Sulphur Dioxide FAQ

Sulphur Dioxide (SO2) is the added ‘preservative’ in many foods and usually represented on the label as 'preservative 220'. It's a colourless, non-flammable gas. In water it can be present in a concentration of up to 12%.

Grapes can be damaged by birds, rodents, foxes, weather and fungi. Harvested indiscriminately they can carry a heavy load of micro-organisms. Grapes are usually warmer than 10°C and consequently these organisms multiply rapidly. In such circumstances unfavourable outcomes are very likely.

At high concentrations SO2 is highly corrosive to mucous membranes, upper respiratory tract, eyes and skin. In dry wine the maximum permissible level is 0.025%. Free sulphite can be detected as a burnt match odour. This can produce air passage restriction in susceptible individuals. Some are intolerant of sulphites. Those susceptible, report headache and bodily discomfort including face flushing, feeling poorly overnight and on the following day although amounts ingested may have been small.

Firstly, due to climatic circumstances in Margaret River and Karridale in particular we begin with very good raw materials. Secondly harvest by hand gives us sound fruit. Thirdly, our grapes have naturally high phenolic content, a natural antioxidant. This is due to the fruit having a greater exposure to sunlight in turn due to our dual row trellising system that disperses the foliage to a much greater extent than other systems of viticulture, especially the more restrictive systems designed for mechanical harvesting. Fourthly we chill fruit prior to crushing. Fifthly we have developed techniques to keep wine cold for long periods of time and to keep it away from the air. None of this is rocket science! But, it is more costly.

The short answer to this question is that these are two very different things. Organic methods relate to the production of the grape whereas the use of preservative is an option after the fruit is picked. Sulphites have no use in the vineyard.

Elemental sulphur is a permitted spray ingredient to control powdery mildew under organic and non organic protocols. Elemental sulphur is not the same thing as sulphite, it volatilises in the vineyard, is not present on grapes at harvest time and even if it were, can not produce sulphites in wine.

All organic protocols allow the use of sulphur dioxide in processing. It is possible to use organic methods without seeking certification. It is also possible to produce wines from grapes grown using organic methods without using sulphur dioxide in the winery. That is the Happs approach with our Preservative Free wines.

We know that undesirable growth of micro-organisms begins as soon as the juice of the berry is exposed. The rate of growth depends on the proportion of damaged fruit that is taken, the temperature and time of exposure. A mechanical harvesting system that produces warm juice in the vineyard is fraught with hazard. It does not discriminate between sound and damaged fruit, taking whatever can be dislodged from the vine. Despite night time harvest, grape temperatures are usually sufficient to promote the growth of unwanted organisms. Sulphur dioxide is a great aid in this circumstance and improves the performance of mechanised harvesting systems so long as it is uniformly distributed. In practice this is difficult to achieve.

The more costly alternative is hand harvest of clean fruit with a low bacterial and yeast load and sufficient time in a chiller prior to crushing so that the fruit is too cold to support spoilage. This is the process used at Happs winery.

In most mechanically driven vineyards, given the potential for premature spoilage the most critical use of sulphur dioxide is undoubtedly as a bactericidal addition to the picking bin in the vineyard or at the winery crusher. It is useful in inhibiting oxidative enzymes. It is a low level scavenger for oxygen picked up whenever wine is exposed to air or kept in poorly sealed or air permeable containers. An abundant supply of oxygen is all that is necessary to degrade flavour and support the growth of acetic acid bacteria.

The second most common place to add SO2 is prior to bottling to assist in promoting stability against further ferment in bottle and to take up some of the oxygen that is absorbed during the bottling process.

Wine is the product of arresting a series of fermentations. Most white wines have no malic acid ferment whereas reds do have this ferment. Given sufficient air, acetic bacteria will ferment any wine to produce vinegar. If a wine goes to bottle with sufficient malic acid to support bacterial activity or sugar to support yeast ferment, and the organisms are also present, these ferments are likely in the bottle. Sufficient sulphur dioxide will prevent the growth of some species of bacteria and yeast but not all.

Because red colour binds sulphites, sugar or malic ferment is still possible in reds after bottling. The usefulness of the ‘preservative’ therefore depends upon the wine type, the presence of fermentable substances, the presence of air and the type of organism present. Low pH helps to keep more sulphur dioxide in the active form. High alcohol and very high sugar contents are independently antagonistic to further ferment.

The ‘antimicrobial’ task that sulphur dioxide performs after bottling is something that can not be relied upon. In practice the only sure method of preventing further ferment is to exhaust that possibility beforehand or filter the wine so that the organisms are not present after bottling.

