Influence of sour rotten grapes on the chemical composition and quality of grape must and wine

In the last post, I wrote about What is Sour Rot, so now that we know all about sour rot, the real issue is, how can sour rot in wine affect it's flavors. The authors of the article, "Influence of sour rotten grapes on the chemical composition and quality of grape must and wine", sought to answer that question through their research.
They took healthy Trincadeira and Cabernet Sauvignon red grape varieties and added grapes affected by sour rot grown during the 2007 and 2008 harvest seasons on the vineyards of the Tapada da Ajuda at the Instituto Superior de Agronomia in Lisbon, Portugal. The grapes were picked clean and then sour rot infected Trincadeira grapes were added to the grapes in calibrated portions.
The grape must was measured for the following analytical parameters:

• glucose–fructose content
• Brix degree
• density
• total acidity
• volatile acidity
• pH
• tartaric acid
• malic acid
• total assimilable nitrogen
• anthocyanins
• total phenols (Folin C index)
• color intensity

Total yeasts, acetic acid bacteria (AAB), and lactic acid bacteria (LAB) counts were performed in four different winemaking stages:

• (1) initial grape musts, before SO2 addition
• (2) after wine fermentation
• (3) after MLF
• (4) after 6 months of bottle aging (2008 wines)

The effect of sour rot on yield:

• sour rot affected grapes resulted in pulp breakdown leading to berry dehydration and loss of berry juice
• large loss in free run must after crushing of grapes
• all sets with sour rotten grapes had higher quantity of stems, when compared with the respective control sets due to the necessity of adding more clusters to obtain the same amount of grapes in each vessel (30 kg/set)
• increase in the percentage of dry parts (skins, stems, and seeds) for the same grape volume leading to a higher concentration of extractable compounds
• after pomace pressing, a slight decrease in wine volume and a clear increase in lees volume were observed after MLF as a function of the proportion of sour rotten grapes
• sour rot contributed to a marked decrease in the final wine yield

Effect of sour rot on grape must chemical analysis:

• increased the Brix and glucose–fructose content and thus the potential alcohol content; observed increase in sugar was likely due to the process of concentration as a consequence of berry dehydration
• a rise in total acidity accompanied with an increase in volatile acidity and a drop in pH values; acetic acid bacteria (AAB) could also lead to a probable accumulation of other sugar degradation products such as lactic, propionic, and butyric acids, resulting in lower pH values
• increase in color intensity, anthocyanin, and phenol quantification, as a probable consequence of berry dehydration and higher skin and stems proportion relative to free juice
• considerable decrease in free SO₂ content with the increase in sour rot

Control set musts containing sound grapes were classified as ‘‘good grapes,’’ while must with 30 and 50% sour rot showed increasing index values, being classified as ‘‘good grapes to slight attack’’ and ‘‘slight attack to severe attack’’.
Two sensory analysis were conducted to determine whether there were sensory differences between sound grape–produced and sour rotten grape–produced wines. During the first sensory session held in 2007, was tasting panel was comprised of 3 women and 5 men during the second sensory session conducted in 2008, the panel was comprised of 5 women and 10 men.
The participants were not informed about the nature of the wine samples. In all cases, 30-mL wine samples were served at room temperature and presented to the panel in individual tasting booths, using transparent International Standards Organization (ISO) wine-tasting glasses, coded with three-digit random numbers. The sample order was randomized across all judges.
The wines were scored for the following sensory attributes:

• color (intensity); 5 point scale (1 = absent; 2 = weakly intense; 3 = intense; 4 = fairly intense; 5 = very intense)
• aroma (intensity, quality, and equilibrium); 5 point scale (1 = poor; 2 = satisfactory; 3 = good; 4 = very good and 5 = excellent)
• taste (intensity, quality, body, equilibrium, persistence, and final taste); 5 point scale (1 = poor; 2 = satisfactory; 3 = good; 4 = very good and 5 = excellent)
• overall quality; 20-point hedonic scale (1 = very bad; 5 = bad; 10 = acceptable; 15 = good; 20 = excellent)

During storage, results obtained in both vintages showed a clear trend for a higher loss of color intensity with sour rot. After 8 months of bottle aging, 2007 wines with 30% rotten grapes showed an average color loss of about 47%, in contrast to the 38% in control wines.
The use of increasing grape sour rot percentages was associated with important changes in the initial chemical grape must composition, expressed by the increase in sugar content, total acidity, volatile acidity, color intensity, anthocyanins, total phenols, and by SO2 binding power.
The bottom line was that when the wines were tasted, after the MLF, rotten wines were not statistically different in terms of color, aroma, taste, and overall quality.
Still, I would not want to many any wine with sour rot.
References:
1. Andre ́ Barata, Alda Pais, Manuel Malfeito-Ferreira, Virgılio Loureiro, "Influence of sour rotten grapes on the chemical composition and quality of grape must and wine", Eur Food Res Technol, (2011), 233:183–194.
2. Illustration from Purdue University, Bruce Bordelon, Grapes: The Sour Rot Situation, August 25. 2016.