Examining Variations in Grape Phenolic Maturity and Ripeness at Harvest and How It Influences Wine Quality

It is well known among grape growers and winemakers that the phenolic maturity of the grapes at harvest significantly impacts the overall quality of the finished wine. Specifically, the aroma, flavor, mouth feel, and astringency (to name a few) are all tied in with the composition of phenolic compounds in grapes and wine, thus are strongly influenced by grape ripeness or a lack thereof. Studies have found that certain phenolics in the skins of unripe grapes are less extractable and certain phenolics in the seeds of unripe grapes are more extractable than those in the skins and seeds of fully matured grapes, thus resulting in altered flavor and aroma of the finished wine.

The current demand from consumers in terms of ideal red wine characteristics are wines with a dark red color, full body, soft tannins, and ripe fruit flavors and aromas. In order to create wines with these types of characteristics, winemakers need to use fully ripened grapes, specifically those grapes that have reached “phenolic maturity”. It is because of this that there has been a lot of work done looking for a method or system to test grapes in the field to determine their ideal harvest date when phenolic maturity has been reached. There has been some progress in this field of research, however, according to the authors of today’s paper, these methods rely on the average values of a sample of grapes in the vineyard, and don’t take into account the variability of the phenolic maturity of grapes within that sample. A large variability in grape phenolic maturities could spell trouble for a winemaker attempting to create the “ideal wine” for consumers.

When grapes ripen throughout the season, each and every grape does not ripen at the exact same rate. A lot of factors go into this variability, from weather to vineyard management practices and even to the specific location on the cluster. As a result of this variability in ripening rates, there is the potential for there to be a large variability in phenolic maturities of the grapes at harvest, even when the average values indicate it’s about time to pick. According to the authors of today’s study, no one has ever looked at this variability (in other words, heterogeneity) of phenolic maturities in grapes, nor has it ever actually been quantified.

Thus, the goal of the study presented today was to evaluate the variability or heterogeneity of the degree of grape ripeness (degree of phenolic maturity) and how this variability affects wine quality and phenolic composition.

Methods

Grapes used for this study were Cabernet Sauvignon from experimental vineyards at Constantí owned by the Rovira i Virgili University in Spain. Grapes were studied during the 2007 and 2008 vintages and were harvested from the two central rows in this experimental vineyard.

600 grapes were collected randomly at 1, 3, 5, and 7 weeks after veraison (i.e. when the grapes start to change color) and analyzed. Sugar content, titratable acidity, pH, weight, and ripening heterogeneity of a portion of those grapes were measured.

Grapes were harvested 7 weeks after veraison for winemaking purposes and were separated into three different groups based on their densities. Grapes were then crushed, and then underwent typical red winemaking procedures (including a 14 day maceration period). After bottling, wines were stored at 15oC until ready for analysis.

The following were measured for grapes: sugar content, probable alcohol degree, titratable acidity, and pH; and the following were measured for the finished wines: ethanol content, titratable acidity, and pH. Also measured were: color intensity, lightness, chroma, hue, red-greenness, yellow-blueness, total color difference, total anthocyanin content, flavanol content (including catechins and oligomeric proanthocyanidins), total phenolic index, and astringency index.

A sensory analysis was performed on all wines by a panel of 10 “expert enologists” from Rovira i Virgili University after a 6 week storage period after bottling. Dark tasting glasses were used so the appearance of the wine color did not have any influence on the panel’s scores. Wines were compared in pairs, with the pairs being made up of two of the following: low density wine, medium density wine, and high density wine. The panel was asked to determine if they could tell a difference between the two samples and also what their favorite sample was and why.

