By: Gillian Grafton
This page is organised into several sections. The principle stages of the fermentation are described, followed by an overview of the cider making process, a discussion of the characteristics of the apple juice, the microbiology of the process, the changes in the composition of the cider during fermentation, and finally a description of how to make your own cider. Don't be put off by the technical detail, you don't need to know any of it to make your own high quality cider - just skip direct to the home cider making section, but if you are interested, I hope that this document satisfies your curiosity. There's a further reading section at the end if you want to know more. I'd be more than glad to have your feedback, questions (although I don't promise to be able to answer them all!) and so on. This document is as accurate as I can make it, but you're on your own - I don't accept any liability for the contents!
Cider is made from apple juice which has undergone two different kinds of fermentation. The first fermentation is carried out by yeasts which have either been added deliberately or which are naturally present on the apple skins. This fermentation converts sugars to ethanol and the higher alcohols (fusel alcohols). The second fermentation, the malo-lactic fermentation converts L(-)-malic acid to L(+)-lactic acid and carbon dioxide. This fermentation is carried out by lactic acid bacteria which are present in the apple juice and also in the area in which the fermentation is carried out. The malo-lactic fermentation can occur concurrently with the yeast fermentation but more often it is delayed until the fully fermented cider reaches 15 C, normally in the late spring or early summer of the year following that in which the cider was made.
The Cider Making Process
Traditional cider making starts with the picking of the apples. These are left to mature for a week and then tipped into a "scratcher" which crushes the apples. In more modern plants the apples are reduced to a pulp in a grater type mill made of stainless steel. The apple pulp is known as the pomace or pommy
Next the pulp must be crushed to extract the juice. This is done in a cider press. Several types of press are used. The traditional type is a rack and cloth press (sometimes known as a pack press). In this type of press a sheet of sisal or hessian is placed across the bottom of a square frame above a trough. A layer of pomace, 4-5 inches deep, is poured onto the hessian. The hessian is folded over the pomace, completely enclosing it. Another sheet of hessian is placed on top of the first and the process repeated until the layers fill the frame. The cider press is then racked down onto the layers and the juice runs into the trough. The pomace is pressed until it is solid and no more juice runs out. The press is then racked up, the layers of pomace are broken up by hand, and the whole lot is re-pressed. In modern plants mechano- hydraulically operated plate presses are used. The pressed pomace is either dried in hot air to 12% moisture and used for manufacture of pectin, or it is directly sold on for cattle feed.
The freshly pressed juice may be fermented straight away. In some commercial operations it is concentrated and stored for later conversion to cider, in which case it is extensively treated to pasturise it and to remove pectin. The fresh juice may be fermented in one of two different ways. Traditionally the juice is run into a wooden pipe (a barrel which can contain 120 gallons) or smaller wooden barrels, and the bung of the barrel is removed. No yeast is added, traditional cider making relies on wild yeasts. The fermentation starts in 1-2 days and continues for several weeks, during which time the barrel is topped up with more cider. When fermentation is over, the bung is replaced and the cider matured for 5-6 months.
Alternatively the juice is treated with sulphur dioxide to inhibit natural wild yeasts, and is then fermented with added pure yeast cultures. This method is used in high output commercial operations. After the initial fermentation subsides, the cider is left for the yeast to settle, and it is either racked and/or centrifuged and placed into storage tanks. Storage may last 12-18 months, and the cider is blended with new and old ciders to moderate any excessive changes thus maintaining a consistent flavour profile year on year. These cider blends are nearly always cleared by centrifugation or kieselguhr filtration. This type of cider is sterilised by sterile filtration or flash pasteurization and is artificially carbonated in the bottle by counter-pressure bottle fillers. Sulphur dioxide is added at this stage to maintain the stability of the cider. The resulting product may be considered analogous to keg beer.
Traditional cider is often served completely flat and may be cloudy. It may also be served a naturally-conditioned cask cider, analogous to real ale. In France, cider is produced by the Charmat process (often used to produce sparkling wines) and is highly carbonated and more like an apple wine than traditional English cider.
Characteristics of Apple Juice
Compared to wort, apple juice has a much lower pH, a much lower soluble nitrogen content, and a virtual absence of any sugars other than mono- and di-saccharides. The composition of the juice varies with the apple variety used. The average composition of cider apple juice in terms of its sugar content is 74% fructose, 15% sucrose, and 11% glucose. There are almost no other sugars present so that there is very little residual gravity left in fully-fermented ciders.
The major acid present is L(-)-malic acid but shikimic, quinic, chlorogenic and p-coumarylquinic acids are commonly present. The juice also contains soluble pectin (polymers of galacturonic acid esterified with methanol). Tannins are present, mainly epi-catechin, dimeric and trimeric pro-anthocyanidin and phenolic acids. These phenolics are the fraction which undergoes oxidation in damaged fruit.
