This is my first try at making cider. I had great results from fermentation. I was short of some things like a hrdometer so it was reacted off into 2 car boys and sealed with airlocks stored in a dark place with a very constant temperature +/- 3 degs. For 3 years. It is a nice clear amber color. What SG should I be looking for or reading my new toy refactor meter Opinions Please ?
Measuring specific gravity can tell you when fermentation has finished, but after several years you can already be quite confident that it has! SG measurements are of no use at all in determining if the product is safe to drink.
The good news is that cider is quite low pH and contains a fair amount of alcohol, so it's not friendly to many pathogens.
If the airlock ran dry, allowing significant oxygen ingress, you will likely find the product has undergone another fermentation process and turned to vinegar. An aerobic (oxygen-requiring) family of microbes called Acetobacter will convert ethanol to acetic acid if enough oxygen is present.
If the product has escaped oxygen ingress, has dropped clear, and doesn't have have a pellicle on the surface or any sign of mould, it is quite likely to be OK. Probably somewhat oxygen damaged as an airlock and bung are not 100% effective at excluding permeation, but drinkable.
If the product had been stored in glass carboys with a proper lid I would be very confident that it would be in good condition.
Your best tool here is your nose, and if your nose leaves you confident, your sense of taste.
First to answer the title of your question. Yes cider can certainly go bad! It will go bad the same way as other wines:
Luckily spoiled brews almost always taste extremely awful, so you'd never be in doubt if your brew has been spoiled.
SG is starting gravity, so it seems a bit late for your 3 year old brew. But given the typical ABV of ciders I'd say a typical SG would be in the range of 1.040-1.070; that would give you an ABV of 5-9% if all the sugars fermented out.
A note to remember regarding refractometers is that they are nice and easy to use with SG, but when there is alcohol in the solution you need to do some calculation on your reading to get the true value.
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