Use 1 teaspoon per cup of water. Can use up to 4 teaspoons in a five gallon batch. Lactic acid comes in various concentrations with 88% being the most popular for home brewing. It is used to acidify the sparge water to a pH of 6 or lower (relative to mash temperature) which helps reduce the chance of extracting mouth-puckering tannins from the grain husks while we sparge slowly. We also use lactic acid to lower the mash pH when the various water adjustment salts we add do not take the pH low enough (and we do not want to add more salts).