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THE SCIENCE OF CIDERMAKING

Part 4 - Customising your cider

In the last article we looked at the production of a straightforward dry, still cider. In this part we shall look at the production of sparkling and sweetened versions, concluding with an outline of a traditional French and English technique which many regard as the pinnacle of the cidermaker's art.

Dry carbonated cider

To get some bubbles into our cider we need to incorporate excess carbon dioxide under pressure and to put the cider into bottles or into a keg which will withstand it. Then, when the pressure is released and the cider is dispensed, out come the bubbles! This is done commercially by chilling the cider and dissolving two or three volumes of carbon dioxide in the liquid using special equipment. Small scale commercial carbonation units can be bought or, on a domestic scale, a bottle or two can be carbonated using a soda-syphon and a 'Sparklets' bulb.

It is more satisfying, however, to allow the fermentation itself to generate the carbon dioxide by 'natural conditioning'. One way of doing this is by racking and bottling the fermentation early, say at a gravity of 1.010, and allowing the cider to finish fermenting and to mature in the bottle. The CO2 produced will dissolve in the cider to produce bubbles when the bottle is opened. A drawback to this technique is that the yeast deposit in the bottle may be rather heavy and coarsely flavoured. An alternative is to rack the cider into bottles after fermentation to dryness, adding a small amount (10 g per litre) of priming sugar to each bottle, and allowing a secondary yeast fermentation of the added sugar to produce the gas. This can be very successful although the bottom of each bottle will inevitably be a little cloudy when poured, because there will always be some yeast deposit which will be roused up when the pressure is released. This problem can be lessened on a domestic scale by storing the conditioned cider in a pressurised keg or barrel similar to those used for home-made beer. In these devices the yeast drops to a below below the draw-off tap and a bulk, sparkling and clear cider is easily achieved. The ultimate way of avoiding the yeast problem is to produce a cider by the 'methode champenoise', in which the yeast is removed by inverting and turning the bottle in stages until it is all collected in the neck. This is then frozen in an ice-salt mixture, the bottle is opened, the frozen yeast plug is forced out by gas pressure, and the bottle is topped up and resealed before the majority of the gas can escape. The quality of such ciders is legendary, although for obvious reasons they are labour-intensive to produce and therefore expensive to purchase!

Any bottles used for carbonated ciders must be designed to withstand the pressure generated by the gas, or there is a serious risk of them bursting and causing injury (not to mention the mess!). Some years ago there used to be quart cider bottles with internal screw threads and special threaded stoppers, but these no longer exist. An alternative is to use glass beer bottles which are sealed with a crown cork - these and the capping tools are widely available from home-brewing suppliers. The industry has now gone over almost entirely to PET (polyethyleneterephthalate) bottles which are lightweight and hold a moderate pressure well. Also, if they do burst, there is no risk of injury from flying glass. If you are making a small amount of cider for home use, you can recover, rinse and re-use these bottles several times if they have previously contained other carbonated drinks. If you cannot scrounge sufficient secondhand bottles or you are working on a larger scale, you may have to buy new PET bottles from a specialist supplier. Non-carbonated ciders can of course be bottled in wine bottles with regular corks if required, but the bottles must be stored on their sides to prevent the corks drying out and the air getting in (which will cause spoilage).  The plastic (PET) bottles do allow oxygen in through their walls and so the flavour of the cider does alter over time compared to natural conditioning in glass.

Sweet ciders

Even the 'dry' ciders described above will have a little residual sweetness, from the small amounts of non-fermentable sugars which exist in the original juice. However, many people prefer an overtly sweeter cider and so some way often has to be found of adding or retaining some sugar without running the risk of it re-fermenting. This is difficult if any yeast remains in the presence of an adequate supply of nutrients, because it will immediately get to work on the added sugar. Commercially, the problem is tackled by centrifugation and filtration of dry cider to remove most of the yeast, followed by pasteurisation to eliminate the remainder after the addition of sugar. This is not so easy to do on a small scale, although disposable-sheet filter units which can give near-sterile filtration (if properly operated) are available. The sweetened cider is then treated through a flow-through heat exchanger operating at 90o C with a residence time of 30 seconds so that the pasteurised cider is filled directly into warmed bottles. Equipment of this sort does not come cheap and can usually only be justified in the context of a commercial operation.  On a domestic scale you can sweeten your still cider to taste and then pasteurise the filled but unsealed bottles in a tank of hot water on a stove top, holding them at  68o C for 20 minutes. Use a digital thermometer in the bottles to be sure that the contents reach the specified temperature for the specified time.  Then cap the bottles, take them out of the water bath and leave them to cool on their sides to sterilise the inside of the bottle neck and the cap. Crown or screw cap glass bottles can be used, but not plastic because it will not stand the heat. 

Traditionally, naturally sweet ciders were made from slow fermentations which are poor in nutrients. Ciders which show an S.G. loss of less than one degree per day may be suitable for this treatment. The cider is racked initially into a new clean tank at say S.G. 1.030, leaving most of the yeast behind. The fermentation will then become even slower, and the sweet cider is racked again (and preferably filtered) at S.G. 1.020 - 1.025. (Racking at S.G. 1.015 will give a medium sweet cider). After this racking it is worth waiting several weeks (under an air lock) to ensure that no further fermentation takes place, before sealing the vat tightly or bottling off.

