The present invention is in the field of the preparation of lactic acid by means of fermentation on industrial scale wherein a concentrated raw beet juice having a Brix of at least 60 is used as fermentation substrate. After dilution to the desired initial sugar concentration and addition of nutrients, the juice is fermented to lactic acid and/or lactate by means of a lactic acid-producing microorganism. Said concentrated raw beet juice is prepared by: washing and cutting sugar beet and extracting the cossettes in water, removing the beet pulp from the resulting raw beet juice, and heat treating the raw beet juice at a temperature between 50 and 90 degrees Celsius, and concentrating the raw beet juice to at least 60 Brix.It was found that concentrated beet juice having a Brix of at least 60 is storage-stable, is not very sensitive to infections, and can be used as fermentation substrate for lactic acid production on industrial scale with the same yield, chemical purity, optical purity, clarity and taste as lactic acid obtained from fermenting white sugar.