Few studies have been reported on lactic acid production by kefir grains. Kefir has been widely associated with probiotic use due to its cell growth in food matrices such as milks, juices and sugary solutions. However, from industrial scale there are no reports of its use in lactic acid production. In this work, we carried out experiments to test and understand how glucose, galactose and lactose, during lactic-acid fermentation, were converted in lactic acid by kefir grains. Given the microbial complexity in coexistence in kefir grains, it is likely that kinetic studies are not the most appropriate for the positive or negative definition of the kefir use as a starter in the lactic acid fermentative production on an industrial scale. It was concluded that, although with a higher Lag phase, lactose was the substrate that best presented a product and cell conversion rate, although glucose and galactose can also be used as a substrate in the production of this carboxylic acid.