact may be limited to barrel aging post-fermentation or it may occur during fermentation. Chardonnay for example is frequently fermented in oak barrels. The kind of Oak used and the processing of the Oak critically influence the oak characters that will be present in the wine. The effects of oak will be discussed in more detail in the lecture on aging. Alternatives to barrels, such as the use of oak chips or staves, can also be used. These techniques have the advantage of being cost effective, but many expert tasters can tell the difference between barrel-fermented wines and those that have been fermented in a stainless steel tank with wood addition. There are lots of reasons why this might be the case that are not dependent upon the characters extracted from the wood exposure. In contrast, many consumers cannot tell the difference, particularly in the fighting varietal price category (7-10$).

IV. Blush and Rosé Wine Production

Blush (white wine made from red grapes) and rose (pink) wines are made using similar production practices for white wines.

Rosé Wine Production:

  There are two ways to produce a rosé wine:
    1. As a blush wine of a red varietal
    2. As a blend of a white wine with a red wine

The pink coloration of these wines may come from limited skin contact with a red variety (blush) or by blending of a white wine with a red wine (rosé).

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