Results

• As expected, during the growing season, sugar content, probable alcohol degree, pH, and berry weight increased, while titratable acidity decreased.
• The 2007 vintage grapes reached a greater phenolic maturity than the 2008 vintage grapes, though both years were considered “normal” years.
• Significant heterogeneity in phenolic composition and maturity of grapes was noticed from the start of the sampling period all the way through harvest.
o According to the authors, this means that are likely many unripe grapes mixed in at harvest prior to the winemaking process, which could increase bitterness and astringency due to lower sugar content, higher acidity, lower anthocyanin levels, and higher seed tannin levels, ultimately lower the quality of the finished wine.
o They suggest that these results indicate a significant influence of grape heterogeneity on phenolic composition of wine.
• As grape density increased, ethanol content and pH of wines increased, and titratable acidity decreased.
• Grape density significantly affected wine color.
o Higher density grapes resulted in wines with higher color intensity, chroma, and red-greenness as well as lower lightness levels.
o Hue and yellow-blueness levels were not consistent from year to year.
• The sensory panel was able to tell the different in wine color of wines from different grape densities just by using the naked eye.
o According to the authors, these results indicate that grape heterogeneity significantly impacts wine color.
• Grape density significantly influenced the anthocyanin content in the wines.
o As grape density increased, anthocyanin content increased (for both free and combined with flavanol forms)
o The authors noted that the presence of unripe grapes (i.e. less dense) at harvest could significantly impact the anthocyanin content of the finished wine.
• As grape density increased, proanthocyanidin concentrations and astringency increased.
o Denser grapes resulted in greater proanthocyanidin levels, which according to the authors could result in greater tannin levels, indicating a greater ability for aging than less dense grapes with lower levels of proanthocyanidins.
• As grape density increased, (+)-catechin decreased and (-)-epigallocatechin increased.
• Grape density significantly influenced proanthocyanidin monomer and oligomer concentrations.
o As grape density increased, monomers and dimers increased.
• Only 50% of the sensory panel could tell the difference between the medium and high density wines, while 100% of the panel could tell the difference between the low and high density wines.
o Overall preference was unanimous for the higher density wines, which the panel indicated had greater fruit and floral notes, as well as having better balance, lower acidity, and less bitterness than low density wines.

Conclusions

According to the results of the study and the author’s interpretations, grape ripeness heterogeneity has significantly more influence on finished wine quality than initially thought. Specifically, less ripe grapes (i.e. indicated by the lower density grapes in the study) can significantly influence the phenolic composition of the finished wine, and ultimately the overall quality of that wine. Basically, lower density grapes (i.e. less ripe grapes) lower the ethanol content, pH, anthocyanin concentrations, color intensity, total phenolic index, and proanthocyanidin concentrations, as well as increase the titratable acidity of the finished wine, resulting in an overall lower quality wine. By mixing in these lower density grapes in with the higher density grapes, the overall quality of the finished wine will most certainly be lowered some.

While only half of the sensory panel could differentiate between the medium and high density wines based on taste alone, 100% of the panel could tell the difference between the low and high density wines, indicating that grape ripeness heterogeneity is an extremely important factor in determining overall wine quality. Even though not all the panelists could tell the difference between the medium and high density wines based on taste alone, they were able to tell the difference based on color, so adding those two factors together could potentially decrease the overall acceptability and likelihood of purchase of even medium density (or not as many unripe grapes) wines (this was not tested—just my theory).

According to the authors, more research on grape phenolic maturity heterogeneity is needed, as well as methods or protocols for filtering out these less ripe grapes either in the field or just prior to the winemaking process. While the average phenolic maturity of the grapes may indicate it’s time to harvest, the fact that the grape to grape variability or heterogeneity is so vast indicates that there will likely be some lowering of quality if those grapes are mixed in with those truly ripe grapes.

Perhaps a method to quickly sort lower density grapes away from the higher density grapes would be most effective at reducing the heterogeneity of phenolic maturity of the grapes destined for making wine, be it through mechanical or digital means. My first thought was some sort of equipment or attachment to existing equipment that allows for density sorting in a similar manner that was done in this experiment. Those grapes with higher density would sink to the bottom of the machine, while those with less density would stay afloat, thus allowing someone to either scoop out the less dense grapes or have them funneled separately to a different container.

Overall, I thought this was an interesting study and certainly one that deserves follow up investigations based on its important findings. The entire concept of grape phenolic maturity variation and heterogeneity deserves more research, as well as applicable methods for sorting out these less ripe grapes either prior to harvest or just prior to the winemaking process in order to improve the chances of making a higher quality wine.

What do you all think of this study? Do you already employ methods for sorting out these less ripe grapes prior to winemaking? Please feel free to share your thoughts and/or experiences!

Source: Kontudakis, N., Esteruelas, M., Fort, F., Canals, J., De Freitas, V., and Zamora, F. 2011. Influence of the heterogeneity of grape phenolic maturity on wine composition and quality. Food Chemistry 124: 767-774.