THE SCIENCE OF CIDERMAKING
Part 3 - Juicing and Fermenting
Materials of construction
Before we go any further it is worth considering the equipment which will be used for milling, pressing and fermenting. I shan't repeat the description of the mills and presses themselves which was given in the first article, but it's worth stressing that only certain types of materials should be allowed to come into contact with juice and cider. Most metals should be avoided, with the noteable exception of food-grade stainless steel which is excellent but costly. Aluminium is permissible for short periods only. Iron and copper should never be in contact with cider or juice because they dissolve in the fruit acid to give strange colours and flavours. Lead is particularly dangerous, because it dissolves to give a sweet compound which is potentially fatal. Indeed in the 18th century the so-called 'Devonshire colic' claimed a number of lives and this was eventually discovered to be caused by cider which became contaminated when standing in lead pipes overnight in pubs and inns. Similarly the old practice of lining juice tubs or press trays with lead sheeting is highly dangerous.Wood is quite permissible and of course for many years was the only practicable material for fermentation and storage vats. It may be difficult to keep clean and free from bacteria but at least it will not poison anybody! Wood coated with modern polyurethane varnish (e.g. for press racks) is much easier to keep clean than is unsealed wood. For fermentation and storage tanks, food grade stainless steel, plastics, fibreglass and epoxy resins are generally preferable to wood, because they contain no pores where undesirable bacteria and moulds can lurk. Glass is also very satisfactory on a small-scale. If you particularly want to use wooden barrels, make sure that they are well scoured, bleached and rinsed or steamed beforehand. They should also be 'sweetened' with 5% sulphur dioxide solution (see Table) before a final rinse with clean water. It should go without saying that all equipment and containers in contact with juice or cider should be well cleaned (and well rinsed) beforehand. Modern non-foaming sterilising detergents such as 'Chempro' are most effective in this role, and should be used according to the instructions given on the packet.Fruit and juice blending
Once the apples have been chosen, washed and milled to a pulp, they must be transported to the press in a suitable container - probably the ubiqitous plastic bucket! In the present article we shall assume that this is done almost immediately without 'cuvage' or 'maceration'. Even so, the juice and pulp will become quite brown in a matter of minutes and it is here that the natural colour of the product is determined. The press juice then needs to be collected in another container and at this point it is convenient to measure its sugar level, acidity and pH so that blending may be corrected with other batches of juice pressed on the same day. A fair amount of sugar still remains in the dry press-cake (or 'pomace') so by adding a litre or two of water to each 5 kg of broken-up pomace before re-pressing, a useful yield of slightly weaker juice may be obtained, which is usually added to the first pressing.Previously we described the composition of the ideal cider fruit in terms of materials such as sugar, acid and tannin. Sugar levels are set largely by the weather - in a good summer we might expect them to be as high as 17%, but in a cool wet summer less than 10% might be achieved. The sugar levels can be measured directly on a drop of juice squeezed out from the fruit, using a hand held refractometer. This equipment is expensive (ca £70), but is often used by grape-growers, who need to measure sugar content daily as harvest approaches. For cider-making, the changes in sugar levels are not so critical and the fruit will usually have been stored for a while to convert all the starch into fermentable sugar anyway. So it is usual to measure the juice 'specific gravity' (S.G.) after pressing, using a hydrometer, which is much cheaper (ca £5). Roughly speaking, 15% sugar corresponds to an SG of 1.070 and a total potential alcohol of 8.5 %; 10% sugar is SG 1.045 and a potential alcohol of 6%. If the juice S.G. is less than 1.045 and you have no sweeter juice for blending, it should be brought up to this level by the addition of sugar or apple juice concentrate. Otherwise the resultant alcohol level may not be sufficient to protect the final cider during storage. To raise the S.G. in 5° steps, dissolve 12 - 15 grams of sugar in each litre of juice and re-test with the hydrometer until the desired level is reached.Acidity and pH
