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compréhension des interactions entre protéines végétales et protéines de blanc d’œuf pour permettre l’amélioration de la « techno-fonctionnalité » des mélanges, notamment pour la stabilisation des mousses et émulsions, l’optimisation de ces fonctionnalités et l’identification des conditions permettant de limiter leur sensibilité aux traitements thermiques.

This study investigated the feasibility of using hyperspectral imaging (HSI) to characterize the diffusion of acid and water within food structures during gastric digestion. Two different sweet potatoes (steamed and fried) and egg white gel (pH5 and pH9 EWGs) structures were exposed to in vitro gastric digestion before scanning by HSI. Afterward, the moisture or acid present in the digested sample was analyzed for calibration purposes. Calibration models were subsequently built using partial least-squares (PLS). The PLS models indicated that the full-wavelength spectral range (550–1700 nm) had a good ability to predict the spatial distribution of acid (Rcal2 > 0.82) and moisture (Rcal2 > 0.88). The spatiotemporal distributions of moisture and acid were mapped across the digested food, and they were shown to depend on the food composition and structure. The kinetic data revealed that the acid and moisture uptakes are governed by Fickian diffusion or by both diffusion and erosion-controlled mechanisms.

The hypothesis is that the characteristics of ingested protein gels influences the subsequent in vivo gastric digestion process. Three egg white gels (EWGs) of identical composition but differing in structure and texture were prepared and fed to pigs. Sampling throughout a 6 h postprandial period, and at different locations in the stomach of the pigs, enabled a detailed spatial-temporal mapping of the pH, dry matter content, particle size and rheological properties. The results showed different gastric acidification kinetics implying an effect of the gel structure and/or texture. The most elastic and cohesive gel resulted in the highest median particle size and the most viscoelastic chyme. Distal and proximal regions of the stomach did not differ in terms of dry matter content, particle size distribution or rheological properties. These results demonstrate the consequences of protein food structure on gastric chyme properties, and thus suggest an effect on the digestion process.

Salmonella Enteritidis is the most prevalent food-borne pathogen associated with egg-related outbreaks in the European Union. In order to colonise eggs, S. Enteritidis must resist the powerful anti-bacterial activities of egg white (EW). Possibly, the major EW antibacterial property is iron restriction, which results from the presence of the Fe3+-binding protein, ovotransferrin. To circumvent iron restriction, S. Enteritidis synthesise two types of catecholate siderophores, enterobactin and salmochelin, that can chelate iron from host iron-binding proteins. However, EW contains a lipocalin (Ex-FABP) that is known to bind enterobactin. Two other lipocalins, Cal-¿ and a1-ovoglycoprotein, are found in EW but their siderophore-binding potential was yet to be exploredThe aim of this project was to study the antimicrobial activity of three EW lipocalins through sequestration of bacterial siderophores synthesised by S. Enteritidis. Among those lipocalins, Cal-¿ and a1-ovoglycoprotein were shown to bind neither enterobactin nor salmochelin. Further, it was confirmed that Ex-FABP binds only enterobactin and not salmochelin. In standard growth media, S. Enteritidis escaped Ex-FABP-mediated growth inhibition thanks to salmochelin synthesis. However, no clear antibacterial activity was observed for Ex-FABP in EW. This surprising observation might be correlated with the weak growth of S. Enteritidis in EW and a lack of requirement for siderophores in persistence., Salmonella Enteritidis est le pathogène le plus fréquemment identifié dans les œufs et ovoproduits au sein de l’Union Européene. Pour coloniser les œufs, S. Enteritidis doit surmonter de nombreux obstacles liés à l’activité antibactérienne du blanc d'œuf (BO). Un des mécanismes antibactériens majeurs du BO est la restriction en fer imposée par l'ovotransferrine (protéine liant le fer ferrique, Fe3+). Pour pallier cette carence, S. Enteritidis est capable de synthétiser deux types de sidérophores, l’entérobactine et la salmochéline, capables d’acquérir le fer des protéines de l'hôte. Cependant, le BO contient également une protéine de type lipocaline (Ex-FABP) connue pour séquestrer l’entérobactine. Deux autres lipocalines, Cal-¿ et l’a1-ovoglycoprotéine ont été identifiées dans le BO, mais leur capacité à séquestrer des sidérophores restait à explorer.L'objectif de ce projet était d'étudier l’activité antimicrobienne de trois lipocalines du BO par leur capacité à séquestrer les deux sidérophores de S. Enteritidis. Les résultats montrent que Cal-¿ et l’a1-ovoglycoprotéine ne sequestrent ni l’entérobactine ni la salmochéline. Nos observations confirment qu’Ex-FABP lie uniquement l’entérobactine et non la salmochéline. En milieux de culture, S. Enteritidis échappe à l'inhibition de croissance induite par Ex-FABP grâce à la synthèse de salmochéline. Néanmoins, le rôle antimicrobien d’Ex-FABP via la séquestration de sidérophores n’a pas été observé dans le BO. Ceci pourrait être lié à la faible croissance de S. Enteritidis dans le BO et au fait que les sidérophores ne semblent pas être nécessaires à sa survie dans ce milieu.

