Tag Archives: oak barrels

The Influence of Water and Temperature on the Volatile Compounds of Oak Barrel Staves

Posted on May 8, 2013 by Becca| Leave a comment

 

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Many of you already know that using oak barrels during winemaking and aging increases the complexity of the finished wine, and often increases the overall quality of the wine. Using oak changes the aroma and color, as well as the stability of the finished wines. The type of aromas and flavors imparted into the wine depends upon a variety of factors, including the type of grape, the type of oak, and even where in the forest from which the oak tree was harvested. When making the oak barrels, heat treatments are frequently employed to help the wood become more pliable and thus able to be bent into the curved position of the barrel.

These heat treatments, referred to as “toasting”, alter the flavors and aromas imparted by the oak into the barrel, though the exact behavior of volatile compound concentration changes in wine is not known due to differing results in the literature. Traditionally, all of the research so far has focused on how toasting or heat treatments affect the aromatic and volatile compounds of wine

Photo By Olivier Colas (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Photo By Olivier Colas (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

when dry wood is used during cooperage, though none have examined getting the wood wet first prior to toasting. Soaking the wood staves prior to heat treatment could have a significant impact on the aroma and flavor of the finished wine, though to date, no studies have examined this until now.

The goal of the study presented today was to examine 6 different aromatic compounds in wood samples that were either wet prior to heat treatment or not, to determine what effect, if any, soaking the wood has on the volatile composition of the wood (and thus potential volatile composition of the finished wine). This study examined several different temperatures and 2 different heat treatment exposure lengths.

Methods

Wood samples originated from one 400-year-old Quercus petreae tree from the “forêt des beaux Monts” in Oise, France. The staves were given by Tonnellerie Seguin Moreau and had been naturally seasoned for two years prior to the experiment. Staves were cut into samples of 70mm x 25mm x 3mm.

Heat treatments were performed in triplicate. Five temperatures were tested (90, 120, 160, 200, and 240 oC) and two treatment time periods were tested (10 and 25 minutes). For the soaked wood treatment, cut stave pieces were soaked in 90 oC hot water for 20 minutes. Unheated samples were used as controls.

After heat treatments, stave pieces were broken down and homogenized into sawdust in order to extract the volatile compounds from the wood. Volatile compounds were analyzed using HS-SPME GS-MS analysis.

Results

• Guaiacol:
o No significant differences in guaiacol levels were found between wet and dry woods for temperatures up to 200  oC.
o Guaiacol was 5x higher in dry woods than wet woods at the 240  oC treatment temperature (significant difference) and 10 minute treatment, and 2x higher for the 25 min treatment at this temperature.
o Guaiacol levels in dry wood at 240  oC for 10 minutes were not significantly different than the levels in wet wood at 240  oC for 25 minutes.
• Eugenol:
o Eugenol values were constant in woods for all temperatures, though were slightly higher at the 25 minute treatment compared with the 10 minute treatment.
o At the 240  oC temperature and the 25 minute duration, eugenol values in dry wood significantly decreased to levels found at the 240  oC temperature and 10 minute duration treatment.
• Furfural:
o Furfural levels in dry wood significantly increased at the 160  oC and 200  oC temperature treatments, and significantly increased further at the 240  oC temperature treatment.
o Furfural levels in wet wood significantly increased at the 200  oC temperature and peaked at the 240  oC temperature treatment.
• Vanillin:
o In all treatments at the 10 minute duration, vanillin levels were similar, with the exception of 240  oC temperature which showed increased vanillin levels in dry wood.
o For the 25 minute duration, there was a significant increase in vanillin in dry wood at 200  oC and a significant decrease in vanillin at 240  oC.
• Cis-whiskey lactone:
o Cis-whiskey lactone levels remained constant in dry wood for all temperatures except for the 240  oC treatment which showed a significant decrease in cis-whiskey lactone levels.
o Cis-whiskey lactone levels were significantly lower in soaked wood compared with dry wood at the 160  oC treatment temperature, similar at 90, 120, and 200  oC, and significantly higher at the 240  oC treatment temperature.
• Trans-whiskey lactone:
o Trans-whiskey lactone levels were significantly lower in wet wood at 90 and 160  oC, similar at 120 and 200  oC, and significantly higher than dry wood at 240  oC.
• General Trends:
o Lower temperatures were not correlated and in some cases negatively correlated with furfural, vanillin, guaiacol, and trans-whiskey lactone in woods.
o Higher temperatures were positively correlated with furfural, vanillin, guaiacol, and trans-whiskey lactone in woods.
o Higher temperatures were negatively correlated with cis-whiskey lactone and eugenol.
o Lower temperatures (particularly in the 25 minute duration treatments) were positively correlated with cis-whiskey lactone and eugenol.
o There was no significant influence of the temperatures 90, 120, and 160  oC on wood volatile compounds.
o Increased temperatures led to greater correlations with furfural, vanillin, and guaiacol and to weak correlations with cis-whiskey lactone, eugenol, and trans-whiskey lactone.

