Characterization of Mexican coriander (Eryngium foetidum) essential oil and its inactivation of Listeria monocytogenes in vitro and during mild thermal pasteurization of pineapple juice.

The aim of this work was to characterize the essential oil (EO) of Eryngium foetidum (EfEO) and assess its activity toward Listeria monocytogenes in broth and during thermal inactivation of the pathogen in pineapple juice. In this respect, EfEO was chemically characterized, and its antilisteria potential in broth as a function of pH, cell load, and EfEO concentration was assessed through a central composite design. Furthermore, the inactivation kinetics of L. monocytogenes in the juice were assessed by combining EfEO and low pasteurization temperatures. A total of 81 compounds were identified from EfEO. The reduction of pH and cell load increased EO activity. The use of only 15 ppm of EfEO during pasteurization of pineapple juice at 60°C reduced the time required for a 4-log reduction in L. monocytogenes CFU/ml by 74.9% (i.e., from 8.5 to 2.1 min) compared with treatment without EfEO. It could be concluded that EfEO activity toward L. monocytogenes increases with the reduction of pH and that it can be used at sublethal concentrations in combination with low temperatures in pineapple juice pasteurization. This study demonstrates that EO-assisted pasteurization is a promising strategy for the reduction of thermal impact during juice production. EfEO is easily available and compatible with many juices and is thus promising for industrial application.

High pressure homogenization vs heat treatment: safety and functional properties of liquid whole egg.

This research investigated the potential of multi-pass homogenization treatment for the inactivation of Salmonella enterica serovar Enteritidis inoculated at different levels in liquid whole egg (LWE) comparing the efficacy of this treatment with a traditional thermal one performed at 65 °C. Moreover, the effects of high pressure treatment (HPH) on structural and functional properties such as viscosity, microstructure and foaming abilities of LWE were investigated. The data obtained suggested that the multi-pass high pressure treatment at 100 MPa of S. enterica serovar Enteritidis inoculated in LWE at 7 and 4 log CFU/ml resulted in a first order inactivation kinetic, while the thermal inactivation curves of S. enterica serovar Enteritidis inoculated at 8 and 4 log CFU/ml presented a non-linear behaviour, with a marked tail after 3 min of treatment at 65 °C. Additionally, HPH treatment caused an increase in foaming capacity of LWE, with respect to the untreated samples, passing from values of 26% of the control to 50% of pressure treated samples.

Acid stress-mediated metabolic shift in Lactobacillus sanfranciscensis LSCE1.

Lactobacillus sanfranciscensis LSCE1 was selected as a target organism originating from recurrently refreshed sourdough to study the metabolic rerouting associated with the acid stress exposure during sourdough fermentation. In particular, the acid stress induced a metabolic shift toward overproduction of 3-methylbutanoic and 2-methylbutanoic acids accompanied by reduced sugar consumption and primary carbohydrate metabolite production. The fate of labeled leucine, the role of different nutrients and precursors, and the expression of the genes involved in branched-chain amino acid (BCAA) catabolism were evaluated at pH 3.6 and 5.8. The novel application of the program XCMS to the solid-phase microextraction-gas chromatography-mass spectrometry (SPME-GC-MS) data allowed accurate separation and quantification of 2-methylbutanoic and 3-methylbutanoic acids, generally reported as a cumulative datum. The metabolites coming from BCAA catabolism increased up to seven times under acid stress. The gene expression analysis confirmed that some genes associated with BCAA catabolism were overexpressed under acid conditions. The experiment with labeled leucine showed that 2-methylbutanoic acid originated also from leucine. While the overproduction of 3-methylbutanoic acid under acid stress can be attributed to the need to maintain redox balance, the rationale for the production of 2-methylbutanoic acid from leucine can be found in a newly proposed biosynthesis pathway leading to 2-methylbutanoic acid and 3 mol of ATP per mol of leucine. Leucine catabolism to 3-methylbutanoic and 2-methylbutanoic acids suggests that the switch from sugar to amino acid catabolism supports growth in L. sanfranciscensis in restricted environments such as sourdough characterized by acid stress and recurrent carbon starvation.

Synthesis of cyclopropane fatty acids in Lactobacillus helveticus and Lactobacillus sanfranciscensis and their cellular fatty acids changes following short term acid and cold stresses.

An implemented GC method to separate and quantify the cell cyclopropane fatty acids lactobacillic (C19cyc11) and dehydrosterculic (C19cyc9) was used to study the adaptive response to sublethal acid and cold stresses in Lactobacillus helveticus and Lactobacillus sanfranciscensis. The comparison of the composition of cellular fatty acids of the two strains and their changes after 2 h of stress exposure under micro-aerobic and anaerobic conditions indicated that the aerobic biosynthetic pathway for unsaturated fatty acids is prevalent in L. sanfranciscensis, while the anaerobic pathway is prevalent in L. helveticus. Indeed in the latter strain, in the presence of a source of oleic acid and under micro-aerobic conditions, C18:1n11 and its post-synthetic derivative C19cyc11 accounted for overall proportion ranging from 52 to 28% of the total FAs. On the other hand L. sanfranciscensis synthesizes by aerobic pathway C18:1n9 and transforms it to C19cyc9. However in this species the cumulative level of these two FAs did not exceed 30%. The relevant proportion of dodecanoic acid in the latter species suggests that carbon chain shortening is the principal strategy of L. sanfranciscensis to modulate fluidity or chemico-physical properties of the membranes.

Effect of alpha-linolenic, capric and lauric acid on the fatty acid biosynthesis in Staphylococcus aureus.

The antimicrobial activity of alpha-linolenic, capric and lauric acids on Staphylococcus aureus was studied in relation to their effect on the de novo fatty acid biosynthesis. Labelled acetate was used as integrated carbon source and traced in the de novo fatty acid by using a GC-Mass spectrometer and the single ion monitoring (SIM) technique. The detection of the incorporation of the labelled carbon into the individual cell fatty acids (FAs) provided an insight into the different effects of alpha-linolenic, capric and lauric acids on the FA biosynthesis. The results suggested that FAs pathway is the major target of alpha-linolenic acid and that other enzymes in addition to FabI are involved in S. aureus response mechanism when medium chain fatty acids are present.