Sulphur Dioxide - the Cidermaker's Friend

Sulphur dioxide or SO₂ is the cidermaker's friend. It has been used for hundreds of years (and possibly thousands for winemaking). A quote I love to use is from John Beale, a contributor to Evelyn's Pomona of 1664. He wrote:

“Lay brimstone on a rag, and by a wire let it down into the cider vessel, and there fire it; and when the vessel is full of the smoak, the liquor speedily pour’d in, ferments the better”

In simple terms what happens is that the sulphur dioxide obtained from burning sulphur (brimstone) inhibits or kills most spoilage yeasts, moulds and bacteria, while permitting the most desirable fermenting yeasts (such as Saccharomyces cerevisiae or uvarum) to multiply and to dominate the conversion to alcohol. Only small amounts of sulphur dioxide are used, and its effectiveness depends quite critically on the pH of the juice (see below). Cider juice, when pressed, contains large numbers of yeast, mould and bacteria which we do not want and only a very few of those that we do. The sulphite levels in the table have been empirically established based on laboratory investigations of the sensitivity of these cider micro-organisms and the likely amounts of natural ‘sulphite binding’ substances in the juice, in order to establish the dominance of benign fermenting yeasts and hence a much reduced chance of taints and off-flavours in the finished product.

Sulphite is quite natural, occurring in the atmosphere in the gaseous form of sulphur dioxide (SO₂) as a result of volcanic processes, and in the human body as a result of the breakdown of sulphur-containing proteins during digestion. Many yeasts also produce it during fermentation, reducing naturally occurring sulphate to sulphite. Of course it can be toxic, even lethal, in the wrong dose and in the wrong place, but so also can common salt and water. The form of sulphite used in cider or winemaking nowadays is as the "Campden tablet", or as sodium or potassium metabisulphite powder (which should not be breathed in as it is irritant). In acid solutions such as apple juice these materials liberate sulphur dioxide directly into the liquid. (You can burn 'sulphur string' or 'suphur candles' in barrels as Dr Beale did, but it is impossible to be accurate with the sulphite dose then). The amounts used are tiny, less than 200 parts per million (ppm), and most of it gets quite rapidly bound up in the cider and is no longer 'free'. Note that a very few people, principally asthmatics, are hypersenitive to sulphur dioxide in the free state which is why it is labelled as a food allergen. Once bound in a beverage, it is no longer a problem. The best documented cases of sulphite sensitivity were from its use (now banned) at high levels on pre-packed salads or pre-cut potatoes, where the high free SO₂ levels could be a severe problem for asthmatics. The data on wine is much more equivocal and where people have been challenge tested in clinical trials far fewer of them prove to be sulphite sensitive than they imagine. There are many other things in fermented beverages which can cause adverse reactions, in addition to sulphite.

So.......how much sulphite should I add?

The effectiveness of sulphite depends very much on the juice pH. The Table below gives an approximate idea of how much sulphite needs to be added to a juice at a given pH before fermentation, depending on whether you intend to add a cultured yeast or allow the wild yeasts to do the job. In both cases, addition of sulphite is important to kill bacteria, moulds and adverse wild yeasts, while allowing the beneficial ones to flourish. If a cultured yeast is added, sulphite addition should be made 12- 24 hrs before the addition of the yeast, or the added yeast will be severely inhibited. Added cultured yeasts will start within a day or two. If you are allowing wild yeast to build up and to start the fermentation, you can use the 'total yeast kill' dose in the left hand column, but the beneficial yeasts that survive this may take several weeks to build up and get going. Things will move quicker if you use the 'partial yeast kill' column, which allows more wild organisms such as 'apiculate' yeasts to survive and will give a different flavour balance. The dose is given both in parts per million of sulphur dioxide (ppm or milligrams per litre) and the equivalent in Campden tablets per gallon (which are formulated so one tablet gives per gallon gives 50 ppm). For more about the origin of Campden tablets click here.

It's often easier to use a stock solution of sulphur dioxide. To make a 5% stock solution, dissolve around 10 grams of sodium or potassium metabisulphite in 100 ml of water. (The metabisulphite salts contain around 50 - 60% of available SO₂ depending on how they've been stored). Then 1 ml of this per litre of juice (5 ml per gallon) corresponds to 50 ppm (parts per million) of SO₂.

Note that the correlation between titratable acid (TA) and pH is only approximate (as the graphs elsewhere make quite clear!). pH is the best measurement.

Sulphite Addition Table

pH	Approx TA (% malic)	For total yeast kill (when adding cultured yeast)		For partial yeast kill (for wild yeast fermentation)
pH	Approx TA (% malic)	SO₂(ppm)	Campden tablets per gallon	SO₂(ppm)	Campden tablets per gallon
3.0 – 3.3	1.2 – 0.8	50	1	nil	nil
3.3 – 3.5	0.8 – 0.6	100	2	50	1
3.5 – 3.8	0.6 – 0.3	150	3	100	2
> 3.8	< 0.3	add more acid!	add more acid!	150	3

A more accurate chart

The chart below gives the same sort of information, but is for those who are measuring the pH accurately and dispensing their SO₂ by volume from a stock solution and not by Campden tablets. The red bars give the amount of SO₂ you need for a more or less total kill of the wild yeasts. If you want a partial kill, use only half the amount.

