Preservation of viability and anti-Listeria activity of lactic acid bacteria, Lactococcus lactis and Lactobacillus paracasei, entrapped in gelling matrices of alginate or alginate/caseinate

In order to control undesirable microorganisms growth in foods, the performance of alginate and alginate-caseinate (an aqueous two-phase system) matrices entrapping lactic acid bacteria (LAB) (Lactobacillus paracasei LAB1 and Lactococcus lactis LAB3) was investigated. Polymeric matrices were initially loaded with LAB cells at ∼108−10 or ∼104−6 CFU mL−1, and were monitored, in liquid and gelled form (beads), for 12 days at 30 °C. In the liquid form, maximum cell density (∼109 CFU mL−1) was reached after 24 h whatever the matrix. Then, the LAB population decreased but remained higher in alginate-caseinate matrices: 107 and 106 CFU mL−1 of LAB3 cells were enumerated after 12 days in alginate-caseinate and in alginate matrices, respectively. Anti-Listeria activity (assayed by agar well diffusion method) did not vary much over 12 days and was also higher for cells entrapped in alginate-caseinate matrices. When matrices were gelled, similar trends were observed: at “Day 12”, LAB3 population was 104−5 and 102−3 CFU/bead, and, LAB1 population was 105−6 and 103−4 CFU/bead, in alginate-caseinate and alginate beads, respectively. Antimicrobial activity of alginate-caseinate beads containing LAB1 cells was quite constant over 12 days. The anti-Listeria activity of LAB cell-free supernatants incorporated in matrices with caseinate was also higher. In fact, the presence of caseinate was shown to promote both the survival of LAB cells and the release of their antimicrobial metabolites. Observation of liquid and gelled matrices by confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) revealed a preferential localization of LAB cells in casein-rich microdomains which could affect favorably the efficiency of bipolymeric matrices.