The Effect of Grape Marc Acidification on the Aromatic Quality of Grappa

We’ve discussed on this blog many different uses of grape marc, the material left over after the juice is pressed out of the grapes in order to make the wine, including but not limited to using grape marc extracts as a health supplement for individuals with diabetes and using grape marc extracts as a natural preservative for meats for human consumption.  One thing we’ve yet to cover regarding grape marc, and the thing that is probably the most common use of grape marc in some wine regions of the world, is the production of distilled beverages from these by-products.

The practice of making distilled beverages from grape marc is most popular in the Mediterranean countries and in Italy specifically, they specialize in making Grappa from these “wine leftovers”.  Typically for Grappa made from red grapes, the sugar from the marc is partially fermented and distilled within a few days.  For Grappa made from white grapes, the marc is completely fermented

during a storage period, which may last anywhere from a couple of days to many weeks.  The storage containers employed during this time are those which favor anaerobic conditions, or in other words, containers that do not allow any amount of air to pass through (concrete, steel, etc).

It is during this storage process that chemical processes are occurring, primarily through interactions with yeasts and more importantly, the microbial community present.  The volatile compounds formed during this process contribute greatly to the overall aroma of the Grappa, the quality of which depends on the exact microbial makeup of the marc as it undergoes the production process.  Often times, certain microbial populations contribute more off-aromas and flavors than desirable ones, which is of course something that those making Grappa actively try to avoid.

In order to avoid these off flavors, distilleries have traditionally employed a variety of mechanisms, including adjusting the pH of the marc, adjusting the temperature, and actively managing the microbial community.  Another technique that has become more popular in recent years is acidification of the marc using diluted sulphuric acid.  According to the authors of the study presented today, even though acidification of marcs is becoming commonplace in Grappa making, there has been very little scientific research into how this practice affects the aromatic signature and overall quality of the beverage and no studies have been done examining the microbial populations of these marcs.

The study presented today aimed to fill in these holes in the literature, and performed a pilot study examining the effect of acidification on grape marc microbial populations and Grappa aromatic quality.

Methods

The grape marc used in this study was from Prosecco grapes and were harvested in 2009 from a vineyard in northeast Italy.  After pressing, the marc was separated into two batches of 350kg each.  One batch was used as the control and the other batch was used for the acidification treatment.  For the acidification treatment, 33% v/v H₂SO₄ was added to achieve a pH of 2.94.

Each treatment was stored in 400kg plastic containers at room temperature.  Samples were taken at the beginning, 15 days into storage, and 43 days into storage.  Distillation (thus experiment ending) was 43 days after the marc was initially placed into the containers.

Yeasts and bacteria were identified from 20g samples collected from both treatments using DNA sequencing.

Volatile compounds were measured and analyzed in the distilled grape marc.

pH, sugar content, and alcohol content were measured for both treatments.

Distillation of grape marc was performed in triplicate.

Results

• After 43 days, the pH of the acidified grape marc was between 2.75 and 2.89 and the pH for the untreated grape marc was between 3.87 and 3.92.
• Reducing sugars were consumed in both treatments within the first 15 days.

Yeast Populations

• At day 0, yeast populations were the same for both treatments.
• By day 43, yeast populations were significantly different between the two treatments, with significantly more yeasts cells (5.1×10⁷ cfu/g) populated in acidified marc and significantly fewer yeasts cells (3.3×10⁶ cfu/g) populated in untreated marc.
• At day 0, the most common yeasts in both treatments were H. uvarum and H. opuntiae.
• By day 15, the most common yeasts found in untreated grape marc were Torulaspora delbrueckii, Saccharomyces cerevisiae, Candida humilis, and Issatchenkia occidentalis, with the majority (60%) being S. cerevisiae.
• By day 15 the most common yeasts found in the acidified grape marc were Pichia galeiformis and Issatchenkia occidentalis, with I. occidentalis making up 50% of the population.
• By day 43, the most common yeasts found in both treatments was P. galeiformia making up between 50% and 70% of the yeast population in both treatments.

Bacterial Populations

• At day 0, there were no significant differences in bacterial populations between treatment groups.
• By day 15, bacterial populations were significantly higher in untreated marc (6.2×10⁷ cfu/g) than acidified marc (exact value not given, but it was said to be 1 log higher).
• By day 43, bacterial populations were at the same level for both treatments.

Aromatic Compounds

• The distillate created from untreated grape marc contained significantly higher levels of 2-methyl-2-butanol, 2-butanol, 1-butanol, and 1-propanol.
• Methanol concentrations were equal between the two treatments.
• Certain esters were found to be lower in concentration in acidified grape marc distillate compared with untreated grape marc distillate, including: ethyl acetate, ethyl lactate, ethyl caproate and ethyl caprylate (solvent, buttery, and fruity aromas, respectively).
• There were no significant differences between treatments in regards to concentrations of isoamyl acetate, phenylethyl acetate, isoamyl alcohol, and 2-phenylethanol.
• Prosecco-specific aromatic compounds of linalool, nerol, β-damascone were found in low quantities and were not significantly different in concentration between the two treatments.
• The Prosecco-specifics aromatic compound of geraniol was significantly higher in untreated grape marc distillates than acidified grape marc distillates.

Conclusions

The results of this study indicate that acidification of grape marc prior to distillation reduces some off-flavors caused by certain aromatic compounds that can be formed in Grappa production.  Specifically, acidification effectively reduced the numbers of bacterial colonies setting up camp in the grape marc early in the distillation process, ultimately reducing the aromatic compounds that are formed as by-products from the bacterial metabolic activity.

As a result of acidification of grape marc, ethyl acetate levels decreased nearly 6 times compared with untreated controls, which effectively enhanced the overall quality of the finished Grappa.  Another important effect of acidification on

grape marc was that the 2-butanol and ethyl-lactate levels were reduced, both which contribute to “off flavors” and undesirable aromas in Grappa.

Though the authors concede that they could not accurately evaluate the relationship between acidification and the presence of particular yeast species, they hypothesized that the change in pH could have influenced the yeast strain population in some way, shape or form, which may have resulted in a metabolic change in the yeast that would effectively alter the aromatic compounds produced by those yeasts.

Overall, I thought this was an interesting study that found that acidification of grape marc appears to improve aromatic quality of Grappa by way of altering the yeast-bacteria population dynamics throughout the distillation process.  Of course, as the authors mentioned, this is just a pilot study, and one of which did not have much replication.  I would like to see this study repeated on a larger scale (i.e. multiple acidification and control treatments), in order to confirm or refute these interesting pilot study results.

I’d love to hear what you all think!  Please feel free to comment!

Source: Bovo, B., Nardi, T., Fontana, F., Carlot, M., Giacomini, A., and Corich, V. 2012. Acidification of grape marc for alcoholic beverage production: Effects on indigenous microflora and aroma profile after distillation. International Journal of Food Microbiology 152: 100-106.