The Influence of Sour Rot on the Chemical Composition and Overall Quality of Wine

Even with the greatest wine maker in the world, if the grapes are poor quality, the wine will likely be poor quality.  Obtaining high-quality grapes is of utmost importance to vineyard owners and winemakers alike, and avoiding disease and rot is high priority.  The two most common forms of rot are gray rot and sour rot.  Gray rot, caused by the mold Botrytis cinerea, is well-documented and well known across the globe, and causes damage to just under the skin of the infected grape.  Sour rot, which is becoming more prevalent either due to better prevention of gray rot or changes in global temperatures which select for the proliferation of this disease, causes heavy crop losses and may also reduce the quality of the wine made from infected grapes.

In the beginning stages, sour rot is difficult to distinguish from gray rot, since both of them start with the same color change characteristic.  Later on, however, sour rot exhibits a browning of the pulp, which is not a characteristic shared by gray rot.  In addition to browning of the pulp, the following characteristics are seen with grapes infected by sour rot:  disintegration of internal tissues, separation of the grape from the pedicel (stem), leaking juice from higher grapes to lower grapes in the cluster, a vinegary (acetic acid) and ethyl acetate odor, and the presence of fruit flies.  At the end of the entire process, the inside of the grapes are empty and the skins are left dehydrated.

Grapes can become infected with sour rot a multitude of ways, including damage to the skin caused by splitting (from insects, birds, other diseases, other mechanical and physiological injuries, etc), fruit flies (which introduce yeasts and bacteria which speed up the process), climate conditions (i.e. high heat and humidity, which are ideal conditions for bacterial growth), and other organisms that colonize the area after the initial injury (yeasts, bacteria, etc).  Studies have shown that climate change may be partially responsible for the increase incidence of sour rot, as warmer temperatures favor this type of rot over gray rot/Botrytis cinerea, which functions better under lower temperatures.

Sour rot typically affects grapes that are in tight clusters and thin skins, even if they are produced with low canopy densities.  One of the most sensitive grapes to sour rot infection is Trincadeira, which is a red grape that is widely planted in Portugal.  When infected with sour rot, these and any other infected variety of grape undergoes a variety of chemical changes, which may be detrimental to the overall quality of the resulting wine.  These changes include the production of high levels of acetic acid (a.k.a. vinegar), glycerol, ethyl acetate, ethanol, acetaldehyde, galacturonic acid, and gluconic acid.  Ideally, these grapes should be selected against after harvest and before fermentation, in order to minimize the impact on the finished wine.  For smaller wineries, this is done somewhat easily by hand, and with larger wineries where manual sorting is not feasible, poor quality grapes caused by sour rot may be removed automatically by using Fourier transform infrared (FTIR) spectroscopy.  FTIR spectroscopy allows the identification of many factors indicating overall grape health, including gray rot and sour rot, as well as yeast and other bacterial activities.

To date, there has been very little study on the contribution of grapes infected with sour rot on the overall finished wine quality, as well as the chemical, biological, and sensory changes of the wine after aging.  Therefore, the overall goal of the study reviewed today, which was published in the journal European Food Research and Technology this summer, was to characterize the wines produced from grapes with varying levels of sour rot, in order to determine the effect on the winemaking process, in addition to the overall quality of the finished wine.

(Just a head’s up:  even with removing a good number of details and results, this is still a long post.  I always bold the important points, so if you don’t have time to read through it all at first, check out the important bold items first).



Vinifera varieties Trincadeira and Cabernet Sauvignon, grown during the 2007 and 2008 vintages at the vineyards of the Tapada da Ajuda at the Instituto Superior de Agronomia (Lisbon, Portugal) were used for the study.  Healthy grapes (both varieties) and sour rot infected grapes (Trincadeira) were manually harvested and brought to the experimental cellar at the Instituto Superior de Agronomia.

Wine Fermentation and Bottling

In 2007, two treatment types were used:  musts containing 30% rotten Trincadeira grapes, and the control 100% healthy Trincadeira grapes.  To evaluate the effect of initial vatting SO2 concentration, each grape must was added with 30 and 100mg/kg aqueous sulfur dioxide(6%w/v).

In 2008, five treatment types were used:  musts containing 30% rotten Trincadeira grapes, musts containing 50% rotten Trincadeira grapes, musts blended with 70% healthy Cabernet Sauvignon and 30% rotten Trincadeira grapes, a control must containing 70% healthy Cabernet Sauvignon and 30% healthy Trincadeira grapes, and a control must containing 100% healthy Trincadeira grapes.  SO2 was also added to these musts as was done in the 2007 vintage.

