Examining the Anthocyanin Content and Antioxidant Capacity of Port Wines

Port wines are created from the Duoro Demarcated Region in northern Portugal, and use several varieties of grapes, including Touriga Nacional, Tinta Barroca, Tinta Roriz, Touriga Francesa, Tinto Cão, and Tinta Amarela.  One important characteristic of Port wines and Port-style wines is that the residual sugar in the finished wine is obtained by stopping the fermentation process early, by way of the addition of brandy (or other wine-based spirit).  The final concentration of alcohol, therefore, in Port wines is around 18% v/v. 

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The aging process is also a very important factor in the creation of Port wines, which includes aging in oak barrels, steel tanks, or within bottle, depending upon the style of Port wine desired.  With Ruby Port wines, they are stored in stainless steel tanks for two years in order to prevent oxidative aging and retention of color.  Late Bottled Vintage (LBV) Port wines are made from a single years’ harvest, and are kept in barrels for 4 to 6 years, obtaining a ruby color.  Tawny Port wines are made from grapes aged in barrels and blended with other grapes from other barrels (of different vintages), which result in the wine being exposed to gradual oxidation and evaporation to produce a golden-brown colored wine.  Tawny Reserve Port wines are labeled without any indication of the age, and are blends of wines that were aged in barrels for at least two years.  As a result of all these different styles of Port wines, chemical composition and color differs markedly, depending upon the winemaking and storage process for each style.

In regards to color, various phenolic compounds in wine that create the colors that we see.  Specifically, anthocyanins are categorized as contributing to the variability of the colors of wines.  Various natural chemical reactions occur amongst the different anthocyanins present within the wines, resulting in the formation of more stable pigments that create wine color, which ultimately changes it to a brick-red color in more aged wines. Not only are anthocyanins very important in regards to wine color, but they also have several potential health benefits as antioxidant agents.

Some researchers believe that due to decreases in phenolic complexity over a period of aging, wine loses its antioxidant capacity, though this hypothesis has seen mixed results.  As a result of these mixed results, and the fact that data on free radical scavenging activity (i.e. antioxidant capacity) and anthocyanin content of Port wines is limited, the study presented today aimed to examine and understand the anthocyanin content of different styles of Port wine, and also to evaluate the antioxidant capacity of these wines.

Methods

Port wines used were made from blends of Touriga Nacional, Touriga Francesa, Tinta Roriz, and Tinta Barroca, from Douro vineyards.  Grape must samples were collected from four different fermentation vats and were collected at two different times during fermentation (2 days and 3 days. 

Commercial Port wines were purchased from a market and included the Port styles of Ruby, Late Bottled Vintage (LBV), Tawny Reserve, Tawny 10, 20 year old Ports.  6 brands of each style were selected, resulting in a total of 30 wines studied.

Anthocyanins and antioxidant activities were measured for each must and each wine.

Results

  • ¬† ¬† ¬† The most abundant anthocyanin in the musts was malvidin 3-O-glucoside (60% in weight)
  • ¬† ¬† ¬† The same anthocyanins were found in Ruby and LBV Port wines as were found in the must.
  • ¬† ¬† ¬† Ruby and LBV Port wines had similar anthocyanin profiles, though there were quantitative differences.

o   A lower percentage of malvindin 3-O-glucoside was found in LBV Port wines.

o   Only traces of anthocyanins were found in Tawny Port wines.

  • ¬† ¬† ¬† Higher anthocyanin content was found in musts with 3 days of fermentation than must with 2 days of fermentation.

o   This result indicates that there is an increase in the extraction of anthocyanins during fermentation.

  • ¬† ¬† ¬† ¬†Anthocyanin content of Port wines was much lower than the content in musts.

o¬†¬† The authors claim this is likely due to the addition of the wine spirit to stop fermentation, as well as the aging process and aging ‚Äúenvironment‚ÄĚ.

  • ¬† ¬† ¬† Higher amounts of anthocyanins were found in Ruby Port wines, followed by LBV Port wines.

o   These wines age in stainless steel tanks and/or sealed glass bottles, which are not exposed to air and undergo reductive aging.  This results in a much slower loss of anthocyanins than other aging methods.

  • ¬† ¬† ¬† Tawny Reserve Port wine had trace amounts of anthocyanins present (if any), with other Tawny Port wines having none at all.

o   Since these styles of Port wines are aged in barrels, the wine undergoes an oxidative aging process due to the permeability of the barrel to the air.  This results in a much faster loss of anthocyanins than other aging methods.

