Development of a Spiral Mesh Bioreactor with Immobilized Lactococci for Continuous Inoculation and Acidification of Milk

A laboratory-scale bioreactor with lactococci immobilized in calcium alginate gel was developed for continuous acidification and inoculation of milk.Cells were entrapped in a calcium alginate film coating a spiral mesh and placed in a column through which milk was recirculated from a reservoir.Steady-state conditions were achieved by addition of fresh milk using a pH controller to maintain the pH at 5.7 and acidified milk was continuously removed during operation periods up to 5 d.Immobilized and free cell bioreactors were compared using both proteinase-positive and proteinase-negative strains of Lactococcus lactis ssp.lactis C2.Productivities were 1.5to 3.5-fold larger with immobilized cell bioreactors than with free cell bioreactors because of higher cell densities, although specific productivities were lower for immobilized cells.Productivity increase was larger for proteinase-negative cells, which do not grow as well as free cells in milk.However, high densities can be immobilized, resulting in productivities of immobilized proteinase-negative cells that were similar to those of proteinase-positive cells.