It is best to choose days on which the barometric pressure is high for these operations, since this will help to keep suspended yeast to a minimum and will retain the maximum amount of dissolved carbon dioxide in the cider. The success of the whole process depends on reducing both yeast and nutrient levels to a minimum so that re-fermentation of the remaining sugar is unlikely to take place. Sweet ciders of this sort may have a slight 'prickle' to them, particularly in bottle, since a slow fermentation may continue to generate carbon dioxide. The procedure described is ideal for single-variety demonstration ciders or for those which need no further blending - the flavour tends to be 'fruitier' since the sweetness is derived from unfermented juice rather than from added sugar. The alcohol level in the cider is of course less than if it had been fermented to dryness because only a part of the sugar has been converted. Ciders which do need blending (see below) are best fermented all to dryness first, before blending and finally sweetening after a further period of storage.

If you want to sweeten dry ciders with added sugar (or with frozen or concentrated apple juice) but you do not want to pasteurise or filter them, it is important that they should be racked and stored for several months after fermentation is complete, to allow the yeast to die out completely before the sugar is added. Otherwise the risk of re-fermentation is considerable; almost inevitable! The chances of re-fermentation can be somewhat reduced by the addition of yeast inhibitors such as potassium sorbate and benzoate at levels up to 200 ppm. (Both these materials occur naturally in rowan berries and cranberries respectively). Potassium sorbate may be bought from home winemaking suppliers. It is most effective if combined with say 50 ppm of SO2 added at the same time. But to be honest I do not advise this route for long keeping of sweetened cider - the risk of instability and explosion is just too great. Other additions, such as saccharin (which is of course not fermentable), may also be made to enhance the sweetness of the cider itself. Although these materials are perfectly safe, they do have certain drawbacks - saccharin has a noticeable aftertaste and sorbate may lead to 'geranium-like' off-flavours in the presence of malo-lactic bacteria. Most small-scale cidermakers will probably wish to avoid them.  The most successful artificial sweetener is the recently introduced sucralose, sold under the brand name "Splenda". The small compressed tablets are each equivalent to one teaspoon of sugar in terms of sweetening power and are ideal for just one glass of cider at a time.

Blending

Blending of ciders, if required, should always be carried out well before the final racking for storage or bottling. This is because the changes in acidity, nutrients and yeast levels, which occur when different batches are mixed, may affect the stability of the bulked cider and allow it to ferment further, even if the individual ciders were stable before blending. Similarly, if clear ciders are blended together they are quite likely to throw down a new haze or deposit which may need time to settle down. The general principle of blending is to ensure a flavour balance which is unobtrusive, particularly in terms of its acidity and tannin. It is best to take small test quantities of the ciders to be blended, and sweeten these first to the S.G. which will be required for the final blend (1.025 for sweet, 1.015 for medium sweet). Then a measuring cylinder should be used to blend equal parts of those ciders which are highest and lowest in tannin. The proportions should be varied until acceptable tannin levels are achieved, and the operation repeated for any other ciders which are unbalanced in tannin composition. If the tannin levels are too low because no bittersweet fruit was used, it is possible to increase them by the addition of grape or other food grade tannin (in 0.1% steps) until a satisfactory level is achieved. Careful note should of course be taken of the volumes used for blending and the amount of any tannin added. If the tannin levels are too high because of a large proportion of bittersweets, then fining with gelatin can be considered. This is covered in the next article.

The trial blends which are now balanced for tannin can be blended for acidity following a similar routine. If the addition of acid is required, malic acid may be used in 0.1% steps. Removal of acid is difficult at this stage, but may be done if necessary by the addition of potassium carbonate - calcium carbonate used here tends to leave a residual chalky flavour in the cider. The trial blends now have the correct tannin and acid balance, and they can finally be corrected for other more subtle flavours and aromas by blending amongst each other. Finally, the main bulk of ciders can be blended according to the proportions determined by the trials but without the addition of the sugar at this stage. For reasons explained above, the ciders must be allowed to stabilise further in bulk store before correcting the sweetness (unless filtration and pasteurisation are used to prevent all possibility of re-fermentation).

French and English tradition

We have now seen how to ferment, blend and bottle a number of styles of cider. It is now worth looking at the production of high quality naturally sweet ciders by the best traditional French and English methods. The objective of these is to lower the nutrient status in various ways so that the fermentation remains slow and the natural sugar can be retained without fear of re-fermentation.

A blend of fully ripened sharp and bittersweet fruit is used for these ciders, taken from mature orchards which are naturally low in nutrients but fairly high in tannin. Dessert fruit is much less likely to be successful here, due to its generally low tannin and high nutrient levels. The sugar level should be at least 12% (SG 1.055). The fruit is stored until a cold day late in the year when the temperature is about 5o C and expected to remain so for a week or more. The fruit is washed and milled in the normal way, but the pulp is then packed into barrels (or, better, plastic containers) to stand for up to 24 hours. This is the procedure of 'maceration' or 'cuvage', terms with no particular English equivalent. During this time, oxidation slowly proceeds which develops the juice colour, and pectin leaches out of the apple cells into the juice. The juice is then pressed out, rich in colour and thick in texture, and is run into clean tanks which are allowed to stand without sulphiting or the addition of yeast (author's note: If the pH is around 4, which it is likely to be, I do actually add 100 ppm sulphite at this stage to provide some inhibition of bacterial infection. The 'official' French recommendation is to burn 10 grams of sulphurated string in the barrel!!)

