Examining the Phenolic Content and Antioxidant Capacity of Grape Leaves: Possible Implications for Recycling and Sustainability in the Wine Industry

The majority of research papers published focusing on wine industry wastes or wine industry by-products have focused primarily on grape marc or grape pomace.  However, in the spirit of sustainability, should we only be considering the fruits themselves as the only source for recycling in the wine industry?  Of course not!  Not only can the fruits (in the form of skins and seeds, primarily) be recycled into other uses, such as for medicinal purposes or for use in other industries (i.e. leather production, food preservatives, additives to wine, etc), but also the water waste produced from wine making as well as the grape leaves from the vines can be utilized and reused for other purposes.

In regards to grape leaves, research has been more limited compared to research on grape pomace.  Studies have shown that the juices obtained from grape leaves can be used for many medicinal purposes due to several biological activities including antibacterial, antifungal, anti-inflammatory, and antiseptic properties.

Grape leaves are also used frequently as food in Greek cuisine, as well as additives in other foods due to the leaves known antioxidant properties.

The ability to recycle the whole plant, and not just from grape skins and seeds, is just another step toward sustainability for the wine industry as a whole.  By reusing and recycling all parts of the plant, the amount of potentially toxic waste entering the environment is dramatically reduced.

The article presented today examined the antioxidant properties of grape leaves in order to confirm if grape leaves are appropriate for use in other industries.

Methods

Leaf samples were collected in July of 2006 from an experimental vineyard in Sendim, Bragança in NE Portugal.  Twenty varieties of Vitis vinifera were studied: 9 white and 11 red.  Each variety was grown in two adjacent rows, all of which underwent the same viticultural treatments (no irrigation or soil treatments).  5 plants in the center of the two rows we selected for each variety, and 4 leaves were harvested from each plant.  Leaves were freeze-dried and then ground into a powder for further analysis.

The following were analyzed for the leaf samples: phenolic content and antioxidant activity (total reducing capacity, reducing power, and scavenging ability).

Results

• Extraction yields were greater for red varieties than white varieties.
  • For whites, the highest yields were found in Chardonnay and Samarrinho.
  • For reds, the highest yields were found in Alicante, Bouchet, Bastardo, and Trincadeira.
• All samples contained the following compounds: trans-caffeoyltartaric acid and trans-coumaroyltartaric acid (both hydroxycinnamic acids) as well as myricetin-3-O-glucoside, quercetin-3-O-glucoside, quercetin-3-O-galactoside, and kaempferol-3-O-glucoside (all flavonoid glycosides).

Why are the leaves be so high in phenolic content?

The authors speculated that the grape leaves were likely rich in polyphenols, specifically flavonoids, since these compounds are known to act as UV filters.  The UV protection characteristic of flavonoids functions to protect the plant cells (particularly chloroplasts) from the damaging effects of UV rays.  In effect, flavonoid function as a part of the plants’ defense mechanisms against harmful UV rays, thus their high levels in the leaves of the plants.

• Total phenolics were found to be higher in the leaves of red varieties than the leaves of white varieties.
  • For red varieties, Tinto Cão had the highest levels of polyphenols whereas Mourisco had the lowest (3.5 times different).
  • For white varieties, Codega had the highest levels of polyphenols, whereas Gouveio had the lowest.
• For all leaf samples, quercetin-3-O-glucoside and quercetin-3-O-galactoside made up 64-73% of the total phenolics.
• Grape leaves appear to have higher polyphenol levels than grapes themselves.

Antioxidant Capacities

• White varieties had higher antioxidant capacities than red varieties.
  • The white varieties showing the highest reducing capacity were Viosinho, Rabigato, Côdega, and Malvasia Fina.
  • The red varieties showing the highest reducing capacity were Tinto Cão and Alicante Bouchet.
• In regards to reducing power, for white varieties, Côdega showed the strongest levels, with Chardonnay and Samarrinho following behind it.
  • For red varieties, Tinto Cão, Rufete, and Touriga Francesa showed the strongest levels.
  • Both Côdega (white) and Tinto Cão (red) showed similar levels, thus there was no clear difference between red and white varieties in terms of reducing power.
• In regards to scavenging activity, there were no clear differences between red and white varieties.
  • For white varieties, Malvasia Rei showed the highest scavenging activity.
  • For red varieties, Tinta Gorda showed the highest scavenging activity.

Why did the varieties with the highest polyphenol content not have the higher antioxidant capacities?

It is typically thought that those items possessing high polyphenol levels will also have high antioxidant capacities.  However, the results of this study found nearly the opposite.  The authors speculated that this result was likely due to the fact that it isn’t all about polyphenols.  There is clearly some other compound or compounds that are interacting with either the polyphenols or other compounds to determine antioxidant capacity.

Conclusions

The results of this study indicate that grape leaves are good sources of polyphenols and antioxidant capacity to be utilized in a multitude of industries.

It was also clear from the results that there was no relationship found between total polyphenol content and antioxidant capacity.  According to the authors, this is likely due to the involvement of one or more other compounds acting either alone, or more likely in concert, with polyphenols or some other compound to affect the antioxidant capacity of the leaves.  These results suggest more work needs to be done to tease out what is the mechanism behind antioxidant capacity in grape leaves.

Overall, this was a straightforward study that clearly shows that grape leaves are an excellent source of polyphenols that could be utilized for multiple industries.  By recycling not only the grapes themselves but also the grape leaves, the wine industry can move one step closer to a more globally sustainable industry.

Source: Fernandes, F., Ramalhosa, E., Pires, P., Verdial, J., Valentão, P., Andrade, P., Bento, A., and Albert Pereira, J. 2013. Vitis vinifera leaves toward bioactivity. Industrial Products and Crops 43: 434-440.