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.