Anreicherung deutscher Weine mittels Umkehrosmose

Abstract

The aim of this study was to examine the analytical and sensory effects of grape must concentration by reverse osmosis on the German vintage wines from various vine stocks and years. Additionally, the economical aspects of this method were investigated.
This new physical method of alcohol concentration is proposed as a possible alternative to the conventional method of saccharose enrichment. To compare these two methods, various grape musts of different vine stocks and from different vineyard regions were concentrated according to the test methods and then made into wine. This study was conducted over a period of three years. The grape musts and the ripened wines underwent comprehensive analytical testing. These tests mainly focused on constituents that either enhanced or impaired the quality of wines. The effects on various technological cellar parameters were investigated throughout the entire process, from wine cultivation, must concentration and fermentation to the ready-to-drink product.
To start, spiral wound membranes with various pore sizes were used in the reverse osmosis tests, and the suitability of these membranes was tested with regard to their ability to retain key must constituents. The results showed that membranes with a nominal cut-off of 100 Dalton are required to achieve sufficient sugar retention.
High levels of potassium and tartaric acid, which were determined in the concentrated musts, decreased over the course of fermentation and further cultivation of the wines due to an elevated tendency to the formation of tartar.
Preclarification required when spiral-wound membranes are used led to a reduction of the inner area and thus to problems with fermentation. However, it was possible to achieve a sufficient degree of final fermentation by using common fermentation-promoting measures.
Following the concentration tests, the musts were used in a pilot-scale winemaking process. After bottling, the musts were subjected to comprehensive analytical and sensory testing.
There were higher levels of ash, sugar-free extract and residual extract in the reverse osmosis wines. The different levels of glycerin, which is often used as a measure of quality, varied only slightly between reverse osmosis and saccharose-processed wines. This was due to the fact that when healthy grapes have been selected, the major portion of glycerin is formed during initial fermentation and is therefore hardly affected by the must concentration. Except for zinc, the level of metals found in the musts had no relevance in the wines tested. This is because the levels of virtually all metals markedly decreased during the process of fermentation.
Reverse osmosis concentration caused red wines to accumulate pigmented constituents.
The sensory tests showed that the higher level of extract and the concentration of phenol compounds had no effect on the overall taste of the wines. In isolated cases, the tasters were able to distinguish between the quality of wines concentrated by the reverse osmosis or the saccharose method. In an extended three-way test, the investigators were not able to verify their preferences. Accordingly, reverse osmosis concentration neither improved nor worsened the taste or the bouquet of the wines.
In 1995, extensive studies of the performance of a reverse osmosis system were completed. Costs were calculated based on the findings of these trials showing that the differences between the costs for reverse osmosis and saccharose enrichment tended to be marginal. However, the reduction in the must volume due to permeate separation plays a crucial role in the cost-effectiveness of reverse osmosis concentration.
Overall, the results indicate that concentrating must by means of reverse osmosis is technically possible and, compared to saccharose enrichment, wines of similar quality can be produced. Nevertheless, it was clearly shown that, regardless of which reverse osmosis method is used, this form of concentration cannot compete with saccharose enrichment from an economical standpoint because of the losses of quantity involved. It remains to be seen whether reverse osmosis concentration will gain in popularity once it receives marketing authorization. This will indeed depend on whether the current legal framework governing concentration can be adapted to changing circumstances.