The operation in global yoghurt market provokes frequently the reject and withdrawn of yoghurt derivates out of shelf life. Yoghurts that have past their 'best before' date constitute a waste that has to be environmentally treated. Its use as a source for lactic acid production by Lactobacillus casei is studied in this work, being also a proposal that can reduce the end-of-cycle impact of the residue. Production of yoghurt generates a residue highly supplemented with sugar and fruits syrups that can be metabolized to lactic acid, but no data are available in the literature about this potential transformation into a commercial valuable lactic acid. Bioconversion of total sugars around 44% was achieved in pH controlled batch fermentation after 34 h. Control of pH and yeast extract supplementation in the preculture stage have shown a remarkable enhancement of fermentation performance due to the reduction in the toxicity and the increase in nitrogen content. Successive bioconversions of lactose, glucose and sucrose have been shown, with a sugar consumption profile giving a hierarchical mode consumption in the order glucose > sucrose > lactose. (C) 2010 Elsevier Ltd. All rights reserved.