Reverse osmosis

Published by Sally on October 1, 2010

A version of this article first appeared in Wine and Spirit in 2006, since merged into Harpers Wine and Spirit.

Osmosis is essentially water molecules passing across a semi-permeable membrane from the side of higher water molecule concentration to lower water molecule concentration. Memories of curly raw potato chips in school science lessons suggest the concept of osmosis, and thus reverse osmosis, shouldn’t be completely alien.

Clearly osmosis is not good for wine or grape juice – they would become diluted. But there are three decent reasons to reverse the passage and selectively take bits out of wine:  

  1. Your vineyard is in a cool climate; you’ve had to wait till rainy late autumn to harvest because that’s when the sugars and phenols (tannins, colour etc) in the grapes are ripest. If wet grapes go straight to winemaking, all that rainwater dilutes the fruit.
  2. Your vineyard is in a hot climate. All the grape sugars are ripe, and continue to accumulate, but the phenols take longer, which means, the wine has ripe tannins, but shed-loads of alcohol too.
  3. Volatile acidity (VA): stuff that’s brilliant at adding complexity and lifting aroma in small doses, but smells like nail varnish remover and vinegar in larger quantities which is a major fault.

The ‘reverse’ bit of osmosis is achieved through high pressure and it separates out different bits of juice or wine in the opposite direction to osmotic flow across a selective membrane. The liquid going through the membrane looks pretty much like water; the winey bit stays on the original side of the membrane. Water is the smallest molecule in wine and passes readily through the membrane. So you whiz some of the wet grapes through an RO machine before the fermentation starts.

When dealing with high alcohol wine, water goes through the membrane, and so does ethanol. The water/ethanol mix is distilled to separate the two.  The watery bit can be added back to the wine to keep the original concentration while the alcohol is removed.  (If the alcohol is added back and the watery bit removed, this increases the alcohol content of the wine).

The molecules responsible for VA are also quite small and can pass through a selective membrane, along with water and a few other things. This mix is treated in a different way to remove the offending acetic acid, and what remains of the mix is returned to the winey portion. Et voilà.


2 Responses to “Reverse osmosis”

  1. Per-BKWine Says:

    And if you are curious how this looks in real life, here are some photos of the machines used to do it:
    or shorter

    Not cheap, but some think it’s worth it. And it’s not always that they want to show the equipment that they use for it.

  2. Sally Says:

    Yes, thanks Per. Not all that exciting to look at (though very shiny), given all the whizz-bangery inside. I have some pics myself somewhere, I must dig them out.

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