Salmonella Enteritidis is the most prevalent food-borne pathogen associated with egg-related outbreaks in the European Union. During egg colonization, S. Enteritidis must resist the powerful anti-bacterial activities of egg white (EW) and overcome ovotransferrin-imposed iron-restriction (the most important anti-bacterial mechanism of EW). Many pathogens respond to iron restriction by secreting iron-chelating chemicals called siderophores but EW contains a siderophore-sequestering "lipocalin" protein (Ex-FABP) that is predicted to limit the usefulness of siderophores in EW. S. Enteritidis produces two siderophores: enterobactin, which is strongly bound by Ex-FABP; and the di-glucosylated enterobactin-derivative, salmochelin (a so-called "stealth" siderophore), which is not recognized by Ex-FABP. Thus, production of salmochelin may allow S. Enteritidis to escape Ex-FABP-mediated growth inhibition under iron restriction although it is unclear whether its EW concentration is sufficient to inhibit pathogens. Further, two other lipocalins (Cal-γ and α-1-ovoglycoprotein) are found in EW but their siderophore sequestration potential remains unexplored. In addition, the effect of EW lipocalins on the major EW pathogen, S. Enteritidis, has yet to be reported. We overexpressed and purified the three lipocalins of EW and investigated their ability to interact with the siderophores of S. Enteritidis, as well as their EW concentrations. The results show that Ex-FABP is present in EW at concentrations (5.1 μM) sufficient to inhibit growth of a salmochelin-deficient S. Enteritidis mutant under iron restriction but has little impact on the salmochelin-producing wildtype. Neither Cal-γ nor α-1-ovoglycoprotein bind salmochelin or enterobactin, nor do they inhibit iron-restricted growth of S. Enteritidis. However, both are present in EW at significant concentrations (5.6 and 233 μM, respectively) indicating that α-1-ovoglycoprotein is the 4th most abundant protein in EW, with Cal-γ and Ex-FABP at 11th and 12th most abundant. Further, we confirm the preference (16-fold) of Ex-FABP for the ferrated form (K d of 5.3 nM) of enterobactin over the iron-free form (K d of 86.2 nM), and its lack of affinity for salmochelin. In conclusion, our findings show that salmochelin production by S. Enteritidis enables this key egg-associated pathogen to overcome the enterobactin-sequestration activity of Ex-FABP when this lipocalin is provided at levels found in EW.

Digestion behaviour of food protein-based hydrogels is greatly influenced by the gel characteristics and in particular the microstructure. Using egg white gels (EWGs) as a model food, this study aimed to explore the real time disintegration by pepsin of different microstructures and subsequent nutrient release kinetics. Using thermal treatment at 80 °C, EWGs with two different microstructures but similar protein concentration (10%) were produced by varying the pH conditions (pH 5 and pH 9). The in situ spatiotemporal disintegration of the microstructure during static in vitro gastric digestion was followed using a high resolution confocal microscopic technique. Tetramethylrhodamine isothiocyanate (TRITC)-dextran (4400 Da) was incorporated into the gels as a model fluorescent molecule of peptide-like size, to trace its release due to the pepsin action. The looser microstructure of pH5-EWG caused the gel to disintegrate more quickly and to a greater extent, leading to a higher rate of TRITC-dextran release. In contrast, the compact-dense microstructure of the pH9-EWG showed slower kinetics of disintegration and (TRITC)-dextran release, likely due to a reduced accessibility of pepsin to its substrates. Pepsin activity being highly pH-dependent, high local pH and high buffering capacity of pH9-EWG may also play a pivotal role in the slower disintegration observed for this gel. In both EWGs, spatial degradation was mainly observed at the gel surface while the interior area fractions remained unchanged. Thus, surface erosion is possibly the underlying mechanism of EWG disintegration by pepsin in these experimental conditions.

