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THE SCIENCE OF CIDERMAKING

Part 6 - Apple Juice and Cider Vinegar

In the first five parts of this series, we have looked at the making of cider. This final article looks instead at how we can stop the process one stage earlier to make apple juice, and how we can take it one stage further to turn our cider into vinegar.

Apple Juice

Apple juice is in some ways more difficult to make than cider, and indeed it can scarcely be regarded as a traditional product at all. Until the recognition late in the last century that fermentation was caused by yeast converting sugar into alcohol, the difference between juice and cider was somewhat obscure and of little practical importance in any case. Not until the invention of pasteurisation was there any practical way of preserving the juice with its full content of sugar. That is the fundamental process of juice-making - to preserve the sugar from the fruit without it turning into alcohol - and this can really only be done by heat treatment to kill the yeasts, or by deep-freezing or chemical additions to stop them growing. All these processes require a relatively sophisticated technology by traditional standards.

The milling and pressing requirements for juice making are no different from cider making, since both begin at the same point. The fruit requirements, however, are rather different. In cider we need high sugar to turn into alcohol, some acid to benefit the course of fermentation and some tannin to give body to the final blend. In juice the most important feature is the 'Brix/acid' ratio, which is the percentage sugar divided by the percentage acid. In the UK and the rest of Northern Europe, a ratio of 15 - 20 would be considered appropriate. In the USA, ratios as high as 30 are acceptable, but the juices would be considered very sweet to a British palate. Up to a point, the absolute values of sugar and acid do not matter so much as the ratio. Thus, a juice with 10% sugar and 0.5% acid would be equally as acceptable as a juice with 15% sugar and 0.75% acid, both having a ratio of 20. From this, it is easy to see that a Bramley juice with 10% sugar and 1% acid gives an unacceptably low ratio of 10, while a sweet cider cultivar with sugar of 15% and acid of 0.2% would have an unacceptably high ratio of 75. A bittersweet cider cultivar, with high tannin levels too, would also be quite inappropriate for juice making.

In practice, good juices can be made from a variety of dessert apples, and those which have interesting flavours in their own right (e.g. Cox or Russett) generally make interesting juices too. Apples which are delicate in flavour, such as Worcester Pearmain, make rather flavourless juices. Sweet apples can always be blended with Bramley to improve their acidity, while Bramley itself can always have some sugar added to improve its B/A ratio even though it will never make a first class juice. A good general starting point is three parts dessert apple to one part Bramley. Although the fruit must be clean and wholesome it can be small or misshapen. Indeed most commercial apple juice is made from such fruit which is cosmetically unsaleable on the retail market. But the fruit must be well-washed and NONE OF IT MUST BE MOULDY! If you wouldn't be prepared to eat it as fresh fruit, then it's not fit for juice-making!!

Cloudy 'fresh' juice

One of the best juices to make, if we are going to the trouble of making apple juice at all, is the pale cloudy juice which has become very popular in the UK in recent years. (Curiously, the basic process was developed in the USA and Canada but it is scarcely used there at all at present). The fruit is chosen, washed, sorted, milled and quickly pressed - fruit blending has to take place before pressing. After screening through a coarse mesh, Vitamin C (ascorbic acid) is added directly to the juice at the rate of 500 parts per million (5 g per 10 litres of juice). Pure powdered ascorbic acid should be used for this - it will be much cheaper and more convenient as a winemaking sundry than as a formulated vitamin from the chemist shop. The ascorbic acid allows certain oxidation reactions to happen in the juice which develop its flavour, but it prevents the browning of the tannins which make it look unsightly and lead to sedimentation.

Now the juice must be preserved without delay. If you have room in your freezer, it can be poured into plastic containers, or into polythene bags packed into cardboard shells. Once frozen, the juice can be withdrawn from the cardboard and stored as frozen polythene bricks. The juice can be thawed for use as required, but it will not keep long after thawing, because the enzymes and yeasts that were present in the juice originally will still be active. Nor will the cloud be 'set' and it may settle out rapidly when thawed. However, freezing is very convenient if you have the space to cope with it.

