Barrel-fermented and -aged white wines

Oak was the primary fermentation vehicle prior to the post-war inroads made by stainless steel tanks, inroads driven by the latter's perceived advantages:

• Provides an anaerobic environment
• Easier to clean, thus reducing the risk of bacterial contamination
• Increased durability
• Allowed fermentation temperature control
• White wines could be fermented cool and thus preserve floral and fruity aromas
• Cooler fermentation temperatures lowered the risk of off-flavor production
• Allowed control of fermentation rate.

With all of these advantages arrayed against it, oak had to have some overriding benefits for winemakers to continue using it as a vehicle. And it did. According to Ibern-Gomez, et al*., "Fermentation in oak barrels leads to wines with much more complex sensory properties, largely attributed to the phenols extracted from oak wood."

I will examine these substances and their impacts on barrel-fermented wine in this post.

Oak Wood
As a result of its "strength, resilience, workability, and lack of undesirable flavor," oak is the wood of choice for most wine cooperage applications.

The oak used in the maturation of alcoholic beverages fall into one of three species: Quercus alba, Quercus robur, and Quercus sessilis.  Q. robur and Q. sessilis, and their respective subspecies, are European white oaks while Q. alba is the source of 45% of the white oak lumber produced in the US.  American oak used in barrel production is sourced from Kentucky, Missouri, Arkansas, and Michigan but there is no apparent regional distinction.  European oak, on the other hand, may have designations which reach all the way to the forest from which the oak originated.  For example, French oak from the department of Alliers may be sourced from a forest named Tronçais.

Sources: enologyinternational.com; Dr. Murli Dharmadikari; Principles and Applications in Wine Science

The journey from oak tree to wine barrel is shown in the graphic below.

Alcoholic Fermentation in Oak Barrels
Grapes are pressed and the resulting juice is deposited into oak barrels (In many Burgundy white wines the grapes are pressed "whole-cluster"). The juice levels do not fill the tank as space has to be left for expansion of the contents during alcoholic fermentation.

In a study on barrel-fermentation of white wines (S. Herjavec, et al., The quality of white wines fermented in Croatian Oak, Food Chemistry, 100, 2007), the authors stated thusly:

One of the practices used to intensify the aroma and flavor characteristics of white wines is to ferment the must in oak barrels, and Chardonnay is one of the most suitable varieties for this. Wines produced by fermentation and maturation in oak barrels have different flavor characteristics to those which have undergone barrel maturation only after fermentation in stainless steel. One reason for this is that actively growing yeasts are capable of transforming volatile flavor components, extracted from oak wood, into other volatile metabolites.

This metabolite transformation results in what Zac Brown, Winemaker at Alderlea Vineyards, describes as "better integration of the oak and softer mouthfeel when compared to a white that is finished and then transferred into oak barrel to age."

In the case of reductive winemaking, we seek to prevent the rich varietal aromas of Riesling, Petit Manseng, and Gewurtztraminer from oxidizing effects. This environment will be subjected to oxygen effects and it is not recommended that these varietal types be barrel-fermented.

Malolactic Fermentation
According to Sauvageot and Vivier (Effects of Malolactic Fermentation on Sensory Properties of Four Burgundy Wines, AJEV 48(2), 1997), malolactic fermentation (MLF) is a bacterial conversion -- most commonly performed by Leuconostoc strains, due to their tolerance of the high acid and alcohol content associated with wine -- of L-malic acid to L-lactic acid and CO₂.

The main effects of MLF on wine are (i) a reduction in titratable acidity (by 0.1 to 0.3%) and an increase in pH (0.15 to 0.30). In addition, dramatic organoleptic changes to the wine are evidenced (Lonvaud-Funel, Microbiology of the Malolactic Fermentation: Molecular Aspects, FEMS Microbiology Letters):

• The specific taste of malic acid disappears
• Sugars are catabolized to produce mainly lactic and acetic acid
• Citric acid is transformed into acetic acid and carbonyl compounds, notably the butter-flavored diacetyl
• Wine taste and color are modified due to the metabolic activity of bacteria on phenolic compounds (tannins, anthocyannins).

By synthesizing anti-bacterial compounds and depriving the wine of nutrients, MLF also contributes to its microbial stability (Lonvaud-Funel).

The process is encouraged (Bauer and Dicks, Control of Malolactic Fermentation in Wine, S. Afr. J. Enol. Vitic. 25(2), 2004): in cooler areas where grapes have high malic acid content; in cases where the wine is aged in oak barrels; and when the wine style calls for long-term aging in bottle. The practice is sometimes forsworn in warmer, lower-acid areas and in the cases where undesirable organoleptic changes or the production of biogenic amines result.

Lees Aging

Murli Dharmadhikan (Yeast Autolysis, extension.iastste.edu) defines yeast autolysis as "... self-destruction of the cellular constituents of a cell by its own enzymes" following its death. Figure 1 below shows the component parts of a healthy yeast cell while Figure 2 shows an overview of the process  -- autolysis -- that occurs once that yeast cell has consumed all of the available nutrients and dies. At a high level, autolysis encompasses (i) the degradation of intracellular materials and (ii) degradation of the cell wall.

The detailed autolysis process is shown in Figure 3 below. The yeast extract, product of the degradation of intra-cellular material, is confined to the cell until such time as the cell wall becomes porous enough to allow the material to seep out. It should be noted that degradation and compound creation continues outside the degraded cell walls.