The soluble nitrogen content is low and is largely made up of asparagine, aspartic and glutamic acids. Apple juice usually contains one eighth of the soluble nitrogen content of wort. The lower nitrogen content is further exaggerated by the much lower pitching rates used in cider making when compared to beer making, usually 5-15 times lower. This means that the apple juice must support a higher degree of yeast growth and thus the fermentation is much protracted. Some commercial operations now add ammonium sulphate to the cider to give rapid and consistent fermentations.
The Microbiology of Apple Juice
Ripe apples have less than 500 yeast-like organisms per g of sound fruit. The main organisms are Aureobasidium pullulans, Rhodotorula spp., Torulopsis, Candida, Metschnikowia, and Kloeckera apiculata. Saccharomyces species and other sporulating yeasts are rarely found. Acid-tolerant bacteria such as Acetomonas spp. are usually present. Lactic-acid bacteria are rare. The amounts of micro-organisms rise if the fruit is allowed to fall naturally or particularly if the skin is damaged. Yeast counts rise due to the indigenous flora of the factory in which the apples are processed. The traditional rack and cloth press is also a major source of contamination.
Apple juice cannot be sterilised by heating since the pectin esterase enzymes in the juice are destroyed by heat, thus the resulting cider will not clear. Addition of sulphur dioxide is the most common way of controlling unwanted organisms. The amount of sulphur dioxide needed depends on the pH of the juice. Between pH 3.0 to 3.3, 75 ppm is needed, between pH 3.3 and 3.5 100 ppm is necessary and 150 ppm between 3.5 and 3.8. In the UK the maximum legal limit for sulphur dioxide is 200 ppm and this may well be lowered by subsequent legislation. Always check with your local authorities! The sulphur dioxide can be added in the form of Campden tablets. The juice is left overnight to allow the different forms of dissolved sulphur dioxide to equilibrate. Aerobic yeasts, and lactic and acetic acid bacteria are generally destroyed. The activity of other yeasts is usually inhibited. If there were substantial amounts of rotten fruits used to make the juice, compounds present in these fruits such as 2,5-D-threo-hexodiulose and 2,5-diketogluconic acid will strongly inhibit the action of the sulphur dioxide. As well as preventing infections, the sulphur dioxide also has an anti-oxidant function producing a cleaner flavour. This is not necessarily an advantage, the use of sulphur dioxide has led to sweeter ciders with a loss of the apple character in the flavour.
The malo-lactic fermentation is carried out by non-slime forming strains of Leuconostoc mesenteroides, Lactobacillus collinoides and very rarely Pediococcus cerevisiae. These bacteria are readily inhibited by the levels of sulphur dioxide used in cider making yet ciders readily undergo malo-lactic fermentation in the spring/summer after they were made. The explanation for this is not certain, possibly lab strains of these organisms are more sensitive to sulphur dioxide than are wild strains, possibly the sulphur dioxide merely inhibits the bacteria and they subsequently recover, or possibly there are other organisms at work.
Changes in Apple Juice Composition During Fermentation and Maturation
The majority of this section refers to ciders fermented with naturally occurring yeasts. It is assumed, but not known, that similar process occur when fermentation with pure cultures is used.
At the end of the yeast fermentation, yeast release nitrogenous compounds into the cider. These include amino acids and peptides. Pantothenic acid and riboflavin are also released along with some phosphorus compounds. The release of nutrients is important since it is necessary for the malo-lactic fermentation to occur.
During the yeast fermentation there is an increase in acidity due to the formation of L(- )-malic acid by the yeast. Gluconic, lactic and succinic acids are also formed. Mono- di- and tri-galacturonides are present from the enzymic degredation of pectin, and keto acids are also formed. Higher or fusel alcohols are formed; unlike beer where they are unwanted compounds, in cider they form important components of the flavour profile. The levels formed depend on apple variety, juice treatment, yeast strain, fermentation conditions, and storage conditions. In general, low pH and low nitrogen levels tend to produce ciders with higher fusel alcohol levels. Use of sulphur dioxide, and centrifugation of the apple juice before fermentation both result in the lowering of fusel alcohol levels. The factor most affecting fusel alcohol levels is the strain of yeast. Aeration is also a factor, aeration reduces fusel production markedly.
The maturation phase of cider production includes the malo-lactic fermentation. In this stage, malic acid is converted to lactic acid and carbon dioxide. The exact type of acid produced depends on pH. At pH 3.6 more lactic than succinic acid is produced, whilst at pH 4.8 only succinic acid is produced. The nearer the pH is to 3.0, the more delayed is the onset of the malo-lactic fermentation. As well as the conversion of malic to lactic acid, this fermentation also sees the production of quinic and shikimic acids both of which are essential for a good flavour balance.
