I saw a recipe (in Croatian, check out if you understand) for making octopus salad that says boil the octopus with a piece of wine cork. The article doesn't explain what effect the cork gives. Do you know what the purpose of it may be when added to boiling octopus?
The muscle fibers of octopus are very thin compared to other species, arranged in multiple layers and alternating rings, which are then even further reinforced with collagen, 3-5 times more than regular fish muscle fiber. It's basically the reinforced concrete in the world of muscle fibers.
There are exactly two ways to get tender octopus:
- Destroy the collagen by force or to cook it for a very long time like a stew
- Barely cook it to a core temperature of not more than 130-135°F/55-57°C. At 140°/60° temperature the collagen layers will contract and you are screwed.
Note that these two methods generate edible octopus, but with different texture.
The wine cork serves no purpose. If it has any effect at all, it will negatively affect the destruction of the collagen, as tannin is actually used to cross-link, a process to make the collagen stable and durable, which is the complete opposite of what you want, when you want tender octopus.
As an unrelated side note, octopus and squids have the least flavorful flesh of all fish and molluscs, as they use Trimethylamine N-oxide for osmotic balance; which happens to be completely tasteless. Other species use tasty amino acids. Unless you happen to have one as free by-catch on your own fisherboat, there is not much reason to invest the time and costs to make a dish out of it.
In Galicia, corks were tied to squids. That made it easier for the cook to raise and lower the squid out of boiling water - the historic recipe calls for doing that to the squid 5-7 times. With modern cooking utensils, that practice became obsolete, and wine cork is thrown in the pot purely as a historic tradition.
I would guess that it is the wine residue in the cork that helps.
In my humble experience, cooking squid or octopus is either very short or very long and slow.
Squid or octopus in any Mediterranean salad is parboiled for a minute or less in acid liquid, water with either lemon, vinegar, wine and then cooled. This results in something 'chewy' for a salad. I doubt large pieces of octopus would work.
If you desire tender, then long and slow is the way to go.