Pasteurisation

The alternative is pasteurisation. For this, the juice must be run into Kilner jars or good quality glass bottles (not plastic!) which can be sterile sealed by heating. Crown capped beer bottles, available from home brewing suppliers, will do quite well, as will good quality screw-capped bottles. The bottles or jars are filled to within an inch of the top and placed in a large pan of water which is put on the stove and gently warmed. The bottles should be as far immersed in the water as possible. Using a thermometer, bring the water up to 77 C and hold it at that temperature for 30 mins. Alternatively, place the thermometer in the centre bottle of the group and continue heating until the temperature of that juice itself reaches at least 74 C. At this temperature, all the yeasts should be destroyed. Take the hot bottles out of the pan and cap them immediately. Do not stand the hot bottles on a cold metal surface or the cold shock may crack them. If using Kilner jars, follow the usual procedure to obtain a sterile vacuum seal. If using beer or screw-cap bottles, seal them tightly and then lay them on their sides to cool slowly, so that the hot juice can sterilise the inside of the cap. Do not hurry the cooling process. Next day, the bottles may be stored at room temperature indefinitely until required, although the juice itself always tastes best if chilled for a few hours before drinking.

Heat treatment of this sort is very satisfactory although the occasional broken bottle may result during pasteurisation. Mould growth very occasionally occurs in the bottles during storage because mould spores can be extremely heat resistant although yeasts are quite easily killed. An advantage of heat treatment is that it actually 'sets' and stabilises most of the desirable apple juice cloud, which does not happen when the juices are frozen. On a large scale, purpose built pasteurisers may be purchased, or a handyman can convert a stainless steel sink with an immersion heater and a false bottom to maintain the correct temperature. Bulk pasteurisation of the juice itself in a tank or a saucepan is a poor alternative to in-bottle pasteurisation, because of the danger of overheating and excessive oxidation, and it is difficult to hot-fill the bottles aseptically on a small scale. Commercially, flow-through heat exchangers are used and the hot juice is filled straight into clean warm bottles.

Clear Juice

To make a clear golden juice it is necessary to destroy the pectin cloud and to allow a certain amount of oxidation for colour development. The most reliable way of doing this is to press out the juice as normal into a clean container, without the addition of any ascorbic acid. Add a pectolytic enzyme and keep the juice cool overnight to prevent yeast growth and fermentation. Next morning, the juice should be golden in colour and should have dropped bright, leaving a sediment at the bottom. If not, it may have to be fined with gelatin/bentonite (see Part 5) and left cool for a further day. Rack or strain the juice carefully into clean containers for preservation by freezing or by heat treatment as described in the previous section. Sometimes the colour becomes rather dark by this method, and a small amount of ascorbic acid (100 - 250 ppm) may therefore be added before enzyming to inhibit oxidation. In some cases the addition of pectic enzyme may be unnecessary since there may be sufficient enzyme and calcium present naturally in the juice for it to 'drop bright' by itself overnight in the cold - but you will not know this until you try it!.

WARNING

JUICES MUST NEVER BE BOTTLED (ESPECIALLY IN GLASS) WITHOUT EFFECTIVE PASTEURISATION ! If all the juice sugar ferments inside a closed bottle, it can theoretically develop an internal pressure in excess of 400 p.s.i. (pounds per square inch). This is more than enough to cause serious damage or injury when the bottle eventually explodes (as it almost certainly will). Just for comparison, even a properly designed champagne bottle is only expected to hold a pressure of about 100 p.s.i.

Cider Vinegar

From a biochemical viewpoint, cider vinegar is the next step after cider itself on the road which converts sugar through to alcohol, thence to acetic acid and finally to carbon dioxide and water. At each step, the organisms involved gain energy - this, after all, is why they do what they do and their metabolism is very little different from our own in many respects. Animals, however, do not stop at the alcohol or acetic acid stage. Some micro-organisms do and we can take advantage of this to provide the products that we want. Vinegar is simply a dilute solution (about 5%) of acetic acid which has been converted from a corresponding quantity of alcohol.

To make cider vinegar we need to start with a fully fermented dry cider with a minimum 5% alcohol content. Sulphur dioxide should not have been added for storage, because this will inhibit the conversion to acetic acid. Contrary to all good cidermaking practice, we then need to leave the cider in a vessel with plenty of access to air and to ensure that Acetobacter are present. These organisms, fatal to good cider, are just what we need for vinegar. The traditional set-up for vinegar-making is known as the Orleans or barrel process and consists of a barrel laid on its side, three-quarters full of liquid with open access to air. The bung hole is lightly plugged or covered with gauze so that oxygen can get in but flies cannot! A 'mother' of Acetobacter floats on the surface and converts the alchol to acetic acid at the rate of roughly 1% per week if the weather is warm enough. It really won't work if the temperature is below about 20 C, and goes much faster if it's warmer. So a cider with an alcohol of 6% could give a vinegar of 6% acetic acid in a couple of months or so. Two-thirds of the barrel is then drawn off as vinegar, fresh cider is added, and the cycle is repeated. Modern vinegar factories do not use this method, because it is far too slow. They use big fermenters with forced aeration and a very high population of acetic acid bacteria, which can convert a wine or cider to a vinegar within a few hours. Efficient as the big fermenters may be, the advantage of the barrel process is that is has no moving parts and virtually nothing to go wrong. You just have to wait a lot longer!

