The Science bit: The picture above shows the primary role of reducing agents such as sulphiting agents in the inhibition of enzymatic browning is to reduce the pigment precursors (quinones) to colourless, less-reactive diphenols. Sulphites serve a multifunctional role in foods. They possess antimicrobial activity and inhibit both enzymatic and non-enzymatic browning reactions. Madero and Finne (1982) proposed that bisulphite exerted a competitive inhibitory effect on polyphenol oxidase, by binding a sulphydryl group at the active site of the enzyme. Ferrer et al. (1989b) on the other hand, proposed that bisulphate inhibition was due to the reaction of sulphites with intermediate quinones, resulting in the formation of sulphoquinones, which irreversibly inhibited polyphenol oxidase, causing complete inactivation.
That's all very nice, I hear you say, but what does all that really mean to those of us without the science degree and who just want a wine for dinner tonight?
Wine has since time immemorial, contained sulphur, it is nothing new. It was made an 'official' additive back in 1487 when a Prussian royal decree officially permitted the use of the wine additive sulphur dioxide (or SO2) for the first time. The reason was, to help preserve their wines during transport, Dutch and English wine traders regularly burnt sulphur candles inside barrels before filling them. It was something that they learned from the Romans who had conducted the same practice before them, so sulphur has been used to help keep wine almost since we first learnt winemaking!
All wines contain at least some small amount of sulphites. They are a natural result of the same fermentation process that turns grape juice into alcohol. Even wines that have not had any sulphites added during the winemaking process contain some amount of sulphites.
The problem is therefore, just having something saying "contains sulphites" is really of no use to anyone anywhere! How much sulphur has been used is of much more use and importance to allow the consumer to make a buying decision. Bad winemakers use excess amounts of SO2 as a coverall for their careless actions or to compensate for bad hygiene or cellar work, or for extra protection of the wine just in case of any of the above!
When SO2 is greatly in excess, it can also produce a pungent aroma in white wines, considered by most to be a fault. The aroma is best described as that of a match that has just been struck. High SO2 can also render the palate of the wine metallic and bitter, which also means that there is often very little fruit character apparent and only harsh acidity. As wine merchants who have high quality control and always taste all our wines before putting them on sale, we regularly taste samples that clearly have had lots of SO2 added to cover up bad winemaking and are rejected by us accordingly.
The problem only comes when retailers just buy on price and not on the quality of what's in the bottle. An interesting new 'discovery', and another point to throw into the cork vs screw cap debate, is recent research shows that wine bottles that use the screw cap or Stelvin method of closure appear to have a higher occurrence of the struck match SO2 smell immediately after opening. This is no bad thing if you know what you're looking at; it simply means you need to leave the wine for a minute or two to let the SO2 dissipate. The only reason SO2 is more apparent in screw capped wines is not because more has been added but simply because the screw cap is so much more efficient in keeping both oxygen out and thusly SO2 in.
A wine with a cork closure allows a tiny ingress of air through it and so allows SO2 to escape. Of course this means that arguably the wine will not keep as long and will mature faster than those sealed with screw caps. Indeed this is why some of the high end Burgundy houses are now sealing their top wines with a screw cap!
In conclusion the addition of SO2 to wine is made to protect it from oxidation in the longer run and is not a bad thing necessarily. It is in fact absolutely necessary in wines made for the longer term. It is only bad when used by poor wine makers to cover up a multitude of mistakes or those who are more concerned about producing quantity rather than quality...
Blog post written by: John Martin
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