EFFECT OF CONTINUOUS PREFERMENTATION OF MILK WITH AN IMMOBILIZED CELLBIOREACTOR ON FERMENTATION KINETICS AND CURD PROPERTIES

The production of fresh cheese involves a complex fermentation of milk with a mixed culture of mesophilic lactic acid bacteria. The aim of t his work was to compare the performance of the traditional batch ferme ntation process with a new process integrating a continuous preferment ation step with an immobilized cell bioreactor. Acidification and coag ulation kinetics, as well as microbiological composition and rheologic al and sensory properties of the final curd, were studied for the two processes during experiments conducted at 22, 26, and 30 degrees C. At 26 degrees C, the prefermentation step reduced the time to reach the draining pH by 10%, and coagulation time was reduced by 30%. The micro biological composition of the inoculum was more reproducible using an immobilized starter culture than a bulk starter culture. However, one strain of citrate-utilizing Lactococcus lactis ssp. lactis dominated, representing 80% of the bacterial population in the prefermented milk and in the resulting curd and 60% of the total population in the curd made from batch fermentation. Compared with the batch process, the con tinuous prefermentation of milk using an immobilized cell bioreactor h ad no significant effect on the rheological properties of the curd (su sceptibility to syneresis, firmness, and modulus of elasticity) or on sensory properties of the final fresh cheese.