Between grape and wine lies a process known as vinification. In its simplest form that's no more than allowing the natural yeasts on the skins of the grapes to digest the sugars within the grapes, what we call fermentation. The result is fermented grape juice, an alcoholic beverage known as wine. But oenology is increasingly a science, something more dominated by the white-coated boys in a laboratory than hard-skinned men treading grapes with their feet.

Amongst the many parts of vinification in a modern winery is a process called 'fining'. This is done to clarify the wine, to make it bright and clear. The idea is simple enough, you add to your wine a substance that has a specific gravity that is marginally greater than the wine. As it slowly sinks through the wine from top to bottom it binds with free particles that are suspended in the wine and precipitates them to the bottom, leaving the wine free of any cloudiness. Isinglass, made from fish swim bladders has been used for centuries, but today many more fining agents are available to the wine-maker, most commonly bentonite and casein. Bentonite is a clay, a compound of aluminium, a mineral that has been linked to Alzheimer's. It's used in large quantities, up to 1 part per 1,000, and we rely on subsequent filtration to remove any residue. It's possible that if filtration isn't properly carried out then some aluminium could remain in the wine. Casein, a milk by-product, is also commonly used and its worth noting that many people have an allergy to casein. Again, improper filtering can leave residues. Both of these fining agents are permitted in 'organic' wines.

Another processing aid is potassium ferrocyanide, which is used to resolve discolouration in wine caused by a combination of tannin and iron. When it was introduced in Germany back in the 1920s it was an instant hit, and Hock glasses no longer needed to be coloured. The wine became brilliant, and clear glass could show this off. It was called 'blue fining', because it formed a blue precipitate called 'Prussian Blue', which was then removed by filtration. But again, should the filtration not be properly done, then any blue precipitate getting into the bottle will decompose into hydrogen cyanide, the gas with the characteristic smell of bitter almonds. No wine has ever been found to have hydrogen cyanide above permitted levels of toxicity, but the use of potassium ferrocyanide is banned in America.

Most organic producers do not filter their wines, although they're allowed to do so within the rules. However the argument as to whether to filter or not is more to do with the final flavour of the wine than with organic viticulture, but that's a story for next week.

Wine of the Week

Prosecco Brut, Ombra

The 'Bellini' was popularised by 'Harry's Bar' in Venice. A great drink for hot afternoons, all you need is a carton of peach juice and a bottle of chilled prosecco, a sparkling Italian wine. About fifty/fifty makes a good Bellini.

Available McCabes, Ashford Food and Wine, Burke's Cabinteely, Murtagh's Enniskerry and The Grapevine Dalkey. RRP. €15.90