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A Technical Guide for Wine Producers
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Effect Of Must Enrichment On Sauvignon Blanc Composition And Wine Quality
INTRODUCTION
There is a serious need for the production of cultivar typical and high quality Sauvignon blanc wines in the wine industry. Promising technology which may contribute to satisfying this need, is the removal of water from must by means of the reverse osmosis process, in order to obtain a more concentrated must and theoretically more typical and higher quality wines. It is therefore possible to harvest grapes at a lower degree of ripeness, where higher levels of methoxypyrazine occur, while still obtaining sufficient alcohol and a balanced wine by means of must enrichment. This process was investigated on an experimental basis in the industry in order for it eventually to be legalised.
The purpose of this investigation was therefore to determine, in the light of chemical and sensorial analyses of wine made from pre- and post-reverse osmosis treatment musts, whether the process was successful and could be used in the wine industry.
MATERIAL AND METHODS
Sauvignon blanc musts of pre- and post-enrichment by means of reverse osmosis (20 litres per sample) were obtained from seven producers during the 2002 season. The trial was conducted in duplicate. The musts were treated under identical conditions at Nietvoorbij, according to standard small-scale vinification techniques, and wines made from them. Yeast strain VIN 13 was used and fermentations conducted at 14oC. The musts were analysed for sugar, acid and pH. The wines were stored at 0oC after bottling, up to the time of chemical and sensorial analyses. The following analyses were conducted on the wines:
The sensorial evaluations of the wines were done by an experienced panel of seven judges. Five of them were producers involved in the trial. The wine characteristics evaluated, were general aroma intensity, Sauvignon blanc aroma intensity and overall quality as a wine, as well as a Sauvignon blanc wine.
The data was statistically processed to determine whether the pre- and post-treatments differed significantly from each other.
RESULTS AND DISCUSSION
The respective producers and general information about the musts and wines are summarised in Table 1. The degree of must enrichment ranged from 4% to 23% and sugar, alcohol, extract and total acid concentrations increased accordingly. Except for obvious increases in acetate ester and higher alcohol concentrations in the cases of wines from Producers 3, 5 and 6, the rest of the wines showed little difference between pre- and post-reverse osmosis treatment (Tabel 2). A possible reason for this phenomenon is that the musts of the latter wines were probably not sufficiently enriched. With regard to statistically significant differences between pre- and post-treatments, the limited extent of the trial meant a lack of sufficient variance which, in some instances, resulted in non-significance. Nevertheless the trends are clearly noticeable. With regard to the cultivar typical 2-methoxy-3-isobutylpyrazine, declines in concentrations were generally noticed. These values were so low, however, and difficult to quantify, that it would be safer not to use them to reach conclusions.
Table 1. General information about Sauvignon blanc wines made from musts that underwent reverse osmosis treatment (2002 season).
Producer and
treatment Date received Must before treatment (L) Water removed (L) and (%) Sugar degree (oB) Alcohol (Vol %) Extract (g/L) Total acidity (g/L) pH Free SO2 (mg/L)* Total SO2 (mg/L)* Producer 1 Must Before 01/02/02 5200 680 (13) 20.5 8.60 3.22 After 23.0 9.80 3.19 Wine Before 12.10 20.2 7.62 3.35 25 72 After 13.80 24.2 8.51 3.35 23 78 Must Before 03/02/02 7700 582 (8) 22.1 7.40 3.31 After 23.3 7.80 3.27 Wine Before 13.04 23.5 6.93 3.30 27 105 After
14.05 25.3 7.25 3.23 26 108 Must Before 04/02/02 9100 1270 (14) 17.7 7.30 3.41 After
20.0 8.30 3.37 Wine Before 10.38 20.6 6.40 3.49 24 96 After
11.83 22.7 7.05 3.54 23 104 Must Before 10/02/02 6200 620 (10) 22.0 6.90 3.60 After