  • ¬† ¬† ¬†¬†¬†The anthocyanin profile of Reserve Tawny, Tawny 10 years, and Tawny 20 years was very similar, and all exhibited the lowest levels of anthocyanins out of all the Port wines.
  • ¬† ¬† ¬† Ruby Port showed high levels of newly formed anthocyanins, and LBV Ports differed in anthocyanin content depending upon the year of production.

o   LBV Ports with 4 years of barrel aging and 6 years of bottle aging had lower anthocyanin levels than samples with shorter aging times in the bottle.

  • ¬† ¬† ¬† Ruby Port wines showed the same anthocyanidins as found in the must, however, the levels of which were significantly lower.
  • ¬† ¬† ¬† Tawny 10 year and Tawny 20 year had similar anthocyanidin profiles when compared with Tawny Reserve.

Antioxidant Capacity

  • ¬† ¬† ¬† Antiradical capacity (indicative of antioxidant capacity) was dependent upon the concentration and style of Port wine.
  • ¬† ¬† ¬† Ruby and LBV Port wines showed higher antiradical activities than Tawny Port wines.

o   This result indicates that reductive aging increases antiradical activity.

  • ¬† ¬† ¬†¬†¬†LBV Port wines showed higher antiradical activities than Ruby Port wines.

o   During aging within the bottle, LBV Port wine composition changes in the reducing environment, with the oxidation-reduction potential decreasing until it reaches a minimum value that prevents any further oxidation reactions.

  • ¬† ¬† ¬† Tawny Port wines had lower antiradical activity than all other Port styles.

Conclusions

Overall, the results of this relatively simple study indicated that the aging process is an important factor that influences the antioxidant capacity of Port wines.  All Tawny Ports showed the lowest levels of anthocyanins, while Ruby and Late Bottled Vintage Ports showed the highest levels.  Similarly, Tawny Port wines displayed the lowest antioxidant capacities, while Late Bottled Vintage Port wines displayed the highest.

The authors did not draw any other conclusions besides these facts (as well as listing some specific anthocyanins present), indicating that this paper was merely a simple stepping stone for future research to be launched.  I’d be curious to see comparisons of different blends within each style, and if there is one particular blend that has superior antioxidant capacities to the rest.  Another interesting comparison would be between Port wines and other wines that are unfortified. 

In a nutshell, it appears, however, that if you love Port wines and are looking for the Port wine with the highest antioxidant capacity (and thus theoretically, a ‚Äúhealthier‚ÄĚ Port), then a Late Bottle Vintage or a Ruby Port is your best bet.¬†

I’m interested in hearing what you have to think about this topic.  What types of research would you like to see done in this field, based on the results of this study?  Feel free to leave your comments below!

Source: Pinho, C., Couto, A.I., Valent√£o, P., Andrade, P., and Ferreira, I.M.P.L.V.O. 2012. Assessing the anthocyanic composition of Port wines and must and their free radical scavenging capacity. Food Chemistry 131: 885-892.

DOI: 10.1016/j.foodchem.2011.09.072
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 “Examining the Anthocyanin Content and Antioxidant Capacity of Port Wines

  1. Clare
    June 27, 2012 at 12:01 pm

    I found your article rather interesting…
    For some time I have been allergic to anthocyanins (this took me a long time to work out!), particularly those found in the skins of red grapes. The resulting migraines are not fun.

    I have recently found, however, that small amounts of balsamic vinegar or other, processed, forms of red wine don't always have the same effect. I was wondering if you might know if some chemical change occurs to the anthocyanins in balsamic vinegar during the later fermentation process?

    I would be interested to know from both a scientific and culinary perspective!

  2. July 4, 2012 at 12:54 pm

    Thanks for your comments, Clare!

    That's terrible that you're allergic to anthocyanins, particularly since they are found in so much plant material (not just grapes!). It's even more terrible that red grape anthocyanins seem to affect you more! I hope you can at least tolerate drinking white wines!

    Good question about the anthocyanin content of balsamic vinegar. I haven't read any entire studies about it (I know they must exist, but I've not had time to hunt one down), but what I have read seems to indicate that anthocyanins are sensitive to pH. Red wine is acidic, but likely in the tolerable range for anthocyanins. However, with vinegar, you get an extremely acidic solution. What I think is going on is that as the pH decreases in the processing of vinegar (i.e. becoming more acidic), the structural morphology of the anthocyanins are degraded and thereby left in such a configuration that will not trigger migraines in your body.

    Think of a lock and key situation–the anthocyanins are the key and the migraines are the lock. Under normal conditions, these two fit together and you get a migraine. If you turn up the pH in the system, the anthocyanins change shape (i.e. are degraded) and the key can no longer fit into the key hole of the lock, thus the migraines are not triggered.

    Again, this is my educated guess, but I think there is a strong possibility that I am correct.

    I hope you're able to manage your allergy without having to completely give up the sweet nectar of the gods! ;)

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