Conclusions

Overall, the results of this study indicated that the temperature of the heat treatment greatly influenced the concentrations of furfural and vanillin, though also had minor impacts on the concentrations of eugenol, cis-, and trans-whiskey lactone. According to the results, furfural was the volatile compound most influenced by the experimental treatments. Also, treating the wood with water prior to the heat treatment appeared to have a significant influence on the concentrations of all the oak volatile compounds studied with the exception of eugenol. The authors concluded that the formation of these volatile compounds may be a combination of the heat treatment influencing the production of the volatile precursors as well as the degradation of the volatile compounds.

After undergoing a wet treatment, it was found that the wood samples in general showed lower concentrations of volatile compounds than the dry wood samples. The authors concluded, and I tend to agree, that the absorption of water by the

Photo By Wmpearl (Own work) [CC0], via Wikimedia Commons

Photo By Wmpearl (Own work) [CC0], via Wikimedia Commons

wood may have some sort of protective effect against the degradation of the volatile compounds, therefore reducing the overall concentration of the compounds found in the homogenized samples. In a way, I would think the water is having some sort of cooling effect, thus delaying the extraction of volatile compounds from the wood.

I would have liked to have seen the authors take this a step further, and actually produce a wine made from barrels undergoing these temperature and water treatments. Do the increases and decreases in volatile compounds noted in the wood change the volatile composition of the wine in the same manner? Or are there other mechanisms involved that result in a different volatile composition of the finished wine? How do these treatments alter the aromatic and volatile composition of different kinds of wine? Do wines made from these types of treatment barrels taste quality and possess higher quality than untreated barrels? All of these questions would make for a great follow-up paper.

What about you all? How did you interpret these results? What experiments would you have liked to have seen done in addition to what was presented here? Any other comments or questions? Please feel free to comment!

Source: Duval, C.J., Sok, N., Laroche, J., Gourrat, K., Prida, A., Lequin, S., Chassagne, D., and Gougeon, R.D. 2013. Dry vs soaked wood: Modulating the volatile extractible fraction of oak wood by heat treatments. Food Chemistry 138: 270-277.

Posted in Chemistry

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The Influence of Oak Chips Added at Various Stages of Winemaking on Sensory Characteristics of Wine

Posted on April 3, 2013 by Becca| 4 Comments

 

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As many of you may already know, using oak barrels in wine fermentation and aging increases wine aromatic complexity and improves overall quality. The use of oak in wine is and has always been very popular, despite the fact that it costs more money to produce an oaked wine than it is to produce a wine made in stainless steel tanks. As a result of this cost differential, some wineries as well as home winemakers have been searching for alternatives to oak barrels that give similar aromatic and quality characteristics to the finished wine without the high costs.

There is some experimental evidence suggesting that application of oak extract to the vines during the growing season may impart oak flavor characteristics into the finished wine, however, it’s a practice that is currently just in the research and development phase, and is not yet widely practiced or accepted. In recent years, the use of oak chips instead of oak barrels has become much more popular, as it has been shown that using oak chips in wine fermented and/or aged in stainless steel tanks results in finished wines that are aromatically similar to wines that are fermented and/or aged in oak barrels.

