April 2009 Issue of Wines & Vines
 
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The Art Of Oak

It starts with the science of oak compounds; part 1 of 2

 
by Dr. Richard Carey
 
 
    HIGHLIGHTS
     

     
  • The author has used oak barrel alternatives for 30 years. In part 1 of 2 he covers oak constituents and how they are manipulated during processing.
     
  • The toasting process and the winemaker's extraction techniques are the keys to using oak alternatives for quality wine that cannot be distinguished from oak-barrel aged wine.
     
  • Information available to winemakers today will allow them to control the specific flavors oak can give to a wine.
     
  • Part 2 of 2 in the May issue will develop a winemaker's palate of techniques for using oak alternatives.

I often tell consumers and clients that winemaking is 50% art and 50% science--the problem is we don't know which is which. Winemakers today approach the "art" of winemaking in many different ways. Some are classicists who agree with the Old World view that if something has been done in a certain way for centuries or decades, then that is the proper way to make wine and everything else is just manufacturing a product. Some winemakers are commercial artists, where the bottom line rules; others in the modern art school of winemaking seize on every new technology and abandon the classic roots of the art; pop art winemakers take the craft to any zany extreme just to put a product out there and see if anyone finds it interesting.

My philosophical approach to winemaking could be defined as neo-classical post-modern. I want to create wines that are based on classical traditions in order to get the most out of the past, but also take advantage of the present--and future--elements of the art and the science of our business. This has been my approach to such elements as crossflow filtration, plastic tanks (see my article, "Advantages of Plastic," in the January, 2009, issue of Wines & Vines) and to oak alternatives.

In today's world, we as winemakers should be concerned with the environmental impact of our traditional ways of making our products, as well as the costs of maintaining those traditions. We as an industry should be looking for ways to make our products in an environmentally sound, sustainable manner--but without sacrificing the qualities we value in our products.

If we keep these goals in mind when we look at wine storage containers, the worst container we could use is an oak barrel. It requires the finest wood, grown in specific environments; it is the most labor intensive container to produce; it takes up more square footage of warehouse space per liter of wine stored than any other type of container; and then it is only good for three to four years before it loses its primary reason for existence--the transfer of oak flavors into the wine.

Figure 1
 
Figure 1
 

If using oak barrels is neither cost-effective or environmentally a good idea, what should winemakers use instead?

First, we must identify the characteristics of an oak barrel as it is used in the wine industry. It is a closed container of indefinite size that transmits oxygen at a known rate per unit area, and it imparts the flavors of the oak from which it is made into the wine in a reasonably consistent manner. Barrels should be cleanable to a level that will allow wine to be stored without being contaminated by the previous lot of wine. With the exception of imparting oak flavors, many other containers have all of these qualities, and should qualify therefore as a "barrel" for the purposes of aging wine.

There are several materials that transmit oxygen, but the most useful for our purposes in wine production is plastic. Polyethylene plastic has a precise rate of oxygen transfer per unit area per mm thickness, and it is for that reason that a properly designed plastic barrel-tank can be constructed that will emulate an oak barrel in oxygen transfer rates per liter of liquid stored per unit of time. While other plastic materials can be used, they are not as readily available as the current polyethylene materials. In purchasing plastic barrel-tanks for a winery, the most important factors to determine are the surface to volume ratio and the actual rate of oxygen transfer for that particular barrel-tank.

Figure 2
 

Direct addition of oxygen into the wine is another procedure that injects the correct amount of oxygen to develop the wine. In the recent past, micro-oxygenation has come into favor with wineries that want to achieve a barrel's results without the labor or expense required by a barrel program. This mechanism is the least expensive process of achieving the level of oxygenation required to simulate barrel-aged wines. However, because a micro-oxygenated wine container does not have to be a sealed container that naturally allows oxygen to transfer into the wine, I do not believe this process should be described as barrel aging.

