The precipitation of alpha-lactalbumin (alpha-La) by acidification of whey and whey protein concentrates (WPCs) was studied in this work. Three different acids (HCl, citric acid and lactic acid) were considered to perform the precipitation step. Sweet whey, WPCs with different protein concentration and whey protein isolates (WPIs) were used as feed solutions. The two organic acids were able to complexate the Ca2+ ions. However, when hydrochloric acid was used, protein precipitation was due to the irreversible denaturation of the proteins. The precipitation process was performed at different operating conditions (protein concentration, pH, temperature and acid/Ca2+ molar ratio) at laboratory scale in order to be optimised. The optimum initial protein concentration was observed to be around 12 g/L. A temperature of 50 degrees C, a pH value of 4.0, and an organic acid/Ca2+ molar ratio higher than 9 were the optimum values to perform the precipitation at laboratory scale. When the precipitation process was carried out at a pH value close to the isoelectric point (IP) of alpha-La and it was combined with calcium ion complexation, alpha-La was observed to precipitate together with BSA and immunoglobulins. However, beta-lactoglobulin (beta-Lg) remained in solution due to the stabilisation of this protein at low Ca2+ concentrations. (c) 2006 Elsevier B.V. All rights reserved.