Searching potential candidates for development of protective cultures: Evaluation of two Lactobacillus strains to reduce Listeria monocytogenes in artificially contaminated milk

Reevaluating a non-conventional procedure to microencapsulate beneficial lactobacilli: assessments on yield and bacterial viability under simulated technological and physiological conditions

BACKGROUND

Maintaining viability of beneficial microorganisms applied to foods still constitutes an industrial challenge. Many microencapsulation methodologies have been studied to protect probiotic microorganisms and ensure their resistance from manufacturing through to consumption. However, in many Latin-American countries such as Argentina there are still no marketed food products containing microencapsulated beneficial bacteria. The objectives of this work were: (i) to obtain microcapsules containing Lactobacillus fermentum L23 and L. rhamnosus L60 in a milk protein matrix; and (ii) to evaluate the viability of microencapsulated lactobacilli exposed to long-term refrigerated storage, mid-high temperatures and simulated gastrointestinal conditions.

RESULTS

The method of emulsification/rennet-catalyzed gelation of milk proteins used in this study led to high encapsulation yields for both strains (98.2-99%). Microencapsulated lactobacilli remained viable for 120 days at 4 °C, while free lactobacilli gradually lost their viability under the same conditions. Microencapsulation increased the resistance of lactobacilli to mid-high temperatures, since they showed survival rates of 95-99.3% at 50 °C, and of 72.5-74.4% at 65 °C. Under simulated gastric conditions, the microencapsulated lactobacilli counts were higher than 8.5 log CFU mL^-1 and showed survival rates between 96.61% and 97.74%. Furthermore, in the presence of bile (0.5-2% w/v) the survival of microencapsulated strains was higher than 96%.

CONCLUSION

The microencapsulation process together with the matrix of milk proteins used in this study protected beneficial Lactobacillus strains against these first simulated technological and physiological conditions. These findings suggest that this microencapsulation method could contribute to secure optimal amounts of living lactobacilli cells able to reach the intestine. © 2021 Society of Chemical Industry.

Control of Listeria monocytogenes growth and virulence in a traditional soft cheese model system based on lactic acid bacteria and a whey protein hydrolysate with antimicrobial activity

"Torta del Casar" is a Spanish soft-ripened cheese made with sheep's raw milk and subjected to a short ripening process, which favors the growth of pathogenic microorganisms including Listeria monocytogenes. The development of strategies to control pathogens and minimize health risks associated with the presence of L. monocytogenes in these products is of great interest. In this regard, the anti-Listeria activity of a whey protein hydrolysate (ProH) alone or combined with six lactic acid bacteria strains isolated from cheese was evaluated in this study as a biocontrol strategy using a "Torta del Casar" cheese-based medium. The most active combinations of lactic acid bacteria assayed induced a reduction higher than two logarithmic units in the growth of L. monocytogenes (serotype 4b) compared to their respective control when they were co-inoculated in "Torta del Casar" cheese-based medium at 7 °C for 7 days. In addition, the observed downregulation of some key virulence genes of L. monocytogenes suggests that the strain Lactiplantibacillus plantarum B2 alone and combined with the strain Lactiplantibacillus spp. B4 are good candidates to be used as biocontrol agents against L. monocytogenes growth in traditional soft cheeses based on raw milk during their storage at refrigeration temperatures.

Analysis of the Bioprotective Potential of Different Lactic Acid Bacteria Against Listeria monocytogenes in Cold-Smoked Sea Bass, a New Product Packaged Under Vacuum and Stored at 6 ± 2°C

The aim of the work was to monitor the presence of Listeria monocytogenes in cold-smoked fish products (trout, salmon, and sea bass) marketed in Italy. Cold-smoked sea bass is a new product that has not yet been commercialized and was collected from the production facility. Monitoring data have shown that cold-smoked products can be contaminated by L. monocytogenes, the presence of which has been highlighted mainly by enrichment culture (presence in 25 g). The isolated Listeria were serotyped and belonged mainly to low-virulence serotypes (1/2c), followed by serotypes 1/2a, 1/2b, and 4b. Furthermore, considering the ability of L. monocytogenes to grow in these products due to their chemical-physical characteristics (pH > 6.0, Aw > 0.97) and long shelf life at 4°C, an additional aim was to verify the activity of different bioprotective starters, including Lactilactobacillus sakei (LAK-23, Sacco srl, Via Alessandro Manzoni 29/A, 22071 Cadorago, CO, Italy), Carnobacterium spp., Lacticaseibacillus casei (SAL 106), and Lacticaseibacillus paracasei (SAL 211), in cold-smoked sea bass. All starters were bacteriocin producers. For this experiment, smoked sea bass samples were intentionally inoculated with a mixture of three different strains of L. monocytogenes and of each starter culture. After inoculation, the smoked sea bass were vacuum-packed and stored at 6 ± 2°C for 60 days, simulating the typical abuse storage temperature of markets and home refrigerators. At 0, 15, 30, 45, and 60 days, the sea bass samples were analyzed to evaluate the effectiveness of the starters against L. monocytogenes. Listeria monocytogenes growth was prevented only by the addition of the LAK-23 starter. Indeed, at the end of the shelf life, the amount of L. monocytogenes observed was similar to that in the inoculum. Consequently, the use of this starter can allow the inclusion of cold-smoked sea bass or smoked fish products in category 1.3 of Regolamento CE 2073/2005, which are products that do not support the growth of this microorganism. Finally, the activity of the LAK-23 starter did not produce an off flavor or off odor in the smoked sea bass.