 

The one function that an oak barrel provides that does not occur in plastic barrel-tanks is the loss of vapor to the outside atmosphere. This is an interesting conundrum. If left to nature, an oak barrel loses a considerable volume of liquid to the outside--in some cases, up to several percent. When needed, this is a good method to concentrate a wine naturally, but using barrels can also concentrate undesirable flavors. Even though winemakers may not be able to allow such a concentration to proceed with either a plastic barrel-tank or a stainless steel tank in which a wine might be micro-oxygenated, they can still accomplish the same purpose--and with greater precision--by bleeding juice off after crush.

Figure 3


If plastic barrel-tanks have the necessary elements to substitute for a traditional oak barrel container, we now must look at ways to get oak flavor into the wine. Forward-thinking oak barrel manufacturers have been developing products for more than 30 years in an attempt to achieve this goal, and gradually more is becoming known about both the physics and the chemistry of this process.

First, it is important to define the different types of oak alternative products: staves, particles and granulate (or powder). Each of these types of oak alternatives transfers oak characteristics into the wine differently and, as a result, they must be used differently.

Staves

As an oak alternative, a stave is any plank-like product that has enough depth to mimic a typical barrel stave used in the construction of an oak barrel. This would include the long planks strung together to ring the inside of plastic or stainless tanks, as well as shorter versions that are small enough to insert into a bung hole to rehabilitate used barrels. It should also be a minimum of 8mm thick and sufficiently long so that the internal mass of the stave allows a much longer time to reach the proper toasted color (see Figure 1 F, G, K).

Particles

This category would include all shapes and sizes that are thinner and smaller than staves, and where the major percentage of its wood is toasted in color. These items are sometimes also known as blocks or dominos. Particles are usually short and virtually uniform in their toasted color throughout (see Figure 1 E, H, I, J).

Granulate/Powder

This category is defined by surface-to-volume ratios. Oak particles that range from less than 1mm in any direction to pieces about 4 to 8mm in their longest dimensions and less than 1 to 2mm in any other dimension would qualify for this category (see Figure 1 A, B, C, D).

One of the first oak alternative products to be marketed to wineries in the United States in the 1970s was Cellulo's Oak-more. This product was sold both as an oak-extracted wine product that could be blended into the primary wine and as a granulated, sawdust-like product that the winery could extract on its own. Just like today, winemakers often did not want to be identified as using this product, but it was sold by the ton to many wineries. I remember visiting wineries in California and, on my tour of the facility, seeing very narrow steeply conical-bottom tanks stuck off in a corner out of the view of the main cellar that were being used for making oak extractions.

Figure 4
 

Some winemakers (and wine writers) claim that they can tell a "chipped" wine by the taste. As with many new techniques, there has been a long learning curve as winemakers worked with these products, and just as a wine can be over-oaked in an oak barrel, misusing oak alternatives can just as easily ruin a good wine. I have been using oak alternatives for more than 30 years, and have had occasions to run side-by-side experiments with barrels and oak alternative treatments on the same wine. The differences between the two have been minimal. I believe that how the manufacturer handles the toasting process, and how the winemaker does the extraction are key to using oak alternatives for quality wines that will never be accused of being "chipped."

The use of oak alternatives evolved slowly, partly because of these issues of acceptance by the industry, but also partly because of the wording of the Code of Federal Regulations nomenclature governing the rules of use for these products. For many years, oak additions were limited to fining additions, and oak alternatives could not be toasted prior to use. Once that rule was modified in the late 1980s, a wide range of products was developed that have improved many wines. In the 1990s research continued on the use of oxygen in winemaking, which resulted in the first micro-oxygenation equipment, and on the sources of oak flavor from barrels.

Compounds Found in Oak Wood

While the work on oak flavor was done on barrels, the conclusions also apply to oak alternatives. It is important to understand the compounds that are found in oak wood, and then what happens during the extraction process. Chatonnet in 1995 and then Leffingwell in 1998 identified more than 70 compounds extracted from oak wood and wine aged in oak barrels.