Addition of sulphite after fermentation

The table and chart above is for use of sulphite as an antimicrobial before fermentation. Sometimes it is also used after fermentation has all finished, at racking, storage and bottling. The reason for this is partly antimicrobial but also because it acts as an antioxidant. Or, rather, it mops up the initial products of oxidation such as hydrogen peroxide and aldehydes, preventing them going on to give sherry-like or 'oxidised' off-flavours. In those cases it is usual to add a fixed amount of 50 ppm each time (up to the total legal limit of 200 ppm when all additions are summed together) with a view to achieving a residual 30 ppm of free SO₂ the next day. This is because the antioxidant properties of sulphur dioxide are not affected by pH.

If you are planning to pasteurise a back-sweetened cider to stop it re-fermenting, the addition of 50 ppm SO₂ at bottling also offers the benefit of blocking the Maillard reaction between amino acids and sugars, which helps to minimse the development of "cooked" flavours from the pasteurisation process.

How does sulphite work so selectively against adverse micro-organisms?

It's an interesting question - why does it act more effectively on the 'undesirable' yeasts and other microbes? The answer is not clear but it seems that microbial sensitivity to sulphite is the norm and resistance is a mutation. Most natural weak acid preservatives (eg vinegar (acetic acid), benzoic and and sorbic acids) are believed to work by being able to enter into microbial cells by molecular diffusion through the cell membranes because they are in part lipophilic (fat soluble). Once inside, they ionise and increase the acidity (lower the pH) and the cell homoeostasis mechanism has to work very hard to pump out protons to restore the pH. Eventually the cells become so exhausted that they run out of ATP (their energy source) to do this and give up and die or at least stop growing. Sulphite is believed to work in the same way as other weak acid preservatives. In addition, it can bind to or disrupt sulphur bridges in cell proteins, inhibit enzymes, and interfere with DNA replication.

Sulphite-resistant organisms appear to have the ability to synthesise acetaldehyde in response, more readily than other microbes. This binds up the sulphite and makes it inactive hence it is neutralised and the cell survives and continues to grow. It cannot be an accident that acetaldehyde is a key intermediate (the last step in the chain) of the synthesis of alcohol from glucose. In other words, the same mutation which confers the ability to be a wine or cider yeast (smooth fermentation of sugar to relatively high alcohol levels) entails the facile ability to generate higher levels of acetaldehyde than normal, which will bind up the sulphite and make it inactive. This is the working hypothesis that most fermentation microbiologists go with.

More information for tecchies!

All the dosage information above is derived from long-standing work at the Long Ashton Research Station and other wine research institutes, which started in the 1950's and culminated in the late 1970's. It is based on the empirical fact that the level of molecular SO₂ required to kill adverse yeasts and bacteria but to allow beneficial ones to flourish is around 1 part per million. To get this level of molecular SO₂you actually need a lot more free SO₂ because there is a pH related equilibrium which keeps most of the SO₂ in the inactive bisulphite ion form. Hence, in the table and chart above, the amount of SO₂ you need to add depends on the pH.

Unfortunately, that's not all the story. When you add SO₂ to juice or cider, some of it becomes bound to juice components like glucose, galacturonic acid, pyruvate etc. Hence the total SO₂ you need to add must also take account of this binding. It is the total SO₂ which is given in the table and chart above. This is not an exact science because it needs to make certain assumptions about the levels of the binding components, which will differ depending on the nature of the fruit, how many rotten apples got in etc etc! So the figures given in the table and chart are necessarily approximate. In the table, the column for total yeast kill is based on a target value of 1 ppm molecular SO₂ and for partial yeast kill is based on 0.5 ppm.

Here's a summary which depicts the various forms in which SO₂ might exist in a fermented cider after sulphite addition. The same general principles apply to a juice before fermentation, though the binders are different. The most important point is that only the small amount of molecular SO₂ is actually effective as an anti-microbial.

If you want to know more, and to find out how that distribution is calculated, and you have software which can read Excel spreadsheets, then download this file on sulphite binding and addition. It contains three worksheets. The first one gives a table of molecular, free and total SO₂ at different pH values (and is more comprehensive than the table above). The second is just a graphical representation of the free and total SO₂ columns (for a 'typical' apple juice) and is where the accurate addition chart comes from. The third worksheet gives more detail on how the binding calculation is carried out. There are also some literature references given to show where the science comes from (and my publications page also contains a couple of downloads of relevant papers which should give some further background, although they are focussed more on the addition of SO₂to cider after fermentation than to juice beforehand ).

Last edited 7th July 2011

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