Standard winemaking procedures were used, of which to save space, I will omit details (though if you need greater detail, feel free to ask).  During fermentation (in stainless steel tanks), caps were punched down twice per day until it remained submerged.  After fermentation, the pomace was pressed and the juice (free run and pressed) was transferred to another tank, after which spontaneous malolactic fermentation occurred.  After malolactic fermentation, the lees were removed and the SO2 was corrected to 40mg/L.  Wines were bottled without filtration, capped with natural cork, and kept at cellar temperature until analysis.

Chemical and biological analyzes were completed at four different stages: 1) vatting (initial grape must); 2) just after fermentation; 3) after malolactic fermentation; and 4) after bottle aging in the cellar (8 months for the 2007 wines, and 6 months for the 2008 wines).  Samples from each stage were frozen and -20oC until they could be analyzed.

Chemical Analysis

Samples were analyzed in a WineScan FT120 spectrometer, and for grape must samples, the FOSS-supplied GrapeScan Calibration was used.  Other standard laboratory methods were used as a complement to this industry-specific methods.

The following parameters were measured for grape must samples: gray rot index, sour rot index, yeast activity index, lactic bacteria activity index, glucose-fructose content, Brix degree, density, total acidity, volatile acidity, pH, tartaric acid, malic acid, total assimilable nitrogen, anthocyanins, total phenols, and color intensity.

The following parameters were measured for the wine samples: alcohol content, dry extract, residual sugars, total acidity, volatile acidity, pH, malic acid, and lactic acid.

Sensory Analysis

Two sensory analysis sessions were conducted, one for the 2007 wines, and a second for the 2008 wines.

In the first session, 2007 wines after 12 months of bottle aging and the 2008 wines after completing malolactic fermentation were tasted and analyzed.  The panel was composed of 3 women and 5 men between the ages of 23 and 56 (average 42), all of whom were panelists of the Enology Laboratory of ISA (includes enology students, researchers, wine technicians, and winemakers, all of which had substantial background in wine sensory training and experience).

In the second session, 2008 wines were tasted after 6 months of bottle aging.  The panel was composed of 5 women and 10 men, between the ages of 24 and 59 (average 36) from the same Enology Laboratory of ISA.

The following sensory attributes were scored for each wine sample: color (intensity), aroma (intensity, quality, and equilibrium), and taste (intensity, quality, body, equilibrium, persistence, and final taste).  Color, aroma, and taste were judged on a 5 point scale, whereas overall quality was judged on a 20 point hedonic scale.  Judges were not told about the nature of each wine sample.  Wine was presented to the judges in transparent International Standards Organization wine tasting glasses, and was presented in a blind and random order.


Effect of Sour Rot on Yield Parameters

  •       The increase in sour rot contributed to a large loss of free run must after grape crushing.

o   For the 50% sour rot grape samples, a loss of 45% juice was observed (thus the grapes were more dehydrated).

  •       After pomace pressing, there was a slight decrease in wine volume and a clear increase in lees volume were observed after malolactic fermentation.

o   Therefore, sour rot contributes to a decrease in the final wine yield.

Effect of Sour Rot on Grape Must Chemical Analysis

  •       Sour rot increased the Brix and gluctose-fructose content and thus alcohol content.

o   Results showed an average increase of around 0.73% (v/v) of potential alcohol content for grape musts containing 30% rotten grapes.

o   Results showed an average increase of around 1.7% (v/v) of potential alcohol content for grape musts containing 50% rotten grapes.

o   Observed increase in sugar was likely due to concentration factors, as a result of the dehydration observed in rotten grapes.

  •       Sour rot showed an increasing effect of total acidity, which was accompanied by an increase in volatile acidity and a drop in pH.

o   There was a moderate increase in musts containing 30% rotten grapes (0.74g/L), and a large increase in musts containing 50% rotten grapes (1.46g/L).

  •       Tartaric acid decreased and malic acid remained relatively constant with an increase in sour rot.
  •       Free SO2 content significantly decreased with increasing sour rot.
  •      Yeast and lactic bacteria activity indices did not differ significantly between control musts and musts containing rotten grapes.

Sour Rot Effect on Wine Chemical Composition


·         Sour rot increased the ethanol content of wines.

o   Wines produced from musts with 30% rotten grapes had an average increase of 0.9% ethanol (v/v), and wines produced from musts with 50% rotten grapes showed an average increase of 2.4% ethanol (v/v).  This is likely a result of higher sugar concentrations.

·         Sour rot increased the dry extract content of wines.

o   Dry extract reached levels of 44% in the 50% rotten grape wines.  This likely contributed to an observed increase in the proportion of skins and stems, and also the higher residual sugar.