Introduction et but de l’étude: La pepsine, sécrétée dans l’estomac, est la première protéase rencontrée par les protéines alimentaires au cours de la digestion. L’estomac acidifie lentement les bols alimentaires et il a été montré que le pH gastrique a un impact majeur sur les cinétiques de protéolyse 1.En effet, il est généralement admis que la pepsine n’est active qu’à pH acide, pour pH ≤ 5, avec un maximum d’activité autour de pH 2 pour beaucoup de substrats 2.Elle est également représentée comme un diacide 3, ce qui semble vrai lorsqu’elle agit sur l’hémoglobine, substrat de référence pour mesurer son activité. Ce comportement en fonction du pH n’a pas été validé sur des mélanges de protéines alimentaires telles que les caséines, principales protéines du lait, ou les protéines du blanc d’œuf. L’objectif principal de cette étude est de comparer les profils d’activité de la pepsine en fonction du pH sur les caséines du lait de vache (CA) et les protéines du blanc d’œuf (BO).Matériel et méthodes: Les cinétiques d’hydrolyse de protéines de BO entier et de micelles de CA par la pepsine, à différents pH compris entre 1 et 8, ont été suivies au pH-stat pendant 2h, avec quantification du degré d’hydrolyse final (DH) par la méthode OPA. Les vitesses initiales ont été calculées à partir de ces cinétiques entre 0 et 5 minutes. Les concentrations en substrat et en pepsine étaient les mêmes pour chaque expérience, et l’environnement salin était celui du protocole INFOGEST 4. Les CA se présentent en structures supramoléculaires, les micelles, qui précipitent à pH acide. Une étape d’acidification à pH 4 des CA a permis d’étudier des suspensions d’agrégats tous formés dans les mêmes conditions. L’aspect et la taille de ces agrégats ont été observés par microscopie confocale et par morpho-granulométrie.Résultats et Analyse statistique: Les cinétiques d’hydrolyse obtenues suivent, dans leur majorité, des profils de type loi de puissance en fonction du temps. Pour les protéines de BO, le DH final est maximal à pH1 (14,4%) et diminue de manière linéaire en fonction du pH,jusqu’à atteindre des valeurs négligeables pour les pH ≥6. A l’inverse, les valeurs de DH finaux sur les CA ne sont pas significativement différentes sur la gamme de pH 1 à 5 (7,5 ± 1,4 %) puis diminuent à partir de pH 6 et atteignent des valeurs négligeables à pH 8.Le rapport entre la vitesse d’hydrolyse initiale et le DH final des CA est supérieur à celui des protéines du BO quel que soit le pH, suggérant une cinétique d’hydrolyse plus rapide pour les CA.Ces résultats montrent que les caséines du lait,dont le pH se situe un peu au-dessus de 6, n’ont pas besoin d’être longuement acidifiées pour que l’hydrolyse par la pepsine puisse commencer dans l’estomac, atteignant une activité quasi optimale de la pepsine dès pH 5.Conclusion: Les cinétiques d’hydrolyse par la pepsine en fonction du pH diffèrent donc entre un substrat de type protéines globulaires comme l’œuf, et les caséines du lait. Le profil d’activité de la pepsine en fonction du pH est également différent de celui du standard hémoglobine. En outre, contrairement à ce qui est généralement admis, la pepsine peut hydrolyser certains substrats à des pH plus hauts que pH 5.

National audience; Changing the structure of foods can modulate their nutritional quality by modifying the digestion process, a complex process that is still imperfectly understood. In particular, digestion dynamics have been poorly studied, although it can has significant metabolic consequences. The objective of this study is to model the proteolytic cascade by pepsin, and to highlight a potential "structural effect". Egg white is an interesting model to meet this dual objective, as it offers the possibility of obtaining gels with different structures. In a probabilistic modeling approach of the peptide cleavage dynamics, we sought to identify leverages for the cleavage of a peptide after a given digestion time. For this purpose, peptides are identified and quantified at different time points in an in vitro digestion experiment. A Generalized Additive Modeling of the probability of a cleavage based on the physicochemical profile of peptides is proposed, considering either all kinds of cleavages made by the pepsin or only those made on preferential peptide bonds. The most significant variables are related to the length of the peptide and its location on the ovalbumin sequence. These results suggest that the action of pepsin depends more on structural criteria than on the presence of specific cleavage sites.