Oak chips may be added to the wine at any stage during the winemaking process, and will result in varied styles of wine depending upon exactly when the chips were added. The goal of the study presented today was to examine the sensory

Agne27 at the English language Wikipedia [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons

Agne27 at the English language Wikipedia [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons

characteristics of one particular type of wine (Bobal) when placed in contact with oak chips at different stages throughout the winemaking process and to identify when during the winemaking process oak chip exposure will create the wine most similar to a wine kept in oak barrels.

Methods

Bobal grapes from a vineyard in the La Mancha region of Spain were used for this study and were harvested at their optimal ripening time. Grapes were separated into 7 batches and were all processed the same up until right after skin maceration. The following treatments were applied to the grape batches:

1. Control Wine: wine without the addition of oak chips.
2. Addition of oak chips during alcoholic fermentation.
3. Addition of oak chips during malolactic fermentation.
4. Addition of oak chips post-fermentation (1 week contact time).

The three oak chip treatments were split into two sub-treatments: 3g/L dose and 6g/L dose (total 7 treatments including control).

The oak chips used were a mix of French and American oak and had medium toast.

The rest of the winemaking process was pretty standard and the same for all treatments: manual punch downs, malolactic fermentation, racking, filtering, bottling, and storing. Finished wines were stored at 16-18oC until sensory analysis was performed. All treatments were performed in duplicate.

For all wines, the following were measured and analyzed: total acidity, ethanol content, pH, volatile acidity, total SO2, and free SO2.

For the sensory analysis, a carefully trained panel of 15 judges between the ages of 24 and 50 years old from the University of Castilla in La Mancha, Spain was selected. Panelists were specifically trained to analyze flavor descriptors in Bobal wines (both oaked and unoaked).

20mL of each treatment wine samples were given to the judges in standard wine glasses and covered with a watch glass in order to avoid volatile loss to the air. Sensory analysis took place in individual booths in a sensory analysis chamber. Panelists smelled and tasted the wines and recorded the aromatic and flavor descriptors they noted in each wine.

Results

• There were no significant differences between samples in regards to total acidity, volatile acidity, and pH.

Nose:
• Aromas on the nose of control wines (no oak chips added) were: red fruit, fresh, liquorish, pepper, sweet spices, leather, tobacco, and cassis.
• Wines with oak chips added at alcoholic fermentation saw a significant decrease in red fruit, liquorish, cassis, and pepper aromas.
• Wines with oak chips added at alcoholic fermentation saw a significant increase in sweet spices and woody notes at the higher 6g/L dose compared with the lower 3g/L dose.
• Wines with oak chips added at malolactic fermentation saw significant decreases compared with all wines in red fruit, fresh, pepper, and cassis aromas.
• Adding oak chips during malolactic fermentation resulted in significantly higher intensities of oak-derived aromas at the 6g/L dose compared with the 3g/L dose.
• Wines with oak chips added post-fermentation for one week were similar in character to wines with oak chips added during malolactic fermentation, however showed decreases in intensity of woody, vanilla, coconut, toast, and toffee notes.
• Wines with oak chips added post-fermentation for one week showed increases in red fruit character compared with wine with oak chips added during malolactic fermentation.
• 6g/L oak chip wines generally showed greater oak character than wines treated with 3g/L oak chips.
• Principle component analysis (PCA) grouped wines most similar to each other into two groups: 1) both wines with oak chips added at alcoholic fermentation and control wines; 2) both wines with oak chips added at malolactic fermentation and both wines with oak chips added post-fermentation for one week.
o The second group showed significantly more oak character than the first group.
o In the second group, the two wines with chips added at malolactic fermentation were greater in oak aromatic intensity than the wines with oak chips added post-fermentation for one week.

