Search

International audience; Cette présentation fait état des avancées en modélisation de la croissance d'une bulle et de l'impact des propriétés mécaniques du fromage sur cette croissance. L'objectif étant de mieux maîtriser les phases d'affinage de fromage industriel qui sont très coûteuses en énergie (maintien de l'affinage à 20°C).

International audience; A dedicated device was developed in order to monitor the growth of a single bubble in a Magnetic Resonance Imager. The bubble was isolated from a cheese block and enclosed inside a sustaining apparatus. A controlled pressure was applied at the bubble–cheese interface through a needle that was inserted into the bubble. The applied pressure (+ 5.3 kPa) was consistent with the natural pressure measured inside the bubbles of a cheese block during ripening. The volume, the horizontal and the vertical radii as well as deformation of the upper surface of the cylinder of cheese surrounding the bubble were determined from image analysis. The uncertainties on the measurements were estimated, and used as an additional criterion at the validation step of the model. A generalised Maxwell model was used to describe the mechanical behaviour of the cheese. The model parameters used for the simulation were determined in a compression-relaxation experiment on the same cheese block as the one used in the bubble growth experiment. Consistently with the literature, relaxation times from 1 to the order of 10,000 seconds were measured. Quite good agreement between the simulation and the experiment was found in the increase in volume. The ability of a linear model to describe the bubble growth which was mainly driven by the viscous part of the viscoelastic model was confirmed as long as low rate of deformation was involved. No time-independent elasticity was identified.

Installation de traitement de produits alimentaires comprenant : - une enceinte de réception des produits à traiter équipée d'au moins une surface support des produits à traiter et de moyens de chauffage, - des moyens d'aspiration des gaz contenus à l'intérieur de ladite enceinte de réception, aptes à créer un vide au moins partiel à l'intérieur de ladite enceinte de réception, - des moyens de production et/ou d'admission de vapeur à l'intérieur de ladite enceinte de réception. Les moyens de production de vapeur et/ou le débouché des moyens d'admission de vapeur à l'intérieur de l'enceinte sont disposés au moins partiellement au-dessus de la surface support des produits à traiter et le débouché à l'intérieur de l'enceinte des moyens d'aspiration des gaz contenus à l'intérieur de l'enceinte est disposé au moins partiellement dans ou au-dessous de la surface support des produits à traiter.

International audience; The growth of gas bubbles is a key phenomenon occurring in the making of Swiss-type cheeses. To achieve a better understanding of the formation of eyes in cheese, comprehensive knowledge of the transport phenomena involved is required. The aim of the present study was to investigate the momentum transport occurring during bubble growth. A generalised Maxwell model was used to describe the mechanical behaviour of the material. A specific method was developed to identify the model parameters by fitting the model to experimental stress relaxation curves. Those parameters were then used in a numerical model reproducing the growth of a single bubble and involving only momentum equilibrium and material behaviour. The driving phenomena for the growth of the bubble were the applied pressure at the bubble–cheese interface and the gravity. Such configuration was reproduced experimentally as closely as possible. A cylinder containing a single bubble was isolated from the cheese block and enclosed inside a sustaining apparatus made of plastic that prevented the external boundaries of the cylinder from moving. The upper surface of the cheese cylinderwas let free. A needle was inserted at the basement of the cylinder and gas was injected inside the bubble at t = 0. Pressure internal to the bubble was then monitored and bubble volume, aswell as deformation of the upper surface of the cylinder, were dynamically assessed by Magnetic Resonance Imaging within the same experiment. These data were compared to simulation with the objective of validating the adequacy of the mechanical model (structure, model parameters) to the case study of bubble growth. The results showed that the proposed model is able to describe the growth of a single bubble satisfactorily. It also appeared that the growth of the bubble is driven mainly by the viscous part of the viscoelastic model. This study encourages further investigation on the other phenomena implicated in bubble growth, such as gas production in the cheese and its transport from the material to the bubble by gas diffusion.

The overall objective of the present work is to locally assess physical changes taking place during the pre-chilling and freezing of part-baked bread and attempt to relate them to the appearance of crust flakes after final baking at retail. To achieve this objective, it was first necessary to develop an MRI device for the continuous non-invasive monitoring of the pre-chilling and freezing steps and to focus on interpreting the MRI signal as precisely as possible. There are many known sources of variation in the MRI signal during the process, including local changes in temperature, water content, porosity, ice content and molecular structure (e.g. under gelification). The present paper presents and discusses the results of this preliminary work. NMR and MRI experiments with non-yeasted dough revealed that during the pre-chilling step the MRI signal in a given voxel was mainly proportional to the local density (or inversely proportional to the local porosity) of the bread and to a lesser extent to the local liquid water content of the crumb, as water vapor is not visible with MRI. The MRI signal is assumed to decrease proportionally to the local ice content during the freezing step.