Setting up the system is the hardest part. Whereas it is easy to go out and buy a good fermenting yeast, it is much more difficult to buy a 'mother'.Traditionally, a vinegar barrel was always started by adding an inoculum of old vinegar from somewhere else. But in most cases it will be no good for you to buy a bottle of supermarket vinegar and hope to use it as a starter, since most  modern commercial vinegars are pasteurised or sterile filtered and the Acetobacter do not survive. However, if you hunt around the specialist food stores, you might find vinegar which contains live mother, and you can use that.  Pieces of 'mother' can also be purchased from home winemaking suppliers in the USA and in Germany, but nobody sells them in the UK, it seems. Acetobacter cultures are now available by mail order from Brouwland in Belgium, however, and although I have not tried them I see no reason why they wouldn't work as a starter to develop 'mother'. 

If you wait long enough, though, wild acetic acid bacteria will almost certainly find their way in. Probably the best plan is to keep an open jar of cider, covered with a coarse mesh, in a warm place for as many weeks as it takes for a 'mother of vinegar' to form. In this case the presence of fruit flies (Drosophila) may be quite helpful since they tend to carry Acetobacter around with them. It may be wise to add about 25% of commercial cider or wine vinegar to the jar to inhibit other non-acetifying organisms. Make sure that any vinegar you add does not contain any SO2 or other preservative - this will be stated on the label. The 'mother' is simply a floating mat of cellulose made by the acetobacter themselves to keep them close to the surface, since air is essential for their existence. Once you are sure you have a genuine gelatinous 'mother' and not a powdery film yeast, and you can really smell the vinegar, you can pitch it into your barrel with the required amount of still dry cider and your Orleans process will be under way. Keep it warm, up to 30 C if you can, for best results. Don't worry if the 'mother' drops to the bottom of the vat when you transfer it - a new 'mother' will soon grow on the top. 

Another method for generating a vinegar starter is to make a heap of fresh apple pomace, keeping it moist and preferably warm for several weeks. During this time it will ferment its residual sugars and natural acetobacter should then proliferate. Once it smells quite vinegary, the pomace can be squeezed out through a muslin bag and the resultant liquor (rich in acetobacter) can be used as a starter which will eventually develop a 'mother'. 

DO NOT, WHATEVER YOU DO, USE THE SAME EQUIPMENT AND VESSELS FOR VINEGAR MAKING AS FOR CIDER. The risk of cross-infection is just too great and it is not worth spoiling your good cider by trying to economise in this way. Keep both operations entirely separate! If you are making vinegar close to your cider, as you probably will be, it is doubly important that your cider-making kit be properly cleaned and sterilised anyway.

Once the vinegar is made it can simply be run into bottles for use. On a domestic scale there is no need for pasteurisation. Cider vinegar from the Orleans process is generally fairly clear but it may develop a further haze on storage in bottle. This is due partly to renewed growth of bacteria and partly to polymerisation of tannin. You can fine the vinegar with gelatin/bentonite if necessary to reduce an existing or a potential haze. If it is then important to prevent further clouding, SO2 at 50 ppm (i.e. 2 Campden tablets per 10 litres) may be added just before bottling, and this will inhibit both types of spoilage process.

Vinegar vats occasionally become infected with vinegar 'eels'. These are small and transparent nematode worms a few millimetres long, which live on the acetifying bacteria and which wriggle ceaselessly at the top of the vat. Although quite harmless they are generally unsightly and people do not like them. They may be destroyed by heating the vinegar to about 50 C, followed by fining or filtration after cooling. Or you can just leave them as a talking point for your guests - they will liven up any salad dressing

If you want to sell your vinegar it must legally be a minimum of 5% acetic acid. You can measure the acid by the method given here.

TAILPIECE

Well if you've got this far you MUST be interested! I hope you've enjoyed what you've been reading, and good luck with your cider, juice and vinegar making!!

 Andrew Lea 1997. Lightly updated 2016


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