Figure 3. Details of yeast autolysis

The lees-aged wine is enriched by the compounds released during the constituent-degradation process. Compounds released during autolysis include (Thierry Binder, Cremant d'Alsace, TONG #13; Dharmadhikan):

• Nitrogenous compounds
• Amino acids -- known to enrich mouthfeel; aroma precursors of acacia honey notes
• Polypeptides -- sweet and bitter taste; precursors of the autolytic aromas of brioche and toast
• Peptides
• Nucleic acid components
• Polysaccharides -- originates from breakdown of cell wall components
• Degradation products are glucose and mannose
• Mannoproteins increase mouthfeel and foam stability as well as contributing to fineness and persistence of bubbles
• Fatty acids -- important for foam stability, mouthfeel, and flavor. Can be involved in the formation of esters, aldehydes, and other volatile compounds
• Volatile components
• Heavy esters
• Terpene components
• Higher alcohols
• Other volatile components.

In order to ensure distribution of the beneficial autolysis products evenly throughout the wine, a process called batonnage -- stirring of the lees -- is undertaken. Batonnage is generally conducted once or twice per week.

Oak Aging
Wine is aged in wooden barrels to: (i) enhance its flavor, aroma, and complexity through transfer of substances from the wood to the wine; and (ii) allow gradual oxidation of the wine.

In the first instance, many of the wood's native aromatic compounds, as well as the aromatic compounds created during seasoning and toasting, are absorbed, and integrated, into the wine, thus contributing to wine richness and aromatic complexity.  For example, hemicellulose will hydrolyze upon exposure to wine, creating, as a result, sugars and acetyl groups.  The sugars are further converted to furanaldehydes and ketones while the acetyl groups are converted to acetic acid during maturation.  A small proportion of lignin will dissolve in wine (these are called native lignins) while some undergo ethanolysis and are oxidized to aromatic compounds.  These compounds have low olfactory thresholds and will, therefore, impact the wine's aromatic profile. As noted by Dr. Murli Dharmadikari, common descriptors of oak-aged wines are oaky, vanilla, smoky, toasty, spicy, and coconut.

In terms of gradual oxidation, wine loss from barrels amount to approximately 2% per year, resulting from the fact that water and ethanol are smaller molecules and will diffuse into the wood and, ultimately, escape as vapor.  If the air in the cellar is dry, more water is lost and the wine is more concentrated in terms of alcohol.  If the environment is too humid then more alcohol is lost, reducing the ethanol content in the remaining wine.  This loss of liquid opens up a space between the wine surface and the barrel which the winemaker generally "tops up" in order to prevent oxidation and acetic spoilage.  During this "topping-up" process, small amounts of oxygen are dissolved in the wine.  Oxygen is also introduced into the wine during winery operations such as filtering and racking.

The oxygen which is now in the wine reacts with resident phenolic compounds in a manner such that: (i) tannins are softened (polymerization and precipitation as well as tannin-polysaccharide combinations); (ii) complex aromas develop; and (iii) there is improvement in the wine's body and mouthfeel.  It should be noted here that the tannin resident in the wine at this time is the oak tannin absorbed from the barrel (30% from the innermost four millimeters of wood).

In the aforementioned Herjavec, et al., study, the authors found that the sensorial characteristics of barrel-aged wines were modified, due to the wood-derived compounds. These wines manifested roundness in taste with a complex retro nasal aroma." Barrel toast also affected flavor perception: aging in medium-toast barrels yielded a smoky, roasted, and raw oak flavor while light toast resulted in a more fruity aroma.

Comparison of Barrel- and Stainless Steel-Fermented White Wines
According to Ibern-Gomez, et al., "wines fermented in wood barrels are distinguished by the cession of oak wood compounds to the wine." The figure below shows the phenolic compounds found in wine fermented in oak barrels.

Further, the authors compared control wines fermented in stainless steel to wines fermented in oak barrels and noted the following differences:

• Total phenolic content was higher for white wines fermented in oak barrels than for wines fermented in stainless steel tanks
• New phenolic compounds which are characteristic of oak wood (syringaldehyde, coniferaldehyde, sinapinaldehyde, scopoletin, 4-ethyl-guaicol, and eugenol (the latter two being volatile phenols)) were found in the white wines fermented in oak
• The gallic acid and 4-vinylguaiacol increased in white wines fermented in oak
• Browning in oak wood white wines was higher than for stainless steel white wines.
• Furfural, 5-methylfurfural, and furfuryl alcohol from thermal degradation of cellulose and hemicellulose were found in the white wines fermented in oak.

As it relates to sensorial analyses, the following was reported:

• Tasters described white wines fermented in oak as having golden hues
• White wines fermented in oak were described as having toasty and spicy aromas
• Probably due to the 4-ethyl-guaiacol and eugenol
• Tasters also described a coconut aroma for the barrel fermented wines
• Probably due to the cis-β-methyl-γ-octalactone detected.

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While the process described herein is identified as being associated with oak fermented and aged white wines, it is not exclusive, in its entirety, to that style of wine. For example, a wine fermented in stainless steel could also be subjected to malolactic fermentation and lees residence in the tank or could have those two procedures completed in oak barrels and subsequent aging in same.

*M. Ibern-Gomez, et al., Differences in Phenolic Profile between Oak Wood and Stainless Steel Fermentation in White Wines, Am. J, Enol. Vitic, 52:2 (2001).

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