BATCH FERMENTATION WITH ENTRAPPED GROWING-CELLS OF LACTOBACILLUS-CASEI - OPTIMIZATION OF THE RHEOLOGICAL PROPERTIES OF THE ENTRAPMENT GEL MATRIX

Growing cells of Lactobacillus casei were entrapped in κ-carrageenan/locust bean gum (LBG) (2:1 or 2.75%:0.25% w/w respectively) mixed gel beads (two ranges of diameter: 0.5-1.0 and 1.0-2.0 mm) to ferment Lactobacillus Selection (LBS) medium and produce biomass. The results showed significant influence of initial cell loading of the beads and bead size on the fermentation rate. The highest cell release rates were obtained with 2.75%:0.25%κ-carrageenan/LBG small diameter gel beads. However, 17 h fermentation of LBS medium with immobilized cells resulted in substantial softening of the gel matrix, prohibiting reuse of immobilized biocatalysts as inoculum in subsequent batch fermentation. A dynamic shear rheological study showed that the gel weakness was related to chemical interactions with the medium. Results indicated that part of the matrix-stabilizing K+ ions diffused back to the medium. Stabilization of the gel was obtained by adding potassium ions to the LBS medium;L. casei growth was not altered by this supplementation. Fermentation of LBS medium supplemented with KCl by L. casei showed higher cell counts in the broth medium with immobilized cells than with free cells, reaching 1010 cells/ml after about 10 h with entrapped cells in 0.5-1.0 mm diameter beads and 17 h with free cells. Counts in the gel beads after fermentation were higher than 1011 cells/ml and bead integrity was maintained throughout fermentation. © 1990 Springer-Verlag.