Hi all.
Following discussion on this in a recent thread, , Michael Pronay sent me the text of an article which appeared in 1996.
In it, Bruno Prat is interviewed by Alain Laliberté.
Here is a digest. If anyone wants the full french text, they only have to ask.
==================After explaining that vines collect water by osmosis and that eliminating excess water by saignée is an old Médoc tradition, M Prats describes the mechanism of reverse osmosis which in effect does the same thing.
An advantage compared to vacuum evaporation is that reverse osmosis is self regulating, in that the stronger sugar solutions need higher pressures up to the sensible technical limit of about 60 atmospheres which would give 14° in the finished wine.
If RO has long been known elsewhere, it has been tested on a large scale in the Médoc since ‘87, Cos d’Estournel since 1989 Only 5 or 6 properties did it in 1996, because it is very expensive. In contrast, the cheaper vacuum evaporation is relatively widespread. However if the latter is not done to excess, the results are the same.
The technique corrects an excess of water in healthy grapes due to abundant rain just before the harvest. It cannot make them ripe, and should never be employed when grapes lack maturity. Thus in ‘91, 92, 93 & even ‘94 RO showed itself invaluable. Those who didn’t have it, used saignée, through which one also loses sugar, aromatics and long colloidal molecules, especially polysaccharides.
He regards RO, therefore as a kind of selective saignée, but carried out before fermentation starts. They remove from 10 to 15% of the volume, which corresponds to an increase of 1 to 1.5°. It is better than the necessary evil of chaptalisation, he feels, because for him, removing excess water is more natural than adding sugar. RO does not increase tartaric acid in the finished wine, but does increase malic acid, and also makes for a fatter wine as all polyphenols are perfectly retained, as are all yeasts and bacteria.
As aromatic elements are less well defined anyway, the effect is harder to judge. The rejected water can have a faintly herbaceous smell, and if botrytrised musts are treated, the water has a faint smell of rot. Although the water is not 100% pure, the 0.5% impurities are largely mineral salts.
He goes on to explain that in the Bordeaux region, the Cabernet Sauvignon is often light in sugar, which has to be increased one way or another. In America, more often problems are caused by an excess of sugar, and the technique is useless.
If Leoville Las-Cases carries out RO on its whole crop, and not one kg of sugar has entered the estate since 1987, in Cos, they take a middle line, ROing 1/3 of the harvest. He predicted that they would increase this rather than continue to chaptalise.
In ‘95 Merlots gave 14°, and RO was out of the question, but Cabernet Sauvignon even in the ripest years never gets above 12.5°. In St Emilion, although less useful due to the proportion of Merlot, perfectionists such as L’Angelus do it. Although Merlot can become diluted by rain, it can still have too high a sugar content to be concentrated.
He pointed out that in the 19th Century Médoc wines contained 10.5°and doubted whether the public would accept this figure today. When asked if RO wouldn’t lead to standardisation, he felt that it would improve some lesser years but have no effect on the best. The state of maturity wouldn’t be altered, but one could eliminate dilution effects . For example the dilute ‘73s made from perfectly mature grapes would have been wonderful if made nowadays. On the other hand in ‘74, which was both dilute and unripe, RO would merely have accentuated the unripeness.
Asked about the legal position, M Prats explained that it was tolerated on an experimental basis. The French Government would press the European Union to consider it as a precedure to be authorised.
Asked what yields they get, M Prats explained that at 10,000 vines per Ha, St Estephe could easily give 60Hl/Ha, which he felt was viable. He doubted that Margaux could expect more than 45Hl/Ha for decent quality.
He explained that because most manufacturers of RO equipment expected to work on very clean water, only one manufacturer had specialised in adapting their equipment for winery work, for example by regularly brushing the membranes. He prefers flat membranes to fibrous ones, because they tend to clean themselves. They have to change about 2% of them each year.
He doubted (in ‘96) that mobile RO machines would catch on, because the concentration takes place on the must before fermentation starts, and therefore has to be done quickly. With the machines they had, it took about
5 hours per 200 hl vat.Finally he emphasized that one reason consumers love wine is because it is a natural product. For that reason, he was deeply suspicious of adding anything to it, and felt on the other hand that RO which simply _removed_ water, was less artificial, if more technical. He disagreed strongly with the thinking of Australian winemakers, who are in favour of adding aromas. On the other hand, he was unconcerned - unlike Robert Parker - about removing matter by filtration as long as it wasn’t done to excess, pointing out that those who don’t filter add things - egg white, bentonite, blood, albumen and asked whether it was more natural to remove something or to add something.