The acidity is controlled more by the variety of fruit than the climate. Acidity has two aspects - total acid and pH - and both are useful to know. The total acid relates well to our perception of acid flavour, while the pH relates better to various aspects of fermentation biochemistry. These two are connected but not in a simple way, although the acidity always goes up as the pH goes down and vice-versa. In terms of total titratable acid (as malic), we should be looking for 0.3 - 0.5% in a cider juice. If the total acid is too low, the pH will be too high and the fermentation will be susceptible to bacterial infections. If the total acid is too high, the pH will be low enough to safeguard against infection but the final cider will be unacceptably sharp to the palate and may never be pleasant to drink. Acidity can be measured by titration - details will be found in any good wine-making book. Kits for measuring titratable acidity are available.Measurement of pH has to be done by a dedicated 'pH meter'. These used to be very expensive, costing several hundred pounds, but modern 'chip technology' has now brought them down to the range of £30 or so. However, beware the very cheap pH meters which are sold in garden centres for soil testing - these are not accurate enough for cidermaking because we need to measure to at least the nearest 0.1 pH unit or it is not worth making the measurement at all! Narrow range 'pH papers' (e.g. pH 2.8 to 4.2) are now available cheaply from some home brewing suppliers and are a good substitute. A desirable juice pH range for cider-making is say 3.2 - 3.8. At higher pH the fermentation will be subject to microbial infection and at pH 4.0 or above this can lead to serious flavour problems. Many traditional bittersweet cider apples tend to be high in pH which is why they need blending with more acid fruit, preferably before fermentation. That is one reason why bittersharp apples, such as 'Kingston Black', have been regarded as near perfection in terms of their composition for single-variety cider making.If you cannot measure the acidity or the pH, taste the juice instead. Trying to ignore the sweetness and the tannin, judge whether the juice is insipid, balanced or sharp. If insipid, and you have no other juice for blending, malic acid may have to be added in steps of 1 gram per litre (0.1%) until the balance is improved. If the juice is too acid, and you cannot blend it out, you may have to encourage a malo-lactic fermentation to reduce it (see later), or you can add a little calcium carbonate to neutralise it, in 1 gram per litre steps.
Other juice parameters, such as tannin, are difficult to measure, but only people using a high proportion of bittersweet fruit are likely to suffer from excessive tannin and this can usually be detected by taste although the juice sugar does tend to mask it. Deficiencies here can be corrected after fermentation, however. The purpose of blending before fermentation is to give a juice as close in composition to the 'ideal' which was described in the previous article. Although this may not always be possible, it is always worth the attempt at least in terms of sugar and acid levels. Blending after fermentation is a worthy and useful art but it cannot correct a gross biochemical imbalance beforehand!
Juice preparation
Apart from the blending corrections described above, you can of course always add sugar, glucose syrup, synthetic malic acid and apple juice concentrate to any desired extent along with water. On a commercial scale there are considerable cost advantages to be be gained by doing so, since sugar and water are much cheaper than apple juice (and many commercial ciders are now made from around 35% juice and 65% glucose syrup) but these have to be weighed up against the ultimate quality of the cider you wish to make. Excessive dilution will make the cider 'thinner' in its overall complexity of flavour and cannot be recommended for a high quality product.The blended juice should now be strained through a coarse plastic mesh into a suitable clean vessel for fermentation. Whatever scale you are working on, you must also have some sort of 'airlock', whch can be fitted before fermentation begins or shortly afterwards, to allow carbon dioxide gas to escape but to prevent air getting in. At this point a number of other additions may be made. If it is important that the final cider should be sparklingly clear, a pectolytic enzyme may be added, which will help to ensure that