A recent beer - In The Pink - has turned out more than a little disappointing and tastes too fruity estery with no balancing tartness as I planned. Based on previous recipes I added enough acid malt to give me a good sourness, but it failed to materialise in the end product so I planned to replace it by the direct addition of lactic acid.
I bought a small bottle of food grade 60% lactic acid solution and added a couple of millilitres to the bottom of a glass before pouring 450ml beer in. The resulting tartness was just detectable. I started again with around 5ml of acid and added the beer. This time the beer had a pleasant sourness to it, without going into extreme Gueuze territory.
The answer to rescuing this beer would seem to be to add around 1% of a 60% lactic acid solution to it.
It would also seem that acid malt doesn't have a quantitative way of measuring the effect it will have on the finished beer.