How much potassium metabisulfite should I add when racking my wine or cider?
Measurements in grams/gallon please. Thanks!
I'm not certain about grams per gallon (why are you mixing metric and imperial?). The general rule for wine (and probably cider as well) is one campden tablet or 1/8th tsp of k-meta per gallon.
According to wikipedia one campden tablet typically weighs .44 grams and 10 campden tablets equals 1 tsp of k-meta so I'd guess between .44 and .55 grams of k-meta per gallon. (1/8th of 4.4g is .55g)
Adding a little extra won't hurt, especially if your wine or cider is done fermenting.
I generally add 1/4tsp / 5 gallons. 1/4tsp == 1.4 grams so 0.28/gallon.
There are a number of factors (ph, white vs red...), but I've used that measurement for years.