A process for producing pure lactic acid from a whey by-product rich in lactose and minerals, for example delactosed why permeate or concentrated whey permeate, is described. The method comprises upstream steps of neutralising the whey by-product with a basic metal hydroxide to form a precipitate comprising calcium and phosphate, and separating the precipitate from the whey by-product to provide a clarified whey by-product. The clarified whey by-product is fermentated by a bacterium capable of bioconversion of lactose to lactic acid to provide a fermentation broth containing a lactic acid salt. In the downstream steps, the fermentation broth is acidified to release lactic acid from the lactic acid salt, precipitate from the broth produced by acidification is removed, and the acidified fermentation broth is treated to recover pure lactic acid by removal of residual salts, and water, and optionally protein. The process of the invention produces lactic acid having a purity of 80-98% and an isomeric purity of >；98% L-lactic acid using a process that employs upstream removal of divalent salts by chemical precipitation, bacterial fermentation of the demineralised substrate, and minimum downstream processing of the fermentation broth. The methods of the invention may also be employed with milk permeates.L'invention concerne un procédé de production d'acide lactique pur à partir d'un sous-produit de lactosérum riche en lactose et en minéraux, par exemple un perméat de lactosérum délactosé ou un perméat de lactosérum concentré. Le procédé comprend les étapes en amont consistant à neutraliser le sous-produit de lactosérum avec un hydroxyde métallique basique pour former un précipité comprenant du calcium et du phosphate, et à séparer le précipité du sous-produit de lactosérum pour obtenir un sous-produit de lactosérum clarifié. Le sous-produit de lactosérum clarifié est fermenté par une bactérie capable de réaliser la bioconversion du lactose en acide lactique pour donner u