The capacity of low field NMR spectrometry to characterize the different behaviours of fat protons in unfrozen ice cream mix and in ice cream was investigated. Various formulations comprising different types of fat, protein and emulsifier were tested. The NMR parameters attributed to fat were sensitive both to the type of fat and also to the components located at the fat globule interface. In the unfrozen ice cream mix and to a lesser extent in ice cream, liquid fat was more sensitive to the type of protein, whereas the solid fat was sensitive to the type of emulsifier. The solid to liquid fat ratio calculated from the solid part of the NMR signal in the unfrozen mix was mainly dependent on the fat and emulsifier types. Changes in the states of liquid fat were detected during cold storage and interpreted as deviation from an equilibrium state recoverable after prolonged storage.

This work aimed at characterizing both the spin-spin (T2) and spin-lattice (T1) relaxations of water in frozen samples. Pure water and aqueous solutions (sucrose and/or casein) were studied, temperatures ranging from −13 to 20 °C. Three relaxation components could be distinguished after signal fitting. For example, the shorter spin-spin relaxation time was only observed at the frozen state and attributed to the ice crystals protons and the longer relaxation time was attributed to the liquid water and some of the sucrose protons. The exploitation of the 'solid` relaxation parameters gave information on ice content (T2) and on ice structure (T1). The method developed for ice content calculation was coherent with calorimetric data. The changes in the spin-spin relaxation of sucrose protons with temperature showed that at low temperature (−13 °C) it could be fully distinguished from the relaxation of water protons and at positive temperature this discrimination was not possible anymore.

Water absorption from the rice crispies was characterised during a water soaking treatment. Different techniques for monitoring the rehydration process were tested and conventional techniques involving phase separation, weighing and oven-drying for water content determination showed great variability due to the delicate operation of phase separation. Likewise a NMR method was developed for this purpose. Indeed this technique offers the advantage of a non destructive, non invasive measurement. Kinetics of water absorption obtained from NMR relaxation amplitudes were within the range covered by the ones obtained from conventional technique.

ATP-dependent protease complexes are present in all three kingdoms of life, where they rid the cell of misfolded or damaged proteins and control the level of certain regulatory proteins. They include the proteasome in Eukaryotes, Archea, and Actinomycetales and the HslVU (ClpQY) complex in other eubacteria. We showed that genes homologous to eubacterial HslV (ClpQ) and HslU (ClpY) are present in the genome of trypanosomatid protozoa and are expressed. The features of the cDNAs indicated that bona fide trypanosomatid messengers had been cloned and ruled out bacterial contamination as the source of the material. The N-terminal microsequence of HslV from Leishmania infantum (Protozoa: Kinetoplastida) permitted the identification of the propeptide cleavage site and indicated that an active protease is present. High similarities (> or =57.5%) with the prototypical HslV and HslU from Escherichia coli and conservation of residues essential for biochemical activity suggested that a functional HslVU complex is present in trypanosomatid protozoa. The structure of the N-termini of HslV and HslU further suggested mitochondrial localization. Phylogenetic analysis indicated that HslV and HslU from trypanosomatids clustered with eubacterial homologs but did not point to any particular bacterial lineage. Because typical eukaryotic 20S proteasomes are present in trypanosomatids, we concluded that the eubacterial HslVU and the eukaryotic multicatalytic protease are simultaneously present in these organisms. To our knowledge this is the first report of a eubacterial HslVU complex in eukaryotes and, consequently, of the simultaneous occurrence of both a proteasome and HslVU in living cells.

This paper reviews our exploration of the dynamics of the Leishmania genome and its contribution to epidemiology and diagnosis. We used as a model Peruvian populations of L. (Viannia) braziliensis and L. (V.) peruviana, 2 species very close phylogenetically, but phenotypically very different in biotope and pathology. We initially focused on karyotype analysis. Our data showed that chromosomes were subject to a fast rate of evolution, and were sensitive indicators of genetic drift. Therefore, molecular karyotyping appeared an adequate tool for monitoring (i) emergence of close species, (ii) ecogeographical differentiation at the intraspecific level, and (iii) strain 'fingerprinting'. Chromosome size variation was mostly due to the number of tandemly repeated genes (rDNA, mini-exon, gp63, and cysteine proteinase genes), and could involve the deletion of unique genes (L. (V.) braziliensis-specific gp63 families). Considering the importance of these genes in parasitism, their rearrangement might have functional implications: adaptation to different environments and pleomorphic pathogenicity. Our knowledge of genome structure and dynamics was used to develop new polymerase chain reaction (PCR) techniques. Amplification of gp63 genes followed by cleavage with restriction enzymes and study of restriction fragment length polymorphism (gp63 PCR-RFLP) allowed the discrimination of all species tested, even directly in biopsies with 95% sensitivity (compared with PCR amplification of kinetoplast deoxyribonucleic acid). At the intra-specific level, RFLP was also observed and corresponded to mutations in major immunogen domains of gp63. These seem to be under strong selection pressure, and the technique should facilitate addressing how the host's immune pressure may modulate parasite population structure. Altogether, gp63 PCR-RFLP represents a significant operational improvement over the other techniques for molecular epidemiology and diagnosis: it combines sensitivity, discriminatory power and prognostic value.