International audience

International audience

International audience; Salmonella enterica causes 93.8 million human cases of gastroenteritis and bacteremia worldwide which subsequently results into 155,000 deaths annually. Salmonella Enteritidis serovar (SE) is the most frequently detected Salmonella enterica in foodborne outbreaks in the European Union. Among such outbreaks, egg and egg products are identified as the most common vehicles of infection. One of the main antibacterial properties of egg white is iron restriction which results from the presence of an ironbinding protein (ovotransferrin). To circumvent iron restriction, SE synthesises siderophores (i.e. enterobactin [Ent] and salmochelin) that can chelate iron from host iron-binding proteins to make it available to the bacterium. The aim of this project is to explore the role that lipocalin-type proteins might play a role in sequestering bacterial siderophores synthesized in egg white.The three lipocalin-type proteins found in egg white were over-expressed, purified and tested for their ability to bind Fe(III)-Ent complexes. Mutants knocked-out for enterobactin synthesis as well as for salmochelin synthesis/export and import were then used to determine whether exposure to lipocalin-type proteins limits SE growth and whether salmochelin secretion overcomes any observed inhibition by lipocalins.It was shown that the Ex-FABP protein, identified as an egg white lipocalin, is able to inhibit bacterial growth under ironlimited conditions. However, Cal-γ and α-1-ovoglycoprotein do not seem to have a role in siderophore sequestration. Overall, this project provides new insight on egg white anti-bacterial mechanisms that would be of high relevance to the sector of egg production and processing.

Food microstructure and gastric digestion: the egg white gel model. STLOpendays

Conference: 11th International Symposium on Biometals (Biometals) Location: Ottawa, CANADA Date: 2018; Salmonella enterica serovar Enteritidis (SE) is the most frequently-detected Salmonella in foodborne outbreaks in the European Union. Among such outbreaks, egg and egg products were identifiedas the most common vehicles of infection. Possibly, the major antibacterial property of egg white is ironrestriction, which results from the presence of the ironbinding protein, ovotransferrin. To circumvent ironrestriction, SE synthesise catecholate siderophores (i.e. enterobactin and salmochelin) that can chelateiron from host iron-binding proteins. Here, we highlight the role of lipocalin-like proteins found in eggwhite that could enhance egg-white iron restriction through sequestration of certain siderophores, includingenterobactin. Indeed, it is now apparent that the egg-white lipocalin, Ex-FABP, can inhibit bacterialgrowth via its siderophore-binding capacity in vitro. However, it remains unclear whether Ex-FABP performssuch a function in egg white or during bird infection. Regarding the two other lipocalins of eggwhite (Cal-c and a-1-glycoprotein), there is currently no evidence to indicate that they sequestersiderophores.

In situ disintegration of protein gels (egg white) by pepsin based on time-lapse confocal microscopy . Première réunion plénière du GDR SLAMM (Solliciter LA Matière Molle)

Egg white is a hostile medium for microorganisms due to its harsh physicochemical properties (alkaline pH and high viscosity), the nutritional restriction it imposes and its arsenal of antimicrobial molecules that may interact with bacteria in a synergistic way. Salmonella enterica serovar Enteritidis is the major pathogen responsible for egg borne infection in humans, due to its exceptional capability for survival under the harsh conditions encountered within egg white. However, above 42 ˚C, egg white exerts a much stronger bactericidal effect on S. Enteritidis than at lower temperatures, without full understanding of the responsible mechanism. Here we describe the first global-transcriptional analysis of the response of S. Enteritidis to egg white exposure under bactericidal conditions, providing a unique insight into egg white antibacterial activity. In order to enable this study, we used an “egg white model medium” (EWMM) already shown to be an appropriate mimic of egg white. Incubation of S. Enteritidis in EWMM above 42°C provoked a major transcriptional response (18.7% of genes affected at 45 min), indicative of major changes of S. Enteritidis physiology. Cell and envelope damage/stress responses were induced (degradation of unfolded protein, maintenance of cell integrity, and translation shutdown), energy metabolism shifted from respiration to fermentation, and micronutrient provision was enhanced (related to iron and biotin restriction).Our investigation of the global expression response of S. Enteritidis to egg white provides a complete genome-wide view of the complex physiological response of this pathogen to egg white-induced bacterial destruction under specific conditions.

