Titulo corto: Evaluacion de la fermentacion del lactosuero acidoTitulo en ingles: Evaluation of acid whey fermentation (whole and deproteinized) using Lactobacillus caseiResumen: El lactosuero acidificado espontaneamente pone en riesgo la salud del consumidor al utilizarlo como acidificante de la leche usada en la elaboracion del queso tipo mozzarella, y ademas reduce la calidad del producto. El objetivo de esta investigacion fue evaluar el proceso de fermentacion del lactosuero acido (entero y desproteinizado) con Lactobacillus casei, utilizando diferentes porcentajes de su inoculo. Se caracterizaron fisicoquimicamente los lactosueros, sometiendolos a fermentacion anaerobica a 37°C y 120 rpm por 96 h. Se evaluo la concentracion de biomasa celular, el consumo de lactosa, la produccion de acido lactico (AL) y se estimaron los parametros cineticos y estequiometricos. Se aplico un diseno experimental completamente al azar con arreglo factorial 2x3, empleando la metodologia de medidas repetidas. Los datos, se procesaron a traves de los paquetes estadisticos SAS® version 9 y Statgraphics Centurion XVI®. El lactosuero, entero y desproteinizado, se encontro dentro de los intervalos fisicoquimicos aceptables para su acidificacion. Los valores de acidez fueron superiores a 120 Grados Dornic (120°D) con la inoculacion de L. casei, excepto en lactosuero desproteinizado, inoculado con 15% de cultivo. El lactosuero entero con 15% de inoculo, alcanzo 120°D en el menor tiempo (34h). Altas concentraciones de inoculo (15%) favorecieron la acidificacion en el lactosuero entero, mostrando la mayor produccion (20.83gAL/l), Y´p/s=0.86, Qp=0.173gAL/l*h; mientras el lactosuero desproteinizado mostro un mayor crecimiento microbiano, menor conversion de lactosa (8.2%) y menor produccion media de acido lactico (8.19g/l).Palabras clave: suero lacteo; acido lactico; queso hilado; parametros cineticos.Abstract: The spontaneously acidified whey threatens consumers’ health and reduces quality of mozzarella cheese. The goal of this research was to evaluate the acidification process of acid whey (whole and deproteinized) by fermentation with different inoculum concentrations of Lactobacillus casei. The whey was physicochemically characterized and anaerobically fermented at 37°C and120 rpm during 96h. Cellular biomass was evaluated, as well as lactose consumption, lactic acid production, kinetic and stoichiometric parameters were estimated. A completely randomized design with factorial arrangement (2x3) was applied by using the repeated measures methodology. Data was processed by using SAS® statistical package, version 9 and Statgraphics Centurion XVI®. The acidification for the whole and deproteinized whey was found within the acceptable intervals. Acidification values were above 120 degrees Dornic (120°D) with L. casei inoculation, except in deproteinized whey inoculated with 15% of inoculum. The whole whey with 15% of inoculum reached 120°D in the shortest time (34 hours). High concentrations of inoculum (15%) aided the acidification in whole whey and showed the highest production (20.83 g LA/l), and p/s=0.86, Qp=0.173 g AL/l*h; on the other hand, deproteinized whey showed the highest microbial growth, lowest conversion of lactose (8.2%) and lowest lactic acid production (8.19g/l).Key words: whey; lactic acid; cheese yarn; kinetic parameters.