Although low- and intermediate moisture content foods are increasingly incorporated in ready-to-eat meals elaborated at the industrial scale, the rehydration operation is still empirically conducted and surprisingly has been the object of very few studies. Recent works carried out on milk powders rehydrated in different aqueous media showed the interest of the NMR technique for better understanding the mechanisms underlying the rehydration process. The present work aims at exploring these potentialities of the NMR technique, with particular focus on the feasibility of multi-scale studies, from granules (several µm) to food pieces (several mm). Puffed rice grains (Kelloggs) were poured into 30mL of water at 40°C. Measurements were performed with a low field NMR spectrometer (0.24 T) operating at 10MHz for the 1H (Minispec PC110, Bruker). T2 was measured from consecutive Carr Purcell Meiboom Gill (CPMG) sequences. Fitting the water relaxation curve of the water-rice system with the maximisation Entropy method showed that it could be represented by a three components whatever the immersion time. Both relaxation times and their corresponding amplitudes on the rice-water system was compared at different immersion times to the ones given by the rehydrated rice grains separated from the immersion water. Results showed that the longer relaxation time (1000-2500ms) could be attributed to the immersion water in which had migrated solutes from rice grains. The two other ones were representative of the water gained by rice: 20-80ms and 100-250ms depending on water content and rice grains porosity. The kinetics of water gain as measured by NMR was compared to the one obtained after drying of rehydrated rice grains separated from the immersion water. The latter technique, so-called "conventional" technique, was time-consuming; the separation of the liquid and solid phases also revealed to be very delicate. In particular, the different protocols of separation tested had a non-negligible effect on the amount of water attributed to rice: a deviation of 0.4 g of water per gram of dry matter (rice) could be observed at the beginning of the process and reached up to 1g/g after 60 min. However the kinetics determined from NMR measurements was within the interval covered by the ones obtained with the conventional technique.

International audience; An in vitro preparation of the crayfish central nervous system was used to study a negative feedback control exerted by the glutamatergic motor neurons (MNs) on to their presynaptic cholinergic sensory afferents. This negative control consists in small amplitude, slowly developing depolarizations of the primary afferents (sdPADs) strictly timed with MN bursts. They were not blocked by picrotoxin, but were sensitive to glutamate non-N-methyl-D-aspartate (NMDA) antagonists. Intracellular recordings were performed within thin branches of sensory terminals while electrical antidromic stimulation were applied to the motor nerves, or while glutamate (the MN neurotransmitter) was pressure-applied close to the recording site. Electrical motor nerve stimulations and glutamate pressure application had similar effects on to sensory terminals issued from the coxo-basipodite chordotonal organ (CBTs): like sdPADs, both stimulation-induced depolarizations were picrotoxin-resistant and were dramatically reduced by non-NMDA antagonist bath application. These results indicate that sdPADs are likely directly produced by MNs during locomotor activity. A functional scheme is proposed.

In order to study the genetic diversity of the #Leishmania$ genus, Random Amplification of Polymorphic DNA (RAPD) was performed on a set of 18 #Leishmania$ stocks representative of the main species of the genus. We tested on these sample one hundred decamer primers in parallel with samples of other microorganisms (#T. cruzi$, #T. congolense$, #Candida albicans$, #M. tuberculosis$). Several lines of results have been reached. Twenty three primers gave easily scoreable multiband profiles. The combined use of a fair set of these primers will make it possible to perform both, highly discriminative strain identification and population genetic analyses. Some of them showed a synapomorphic specificity, that is to say : they can specifically identify given phylogenetic subdivisions, either at a subspecific or at a specific level. For example, one primer showed a profile specific of #Leishmania peruviana$. Other primers revealed different profiles between #L. guyanensis$, #L. braziliensis$, #L. lainsoni$ and #L. chagasi$. Lastly, some primers appeared to be linked with specific virulence patterns. Some of them, for example, seemed to be able to distinguish the stocks isolated from mucocutaneous forms from the stocks isolated from cutaneous forms for #L. braziliensis$. The great interest of the RAPD method in comparison with isoenzyme analysis is to allow the purification and sequence characterisation of those fragments which are of a specific interest, for designing probes and PCR diagnoses. (Résumé d'auteur)