National audience; Salmonella outbreaks are a serious threat to public health. One of the main vehicles of infection are eggs and egg-products, with Salmonella enterica serovar Enteritidis being the most frequent contaminant. Egg contamination occurs despite the impressive antibacterial properties of egg white and it is notable that S. Enteritidis is able to survive in egg white in contrast to other serovars and species. It is generally accepted that the major egg white-factor limiting growth of Salmonella is the highly iron-restricted environment resulting from the presence of ovotransferrin, an iron-binding protein. To circumvent iron restriction, many bacteria synthesise siderophores that can chelate iron from host iron-binding proteins for delivery to bacteria. The iron starvation response genes are regulated according to iron availability via the transcriptional regulator, Fur. Evidence for a relationship between temperature and iron sequestration ability has been reported by several groups. For Salmonella Typhymurium, little or no siderophore is secreted at 40-42°C in iron depleted medium indicating a temperature regulatory effect that is independent of Fur/Fe control. The aim of this study is to determine if egg white exposure induces the iron-acquisition genes and siderophore production in the highly-egg-white adapted Enteritidis serovar at ~40°C, the natural hen body temperature that Salmonella would encounter when infecting the egg during egg formation in the oviduct or during hatching. In order to easily recover of S. Enteritidis, we used egg white model medium (EWMM) in place of viscous egg white. EWMM consists of egg white filtrate with 10% egg white and numerous studies have validated this model medium. We measured the expression of 15 genes involved in the ironrestriction response by qRT-PCR after S. Enteritidis incubation of 45 minutes in EWMM at 37, 40 and 42°C. We observed a strong expression of genes encoding proteins involved in biosynthesis, export, uptake and utilisation of enterobactin and salmochelin (the two siderophores of Salmonella) and ferric-hydroxamate uptake. A statistical analysis shows that iron-acquisition genes are better expressed at 40°C than at 37 or 42°C. The production of siderophores was quantified during incubation using the Calcein blue test. Siderophores released by Salmonella in EWMM were detected at around a 1 μM desferral equivalent within 6h of incubation at 37, 40 and 42°C. The results of this study suggest that S. Enteritidis is able to induce its iron acquisition systems to cope with the iron-restricted conditions imposed by egg white even at the elevated body temperature of the hen.

National audience; Salmonella enterica serovar Enteritidis is the major pathogen responsible for egg-borne infection in humans due to its exceptional ability to survive in egg white in contrast to other serovars and species. Comprehensive overview is needed regarding the mechanisms which enable Salmonella to survive in egg white despite the antimicrobial activities of this medium. It is generally accepted that the key anti-S. Enteritidis activities of egg white are iron deficiency and bacterial-cell membrane damages due to egg white antimicrobial proteins. However, the ability of S. Enteritidis to maintain cell-envelope integrity could take part in its resistance to egg white. The alkaline pH of egg white (rapid increase from 7.8 up to 9.3 in a few days after laying) was also highlighted in several studies as an anti-S. Enteritidis factor, given that the maintenance of intracellular pH around 7.8 is essential for many biological functions and for the status of cell membranes. Thus, an alkaline pH mainly affects the bacterial cells which must preserve the proton motive force and the electrochemical potential across their membranes. One of the objectives of this study was to investigate if incubation of S. Enteritidis in egg white results in the depolarization of the cell membrane. The role of egg white proteins and alkaline pH in such modifications was also assessed. Membrane depolarization was measured using a method adapted from Epand et al. (2010) based on fluorescence increase of DiSC3 dye when membrane depolarization occurs. In order to investigate the effect of egg white proteins, relative fluorescence intensity was measured on bacteria cells incubated either in egg white (around 10 g/L proteins, reference “100 %”), in egg white filtrate obtained using 10 kDa cut-off ultrafiltration membrane (“0 %” proteins), or in egg-white filtrate plus 10% egg white (“10%” proteins). In order to assess the role of egg white pH, the relative fluorescence intensity was also measured on cells incubated in “10%” egg white at pH 7.8 or 9.3. The results showed a significant increase of DiSC3 fluorescence when cells were incubated with “10%” and “100%” egg white proteins, unlike those incubated with “0%” proteins. No difference was observed between pH 7.8 and 9.3. Thus, egg white proteins are involved in the disruption of S. Enteritidis membrane potential and could participate in the bacteriostatic effect of egg white towards this serovar. On the contrary, the alkaline pH of egg white does not appear to be a key parameter.