The requirement for an antioxidant depends upon the porosity of the container and closure to air. Plastic materials admit air whereas glass and screw caps are relatively impervious. Corks admit variable quantities of air giving rise to the old adage that there are no great old wines, just great bottles of old wines. However, depending upon its constitution, a wine needs variable quantities of air to facilitate the aging process. Without this air many wines become reductive and smelly. The greater the natural tannin in the wine the more protection it has against oxidation. This is why reds age longer than whites. When white grapes are exposed to more sunlight more phenolics are generated and that helps a wine resist oxidation. This is the rationale that we follow. The phenolic (tannin) content of a wine is linked with its ability to protect against heart damage because phenolics are great antioxidants. The long lived wines have their own natural protection against oxidation and they are also the wines that will reduce heart disease.

The natural preservative in wine is alcohol, tannin (phenolics) and natural acidity. A good natural acidity is the product of cool ripening conditions and is usually accompanied by a good depth of flavour. The presence of sulphur dioxide after bottling is largely irrelevant to a wine of healthy constitution except that it will assist in taking up oxygen that can be picked up during bottling or via a container or a stopper that is permeable to air. To suggest that sulphur dioxide is ‘the preservative’ in wine overstates the case. The aging capacity of the wine after bottling is conditional on its depth of flavour, freshness at bottling, the bottling technique, container type, closure type, the presence of fermentable substances and the presence of organisms capable of degrading the wine. Unless these latter conditions are met no amount of added ‘preservative’ will assist. If these conditions are met very little if any ‘added preservative’ is required.

The limit is defined in terms of the amount that is present by analysis in finished wine and depends upon the type of wine made. In practice these legal limits are never approached in modern winemaking except when accidents or miscalculations occur. Dry wines are allowed 250 parts per million but rarely exceed 150 parts per million. Happs whites rarely exceed 100 p.p.m. at bottling and reds 50 p.p.m. At release, levels are frequently one third less.

With the widespread use of refrigeration and inert gas to prevent exposure to air, and the availability of effective filtration, the amount of sulphur dioxide encountered in wine is much less today than it was 50 years ago. SO2 binds substances that evolve during ferment and the more that is used in the beginning, the more that is required in the end. The advantage for wine makers lies in reducing its use because a more palatable wine will result. I believe that, as we discover that we can do a good job with other tools we will rely less and less on this antiseptic. The sounder the grapes and the more considered the winemaking strategy, the less is needed.

A minority of yeasts can produce sulphite under difficult fermentation conditions that apply when grapes have a low nutrient status. These fermentations commonly produce sub standard wines. In fact, the production of sulphite by yeast is anathema in modern wine making and is a recognised difficult situation that will be avoided wherever possible. I do not regard this proposition as a legitimate defence of the practice.

When added to wine the result is SO2, HSO3 (bisulphite) and H2SO3 (sulphurous acid). These are collectively called ‘sulphites’. The active bactericide (SO2 and HSO3) is readily absorbed into some organisms. However the bactericide is present in fractional amounts depending upon pH and the constitution of the wine. The presence of compounds that bind strongly to sulphite including acetaldehyde, pyruvic acid and anthocyanin (red wine colour) leaves only a tiny portion that is active against micro-organisms or oxygen.

Sulphite inhibits the action of bacteria that metabolise malic acid. However it has little activity, when present at permissible levels, against sugar fermenting yeasts, Brettanomyces or acetic acid bacteria. It should be noted that these are the major spoilage organisms of wine. The best safeguard against these organisms is to make sure that there is nothing further to ferment, and if there is, to filter the wine clear of the organism or to keep it cold so as to inhibit activity. Yeast, including Brettanomyces species will not grow appreciably if the wine is free of fermentable sugars. Acetic acid bacteria will not grow in the absence of air. Malic bacteria will not grow unless malic acid is present.

Sulphur dioxide is usually purchased by wine makers in a gaseous or powder form and is readily soluble in water. The powder used is potassium metabisulphite.

The joint FAO/WHO Expert Committee on Food Additives in 1974 established an acceptable daily limit of 0.7mg per kg of body weight. This translates to 56mg for an 80kg individual (0.056grams). A 750ml bottle of wine containing 150 parts per million sulphites contains 0.225grams. On that basis the daily allowance for such an individual is met by consumption of 187 mls of such a wine. After three years in bottle the sulphite level in a white wine may drop by half. Then, twice as much wine may be consumed without exceeding the daily allowance. Red wines start their bottle life with perhaps half the level of sulphite as a white wine and may also be consumed in twice the quantity. However, it should be noted that wine is not the only food containing sulphites.

The following is a list of foods and the level of preservative (in parts per million) that is permitted by law in Western Australia.