Influence of Toasting on Antioxidant Capacity and Phenolic Composition of Oak-Aged Wine

There have been a significant number of studies focused on examining the antioxidant power of wine, studies of which are no stranger to The Academic Wino.  Antioxidants and other polyphenols in wine have been shown to be linked to a wide variety of health benefits, including potentially alleviating the symptoms of essential tremor, dementia/Alzheimer’s, and decreased risk of cardiovascular disease.  Red wine has frequently been studied and has been confirmed to have many health benefits due to the high concentrations of polyphenols within it.

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One enological method employed to increase the antioxidant capacity in wine is the aging process.  Antioxidants and polyphenols from the wood are leached out into the wine, which when increase the overall concentrations of these compounds in the final wine.  Other enological techniques also change the antioxidant components of the wine, thereby making the determination of exactly how much antioxidant capacity is provided by the oak barrels extremely difficult if not impossible.  As a result of these difficulties, there have been no studies investigating the levels of antioxidants provided by the oak barrel itself.

The study under review today, which was published earlier this year by Alañón et al in the journal Food Chemistry, aimed to investigate the antioxidant capacity taken up by wine from the oak cooperage for aging, and how toasting the barrels affects these levels.  The study also employed four different methods for analyzing the antioxidants, in order to validate previous studies and to determine how correlated each method was to one another (i.e. are results consistent across studies that use different data collection techniques?).

Methods

Oak samples were created by taking 2cm x 1cm x 0.1cm shavings from American, French, Hungarian, Romanian, and Russian wood.  All oak wood shavings were supplied by the Magreñan S.L. cooperage in La Rioja, Spain.  Oak wood shavings were seasoned naturally in the open air, with half of the samples remaining untoasted, and the other half undergoing the toast treatment.  A medium intensity toasting treatment was applied to those shaving slated to undergo said treatment.

For the wine, a model wine solution was created, in order to control for all other vinification techniques and to get a sense of how oak wood ONLY affects antioxidant levels in wine.  The model wine solution was a mixture of 12% ethanol v/v adjusted to a pH of 3.5 with tartaric acid.  14 grams of each wood sample was added to a liter of the model wine, prepared in duplicate; and shaken daily.  The model wine was filtered after 3 weeks, and then kept in the refrigerator until analysis.

Phenolic compounds were measured using a UV-vis spectrophotometer.  Phenolic acids were analyzed using HPLC methodologies.

Antioxidants were determined via four methods:  the DPPH assay; the ABTS assay; the FRAP assay, and the ORAC assay.  (If you’d like to know more science behind each assay, just ask).  Each assay was measured four times, in order to test the reproducibility of each assay.

Results

  •       Non-toasted oak samples resulted in model wine containing higher antioxidant capacity values than model wine made with toasted oak samples.

o   Basically, what this means is that toasting oak barrels results in a reduction of antioxidant capacity of the barrel.

o   The DPPH, ABTS, and FRAP methods all found these significant differences, however, the ORAC assay did not find any significant differences of this nature.  The latter assay results are the result of a difference in chemistry principle of that particular method, compared to the rest, the results of which are all highly correlated with one another.

  •       The total phenol index showed a highly positive correlation with antioxidant capacity.

o   i.e.; the greater the antioxidant capacity, the higher the total phenol index (the higher the concentration of phenolic compounds).

o   This result indicates that the total phenol index in an important contributor to antioxidant capacity in model wines treated with oak wood samples.

  •       The toasting process caused major changes in the phenolic composition of the model wine.

o   After toasting, increases were observed for the following phenolic compounds: protocatechuic aldehyde, vanillin, syringaldehyde, coniferaldehyde, sinapaldehyde, vanillic acid, ferrulic acid, sinapic acid, and ellagic acid.

§  These increases are due to thermal degradation of lignin in wood, which occurs during the toasting process.

o    After toasting, decreases were observed for the following phenolic compounds:  gallic acid, protocatechuric acid, caffeic acid, and scopoletin.

§  These compounds are very sensitive to thermal exposures, and levels significantly decrease after toasting.

§  Toasted oak samples showed significantly lower levels of ellagitannins than non toasted oak samples.

  •       No correlations were found between antioxidant capacity and phenolic aldehydes (i.e.; protocatechuic aldehyde, vanillin, syringaldehyde, coniferaldehyde, sinapaldehyde).

o   This means that these compounds do not contribute to the overall antioxidant capacity of oak wood.

  •       There was a significant correlation between antioxidant capacity and some phenolic acids (i.e.; gallic acid, protocatechuric acid, caffeic acid and p-coumaric acid).

o   This means that these compounds are major players in the overall antioxidant capacity of oak wood.

  •       There were significant correlations between antioxidant capacity and all ellagitannins.

o   This result leads the authors to believe that ellagitannins are the compounds most responsible for the antioxidant capacity of oak wood samples.

What does this all mean?

Based on the variability in some of the collection assays, the authors suggest that multiple methods be used for this type of analysis, in order to show consistency in results and thus make findings more credible.

To sum up the main results of this study, they show that the antioxidant capacity of oak wood samples is strongly correlated with the phenolic composition of the wood.  Model wines made with oak that did not undergo toasting treatment was shown to have more antioxidant capacity (or “power”, in their words) than model wines made with oak that did undergo toasting.  Therefore, if a winemaker wants to maximize the antioxidant potential of their wines, using non-toasted oak barrels for the aging process would be ideal.

One piece of data that was never really mentioned, and a piece of the puzzle that I’d be interested in, is whether the different types of oak used had similar antioxidant capacities, or if certain types of wood from different parts of the world provided different antioxidant levels than others.  In the methods, the authors indicated that they used America, French, Hungarian, Romanian, and Russian wood, though they never mentioned it again for the remainder of the study.  It is possible that they did not have enough replication to run any statistical analysis.  I think that would be an interesting next step in this line of research.

This was a very simple study that provided some very interesting results regarding the influence of oak phenols on the antioxidant capacity of wine.  Using the model wine, while extremely important in determining the exact influence of oak wood phenols ONLY in wine, it does not allow the results to be completely applied to real world situations.  The results show that non-toasted oak barrels will produce wines with higher antioxidant levels, however, that’s if (and ONLY IF) all other factors are equal (i.e. vintage, grape variety, vinification techniques, etc).  It is understood that wine is extremely variable in regards to the grape growing conditions and overall vinification methods, thereby in some cases a wine aged in toasted oak barrels may in fact contain higher antioxidants than wine aged in non-toasted oak barrels.  In the end, however, it is an important study in that the authors found that all other things being equal, the toasting of a barrel has a significant influence on the antioxidant capacity of wine.  It’s only afterwards, when all other things are not equal, that this relationship may become more muddled.

I’d be curious to hear what you all think about this topic!  Please feel free to leave your comments below!

Source: doi: 10.1016/j.foodchem.2011.04.005

 

I am not a health professional, nor do I pretend to be. Please consult your doctor before altering your alcohol consumption habits. Do not consume alcohol if you are under the age of 21. Do not drink and drive. Enjoy responsibly!

2 comments for “Influence of Toasting on Antioxidant Capacity and Phenolic Composition of Oak-Aged Wine

  1. October 5, 2011 at 10:30 pm

    What a captivating blog and I'm going to have to check out your post on alcohol and dementia.

  2. October 5, 2011 at 10:39 pm

    Hi Sandra!

    Thanks for reading and thank you for your kind compliments! It makes me happy knowing my blog is so well received by others!

    I hope you continue to enjoy my posts, and if there is anything wine-related that you are curious about, let me know and I'll try to find some research related to it to review!

    Cheers!

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