24.0 7.20 3.56 Wine Before 12.95 22.3 5.95 3.68 23 86 After 14.36 23.4 6.39 3.74 22 100 Producer and treatment Date received Must before treatment (L) Water removed (L) and (%) Sugar degree (oB) Alcohol (Vol %) Extract (g/L) Total acidity (g/L) pH Free SO2 (mg/L)* Total SO2 (mg/L)* Producer 5 Must Before 14/02/02 8400 1900 (23) 18.4 10.60 3.21 After 21.2 12.20 3.17 Wine Before
10.50 22.3 8.91 3.41 24 120 After
12.25 26.3 10.08 3.38 21 114 Must Before 15/02/02 16500 1700 (10) 22.5 8.20 3.21 After
23.5 8.90 3.28 Wine Before 13.43 22.6 7.32 3.50 22 94 After 14.36 24.0 7.88 3.53 21 92 Must Before 18/02/02
11000 475 (4) 21.1 8.30 3.10 After
22.5 -- 3.11 Wine Before 12.58 23.4 6.52 3.59 20 91 After
13.20 24.6 6.64 3.59 18 90
With regard to wine quality, the panel was asked to judge each wine as a Sauvignon blanc, as well as a general, non-cultivar wine, so that, should Sauvignon blanc character be lacking, a score for wine quality would not be compromised as a result thereof. With the exception of two instances (Producers 6 and 7), increases in wine quality (both for general wine and Sauvignon blanc wine) as a result of must enrichment were noticed (Fig. 1). According to the judges' comments, the process went hand in hand with an increase in aroma intensity, body and complexity, which had a positive effect on wine quality. In most instances the typical Sauvignon blanc aroma intensity of the control wines was so low, however, that an increase thereof did not increase the overall quality of the wines sufficiently. This is in agreement with the low 2-methoxy-3-isobutylpyrazine concentrations observed in the majority of cases (Table 2). In the case of wines with high acids, such as the wine from Producer 5, the wine quality of both the pre- and post-treatments was influenced negatively (Table 1 and Fig. 1). In some instances the enrichment process, according to the commentary of the judges, caused an increase in phenolic aroma (e.g. the wine from Producer 3). In the two instances (Producers 6 and 7), where decreases in wine quality were observed, the overall wine aroma increased to the extent that the typical Sauvignon blanc character was quite masked.
CONCLUSIONS AND RECOMMENDATIONS
Reverse osmosis treatment produces higher quality, more full-bodied and complex wines. When Sauvignon blanc character is insufficient in the basis material, however, it cannot be expected that must enrichment will necessarily enhance it. The reverse osmosis treatment can therefore, in the context of this investigation, only succeed if the must contains noticeable levels of the typical character and if the degree of enrichment is sufficient. In addition to the purpose of this investigation, the process could also be applied to the musts of other cultivars. Furthermore the purpose of the process is of course also to use the wines made from enriched musts for blending purposes and it should therefore be seen as the building blocks of a larger product. Aspects still surrounded by some uncertainty, and which should be investigated to optimise the process, are the most suitable degree of ripeness at which a must should be enriched, as well as the optimal degree of enrichment.
Taking everything into account, it is recommended that the application of the reverse osmosis process in the wine industry be legalised.
Table 2. Effect of must enrichment on the composition of Sauvignon blanc wines (2002 season).
Producer and treatment Total acetate
esters (mg/L) Total ethyl
esters (mg/L) Total higher
alcohols (mg/L) iso-Butylme-thoxypyrazine (ng/L) Before 7.544a 4.935a 177.761a 4.175a After 7.447a 4.912a 176.809a 3.490a Before 6.759b 4.178a 173.832a 5.305a After 7.732a 4.289a 168.387a 3.670a Before 7.761a 4.452a 146.055a 4.680a After 9.165a 4.665a 149.315a 4.680a Before 15.204a 4.776a 153.371a 1.400a After 11.178b 3.994b 143.372b 0.400a Before 10.128b 4.376a 139.892b 3.210a After 14.927a 4.549a 185.096a 2.810a Before 13.775a 5.220a 146.407a 4.160a After 15.011a 5.007a 159.739a 3.610a Before 12.785a 5.234a 155.201a 4.330a After 12.468a 4.726a 160.755a 5.980a
The funding of this research by Winetech and input by Philippe Dietrich and the respective participating producers are much appreciated.
Further enquiries may be addressed to: Dr. Johann Marais (tel: 021 809 3096; e-mail: johannm@infruit.agric.za)
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Dr. J. Marais - ARC Infruitec-Nietvoorbij, Stellenbosch.
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