Seasonal and regional occurrence of heat-resistant spore-forming bacteria in the course of ultra-high temperature milk production in Tunisia

International audienceSpore-forming bacteria, principally Bacillus species, are important contaminants of milk. Because of their high heat resistance, Bacillus species spores are capable of surviving the heat treatment process of milk and lead to spoilage of the final product. To determine the factors influencing the contamination of milk, spore-forming bacteria occurrence throughout the UHT milk production line during winter, spring, and summer was studied. The obtained results confirm that the total viable rate decreases rapidly throughout the production line of UHT milk showing the efficiency of thermal treatments used. However, the persistent high rate of spore-forming bacteria indicates their high heat resistance , especially in spring and summer. In addition, a significant variation of the quality of raw milk according to the location of the collecting centers was revealed. The molecular identification showed a high degree of diversity of heat-resistant Bacillus species, which are isolated from different milk samples. The distribution of Bacillus species in raw milk, stored milk, bactofuged milk, pasteurized milk, and UHT milk were 28, 10, 16, 13, and 33%, respectively. Six Bacillus spp. including Bacillus licheniformis (52.38%), Bacillus pumilus (9.52%), Bacillus sp. (4.76%), Bacillus sporothermodurans (4.76%), Terribacillus aidingensis (4.76%), and Paenibacillus sp. (4.76%) were identified in different milk samples

The combined effect of nisin, moderate heating and high hydrostatic pressure on the inactivation of Bacillus sporothermodurans spores.

International audienceAIMS: To investigate the combined effect of hydrostatic pressure (HP), moderate temperature and nisin on the inactivation of Bacillus sporothermodurans spores which are known to be contaminant of dairy products and to be extremely heat-resistant. METHODS AND RESULTS: A central composite experimental design with three factors, using response surface methodology, was used. By analysing the response surfaces and their corresponding contour plots, an interesting interaction with the three factors was observed. The inactivation observed was shown to be well fitted with values predicted by the quadratic equation, since the adjusted determination coefficient (R(adj)(2)) was 0*979. The optimum process parameters for a 5-log spores ml(-1) reduction of B. sporothermodurans spores were obtained, 472 MPa/53°C for 5 min in presence of 121 UI ml(-1) of nisin. CONCLUSION: Nisin and temperature treatments improve the effectiveness of pressure in the inactivation of highly heat-resistant spores of B. sporothermodurans. SIGNIFICANCE AND IMPACT OF THE STUDY: This study shows the potential of using high HP for a short time (5 min) in combination with moderate temperature and nisin to inactivate B. sporothermodurans spores in milk. Such treatments could be applied by the dairy industry to ensure the commercial sterility of UHT milk

In vitro antimicrobial activity of Pistacia lentiscus L. edible oil and phenolic extract.

International audiencePistacia lentiscus L. is known in some Tunisian forest area by its fixed oil used in traditional medicine as an antiseptic product. This investigation is the first to study the antimicrobial activity of P.lentiscus edible oil and its phenolic extract. Oil was extracted from fruits harvested from six provenances located in Tunisia. The antimicrobial activity was tested using disc diffusion assay and the broth dilution method. Kbouch and Sidi Zid oils were most efficient (p < 0.003) against, respectively, Staphylococcus aureus and Aspergillus niger with an inhibition zone of 9.33 mm. The phenolic extract had the largest spectrum of sensitive microorganisms. The minimum inhibitory concentration and minimum bactericidal concentration results showed that all strains were inhibited by both oil and extract

Optimization of pressure-induced germination of Bacillus sporothermodurans spores in water and milk.

Bacillus sporothermodurans produces highly resistant endospores that can survive ultra-high-temperature treatment in milk. The induction of endospore germination before a heat treatment could be an efficient method to inactivate these bacteria and ensure milk sterility. In this work, the rate of spore germination of B. sporothermodurans LTIS27 was measured in distilled water after high-pressure treatments with varying pressure (50-600 MPa), treatment temperature (20-50 °C), pressure-holding time (5-30 min) and post-pressurization incubation time (30-120 min) at 37 °C or 4 °C. The results showed that pressure-induced germination was maximal (62%) after a treatment at 200 MPa and 20 °C and increased with pressure-holding time and post-pressurization incubation time. Treatment temperature had no significant effect on germination. A central composite experimental design with three factors (pressure, pressure-holding time, and post-pressurization incubation time) using response surface methodology was used to optimize the germination rate in distilled water and in skim milk. No factor interaction was observed. Germination was induced at lower pressure and was faster in milk than in distilled water, but complete germination was not reached. The optimum germination obtained with experimental data was 5.0 log cfu/mL in distilled water and 5.2 log cfu/mL in milk from 5.7 log cfu/mL of spores initially present in the suspension. This study shows the potential of using high hydrostatic pressure to induce the germination of B. sporothermodurans spores in milk before a heat treatment

<i>In vitro</i> antimicrobial activity of <i>Pistacia lentiscus</i> L. edible oil and phenolic extract

<div><p><i>Pistacia lentiscus</i> L. is known in some Tunisian forest area by its fixed oil used in traditional medicine as an antiseptic product. This investigation is the first to study the antimicrobial activity of <i>P.</i><i>lentiscus</i> edible oil and its phenolic extract. Oil was extracted from fruits harvested from six provenances located in Tunisia. The antimicrobial activity was tested using disc diffusion assay and the broth dilution method. Kbouch and Sidi Zid oils were most efficient (<i>p</i> < 0.003) against, respectively, <i>Staphylococcus aureus</i> and <i>Aspergillus niger</i> with an inhibition zone of 9.33 mm. The phenolic extract had the largest spectrum of sensitive microorganisms. The minimum inhibitory concentration and minimum bactericidal concentration results showed that all strains were inhibited by both oil and extract.</p></div