Taste:
• Control wines (no oak chips) had flavors of red fruit, liquorice, clove, pepper, leather and tobacco.
• All oak chip wines had significantly decreased red fruit flavors, with the wine treated with oak chips at malolactic fermentation having the least red fruit flavors.
• All oak chips wines had flavors of cinnamon, vanilla, wood, toast, and chocolate that were not present in the control wines.
• Wine treated with 6g/L of oak chips during malolactic fermentation had the most oak-like qualities compared with all other wines.
• All oak chip treatments reduced astringency in the wines.
• Wines treated with oak chips during malolactic fermentation had greater body than all other wines.
• Principle component analysis (PCA) grouped wines most similar to each other into two groups: 1) both wines with oak chips added at alcoholic fermentation and control wines; 2) both wines with oak chips added at malolactic fermentation and both wines with oak chips added post-fermentation for one week.
o The second group showed significantly more oak character than the first group.

Conclusions

The results of this study showed that the addition of oak chips at different times during the winemaking process resulted in finished wines with significantly different aromatic and flavor profiles. The addition of oak chips at any time resulted in wines with more oak-like character, however the amount of oak character was dependent upon the dose of oak chips as well as when during the winemaking process the oak chips were added. It was noted that astringency was also reduced when oak chips were added, and body was only affected when the oak chips were added during malolactic fermentation.

The authors suggested that any of the treatments would be acceptable alternatives to using an oak barrel, however, depending upon what style of wine

By Agne27 (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

By Agne27 (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

you wish to create will determine how much and when the oak chips should be added. According to the results, adding 6g/L of oak chips during malolactic fermentation produced the wine with the most intense oak flavors, however, all oak chip treatments possessed some oak-like character.

One treatment that I feel was missing from this experiment was the oak barrel treatment control. The results showed that oak chip treatments resulted in “oakier” wines compared with the stainless steel control, however, how does it compare with wines that are actually fermented and/or aged in a barrel? Is it comparable? A lot less?

It would also be interesting to see how wines made from other grape varieties are influenced by oak chip addition at different stages during the winemaking process.  Will we see similar results?  Or will we see different treatments emerging as the “oakier” style wines?

One other thing I would have liked to have seen is preference scores by the sensory panel. They described the aroma and flavor characteristics of each sample; however, they did not score whether or not they preferred one particular wine over another. Everyone has different tastes and preferences, of course, but it would have been at least somewhat interesting to see if the panel actually liked these wines or not.

I’d love to hear what you all think of this topic! Do you have any personal experiences you can share regarding oak chip-treated wines? What sort of future research would you like to see coming out of this study? Please feel free to comment!

Source: García-Carpintero, E.G., Gómez Gallego, M.A., Sánchez-Palomo, E., and González Viñas, M.A. 2011. Sensory descriptive analysis of Bobal red wines treated with oak chips at different stages of winemaking. Australian Journal of Grape and Wine Research 17: 368-377.

Posted in Enology

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Using Microwave Technology to Eliminate Spoilage Organisms in Oak Barrels: A Novel Approach

Posted on February 18, 2013 by Becca| 6 Comments

 

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As many, if not all, of you reading this blog already know, the use of oak in winemaking is a very common practice for adding complexity and quality to a wine. Using oak is typically more expensive than using stainless steel tanks, as the cost of the equipment is simply much higher. As a result of this added cost of using oak barrels, winemakers will often use the same barrel for multiple vintages. Of course, using older barrels is also a style choice and not always a financial necessity, as older barrels give a more delicately oaked wine that is well desired among many wine consumers.

All that being said, using a barrel multiple times brings up the issue of having to clean it properly in order to avoid contaminating the next wine that resides inside of it. The porous structure of wood easily allows microbial and other spoilage organisms to “set up camp”, if you will, and is extremely difficult to remove via the current cleaning systems in place at wineries all over the globe. In fact, the washing process will typically only remove the larger particles and

By Maxdesbacchus (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

By Maxdesbacchus (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

organisms on the surface of the oak, and can easily miss those organisms that are living deeper inside the wood staves. Many organisms, such as the spoilage critters Brettanomyces, can develop into very large colonies while only starting from a very small number of cells. So, even if you think you are cleaning the barrel thoroughly, you could miss just a couple of little buggers and they’ll still propagate and thrive to spoil your next batch of wine by causing off-aromas and unpalatable sensory characteristics.