all the pectin is broken down. Pectin is a sort of natural glue which sticks the apple cells together. Although it is water-soluble it is precipitated by alcohol, so it tends to lead to persistent hazes by the end of fermentation. Dessert fruit, or long-stored fruit, tends to suffer more from pectin release than does bittersweet fruit and will often give a very cloudy cider unless depectinised. Although there are natural enzymes in both apple and yeast which will break down the pectin during fermentation, these enzymes are often rather weak and require some assistance. The dosage rates for the commercial enzymes are given by the suppliers.The next addition is that of vitamins and yeast nutrient. These may be bought as such or may be added as thiamine and ammonium sulphate (or phosphate) respectively. The dosage rate is up to 0.2 milligrams per litre of thiamine and up to 300 milligrams per litre of ammonium salt. This is what was meant by 'amino nitrogen' in Table 1 of the previous article, and it is needed by the yeast to make protein and amino acids for its own growth. (This is not unlike human and animal nutrition - the yeast's carbohydrate or energy source is of course the apple sugar which is not in short supply!) Apple juices are generally very low in yeast nutrients (unlike beer worts or grape musts) and so your fermentation rate will probably be much improved if you add these. The fermentation is also much less likely to 'stick' or to grind to a halt before completion. The cider can therefore be racked and bottled sooner, reducing the chances of spoilage in store. On the other hand, it is undeniable that some of the finest ciders are fermented very slowly without the addition of nutrients, but the risks of failure are correspondingly greater. You pays your money and you takes your choice! Traditional cider-makers used to hang a leg of mutton or a side of beef in the fermenting vat to boost the nutrient levels. The meat broke down slowly in the acid juice, releasing soluble amino nitrogen which the yeast could use for growth. The supposed requirement of a few dead rats in every vat is a more colourful manifestation of the same idea!
Sulphur Dioxide
The next addition is that of metabisulphite, sulphur dioxide or SO2, which are all synonyms for the same thing. This topic always inflames great passions amongst the purist cidermaking lobby, who regard it as dancing with the devil - perhaps it is the connection with brimstone which worries them! However, it has a long and honourable history and the use of burning sulphur candles as a sterilant in wine-making is supposed to date back as far as Homer. Certainly it was in use for cider-making from Elizabethan times, and the controlled addition of metabisulphite is far more accurate than the haphazard application of sulphur candles could ever be.In simple terms what happens is that the sulphur dioxide inhibits the growth of most spoilage yeasts and bacteria, while permitting the 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 on the pH of the juice. The Table shows the appropriate levels to use when a cultured yeast is being added for the fermentation. Lower levels are needed if a 'wild' Saccharomyces fermentation is required (see below), or there is a danger that all the wild yeast will be killed. In the absence of sulphur dioxide, the fermentation is much less likely to be 'clean' although with care it is possible to do without it. A great deal of the concern about sulphite derives from its excessive use at bottling not during fermentation, and from the fact that a very few people are hypersensitive to it in the free state. However, it must be stressed that no sulphur dioxide remains free by the end of fermentation, since it becomes bound to various intermediate chemicals (principally acetaldehyde) which the yeast produces on its route from sugar to alcohol. I would always advise the beginner to use sulphur dioxide to minimise the risk of taints and infection. Later on, the experienced cidermaker can omit it at his discretion and see what difference it makes.| Above 3.8(insipid) | .....Lower pH to 3.8 with addition of malic acid..... | |
| 3.8 - 3.5 | 150 | 3 |
| 3.5 - 3.3(balanced) | 100 | 2 |
| 3.3 - 3.0 | 50 | 1 |
| Below 3.0(sharp) | None | None |
1. If a pH meter is not available, use the taste of the juice as a guide
2. To make a 5% stock solution of sulphur dioxide, 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.
3. Campden tablets are formulated with metabisulphite to give the equivalent of 50 ppm sulphur dioxide when each is dissolved in 1 gallon of liquid.
