Effect of inoculum composition and low KCl supplementation on the biological and rheological stability of an immobilized-cell system for mixed mesophilic lactic starter production.

Two strains of Lactococcus lactis subsp. lactis (L. lactis KB and KBP) and one of L. lactis subsp. lactis biovar. diacetylactis (L. diacetylactis MD) were immobilized separately in kappa-carrageenan-locust bean gum gel beads. Continuous fermentations were carried out in supplemented whey permeate in a 1-L pH-controlled stirred tank reactor inoculated with a 30% (v/v) bead inoculum and a bead ratio of 55:30:15 for KB, KBP, and MD, respectively. The process demonstrated a high productivity and microbial stability during the 7-week continuous culture. Compared with previous experiments carried out with an inoculum bead ratio of 33:33:33 for KB, KBP, and MD beads, respectively, the modification of the inoculum bead ratio had apparently little effect on free and immobilized, total and specific populations. A dominant behavior of L. diacetylactis MD over the other strains of the mixed culture was observed both with free-cell populations in the effluent and with immobilized-cell populations. Additional experiments were carried out with other strain combinations for continuous inoculation-prefermentation of milk. The data also confirmed the dominance of L. diacetylactis during long-term continuous immobilized-cell fermentations. This dominance may be tentatively explained by the local competition involved in the development of the bead cross-contamination and in citrate utilization by L. diacetylactis strains. The gel beads demonstrated a high rheological stability during the 7-week continuous fermentation even at low KCl supplementation of the broth medium (25 mM KCl).

Adaptation of Lactobacillus alimentarius to environmental stresses.

Lactobacillus alimentarius BJ33 has been tested for its biopreservative capacities to improve quality and safety in many meat products. The combination of different preservatives such as NaCl, glucono-delta-lactone and citric acid with this protective culture during the manufacture of sausages represent an interesting alternative to control microbial spoilage and to extend product shelf life. The use of these preservatives may also limit the growth of L. alimentarius. In this study, the sublethal doses of these preservatives were determined and tested in combination to verify if the organism was able to adapt to these stresses. The sublethal doses of gluconic acid, citric acid, and NaCl were 100-110 mM, 50-55 mM and 8%, respectively. When the culture was first grown in MRS broth containing citric acid (50 or 55 mM) or gluconic acid (100 or 110 mM) and then transferred in MRS broth containing NaCl (8%), only limited growth was observed (O.D.(600 nm) = 0.2-0.3) after 6 days at 30 degrees C. However, when the culture was first grown in NaCl and then transferred in MRS broth containing gluconic or citric acid, growth was observed after 1 day (O.D.(600 nm) = 0.4-0.5) and after 5 days an O.D.(600 nm) of 0.8 was reached. Cell filamentation was also observed under electron microscopy when cells were grown for 2 days in presence of gluconic and citric acid at their sublethal doses and with a combination of 18 mM gluconic acid and 37 mM citric acid, but cellular elongation was not observed with cultures exposed to 8% NaCl. These results suggest that two different adaptation mechanisms are induced in L. alimenatrius when treated with organic acids and NaCl.