Figure 5
 

Based on radar plots of flavor and aroma descriptors perceived by panels of tasters, Eric Herve of ETS Laboratories narrowed the discussion to nine of the most important compounds contributing to oak influence in wine making. Figure 2 (above) shows radar plots of two barrel types that demonstrate how divergent the aromatic perception of the same wine can be when the same cooper uses different wood sources. Figure 3 shows the same wine in similar barrels from different coopers.

The nine important oak flavor and aroma compounds are:

Eugenol and Isoeugenol: In oak, eugenol and isoeugenol show the same extraction kinetics, and seem to have additive and cumulative sensory effects.

Guaiacol and 4-methylguaiacol: Wood lignin degradation at very high temperatures results in formation of a wide range of volatile phenols including these two compounds. Both guaiacol and 4-methylguaiacol are close to their sensory thresholds in oak; however, their combined concentration has a perceived sensory effect synergistically.

Cis-oak lactone and trans-oak lactone: The cis-isomer is more powerfully aromatic than the trans-isomer. The sensory threshold found by Chatonnet in 1995 shows that trans-oak lactone aroma has an impact approximately 4.5 times less than cis-oak lactone.

Furfural and 5-Methylfurfural: These compounds result from degradation of carbohydrates by heat. In the wood in barrels, the carbohydrates cellulose and hemicellulose are degraded during barrel toasting. Furthermore, they react with other wine constituents during the aging process.

Vanillin: The quantities of vanillin reported in wine vary with oak species and seasoning. Vanillin increases with medium toast levels, but decreases with very high toast.

Oak Aroma Descriptors
Eugenol
and Isoeugenol
Eugenol and isoeugenol possess a similar spicy, clove aroma. Eugenol is present in raw oak and is reported to increase during open-air seasoning.
Guaiacol
and 4-methylguaiacol
Guaiacol and 4-methylguaiacol have smoky aromas and are markers of the smoky character imparted by heavily toasted oak. Guaiacol has more of a char aroma, while 4-methylguaiacol has char and spicy characters.
Cis-oak
lactone/ trans-oak lactone
The main aroma constituents of raw oak are these two isomers of oak lactone. Their associated sensory descriptors are fresh oak and coconut.
Furfural
and 5-Methylfurfural
Furfural and 5-methylfurfural possess sweet, butterscotch, light caramel and faint almond-like aromas.
Vanillin Vanillin, the main aroma compound in natural vanilla, is also present in raw oak.
Compounds that affect flavor and aroma, from the Alchemy of Oak Add-Ins by Nadalié Cooperage.



Oak Extraction

Once the compounds in the oak were identified, the next step was to understand the dynamics of the extraction process. The barrel company Nadalié has done a series of studies comparing different types of oak alternatives made of different types of oak and with different coopers. The website nadalie.com gives an extensive report on many of these variables. It is interesting to look at the changes that happen over time with the different alternatives and to reflect on the possible implications of those results.

Figures 6 and 7 represent the chemical analysis of what happens when a winemaker uses chips. Figure 4 has data for medium toast, French oak chips; Figure 5 for heavy toast, French oak chips. Both graphs show a steady rise in extraction up to four months. At that point, there are notable differences in the extractable compounds between medium toast and heavy toast. With the medium toast, the vanillin extraction actually drops off.

Figure 6
 

The next two figures show similar results for oak cubes. Figure 4 shows data for medium toast plus, French oak cubes; Figure 7 for heavy toast, French oak cubes. If Figures 4 and 6 are compared, we see that the rate of extraction for chips is faster than for cubes, which is not surprising. Cubes have a larger mass, so the compounds should tend to reach a higher average concentration. Once again, the level of vanillin peaks early--in the heavy toast, it levels off after four months, while the other compounds continue to rise for 12 months. With the medium toast, the vanillin peaks at eight months and guaiacol levels off, while the other compounds continue to rise gently from four to 12 months. One other interesting point is the general nature of vanillin to decrease somewhat from months four to 12.

