★★★★★
So that's confirmation, lactic acid is the key ingredient that alleviates sunburn. This can be found in:
Sour yoghurt (eg Greek Yoghurt), sauerkraut, kombucha, kimchi, kefir, and other sour cultured dairy such as sour cream
Yogurt (and Lactic Ferments)
★★★★★
I was in quite a bit of pain from sunburn and a lady near to me said she used to get sunburnt all the time when working in the construction industry (one of the most male dominated professions I imagine..). She said their remedy of choice was Greek Yoghurt. She was quite specific in that it should be real Greek Yoghurt and none of that sweet junk if you want this to work. She said it works straight away. I reckon it should be sour so as to maximise lactic acid, so the older batches are presumed to work better.
Working on the principal that it was lactic acid, I applied kombucha to the sunburn, using tissue paper to dab it on (as I didn't have yoghurt in the house). Success! I took photos and could see a reduction in redness within 10 minutes. It is really quite noticeable. The swelling has greatly improved too.
I suggest that sauerkraut juice would also be a nice and easy one to try.
(PS: The she also recommended aspirin for reducing swelling. But I really wanted to focus on cultured foods as that is more in keeping with the spirit of this website.)