|The vinification process is no longer a spontaneous natural process whereby crushed grapes and the juice that is subsequently derived will ferment spontaneously and eventually produce wine.
The application of pure yeast that used to be revolutionary, is already a forgotten standard practice. Wine cellars are continuously exposed to new technology and have to decide whether the technology in question will be used or not. The decision is obviously not only a technical one, but is also influenced by factors such as market requirements and prescriptions, local and international legislation and financing. Techniques whereby certain components are removed from wine and a certain part thereof added again, are the order of the day and an industry has to decide to which extent new technology is acceptable or likely to impact on the ethics of vinification.
Different techniques such as reverse osmosis, spinning cone, nanofiltration and ultrafiltration may be used to separate different fractions of wines. The fractions that are thus separated, may be treated in different ways to change the composition of the final wine. It is often alleged that the nature of these processes is revolutionary, but the principle of wine fractioning is not strange if the following is taken into account:
The removal of part of the free run juice of red grapes before fermentation in order to obtain a rosé and a full colour red wine from the same grapes. The separation of free run juice from white grapes in contrast to a phenol rich juice that had been exposed to concentrated skin contact. The two fractions will obviously be treated differently and may possibly be blended again afterwards.
The Australian company Wine Network Technology (WNT) offers services to the industry that may be defined as recombined reverse osmosis techniques.
Reverse osmosis is a liquid phase, pressure driven separation process whereby transmembrane pressure causes the selective movement of a solution against the osmotic pressure difference. Reverse osmosis is therefore a kind of filtration by means of a selective membrane at a pressure exceeding 4 000 kilopascal. The high pressure is required to overcome the osmotic differences of various concentrations. The membranes that are used, are so dense that they will soon become blocked in a single flow direction. In order to prevent this from happening, a large volume of the product is recycled over the surface of the membrane, thus removing the blockage. This type of filtration is therefore known as cross flow filtration.
In the case of reverse osmosis, the permeate that moves through the membrane is colourless and flavourless. The wine character is retained in the unfiltered retentate. The membranes used by WNT have a nominal molecular mass separation specification that is able to determine the amount of specific molecules that pass through the membrane. Water, the smallest molecule in wine with a molecular mass (MM) of 18, will move through the membrane easily, while components such as tartaric acid (MM 150) or malic acid (MM 134) and others such as sugars, esters and colourants will hardly move through the membrane.
As a result of the selective permeability of grape components through the membrane, there are various possibilities for application:
If grape juice is passed through a reverse osmosis apparatus before fermentation, the water will move through the membrane and the retentate will therefore be a more concentrated grape juice with a higher sugar content.
Volatile acid removal
The process consists of two stages. Firstly the water, acetic acid and ethyl acetate is removed from the wine as permeate by means of a reverse osmosis membrane. This solution is passed through an anion exchange resin, thereby removing the acetic acid as acetate, while the remainder is passed back to the wine together with the retentate.
As with the removal of volatile acid, this consists of two stages. Components with an MM equal to or less than ethanol are removed from the wine via a reverse osmosis membrane. With the aid of distillation the alcohol is removed from the permeate and the remainder of the permeate is again added to the treated wine.
Removal of off-odours as a result of fires
Smoke components deriving from veld fires may be absorbed in the wax layer on the berry and eventually end up in the wine. The guaiacol content of wine may serve as an indicator of such contamination. By using the right membranes, guaiacol may be removed from wine to a level below the sensorial threshold value thereof.
Wollan, D. 2003. Separation and recombination in modern day winemaking. Wine Industry Journal 18 (4): 89 – 93.