I think your "already damaged" thought is correct. I would think of oxidation like rust. If you take steps ahead of time, you can prevent the rusting, but once it has occurred you cannot just reverse the damage.
I am not 100% sure what causes the oxidation with LME. LME will darken with age in the package and I doubt there is much oxygen present. I would note that while oxygen is a strong oxidizer, oxidation can occur without the presence of oxygen.
One definition: "Oxidation is a chemical process. It is defined as a process that occurs when atoms or groups of atoms lose electrons. Another way to define oxidation is when a chemical species gains oxygen or loses hydrogen. When these events occur, oxidation takes place."
It could also be that the darkening is something else, such as Maillard reactions.
i disagree.
prehopped wort extract is not always already damaged. prehopped wort has a very bad rep around here. due to low turnover rate and long shelf storage.
in country's like the uk and aus there is higher turn over and less risk of old warm stored cans. coopers diybeer offers direct to customer and is always fresh IMO. i never got a stale can from them.
a fresh can of coopers will make good beer.
a few things tho:
dont boil it its already done for you just reconstitute it. ascorbic and brewtan will do nothing for your kit,
the best way to improve (change) color in any beer is steep specialty grains.
im not sure what you mean by improve the color do you mean make it darker or lighter or clearer?
tannins are added for wine not beer
for taste improvement try the following:
to make good kit beer follow these rules:
use spring water not tap (you could prolly use distilled also since lme has plenty of nutrients and minerals already.)
use better yeast . either so5 or bry 97 or some other very clean ale yeast.
MOST importnat IMO is temperature control dont ferment anything over 68 degrees. you will be safer. (maybe 70 highest)
brew to 4 or 5 never 6 gallons.
add hop tea for both bittering and flavor/aroma. 1/2 oz of aroma hops steeped in 170 degrees for 10 mins. works well you can change the amoutn or the time in order to change the hoppiness.
Dont use pure sugar unless you are doing the lager or pilsner kit. those can handle the dryness of sugar but not a kilo thats too much maybe 1 lb at most for 5 gallons.
i almost always add brewers crystals or brew booster or some other maltodextrine containting mix to soften up the mouthfeel. or just MD and sugar 50/50 mix.
if making an ale porter or stout ( ale kits are usually not good IMO) add some extra DME for body. theres a lot of threads on making kits good. homebrewforumuk.com has a whole forum dedicated to kit brewing.
these steps will add flavor in the form of hop tea and steeped grains and body from either more dme less sugar or Maltodextrins.
it took me a long time to figure this out.
good luck