An inquiry into the bitterness and hardness of local wines and the prevention of this problem through cellar technological manipulations

Project Number
WW 08-09

Project title
An inquiry into the bitterness and hardness of local wines and the prevention of this problem through cellar technological manipulations

Project leader
Louw, A

Team members
Van Wyk, C J

Project description
There is a perception that local wines and red wines specifically, have an unacceptable bitterness and/or hardness when compared to overseas wines. A number of prominent winemakers agree that a problem exists, but there are differences in opinion as to what the cause of this problem is. The initial aim of this project was to identify the problem and to solve it by means of adapted cellar practices. The project was eventually replanned prior to the 1998 harvest and more attention was given to the specific phenolic composition of the wines and the correlation thereof with bitterness, rather than the problem of hardness.

In 1994, 1995 and 1996, wines were made from Sauvignon blanc and Merlot. Merlot fermented at 30°C was more bitter than the control fermented at 20°C for the first two harvests. Only in 1995 a similar result was found with Sauvignon blanc where the treatment fermented at 20°C was more bitter than the control fermented at 15°C. Out of line with the results of the previous two years, Merlot of 1996 fermented at 30°C, was the least bitter wine.

In 1997 the experiment was done in cooperation with Distell. Sauvignon blanc that received skin contact at 20°C and fermented in stainless steel, resulted in the least bitter wine. No significant differences were observed with Merlot.

In 1998, after replanning and expansion of the experiment, bitter enhancing and reducing winemaking practices were introduced, water stress was investigated during ripening of the grapes. A variety of cultivars were investigated. The only winemaking practice that led to enhanced bitterness in Gewürztraminer wines, was the addition of seeds to juice prior to fermentation. Bitter reducing treatments in Muscat d’Alexandrie wines included extended lees contact and whole bunch pressing. Water stress during ripening caused bitterness in Sauvignon blanc. Extended pomace contact resulted in more bitter Pinotage wines in comparison to treatments where juice was removed prior to fermentation. Both treatments did not differ significantly from the control.

In 1999 wines were made of Pinotage and Chenin blanc, but no significant differences in bitterness were found among treatments. In 2000 significant differences occurred only in Chenin blanc where wines of the skin contact treatment at 20°C were more bitter than those prepared from whole bunch pressing. This confirms results found in 1994, 1995 and 1998. The total hydroxicinnamic acids and total flavonoid phenols in these wines were also higher and confirm tasting results of bitterness found in these wines. Analyses were carried out on the 2000 red wines, using high performance liquid chromatography. (+)-catechin and (-)-epicatechin appeared in relatively low concentrations in these experimental wines. These results confirmed those found in the literature. The possibility that these substances are responsible for bitterness in South African red wines is hereby probably eliminated.

In conclusion, we can say that bitterness could not always be successfully changed by cellar practices. Bitterness of wines did not correlate with total phenol concentration in both white and red wines. In white wines a better correlation with bitterness was found in total hydroxicinnamic acids and total flavonoid phenols. Although panels were trained and selected to taste bitterness, variances among individuals do occur which complicates the study of bitterness. The introduction of new tasting techniques as described in the report, sharpened their identification of bitterness and made it possible to evaluate the consistency of the tasters. Further research should be done on bitterness as a result of the actions of micro-organisms, as well as on the role of individual phenols in the occurrence of bitterness.

It therefore seems that factors such as ripeness, pH, ethanol concentration, fermentation temperature, grape seeds, water stress during ripening, extended lees contact and extended pomace contact may all influence bitterness in wine. Recommendations to the industry are that bitterness occurring in South African red wines is caused by microbiological activity (Rentschler and Tanner, 1951). Initial tests confirm the role of acrolein in bitterness of red wine. The role of micro-organisms in bitterness of wine will be investigated in project WW 10-14. Until the project produces results and recommendations, the wine maker is advised not to harvest over-ripe grapes. Low pHs are necessary for optimal inhibition of bacteria by sulphur dioxide. Very high fermentation temperatures must be avoided. Red wines must be racked and filtered as soon as possible after malolactic fermentation. A pH of below 3.8 must be maintained in the wine and sulphur dioxide must be added at sufficient levels for the pH of the wines. These practices will greatly reduce microbiological spoilage of wine.

FinalReport.pdf

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