Figure 7
 

Figure 8 represents the chemical analysis for Winewood, Nadalié's "wine slat" product. Note that there are big differences in the concentration of the compounds between the chips and Winewood, especially for guaiacols and vanillin. This is probably due to the treatment of the products during production. It also demonstrates the importance of trials using different oak alternatives in a winery before commercial use is undertaken. Other products in the group show different ranges of compound differences from these, although not as dramatic.

x The chemical components of oak are a complex mixture. The benefit of the original work by Chatonnet was in the identification of more than 70 compounds contributed by oak barrels. Narrowing the list to the nine compounds presented here helped to simplify and give focus to compounds present, while not getting lost in a minutia of detail. I believe that several other compounds based on syringone are also important to examine. Syringone and its analogs are the basis of maple syrup aroma and flavor, a component of oak that is referenced many times in wine and is an important result of the toasting process. Unfortunately, this is one group of compounds that the different oak alternative suppliers are not addressing.

Figure 8
 

The kinetics of the extraction process demonstrated by these figures is shown in static form. Observing the rates of extraction in the oak in a dynamic way emphasizes why oak alternatives are important to the wine industry. Fine Northern Oak has shown the rate of wine infusion into a typical oak stave that is closed to the grain (i.e., penetration perpendicular to the xylem vessels) is 0.198mm/month (Figure 9). For the stave-like alternatives such as slats, the penetration rate is 2X, since penetration happens on both sides of the stave. In this case, the penetration rate is 0.373mm/month.

The interesting innovation by Fine Northern Oak is that they construct and toast a stave and then crosscut it in either 13 to 15mm intervals or 18 to 20mm intervals. This opens up the xylem vessels to become channels for extraction of the toasted oak from the exterior of the stave at a rate of 3.17mm/mo, so that the stave is completely extracted in four to six months, depending on the crosscut interval. To experience the dynamic nature of this process, see the animation:



Returning now to the Winewood graph, this process shows no signs of the extraction rate slowing down after 12 months, and so a wine made with this type of product will take much more careful observation to assure that the wine remains balanced.

Between the data presented by these two suppliers, it is clear that the various oak alternative products have widely divergent kinetics in the extraction of their principle components, and that these products are even more different in the kinetics than their standard oak barrel cousins.

Extraction kinetics dictates that a compound's mass flow diffusion out of a matrix drops by the logarithm of its initial surface rate of mass transfer. The principle reason barrels only last three years with respect to oak flavor transmission is due both to this rate of diffusion and the fact that the whole stave cannot be toasted through. This results in progressively lower amounts of toasted oak compounds available to be extracted from the oak. It does, however, allow for variance in the types of compounds extracted, and thus can add complexity to the final wine.

A better solution is to control the toasting of the final product and then allow for a quicker extraction of the oak flavors. We now have the tools to create designer oak alternatives through these technologies. However, that is only the beginning of the process from the winemakers perspective. If your winemaking style requires more (add your compound here) then you want a supplier who uses techniques that give that result.

ETS laboratories can provide you that information with its GC Mass Spectrometer analysis of oak. The cost is somewhat less than $200 per sample. Nadalié conducted a detailed series of experiments to find out what people perceive, and what they and others have in their oak.

Now Fine Northern Oak has made a commitment to carry this knowledge one step further, to understand the production process and thus provide a better analysis of the products. Fine Northern Oak's quality control is on the cusp of being able to provide oak alternatives based on component analysis rather than medium or medium plus toast. Its solution will be to emphasize high spice/clove and low smoke by adjusting toast level and then verifying the product concentrations for the winemaker.

Our next issue will examine the sensory component of oak alternatives.

 
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