- should I transfer this stuff into some growlers to age and for how long?
I would advise against storing anything in growlers for the purpose of aging. They're too oxygen permeable and you have no good way to test out a small bit of the batch without opening and affecting a whole growler. However, cider gets better with age, and you should age it about as long as you can stand. At least a month after fermentation.
- I've read about backsweetening with apple juice concentrate to sweeten it but is this added before aging and How much do you add?
This is really the tricky part. Backsweetening cider has a lot of caveats- unless you can keg in in which case you can cheat a bit. I'll assume you aren't kegging this. You can backsweeten your cider with anything, but if you add fermentable sugars, they will ferment unless you add enough sorbates to stop fermentation- and in doing so you will prevent your ability to carbonate your cider. Cider tends to ferment almost all of its sugars and will typically terminate around 1.000 given enough time. How much to add is really up to you, but ciders typically turn out much dryer than new brewers expect in flavor- it won't be anything like an Angry Orchard. It will be bone dry when fermentation is done.
- methanol isn't an issue unless fermenting with apple chunks present, correct?
I've never had any concern of methanol in fermenting cider in any state- whether from store bought or fresh pressed with pulp. I don't think you could generate enough naturally to cause you ill effect unless you freeze distill it- and in that case the alcohol will probably hurt your brain more than the methanol content.
- Is the refractometer correction for cider the same as beer (eg: 1.04)?
Refractometer correction seems to be half math, have observation so it's really hard to answer this with confidence. It can vary from device to device, the starting gravity is relevant, and because the liquid density surely is different- I can't assert that it is the same. I can say that if you give your cider enough time, it will be basically 1.000.
- is oxygen just as much of a concern (transferring, etc) with cider as it is with beer?
Oxidation is still an issue- but not to the extent that is in beer. This is mostly personal experience..I can smell an oxidized beer a mile away. Ciders have to be pretty low quality, and horribly oxidized for me to pick it up. Obviously- prevent oxidation when possible but unless you're really abusing it I don't think you should worry too much.
- any other helpful tips for cider?
The hardest part of making cider is also the most important: have patience. Even if you have a kegging system, making good cider takes time. Conditioning is massively important and will do wonders for your cider. I know bottling 1 gallon in 12oz bottles is a pain- but I really recommend it so you can see how much your cider changes over the course of a few months.