Reverse Osmosis

Reverse Osmosis

MemStar (Membrane Separation, Treatment and Removal) Reverse Osmosis is an advanced membrane technology for the selective removal of targeted molecules from wine with minimal changes to the rest of the wine’s composition. MemStar Reverse Osmosis is a mobile service offered in New Zealand exclusively by Vintech Pacific.

In nature, osmosis occurs when molecules of a solvent pass through a semi-permeable membrane (such as a cell wall), from a region of high concentration to a region of lower concentration.

In reverse osmosis, the passage of the target molecule through the semi-permeable membrane (in this case a polyamide with a known porosity) is opposite to that of osmosis when subjected to a hydrostatic pressure greater than the osmotic pressure.

The reverse osmosis of wine has three stages which all occur concurrently.

  1. The wine is split into a concentrate stream and a permeate stream.
  2. The permeate stream is treated to remove the targeted molecule.
  3. The concentrate and the treated permeate stream are recombined.

The membranes used in reverse osmosis to separate the concentrate and permeate streams will depend on the molecule being targeted but are referred to by their Molecular Weight Cut Off (MWCO). That is the molecular weight of the largest molecule that can pass through the membrane. ‘Tight’ RO membranes have a MWCO of about 90 and so molecules with a molecular weight of 90 or less, will pass through the membrane.

Examples of molecules with a molecular weight of 90 or less are…

  • Water (MW = 18)
  • Carbon Dioxide (MW = 44)
  • Acetaldehyde (MW = 44)
  • Ethanol (MW=46)
  • Acetic Acid (MW = 60)
  • Ethyl Acetate (MW = 88)
  • Lactic Acid (MW = 90)

‘Loose’ RO membranes have a MWCO between about 180 and 250 and so with these membranes, molecules with a molecular weight of 180 or 250 or less, will pass through the membrane.

Examples of molecules with a molecular weight of 180 or less are…

  • All those shown above (MW < 90)
  • Malic Acid (MW = 134)
  • Tartaric Acid (MW = 150)
  • Volatile Phenols (MW = 120 – 150)
  • Glucose/Fructose (MW = 180)

Importantly, flavonoid compounds have molecular weights starting at about 300 and so these molecules will not pass through the membranes.

In the second stage of reverse osmosis, the permeate stream, which now contains the molecule targeted for removal is treated to capture that molecule. In the case of alcohol adjustment, the permeate stream is passed through a special perstractive membrane contactor on the other side of which is a counter-flow of treated strip water. Alcohol passes through the membrane from the permeate into the water. The dealcoholised permeate is then recombined with the wine from which it was extracted, thus lowering the alcohol of the blend.

For the removal of other compounds such as Brettanomyces, volatile acidity or other taints, the permeate stream is passed over specific resin contactors which capture the target molecule before the permeate is recombined with the wine.

MemStar Reverse Osmosis can also be used to adjust the concentration of juice or wine by the removal of water.
The results of reverse osmosis can be accurately demonstrated on a five-litre sample of the wine to be treated using Vintech Pacific’s MicroStar benchtop RO Laboratories.

[This article was originally published at www.vintechpacific.co.nz]



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