Since the current method of cleaning and sanitation of wine barrels is relatively sub-par, a method that allows for the removal of all organisms from all crevices of the barrel needs to be found. There is some research on the topic; however, none of the methods so far have been completely effective in reducing spoilage organisms in the oak barrels. The authors of the study presented today introduced the use of microwave technology in order to remove the spoilage organisms from the cracks and crevices of the oak barrel. They claim that this technology could not only potentially solve the spoilage microorganisms problem in oak barrels, but could also reduce the levels of SO2 needed in wine

By Mk2010 (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

By Mk2010 (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

since it would no longer be needed to prevent the spoilage usually caused by oak barrel contamination. They also mention that using microwave technology to kill the spoilage organisms in oak barrels would result in the lowering of water consumption and energy costs since the usual high-energy blasts of water would no longer be needed to remove the spoilage organisms.

Methods

The equipment used to sanitize the oak was based off a “pulse train generator of high frequency microwaves”. Oak was exposed to the microwaves for 3 minutes at 3000 watts. Over the three minutes, the following treatment was applied: 0-90% for 10 seconds, followed by 90% for 40 seconds, followed by 90-0% for 10 seconds, all repeated 3 times for a total of 3 minutes. The authors noted that the maximum temperature that occurred on the oak was 48oC.

The equipment used in the experiment was only large enough to treat 30cm length oak staves, though ideally the commercial product would be large enough to treat the entire barrel.

The barrels used in this experiment were: a 3 year old American oak barrel, and a 2 year old French oak barrel. Both were confirmed to be highly contaminated with spoilage organisms from previous winemaking and storage. The barrels were first washed with hot water vapor pressure and then were broken down into staves. Half of the staves were randomly assigned to the microwave treatment, while the other half were assigned to the control treatment (no microwave exposure). Treatments were performed in duplicate.

Microbial analysis was performed by collecting wood scrapings from each of the staves to a depth of 8mm.

Results

• The most prominent organism found in both microwave-treated oak staves and control oak staves was Brettanomyces bruxellenxis, representing 99.6% and 98% of the respective microbial populations.
• Even after being washed with hot water vapor pressure, the staves from both American and French staves in the control treatment had a high level of microbial contamination.
o Total cell counts found in control oak staves were: 4.16 log units of total yeasts, 3.27 log units of Brettanomyces, 2.15 log units for lactic acid bacteria, and 2.48 log units for acetic acid bacteria.
• The microwave treatment on the oak staves resulted in significantly fewer microbial population counts than the control treatment.
o Total cell counts found in the microwave-treated oak staves were: 3.24 log units of total yeasts, 2.74 log units of Brettanomyces, 0.20 log units of lactic acid bacteria, and 0 log units of acetic acid bacteria.
• The microwave treatment resulted in a 36-38% reduction in the yeast population, a 35-67% reduction in the Brettanomyces population, a 91-100% reduction in the lactic acid bacteria population, and finally a 100% reduction (i.e. total elimination) of the acetic acid bacteria population.
• The percentage of reduction of microorganisms was larger in American oak barrel staves, which the authors deduce is possibly a result of the greater porosity of French oak barrel staves allowing for greater wine infiltration and thus greater difficulty in cleaning and sanitizing the barrels.
• The microwave treatment did not affect the chemical composition and quality of the wood itself.

Conclusions

The results of this study are fascinating in that microwave treatment of oak barrel staves effectively reduces yeast populations inside the pores of the wood, and nearly eliminates the lactic and acetic acid bacterial populations. While it doesn’t completely eliminate all of the spoilage microorganisms responsible for creating off flavors and aromas in wine (i.e. Brettanomyces yeasts), it does significantly reduce them, and in certain cases remove them all together (i.e. lactic and acetic acid bacteria). According to the authors, this microwave treatment is a significant improvement over any of the current cleaning and

Photo by Bernt Rostad: http://farm4.staticflickr.com/3457/3246033875_f9556cf61b.jpg

Photo by Bernt Rostad: http://farm4.staticflickr.com/3457/3246033875_f9556cf61b.jpg

sanitizing methods for removing potential contaminants in oak barrels.