The Yeast
This brings us to the final addition, that of yeast. There are so many good dried wine-making yeasts on the market today that it is well worth considering their use. All of them will get a fermentation off to a good start within hours, by providing a massive inoculum of healthy yeast cells which will multiply quickly and swamp out anything undesirable. Some of these are more cold-tolerant than others and are capable of fermenting even down to 5° C, which can be a great boon to a British cidermaker whose raw material may not be ready until early November. Some yeasts claim to confer specific flavours e.g 'Burgundy', 'Champagne' but these claims should be taken with a pinch of salt and in any case are probably not relevant to cidermaking. Stick to a good general purpose wine yeast - not a brewer's yeast and never a baker's yeast, since these have been selected to have other properties which we do not require. There is no need to select a yeast with a high alcohol tolerance since the natural sugar of apples will rarely produce more than 8% alcohol. If you fortify significantly with sugar and you want alcohol levels up to 12%, then you are making apple wine - not cider! Large commercial cidermakers do just that (known as 'chaptalisation') and then dilute the cider with water for retail sale, but this series is not concerned with that sort of business.Small quantities of branded wine yeasts can be purchased from home winemaking suppliers. On a larger scale, you can buy specific strains of S. cerevisiae, bayanus or uvarumwhich are mostly produced overseas for the wine and fruit wine industries there. Modern dried yeasts are usually 'pitched' direct, but sometimes the yeast is grown up overnight as a 'starter' in sterile juice or sugar solution, and then pitched into the main bulk the next day. Sometimes the yeast only needs hydrating for 20 minutes or so. Whatever the case, it is important to follow the yeast supplier's directions. If sulphur dioxide is used, it is also important to wait overnight before adding the yeast culture. This is because the sulphur dioxide needs time to act against the wild organisms, and it will also inhibit the added yeast too strongly if they are all added together. By standing overnight, the free sulphur dioxide largely disappears once its work is done, giving the added yeast a chance to get away without significant inhibition.Fermentation should commence within 48 hours if an active yeast culture is used. As an alternative, it is possible to rely on the few wild Saccharomycesyeasts which will be present in the juice after sulphiting, and allow them to multiply to sufficient levels to start the fermentation, but this may take up to a fortnight. In this case you might prefer to use around half the addition of sulphite given in the Table. This is equivalent to the traditional practice of burning a 'sulphur candle' in the barrel before adding fresh juice. If neither sulphite nor yeast are added, the juice will probably start to ferment within a day, but the wild yeasts which multiply under these conditions cannot be guaranteed to produce desirable flavours. In any case, they will begin to die after a few days as the alcohol level rises, leaving the fermentation at the mercy of any other dominant organism which has been able to establish itself. If you are lucky, this may be a useful Saccharomyces species - if you are unlucky, you have only yourself to blame!
In summary, therefore, I recommend the beginner to use a pectolytic enzyme, to use sulphur dioxide and to add a cultured wine yeast after standing overnight. Later on you can try out a 'wild yeast' fermentation. You can perhaps skip the nutrients unless the fermentation begins to 'stick' or unless you know that your fruit comes from big old trees with very low nutrient levels and you are not prepared to wait a few months. The progress of the fermentation should be monitored every few days with a hydrometer and the fall in S.G. plotted on a graph against time (a fall of one degree S.G. per day is pretty reasonable). This makes it much easier to see whether sticking is occurring, and the nutrient and vitamin can be added then if necessary.
Conduct of the fermentation
In the initial stages of fermentation, there is considerable frothing and evolution of carbon dioxide as the yeast multiplies and begins to break down the sugar into alcohol. There may be as many as 10 million yeast cells per single ml. of juice at this stage, so it is easy to understand that there is a lot of microbiological activity going on! A loose plug and the outpouring of gas will probably ensure that nothing undesirable can creep back into the fermentation vessel, be it a demijohn, a barrel or a 5,000 gallon stainless steel tank. When the initial frothing subsides, however, it will be worth topping up the vessel with a 10% sugar solution and fitting a fermentation lock to ensure that the flow of gas remains one-way. As you follow the drop in S.G. with time, it will begin to level off and you should consider the first racking of the cider from its yeast at an S.G. of 1.005. If it stops fermenting at an S.G. much higher than this, then it may be 'stuck', and nutrient addition together with twenty minutes vigorous aeration may help the yeast to grow again (the yeast does need some oxygen for growth). It may also stop if the temperature falls too low, but this should need no attention from the cidermaker. When the weather warms up again, the fermentation should re-commence. In fact, a cool fermentation (ca 15° C) is generally preferred for cider and there is no need to keep the fermentation especially warm.If the cider is particularly acid at this stage, the first racking may be delayed for a month or so to encourage the 'malo-lactic fermentation' which is described below. In general, however, it is regarded as bad practice to leave a fully fermented cider on its yeast lees for more than a few weeks.The first racking should be into another clean vessel, trying to leave behind as much yeast as possible and with the minimum of aeration to the cider. This is generally done with a clean plastic syphon tube fixed to a plastic rod so it rests just above the yeast deposit or, on a larger scale, with a suitable pump. The transferred cider should be run gently into the bottom of the new vessel without splashing. Now that there is much less carbon dioxide to protect the cider, it is important to minimise the headspace and to prevent air contact as much as possible. This is partly to keep out any undesirable film yeasts or bacteria, and partly to prevent 'oxidation' which leads to flat dull flavours and a loss of freshness. This is why some people add 50 ppm of sulphur dioxide at every racking, although at the first racking this is probably unnecessary because of the remaining carbon dioxide. Sulphite added at this stage will almost certainly inhibit the malo-lactic fermentation, which may or may not be required (see below).