·         At the end of malolactic fermentation, volatile acidity was slightly higher in wine samples made with rotten grapes.

o   However, after aging, there was no difference between control wines and wines made from rotten grapes in regard to their volatile acidity content.

·         Sour rot did not significantly alter pH values.

·         Sour rot influenced the levels of anthocyanins, total phenols, and color intensity, which is likely due to the higher skins plus stems/free juice ratios.

o   Wines produced from rotten grapes showed higher values of total phenols, with the average increase of 13% and 19%, for 30% rotten grape wines and 50% rotten grape wines, respectively.

o   There was an average increase of 12% in anthocyanin content in wines made from grapes that were 50% rotten.

o   Wines produced from rotten grapes initially showed higher color intensity.  However, after aging, both the 30% rotten grape wines and the 50% rotten grape wines showed an increase in color intensity loss over time compared to the color loss in control wines.

Sensory Analysis

  •        2007 wines:  color intensity and overall quality changed significantly.

o   No significant differences were found between the 30% rotten wine with the intial 30mg/kg SO2 vatting and the control wines.

o   When the SO2 vatting was increased to 100mg/kg, there was a significant decrease in color intensity and overall quality in the rotten wine.

§  These results indicate there may be an effect of SO2 on the decrease in color intensity after aging.

  •       In 2008 wines before aging, no significant differences in any sensory attributes were noted between all wine types.

o   In the Cabernet Sauvignon plus rotten Trincadeira blended wines, rot induced a significant effect on color intensity (decrease).

o   This decrease in color intensity was not correlated with an overall decrease in wine quality.

Effect of Sour Rot on Wine Aging

  •       Color intensity significantly decreased in all wine samples after 6 months of bottle aging, with significantly higher decreases occurring in wines with increasing sour rot proportions.
  •       No significant differences were found regarding the attributes of aroma and quality for any varietal or blended wines.

o   Sour rot only influenced changes in intensity-, quality-, and equilibrium-taste related attributes.

  •       Interestingly, the Trincadeira wine with 30% rotten grapes actually significantly increased the quality and equilibrium taste attributes of the wines.


The results of this research indicate that the presence of 30% rotten grapes (by sour rot) do not have significant detrimental effects on the overall quality of the finished wine.  The presence of 50% rotten grapes, while with some attributes was not detrimental, might be avoided, since some attributes were found to have negative effects on overall quality (primarily with color intensity after aging).  Chemically, wines produced from rotten grapes contained higher values of alcohol, dry extract, reducing sugars, total and volatile acidity, anthocyanins, total phenols, and initial color intensity.  However, despite these differences, there doesn’t appear to be any significant differences in the sensory attributes of wines produced with rotten grapes, compared to control wines with 100% healthy grapes.

One thing I find fascinating is that the total phenol levels are significantly increased in wines produced from rotten grapes.  Being that many phenols are associated with many of the health benefits of red wine, could it be that wines produced from sour rot grapes (up to 30% sour rot grapes, anyway) are actually healthier for you than wines produced from 100% healthy grapes?  Who knows!

There were a lot of results presented in this study, and surely there is much more work that needs to be done, however, I feel as though the results presented here should at least put winemakers a little more at ease.  Sour rot is responsible for major yield and crop losses in the vineyard, however, by having up to 30% rotten grapes in the must, there is likely to be no significant negative effects on the overall quality of the wine produced (up to one year bottle aging anyway).  At the very least, winemakers need not worry about removing every rotten grape, and the small amount that makes it into the must (up to 30%) should have no detrimental effects on the overall quality of the wine.

I’d love to hear what you all think.  Do you have any experience with sour rot in the vineyard and subsequent finished wine?  Please feel free to comment below!

Source: DOI: 10.1007/s00217-011-1505-x
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 “The Influence of Sour Rot on the Chemical Composition and Overall Quality of Wine

  1. Greg
    September 17, 2011 at 12:15 pm

    Thanks for posting this paper. I'd be interested to find out how rotten the "rotten grapes" were. We're these end-stage (dried up) clusters or were they at the height of the rot – fruit flies everywhere, ethyl acetate aroma overwhelming?

    Thanks again!


  2. September 17, 2011 at 12:50 pm

    Hi Greg!

    That's a great question–this is the only thing written in the methods about when the grapes were picked: "Healthy grapes from the two cultivars and Trincadeira bunches affected by sour rot were manually harvested at their technological maturity…". As far as they state of their rottenness, there doesn't seem to be any indication of that in the paper. If they made note of this characteristic at all, they didn't publish it. A nice follow-up experiment could include a rotten grape treatment where they test varying levels of rottenness on the quality of wine produced.

    Thanks for reading and commenting! I hope you continue to enjoy my posts!

Comments are closed.