Since the temperature is low, no significant yeast fermentation takes place, but the natural pectic enzymes in apple juice slowly change the pectin to pectic acid. This forms a gel with the natural calcium in the juice and a 'brown head' (the 'chapeau brun') rises slowly to the surface. Some of the pectin also combines with juice protein and tannin and falls as a sediment to the bottom, leaving a clear juice between the two. To make this process more reliable, a mixture of calcium carbonate (3 g per 10 litres) and sodium chloride (4 g per 10 litres) is often added to the fresh-pressed juice - the calcium helps to form the gel, while the chloride helps to inhibit the growth of any yeast (author's note: I add 400 ppm (4 g per 10 litres) of calcium chloride which is a one-shot way of achieving the same thing). A specially prepared pectin methyl esterase enzyme (which is not the same as a regular pectinase) can also be added as described here. This process is known in French as 'debourbage' or 'defecation' (for obvious reasons!) and in English as 'keeving', and generally takes about a week. If things go wrong, and a yeast fermentation starts too early, a 'white head' (the 'chapeau blanc') is formed. This means that the whole vat has become turbulent and the keeving has failed!

If the keeving has been successful, however, the clear juice between the top cap and the bottom sediment is very carefully pumped or syphoned into a fermentation vat. It is now allowed to ferment under an air-lock in the normal way (with its own yeast), but this fermentation will be very slow because most of the nutrients in the juice will have been left behind in the 'brown head' and in the sediment. In fact, scientific study has shown that the pectin and the amino nitrogen nutrients are reduced by at least 50% during keeving. With such a slow fermentation it should be no problem to make a naturally sweet cider, by racking at S.G. 1.030 and proceeding as described earlier. Preferably the ciders should be bottled in crown-cap beer bottles which are stored in a cool place for maturation -they CAN get quite fizzy if allowed to warm up too much during the summer!

The advantage of this process is that it can produce a naturally sweet and well-coloured cider, brilliantly clear due to the removal of pectin during keeving, and full of flavour because of the low nutrient levels during fermentation. The disadvantage is that a lot of it depends on luck - the correct fruit, cold weather, benevolent strains of wild yeast and freedom from bacterial infections! What actually happens is that the fermentation begins with so-called apiculate' yeasts from inside the apples predominanting - these then slowly die out as the alcohol level rises and the Saccharomyces ( wine yeasts') slowly take over to complete the job. If you get the chance to look at the yeasts under the microscope, as I have done, you'll see all shapes and sizes of organisms imagineable. This is quite different from a fermentation with an added yeast, where all the cells are identical.

(Author's note: In my experience, the flavour immediately after even a well-conducted 'natural' fermentation is heavily dominated by ethyl acetate, and can be really quite unbalanced and unpleasant to drink. A few months storage in bottle, however, can work wonders in blending out these flavours to something really magnificent. Not everybody likes it, though - a friend of mine described my best quality cider as 'tasting like smoky bacon' (which is due to the natural tannins breaking down to give 'spicy' flavours)!!)

There is plenty of scope here for anyone who wishes to experiment with different parts of the process. For instance, the final colour and clarity of the cider is critically dependent on the length of 'cuvage', during which the oxidising enzymes produce colour from the tannin, and the pectin slowly migrates out of the fruit into the juice. If the pulp is held too long a time, the rich orange colour of the final juice will actually diminish and the flavour will become insipid due to excessive adsorption of oxidised tannin back onto the pulp. If the pulp is held too short a time, not enough pectin will migrate out into the juice to form a good 'head' during subsequent keeving and so, paradoxically, effective clarification and nutrient removal will not be achieved. If the pulp is tightly packed and too little air is present during 'cuvage', too little colour will be developed - but if the pulp is loosely packed and too much air is present, spoilage organisms (vinegar bacteria) will quickly take hold. If the weather is too cold, none of the desirable enzyme activity will take place - but if conditions are too warm, yeast fermentation will begin too early and the keeving will fail. If the pH of the fruit is too low (less than 3.6) the natural pectinase activity may be too slow to form a successful 'brown head' - but if the pH is too high (greater than 4) undesirable film yeasts will develop to the detriment of the required Saccharomyces species.

Balancing these factors (and many more) relies on judgement and experience and was part of the skill of the traditional cider maker, although he knew nothing of the biochemistry behind it. The new traditionalist should be able to build on this skill the better, if he only bears in mind the scientific principles of what he is trying to do!

So far we have considered cidermaking when everything goes according to plan. In the next part of the series we shall look at some of the more common things that may go wrong, and what we can do about them.

Andrew Lea 1997. Lightly updated 2009

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