I agree with the authors when they suggested perhaps combining the microwave treatment with another method for microorganism removal, such as laser, ultrasonics, or UV radiation, all of which are currently being studied as possible methods for contaminant removal. Perhaps the combination of one or more of these methods would create a synergy that could remove significantly more of the microbial population than either one on their own. Microwave treatment appears to nearly eliminate all of the lactic acid and acetic acid bacteria in the oak barrel staves, so combining it with a method that effectively reduces the yeasts populations may result in the “perfect storm” of spoilage organism removal techniques.

Determining if a system could be created that would effectively reduce or eliminate the spoilage organism populations in whole oak barrels, and not simply oak barrel staves, is of utmost importance. Real world applications of this technique will occur in the whole barrel, so it is important to determine if this method will work in this scenario, or if the system needs to be altered or tweaked in any way, in order to remain effective in this different scenario.

The authors mentioned that the microwave treatment did not affect the chemical composition of the wood itself, but only affected the microbial population. I would like to see a sensory analysis of wine fermented and/or stored in these microwaved barrels (compared to controls) in order to confirm that, in fact, the chemical composition of the barrels did not change.

If this technique can be applied on a larger scale (i.e. at the intact whole barrel level), it could be a very good investment for the wine industry, particularly if a combination treatment is found to eliminate not only the bacteria populations but also the trouble yeasts. Not only would the winemaker not have to worry about spoilage in their wines (theoretically), but they would not have to use as much SO2 for protecting against these potential contaminants, and they would not be wasting nearly as much water on the hot water vapor pressure washing process. Good for the winemaker, good for the consumer, and good for the environment (theoretically!).

I’d love to hear what you all think! Please feel free to leave your comments!

Source: González-Arenzana, L., Santamaría, P., López, R., Garijo, P., Gutiérrez, A.R., Garde-Cerdán, T., and López-Alfaro, I. 2013. Microwave technology as a new tool to improve microbial control of oak barrels: A preliminary study. Food Control 30: 536-539.

Posted in Enology

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Applying Oak Extract to Petit Verdot Vines: Making Oaked Wines Without Oak Barrels?

Posted on February 4, 2013 by Becca| Leave a comment

 

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Welcome to The Academic Wino!  If you are new here, please read the “About Me” page to find out more about myself and the blog. If you would like to receive free updates on articles like this by email, then sign up here or you can subscribe to the RSS feed. Also, check us out on Twitter, Facebook, Google+, and or Pinterest. Thanks for visiting!

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About a year and a half ago, I reviewed an article that examined the effect of the application of an oak extract to Verdejo grapes on the potential oak flavor of the finished wine, in lieu of oak barrel fermentation or ageing, or the use of oak chips in stainless steel tanks (to read that full article, click here).  The main take-away from that article was that oak extract application on Verdejo grapes does, in fact, produce a finished wine that mimics the flavors and aromas of a wine that has been aged in oak barrels.

I had one reader comment: “who would want oak on Verdejo anyway?”; which is certainly a valid question!  Who knows! Maybe Verdejo was what the authors had at their disposal at the time.  Anyway, those same authors about a year later published a very similar experiment, however this time, focusing on a grape that is almost exclusively oaked: Petit Verdot.

By Eric 先魁 Hwang [CC-BY-2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons

By Eric 先魁 Hwang [CC-BY-2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons

Studies (and experience) have shown that grapes exposed to smoke from wild fires result in wines with a burnt or smoky flavoring.  Taking this concept further, we’ve seen in the Verdejo study mentioned previously and other studies that certain compounds sprayed onto the grapes during the growing season (such as oak extract) result in wines with similar aromatic characteristics.  In addition to the grapes themselves, alcoholic fermentation, malolactic fermentation, and the aging process all have influence on the aromatic profile of the finished wine.

In regards to the aging process, oak aging in particular has a significant influence on the aromatic development of the wine (compared to the stainless steel alternative).  Sometimes aging in oak barrels can be problematic for some vineyards, due to the relatively high cost of a single barrel and the long periods of aging time required to produce a wine with certain aromatic characteristics.  Smaller wineries may not have enough money and/or time to be able to age their wines in oak barrels for these very reasons.

