C. S. Du Plessis

Nutritional Deficiencies of Clarified White Grape Juices and Their Correction in Relation to Fermentation

Juice filtrates of grape samples of the cultivars Cape Riesling, Chenin blanc and Colombar from different wine regions of South Africa, were fermented under standardised anaerobic conditions at a temperature of 15°C. Juice clarification reduced fermentation rates but marked differences between the cultivars were observed. Whereas most of the Cape Riesling filtrates fermented dry at a moderate rate, fermentation of the Colombar juice filtrates were strongly retarded or lagged. The Chenin blanc juices fermented at a rate intermediate to these two. It was established that all slow fermentations were caused by nutritional deficiencies of the juices as a result of filtration and that fermentation could be restored to normal by addition of combinations of er~osterol and unsaturated fatty acids.

The effect of juice clarity and several conditions promoting yeast growth on fermentation rate, the production of aroma components and wine quality

In cellar practice grape juice is preferably made as clear as possible, and fermentations take place under strictly anaerobic conditions. In the laboratory, however, the fermentation rate of EK-filtered and de-aerated juices was always so much slower than that of juices obtained by normal cellar routine that no dry wines could be prepared unless fermentation stimulating methods were applied. Apparently in most cellar fermentations sufficient amounts of oxygen and/or of yeast growth promoting steroids from the grapes are present to complete fermentation, but the exact amounts present are unknown. In practice problems with lagging fermentation could occur in cases where clarifying and de-aeration of juice were performed too rigorously. Grape juice was clarified by diverse methods, and white wines were prepared from the juices under various conditions in order to study the effect of de·aeration, addition of settled juice with residual turbidity, micro-aeration and application of ergosterol suspensions on fermentation rate. EK-filtered juices were used to experiment with methods for reactivating lagging fermentation and also for investigating the effect of fermentation conditions on the production of wine volatiles. With the aid of the above-mentioned fermentation stimulating means the fermentation of EK-filtered juices could be completed within an acceptable period of time. Steps to reactivate lagging fermentations should be taken as early as possible, although additions of settled juice were often successful, even when applied at a late stage during fermentation. Filtered juice from different cultivars and origins behaved in a markedly similar way with regard to fermentation and the production of volatile components. Filtration of juice caused partial loss of the cultivar character of the wine but wines from filtered juice with additions of ergosterol were nevertheless allotted the highest scores during organoleptic evaluation. The production of higher alcohols depends on the degree of juice turbidity and fermentation temperature. In clear filtered juice, however, the production of higher alcohols was independent of fermentation temperature. The formation of acetates of higher alcohols is greatly enhanced, by adding ergosterol to the filtered juice.

Optimum Maturity and Quality Parameters in Grapes: A Review'

The quality of wine is without doubt largely dependent on the composition of the berries (Biol, 1977). In studies on grape quality, aspects of physiplogical, technological and industrial maturities have been covered by Kourakou (1974, 1977) and Marteau & Schaeffer (1978). These maturities refer to stages in the grape which are respectively, those where sugar production in the berry has generally ceased, where the grape has reached optimum quality and finally where grapes would give the most economical return. Burger (1977) pointed out the necessity for specifying parameters which could be used to classify grape quality. This also includes the aspect of grape maturity which is a most important facet of grape quality (Carrol el al., 1978) and regarded by some as being even more critical in this respect than viticultural practices (Slesinger, 1975).

Grape Maturity and Wine Quality

Several indices calculated from grape juice analyses were submitted to statistical analyses in order to determine correlations of these indices with quality of wine made from grapes at different stages of maturity. Curvilinear relationships were obtained enabling an optimum point to be calculated relating grape maturity to maximum wine quality. The application of skin contact in the case of white wine cultivars resulted in earlier maxima for wine quality, thus compensating to an extent for earlier harvesting. The cultivars Chenin blanc, Colombar, Pinotage and Cabernet Sauvignon were examined as outlined above over several years.

Differentiation between Wines Originating from Different Red Wine Cultivars and Wine Regions by the Application of Stepwise Discriminant Analysis to Gas Chromatographic Data

Stepwise discriminant analysis (SDA) was applied to gas chromatographic data of some volatile compounds extracted with freon from red wines. By means of this statistical method, Pinotage and Cabernet Sauvignon wines (1978 vintage) could be classified into separate groups, the most effective variables being hexanol and i-amyl acetate. Pinotage wines from three, and Cabernet Sauvignon wines from two localities (1979 vintage) were subjected to classification according to origin. Successful classifications were obtained using the variables ethyl acetate, ethyl caprate, i-amyl acetate as well as a compound tentatively identified as i-valeric acid.

Initial Study on the Guava Aroma of Chenin Blanc and Colombar Wines

An initial study indicated the importance of mercaptan on the aroma of Colombar and Chenin blanc wines. A synthesized mercaptan viz. 4-methyl-4-mercapto-pentan-2-one when added to neutral wine effected a Colombar or Chenin blanc wine aroma. The importance of sulphur components on wine aroma is emphasized.

Interactions between Grape Maturity Indices and Quality for Pinotage and Cabernet Sauvignon Wines from Four Localities

Three grape maturity indices viz. degrees Balling (OB), OB total titratable acid (TT A) ratio and the OB.pH product of musts were compared regarding their ability to predict optimum quality for Cabernet Sauvignon and Pinotage wines. Comprehensive analytical data from 128 musts and corresponding wines for the 1979 and 1980 vintages from the Stellenbosch, Durbanville, Lutzville and Robertson areas were subjected to principal component analysis using variables and variable ratios selected by correlation to quality weighting. The results indicated that 0B alone could not perform the function of a grape maturity index for predicting optimum quality. In the case of the OB/TT A index, the range wherein maximum wine quality occurred was too wide to be of practical value in this instance. The 0B.pH index gave a narrower optimum range, and in contrast to the two other indices gave similar results for both cultivars.

Interactions between Grape Maturity Indices and the Quality and Composition of Chenin blanc and Colom bar Wines from Different Localities

Three grape maturity indices viz. degrees Balling (0 B), total titratable acidity (TT A) and the 0 B.pH product of musts were compared regarding their ability to predict optimum quality for Chenin blanc and Colombar wines. Comprehensive analytical data from 168 musts and corresponding wines for the 1978, 1979 and 1980 vintages from the Stellenbosch, Robertson and Lutzville areas were accumulated, and subjected to principal component analysis using variables and variable ratios selected by correlation to quality weighting. These results indicated that the two cultivars differed appreciably with respect to the value of all three measured indices. From this study it was not possible to select a specific index to predict optimum maturity in all cases. Optimum values for each index differed according to geographic locality.

Cadmium Content of South African Table Wines

Red and white South African table wines from the major wine producing districts were analysed for cadmium content. Only three wines had a slightly higher concentration than 0,010 mg/ t The average cadmium concentration was 0,003 mg/ f, and no relationship between cadmium content and origin could be determined.