Measuring and affecting alcohol content, as part of the Cider makers FAQ
What are the methods for calculating alcohol content?
What is Specific Gravity and why do I need a Hydrometer?
For most amateurs and small scale cider / perry producers, a good quality hydrometer is an essential piece of kit. A hydrometer is used to measure the Specific Gravity or SG of liquids; in essence the hydrometer is measuring the liquids density. The denser the liquid, the higher the hydrometer will float.
Sugars are quite large, heavy molecules and the more of these present within the juice, the denser it will be. When the hydrometer is placed in a sample of fresh-pressed juice, the sugars that form a (hopefully) large part of this juice will cause it to float quite high in the liquid. A reading should be taken at this point before any fermentation has taken place.
Alcohols are much lighter molecules and the presence of these will make the liquid less dense, so the hydrometer will start to sink and sit at a lower level as fermentation progresses, and the fermentable sugars are used up. Regular readings will enable you to keep an eye on how the fermentation is progressing.
Pure water has an SG of 1.000, so if at the end of your fermentation your cider/perry has an SG of less than 1.000, you have produced a dry cider/perry which contains lots of alcohol - and all of the fermentable sugars will have been used up. Well done! Target achieved!
Take a note of this final reading and compare it with the very first reading and then you will be able to calculate the percentage of alcohol present (ABV). See Calculation below.
- Ahh, but....
Yeah, ok. For the purists out there, there are a number of other factors that can affect your hydrometer readings and if you are trying to be as accurate as possible, (particularly if the tax man may be involved...) you need to be a little more pedantic.
- Dissolved non-fermentable substances and other solids (debris, fruit particles, tannins, pectins, etc.) - These will give you a higher initial reading. Allowing time for the juice to settle and for some of these things to fall out of suspension will give you a more accurate reading. Draw off some juice (a bottle full should do) and after due time for settling out, use this for your readings. Better still, filter this sample and your final sample to ensure there are no bits floating around to affect your readings.
- Temperature - Colder liquids are more dense than warmer ones and can significantly affect readings. Try to ensure that the liquid you take the readings from is always at roughly the same temperature; I believe that most hydrometers are calibrated to be accurate at a given temperature which is usually marked on the hydrometer or printed in any instructions that come with it.
- Meniscus (what...?) - The meniscus is the way that the surface tension that affects the surface of liquids, causes the surface to 'creep' (or curve) up any vertical surface in contact with it. What this means in practice is what you see may not necessarily be the actual reading... So always use a clear glass hydrometer jar / cylinder to get a clear as possible reading, sighting along the actual flat surface of the liquid and not the meniscus.
- Bubbles... - Any dissolved gasses can attach themselves to the hydrometer like little water-wings and again cause it to float higher than it should; always give the hydrometer a good spin before taking a reading.
Always keep the hydrometer clean and store it in a dry, protected place. Wash it thoroughly before and after use in warm (not hot!) water, and give it a rinse in some sterilising solution before you use it in any sample juice that will be put back into the bulk.
If you measure the specific gravity of the juice before and after fermentation then a calculation can be performed to give the alcohol content. Roy Bailey uses the following method to assess alcohol by volume (ABV):
subtract the final gravity from the original gravity and multiply by 0.1285.
Worked example: 1050 - 998 = 52. 52 x 0.1285 = 6.682% ABV
Or you may like to use this handy Online Alcohol Calculator c/o Stonehelm Brewing and Vinting Supplies of Oxford.
These measure the sugar content of a liquid by using the way the concentration of sugars refracts light. A few drops of the juice are placed into the refractometer and the sucrose (sugar) content can be read. Some of these instruments have automatic compensation for changes in temperature. These are serious pieces of kit - and are priced accordingly. A quick search on the internet will come up with prices and suppliers.
Professional testing services : http://www.hamstead-brewing-centre.co.uk/msg5.htm
Rough guide to potential ABV
If you are using a hydrometer, the following initial SG readings could give these potential ABV's:
SG ---- %ABV
It is uncommon for an SG of 1070 or over to be achieved without adding sugar, but it is not unheard of in a very good year!
Raising the SG (and potential ABV)
SG's of less than 1040 mean it's unlikely for the cider to keep well - alcohol is a good preserver and a sufficiently high ABV is desirable for this reason. Adding ordinary white sugar is probably the best way to raise the ABV to a more desirable level (to say 1050/1055) without affecting the taste. Add the sugar in small quantities, for instance 100g. per gallon at a time (preferably as a solution) and frequently check the SG until the desired reading is achieved.
Raising the SG and potential ABV by adding sugar is euphemistically called chaptalisation, though many purists would call it bastardisation... This is named after the 18th/19th century French chemist Jean-Antoine Chaptal, who suggested the addition of small amounts of beet sugar to wine musts in poor years when the SG was not high enough. This is still permitted under French wine law today, but only under strictly regulated conditions. Most commercial UK ciders are now made by gross chaptalisation of juice with glucose syrup to an SG of maybe 1.110. This gives a 'base cider' ca 14% alcohol, which is then diluted with water before retail sale.
Of course you could also add any other source of fermentable sugars, such as fruit juices, honey, etc. but then you would not end up with cider / perry, but something that tastes completely different and that also has a different name. Also, remember that if you add anything other than sugar, apple or pear juice, you would fall foul of Customs and Excise regulations if you tried to sell anything made this way without consulting the tax man first...