One alternative approach is to use oak chips in stainless steel tanks, though this practice is still thought of as poor-quality and less desirable (though in reality, most people can’t tell the difference).  Another alternative, which is the focus for today’s paper, is to instead apply oak extracts directly to the grapevine, which could result in wines retaining the characteristics of these extracts, thus eliminating the need to age the wines in relatively expensive oak barrels.  This

Photo by Bernt Rostad: http://farm4.staticflickr.com/3457/3246033875_f9556cf61b.jpg

Photo by Bernt Rostad: http://farm4.staticflickr.com/3457/3246033875_f9556cf61b.jpg

alternative is still relatively novel, and is still in a more experimental phase than a widely practiced phase.

Methods

The oak extract used was food quality French toasted aqueous oak extract from Protea France S.A.S.

The grapevines used were the Petit Verdot variety grown in La Mancha region of Spain (southeast Spain).

There were 4 different treatments used in this experiment: 1) a 25% dilution of the oak extract applied once on the 7th day after veraison (when the grapes start changing color); 2) a 25% dilution applied 4 times; on the 4th, 7th, 10th, and 13th days after veraison; 3) an undiluted (100% concentrated) oak extract solution applied once on the 7th day after veraison; and 4) an aqueous solution of eugenol and guaiacol standards applied once on the 7th day after veraison.  An untreated control was also used for this experiment.

Treatments were applied to 10 plants in the same row, with 5 plants left untreated between treatments in order to avoid contamination.  250mL of each treatment was sprayed on the leaves of each plant at around 7am and when the temperature was below 20oC.

After grapes were harvested at maturation, grape yield was calculated.  Winemaking took place in a multitube fermenter, which has been shown to reproduce actual cellar and winemaking conditions.  Wines underwent both alcoholic and malolactic fermentation, and were done in duplicate.  After malolactic fermentation was complete, wines were stored bottles at 14oC for 8 months.  Samples were taken at each fermentation step, as well as after the 8 months of storage time.

The following enological parameters were measured for each sample: yeast assimilable nitrogen (YAN), oBaumé, reducing sugars, alcohol degree, titratable acidity, volatile acidity, pH, malic acid, and lactic acid.  All parameters were measured in triplicate.  Volatile compounds were also measured and analyzed.

Results

Grapes

  • YAN, oBaumé, probable alcohol, pH, titratable acidity, and the ratio of oBaumé to titratable acidity of grapes from the oak extract treatments were not significantly different than the control grapes.
    • YAN levels were higher and oBaumé, pH, and probably alcohol levels were lower in grapes treated with eugenol and guaiacol compared with the controls.
  • No oak lactones or 6-methoxyeugenol were found in any of the grape samples.
  • Furfural levels were higher in oak extract treated grapes, with the 25% dilution extract treated four times throughout veraison having higher levels than the 25% dilution extra treated only once during veraison.
  • The lowest levels of acetovanillone, methyl vanillate, and guaiacol were found in the undiluted (100%) oak extract treatment.
  • The 25% dilution extract treated four times throughout veraison and the undiluted (100%) oak extract had lower levels of 4-vinylguaiacol and 4-ethylphenol compared with the control.
  • Syringol was lower in oak extract treated grapes.
  • The eugenol and guaiacol treatment grapes showed higher levels of eugenol and guaiacol (surprise surprise), as well as furfural, 4-vinylguaiacol, 4-ethylguaiacol, and syringol.
  • Discriminate analysis could not distinguish between the oak extract treated grapes and the control grapes, but could distinguish a difference in regards to the eugenol and guaiacol treated grapes.

