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The influence of pressing intensity and juice treatments in rebate making on the quality of brandy
Material and methods
This experiment was conducted with Chenin blanc grapes. The juice treatments were conducted at Ashton Co-op Wine Cellar in 1997 and repeated in the experimental cellar at Nietvoorbij in 1998.
The juice treatments are given in Table 1.
TABLE 1: Juice treatments1
1Juice analysis - 1997: 19,0 °B TS 7,6 g/l pH 3,01
All the juice samples were fermented dry with the yeast strain WE228 at 18°C, except for treatment 2 which was fermented with NT116. Duplicate samples of 35 litres of juice per treatment were fermented in 40 litre stainless steel containers in the Nietvoorbij experimental cellar. Except for treatment 5, no SO2 was added at any stage during the winemaking process.
Distillation was conducted on a small scale at the Department of Viticulture and Oenology at the University of Stellenbosch. The rebate wine samples were distilled to raw brandy at 28 vol% alcohol in copper stills with 15 litres distilling capacity. The raw brandy was then distilled to rebate brandy at 68 vol% alcohol. Equal volumes of a pre-prepared oak extract were added to each rebate brandy sample, which was then matured for one year.
The quality of the rebate brandies, as well as the matured brandies of 1997 and 1998, was evaluated in duplicate by a panel consisting of 10 and 11 brandy experts respectively. The evaluation was done on an unstructured line scale tasting score card and the results were processed statistically. In addition the matured brandies were evaluated with regard to their acceptability as bottled product.
Results and discussion
The quality of the 1997 rebate brandies did not differ significantly from each other (Fig. 1). In the case of the 1997 matured brandies no must treatment produced products that differed significantly from that of the control (treatment 1). It is interesting to note that the quality of the products made from extra high pressure press juice did not differ significantly from the quality of the control (treatment 1). According to Fig. 2 it seems that the acceptability of brandies that had undergone certain treatments (6, 8 and 9) was lower than that of others (treatment 7). According to Fig. 1 the brandies of the first three treatments were significantly inferior to that of the latter. However, there is no meaningful explanation in terms of juice treatments for this phenomenon.
In the case of the 1998 rebate brandies, the quality of the rebate brandy made from the hyperoxidised and high pressure press juice fined with gelatine and bentonite (treatment 13) was significantly superior to the control (treatment 1, Fig. 3). This can possibly be ascribed to a more effective removal of phenols and possibly also to improved juice clarification by the fining agents after hyperoxidation. The other treatments did not differ significantly from the control. With the exception of the brandy made from hyperoxidised, mild pressure press juice (treatment 4), there were no significant differences in quality between the products of the control and the other must treatments. It is not clear why the brandy of treatment 4 (which was made from hyperoxidised mild pressure press juice) was less acceptable than that of some of the other treatments. As in 1997 the brandy made from unclarified high pressure press juice was classified as acceptable.
The quality of the 1997 and 1998 matured products was superior throughout compared to the rebate brandies that had not been matured, except in the case of treatments 4 and 8 (where the quality of the rebate brandy and the corresponding brandy was the same) and treatment 6 (where the brandy was of an inferior quality to the rebate brandy). In the case of the 1998 products, it seems as though the significant differences in the quality of the rebate samples that had not been matured, were to a large extent masked by the maturation thereof with oak extract. Brandy is known as a considerably more flavourful product compared to rebate brandy that has not been matured. The quality of the brandies does not differ significantly, but there were a few differences in the rebate brandies. This confirms the importance of oak extractable compounds and the contribution of maturation to the quality of brandy.
It is interesting that in general those treatments that usually result in clear must produced better products (the brandy of treatment 7 and the rebate brandy of treatment 13). Unfortunately no record was kept of the degree of clarity of the juice. The treatment where hyperoxidation and free run juice without enzyme only were applied (treatments 4 and 12, Fig. 4), probably produced less clear must and also inferior products compared to the first-mentioned treatments. It appears as though the reduction of the phenol content, through the use of gelatine in combination with bentonite (treatment 7), may have contributed to the improved quality of the products.
The yeast strain NT116 was used in this experiment to determine whether it can produce a better brandy than the traditional WE 228. This yeast is known for forming larger quantities of esters during fermentation. However, there were no significant differences in the quality of the products of the control (treatment 1) where WE 228 was used and treatment 2 where NT116 was used as yeast strain.
The methanol concentrations of the various rebate brandies are given in Fig. 5. The methanol concentrations of the 1997 rebate brandies were higher (except for the rebate brandy of treatment 9) in the case of the products made from higher pressure press juice, as well as press juice that had not been clarified before fermentation. However, the concentrations were all lower than that allowed in bottled brandy, i.e. 2000 mg/l absolute alkohol (AA). It is not clear why similar differences between treatments did not occur in the 1998 products.
The fact that the methanol concentration of the 1997 high pressure cloudy press juice was considerably higher than that of the other treatments, is probably due to the higher pressing action, which would have caused the must to release more pectin containing grape matter from the skins, thus resulting in higher methanol concentrations during fermentation. Removal of these particles through sedimentation before fermentation could result in lower methanol concentrations.
Conclusion
Matured brandies of acceptable quality may be made from press juice and even extra high pressure press juice, since there are relatively few quality differences between products made from the press juice fractions and those made from the free run juice. It looks like it is not essential to apply phenol reducing fining treatments to the juice in order to make acceptable quality products. However, better quality brandies are obtained when the juice is clarified before fermentation by using phenol removing fining agents.
The addition of oak extract substances and the maturation of brandies had a levelling effect on significant quality discrepancies that occurred in the rebate brandies. Brandies made from treated and untreated press juice fractions were, with a few exceptions only, comparable in quality to that of products made from free run juice.
Although the methanol concentrations of wine usually increase with pressing intensity, levels remained below the acceptable maximum in all the products of this experiment.
For any queries contact Ms A. Louw (Tel.: 809 3091) or Prof. C.J. van Wyk (Tel.: 883 3961).
Acknowledgements
1.Winetech for funding the project.
2.Mr. H. Haarhoff of Stellenbosch Farmers’ Winery (Research and Development Department) for supplying the oak extracts.
3.Mr. W.A. Joubert of Ashton Co-op Wine Cellar for assistance with the 1997 experiment.
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Juice yield per ton of grapes is considerably increased by the use of contemporary membrane presses. Usually only free run juice and low pressure press juice are used to make rebate, therefore it was decided to investigate the application of press juice fractions for the preparation of rebate and brandy of acceptable quality. In this experiment various press juice fractions were used to make rebate for pot still distillation to brandy. The effect of various polyphenol reduction treatments of press juice before fermentation was also investigated. 
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