Maturation and Bottling
After the first racking the air-lock is re-fitted until it is clear that gas evolution has ceased, when the vessel should be topped up with water or cider and tightly closed. A second crop of yeast will be thrown as the cider settles down. The cider may remain in this state for several weeks or months, before a final racking to a closed container for bulk storage or directly into bottle. It is important that it should not sit for long on a heavy crop of yeast, because the dead yeast will 'autolyse' which tends to give unpleasant flavours. However, a small amount of autolysis from the second crop may be helpful, because this releases nutrients which stimulate maturation through the so-called 'malo-lactic' fermentation. This phenomenon is due to a specialised group of bacteria (Lactobacillus or Leuconostoc species) which convert the malic acid of the apple to lactic acid, giving off more carbon dioxide in the process. Often, this happens in the spring when the trees are flowering, giving rise to the notion that somehow the trees and the cider are working in sympathy! Generally the malo-lactic fermentation is to be welcomed, since it lowers the acidity and gives additional rounder smoother flavours, although in very low acid ciders it can reduce the acidity too far. In bittersweet ciders it produces characteristic 'spicy' notes (often detectable in ciders from Normandy). It may be recognised by the evolution of gas without renewed turbidity (if a yeast re-ferments a sweet cider it becomes cloudy because the yeast cells are so large (typically 10 microns). Malo-lactic fermentations, unless very heavy, tend to remain clear because the bacteria are so small (typically 0.5 microns).The malo-lactic fermentation is difficult to produce at will although some strains of lactic bacterial cultures are now available commercially for use in the wine industry and can be used in cider. It may definitely be prevented by the additional use of sulphur dioxide at racking. Sometimes it reduces the acidity too far and sometimes the 'wrong' organisms take hold, producing other defects such as 'ropiness' (which will be covered in a later article). But if the original juice pH was no higher than 3.8, the chances are that this fermentation will be beneficial if it happens at all. Even if it does not, the cider will mature for several months as its flavour balance stabilises and the harsher notes are smoothed out by slow chemical and biochemical reactions.However, ciders do not generally profit by extended ageing and by late spring or early summer the cider will be ready for bottling and drinking, or for a second racking into bulk store. The golden rule at this stage is to minimise air contact whenever the cider is handled - it is a matter of preference whether you wish to add sulphur dioxide (ca 50 ppm) to help with this, but in any case you should not exceed a total addition of 200 ppm SO2 to any cider when all additions at fermentation and bottling are summed up. A dry cider with no added sugar and sufficient alcohol should be quite stable in clean, closed and well-filled bottles, and should stand a minimal risk of any unwanted conversion to vinegar!
We have now looked at the steps in producing a still, dry cider which is the easiest sort to make. In the next article we shall look at variations of this process to produce other types of cider.
© Andrew Lea 1997. Lightly updated 2009
Source: http://www.cider.org.uk/frameset.htm