Wine

  • Eugenol and guaiacol treatment resulted in the wine with the lowest alcohol level.
  • After alcoholic fermentation, titratable acidity was higher in all treatments compared to the control, however, after 8 months, titratable acidity was the same for all wines.
  • After 8 months, eugenol and guaiacol treatment wines had a lower pH than the control.
  • After alcoholic fermentation, volatile acidity was highest for control wines and lowest for eugenol and guaiacol treatment wines.
  • Malolactic fermentation progressed equally in all wines.
  • There were no oak lactones found in the control wines.
  • Oak lactones were found to be present in all oak extract treatment wines.
  • After alcoholic fermentation, the wine with the highest levels of oak lactones was the 25% dilution that was applied 4 times throughout veraison.
    • After 8 months, these levels decreased in these wines as well as the wines from the 25% dilution that was only applied once.  These levels increased for undiluted (100%) oak extract treatment wines.
  • Concentrations of oak lactones never reached sensory perception thresholds in any of the wines.
  • Furfural and 5-methylfurfural levels were higher in oak extract treatment wines compared with the control wines.
    • Over time, these levels decreased in all wines.
    • Levels of both these compounds were below their sensory thresholds in all wines.
  • After alcoholic fermentation, eugenol levels were higher in oak extract treatment wines compared with the control wines.
    • After 8 months, eugenol levels were actually higher in control wines than oak extract treatment wines.
  • 6-methoxyeugenol levels were higher in oak extract treatment wines than control wines, with the highest levels in the undiluted (100%) oak extract treatment wine. 
    • These levels increased during malolactic fermentation in all wines.
  • After alcoholic fermentation, guaiacol levels were higher in all treatment wines compared with the control wines.
    • These levels increased in the treatment wines after malolactic fermentation.
    • After 8 months of storage, guaiacol levels were 3x higher in treatment wines compared with the control wines.
    • Guaiacol levels were higher than their sensory threshold levels in all wines.
  • 4-ethylguaiacol levels were higher in oak extract treatment wines compared with the control wines.
    • Levels were found to be below sensory threshold in all wines.
  • Syringol levels were higher in oak extract treatment wines than in control wines (recall: this trend was the opposite in regards to the levels in the grapes).
    • Levels were found to be below sensory threshold in all wines.
  • Vanillin, vanillin derivatives, acetovanillone, and methyl vanillate were all higher in oak extract treatment wines compared with the control wines.

Conclusions

According to the results of this study, applying oak extract on Petit Verdot grapes during veraison resulted in changes in the aromatic profile of the finished wines.  Finished wines appeared to have many of the volatile compounds frequently found in wines that are fermented in aged in oak barrels.  It’s interesting to note that nearly all of the oak volatile compounds were not actually present in the grapes themselves, but were present after the fermentation and storage processes.  What is happening here is likely that the oak volatiles are present in grapes as non-volatile precursors, which undergo changes during the fermentation and storage processes that result in the formation of their volatile counterparts in the wine.

Photo by inspector_81: http://farm6.staticflickr.com/5165/5218552773_478a515be5.jpg

Photo by inspector_81: http://farm6.staticflickr.com/5165/5218552773_478a515be5.jpg

In addition to the oak extract treatments, I would have liked to have seen two more treatments:  1) fermentation in oak barrels and 2) fermentation using oak chips.  It would have been interesting to compare the oak volatile compounds from the oak extract treatment wines directly to those from oak barrel fermented or oak chip fermented wines.  How do the levels of the oak volatile compounds in these oak extract treatment wines directly compare with wines fermented in oak barrels or using oak chips?  Sure, the levels are increased with respect to the controls, but is it enough to produce a wine that would taste the same (or similar) to a wine produced in more traditional ways?

The above being said, it would have been interesting to see a sensory analysis of these wines, in addition to the chemical analysis that they performed.  How did the wines actually taste?  How do they actually taste compared to wines made in a more traditional manner?

The results of this study provide some evidence that the use of oak extracts on grapevines during the veraison process yields aromatic profiles similar to that of wines fermented in oak barrels, and thereby could be a good alternative to the more expensive and time consuming oak barrel fermentation methods.  However, before I can say that previous statement with complete certainty, I’d like to see a few more treatment comparisons.

How about you all?  What do you think of this study?  Please feel free to comment!

Source:  Martínez-Gil, A.M., Garde-Cerdán, T., Zalacain, A., Pardo-García, A., and Rosario Salinas, M. 2012. Applications of an oak extract on Petit Verdot grapevines. Influence on grape and wine volatile compounds. Food Chemistry 132: 1836-1845.

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