Home Cider Making
First pick your apples. They should be fully ripe, windfalls are excellent. Do not use heavily bruised or damaged apples. After picking, keep in a cool place for 1-2 weeks to soften the skins. Do not wash or sterilise the apples if you wish the cider to be fermented with wild yeasts. You can perform this step if you want to ferment with a specific yeast strain, however treatment with sulphur dioxide (see below) will get rid of wild yeasts. If you have apples which have small amounts of damage you can cut these parts out, but it is not essential and many traditional cider makers avoid this step.
Having matured the apples, you will need to press them. A domestic fruit juicer will achieve this but I know from bitter experience that this is a laborious time-consuming process, and the return in terms of juice per pound of apples is poor. Much better to buy yourself a wine makers fruit press, the sturdier the construction the better. Alternatively you can build your own press.
Once the juice is separated from the pulp you must check the pH. If the correct balance of apple varieties is used, this step may be omitted. Few of us are fortunate enough to obtain the correct types so some compensation must be made to ensure that there is sufficient sharpness but that it is not overdone. pH should be in the range 3.9 to 4.0. To lower the pH add malic acid (the principal acid in cider). To raise the pH add precipitated chalk. 1 tsp of pectolase per gallon of juice may be added at this stage to ensure that the cider clears. Traditional ciders shun this step and some can look like cloudy apple soup. Never fear, they still taste great.
If a correct balance of cider apples has not been available it may be that you need to compensate for a lack of sweet apples. Only experience with the particular varieties available to you will tell. Measure the O.G. (this may be difficult if the juice was not sufficiently well separated from the pulp). The target O.G. should be around 1055. If not, add sugar to bring it to this level. A good guide to how much to add is 2 1/2 ounces of sugar will raise the gravity of 1 gallon of juice by approximately 5 degrees. You can either dissolve the sugar in a small quantity of juice and add to the bulk of the juice, or if very fine (caster) sugar is used, stir it directly into the bulk of the juice. DO not heat the juice or you will get a cooked apple flavour which will ruin your cider.
Place the apple juice in a fermenting vessel. Traditionally this is a wooden barrel. If these are not available, any suitable wine fermenter would be fine. Put under an airlock and leave to ferment. Cider is traditionally fermented at whatever is the outside ambient temperature, however, if you are fermenting with a pure yeast culture it may be better to ferment at the temperature specified with the culture. There are wild yeasts present on apple skins (so long as they are from an unsprayed orchard) which will ferment the cider naturally. If you wish to ferment with a specific yeast, add 1 crushed campden tablet per gallon of juice and leave to stand, covered, for 48 hours. This will see off the wild yeast. Then pitch with a yeast of your choice. For a traditional style English cider, use an ale-type yeast. For a Normandy style cider use a wine yeast. Kitzinger, Hock, and Champagne yeasts all give good results. The finished product is paler than English cider and tastes closer to apple wine than does English cider.
Check the gravity regularly. There is a tendency to go on fermenting after the desired gravity has been obtained. To prevent this, you can add a crushed campden tablet to the cider when the desired gravity is reached.
Once the desired gravity is obtained, the cider is ready to mature. Store the cider in glass carboys or other similar container, under airlock. Cider is usually left in outbuildings to mature. The fluctuations in temperature are not detrimental. In the late spring or early summer following the making of the cider, it will undergo a malo-lactic fermentation. This will occur when the temperature reaches approximately 15 C. This has the effect of mellowing the cider, it will lose much of its sharpness. You can add malic acid or acid blend at this point if the cider is not sharp enough for your taste. Traditional English cider is flat, no attempt is made at a secondary fermentation. English cider may also be served slightly carbonated analogous to real ale. The target carbonation in this case is 1 volume of carbon dioxide per volume of cider (partial pressure of carbon dioxide of 1 atmosphere). If the cider is to be served slightly carbonated, bottle in beer bottles with 1/2 teaspoon of sugar per pint of cider (dissolve the sugar in water and add to the cider before bottling). Normandy cider is refermented in Champagne-style bottle in a manner similar to Champagne making and is highly carbonated. Consult a good wine making guide for details on how to do this.
And finally - enjoy your cider!
These are the sources I used in writing this document. I strongly advise you to read them!
Cider Making and Cider Research: A Review by F.W. Beech. Journal of the Institute of Brewing volume 78 pages 477 to 491. 1972. Despite being published in a technical journal this is a highly readable account of cider making accessible to anyone although you'll get more out of it if you have a biology background.
Making Cider by Jo Deal. Published by the Amateur Winemaker Publications Ltd. ISBN 0 900841 45 1, 1976. This is a tremendous little book! It tells you how to make cider, describes a cider press you can make at home, gives you recipes for cider, cider punches, and a terrific section on old English food recipes using cider. There are also a couple of good cider drinking songs. Only the brief section on the history of cider making lets it down. I strongly recommend that you buy this.