Reverse osmosis separation of wine is based on the tendency of smaller molecules to pass through a semi-permeable membrane more readily than larger molecules.
Wine is separated by reverse osmosis into concentrate and permeate streams. The alcohol rich permeate is degassed, heated and passed through a proprietary perstractive membrane array on the other side of which is a counter-flow of filtered, deoxygenated strip water.
Permeate – the flow containing the low molecular weight components which pass through the semi-permeable membrane
Concentrate – the bulk wine flow which is retained by the membrane and contains almost all of the vinous components
Alcohol passes through the membrane from the permeate into the strip water at a rate which depends on the alcohol concentration gradient across the membrane and the temperature. The dealcoholised permeate is then cooled and recombined with the wine from which it was separated, resulting in a lower alcohol blend.
According to the wine composition table below, water passes most readily and other, larger wine components progressively less so. As molecules get larger, there is a point where the passage of a particular sized compound becomes insignificant. This is the ‘molecular weight cut-off’ (MWCO) of the membrane.
For ‘tight’ RO membranes, this nominal MWCO is about 90. Water, carbon dioxide, ethanol and acetic acid pass through the membrane into the permeate reasonably easily; lactic acid only marginally and other components hardly at all.
With another, ‘looser’ membrane type all components pass more readily including more of the larger compounds up to about molecular weight 180.
Which membrane is used will depend on the specific Memstar application.
Some wine components and their molecular weights
|Wine Component||Molecular Weight|
|Lactic acid||90||“tight” RO ↑|
|Volatile phenols||120 to 150||“loose” RO ↑|