In the present study, composite egg white/gelatin hydrogels were produced and their porosity was increased through the subsequent removal of gelatin by leaching into water. The composite gel with 0.5% gelatin showed a higher degree of swelling than did the control gelatin-free sample after 60 min of immersion in an aqueous medium which was ascribed to the formation of capillary channels due to gelatin leaching. The composite gel containing 0.3% gelatin showed the highest water-holding capacity and firmness indices among all samples. Gel porosity decreased with increasing gelatin content. However, after gelatin depletion, higher concentrations of gelatin yielded hydrogels with higher porosity, as confirmed by scanning electron microscopy. Based on Fourier transform infra-red spectroscopy, it was concluded that the count of hydrogen bonds decreased after gelatin depletion. X-ray diffraction analysis indicated that intermolecular interaction between gelatin and egg white proteins had taken place in the amorphous phase.

The present study investigated proteolysis combined with heat treatment to make hen Egg White (EW) an efficient emulsifier. EW was hydrolyzed by protease (Thermoase®) at various Enzyme Concentrations (EC) (w/w, 0.1%, 0.2%, 0.4%, 0.8%), followed by heating at 90°C for 8 min. Results showed that optimal emulsifying ability and stability, determined by measurement of emulsion turbidity, were obtained when EC was 0.4%, followed by heat treatment at 90°C. The hydrolysate thus prepared had higher emulsifying ability and stability than either native egg white (nEW) or small molecular weight EW peptides (Runpep®), close to the properties of Egg Yolk (EY) which was a reference as a food emulsifier. Surface hydrophobicity (H0) was found to be linearly related to the emulsifying activity and stability of hydrolyzed egg white proteins.

Selenium (Se) is a key constituent of selenoproteins, which are enzymes that play a major role in the antioxidant system. The present study aimed to evaluate the effect of different selenium sources on the technological properties of the egg white. Freshness (Haugh units), white viscosity and foam cohesion were assessed. 1560 Rhode Island layers were randomly assigned one of the two following diets: control diet with 0.3 mg/kg Se as Sodium Selenite or experimental diet with 0.3 mg/kg Se as hydroxy-selenomethionine (OH-SeMet or Selisseo®) from 53 weeks of age. Laying performance and egg quality were assessed at 55 and 60 weeks of age. While the source of selenium did not affect the laying performance, the organic selenium tended to improve shell quality through enhanced static stiffness and fracture force. OH-SeMet also improved the egg freshness after 8 days of storage. Thick white from the experimental group had a significantly higher viscosity under a 0.1 s-1 and 1s-1 shear pressure. OH-SeMet numerically improved egg white foam cohesion. This study demonstrates that supplementing OH-SeMet to laying hens is of interest to improve the technological properties of egg white.

Food structure is an important factor in the nutritional quality of foods. With regard to dietary protein, it may have an impact on protein digestibility, proteolysis kinetics, gastric emptying and amino acid bioavailability, all with potential metabolic consequences. Egg white is an appropriate food model to investigate these issues, since very different structures and textures can be produced while keeping the chemical composition constant. Recently, we established that protein matrix structure impacts the extent of proteolysis and the nature ofthe peptides generated during in vitro digestion of egg white protein aggregates and gels. Our question here is “do such differences also exist in vivo, and do they have any biological consequences?” To address these questions, a smooth-rigid egg white gel, a granular-spongy gel and an intermediate gel were prepared. Each gel was fed to pigs (n=33) as a single meal, and gastric chyme samples were collected over a 6h-postprandial period in order to measure pH, proteolysis and particle size along digestion at 10 different stomach locations. Blood samples were also collected for plasma amino acid analyses.Acidification of the chyme started in the pylorus regin regardless of the type of gel, but acidification kinetics depended on the gel type, which suggested that gel structure impacts on HCl distribution in the chyme. In contrast, proteolysis commenced in the proximal region, consistent with the location of pepsinogen secretory cells. But from 1h-postprandial, proteolysis profiles greatly differed depending on the gels: the higher proteolysis was 2 observed in the pyloric region for the granular-spongy gel (acidity and proteolysis gradients were consistent), while maximum proteolysis was observed in the proximal region for the smooth-rigid and intermediate gels (acidity and proteolysis gradients were opposite), suggesting that pepsin diffusion profiles depend on the type of gel. Proteolysis kinetics weremore rapid with granular-spongy gel than with intermediate gel, while the smooth-rigid gel was the most resistant to proteolysis, which was consistent with the particle size observed during digestion. Finally, amino acid absorption was faster for the granular-spongy gel than for the two other gels.

Chicken egg white protects the embryo from bacterial invaders by presenting an assortment of antagonistic activities that combine together to both kill and inhibit growth. The key features of the egg-white anti-bacterial system are iron restriction, high pH, antibacterial peptides and proteins, and viscosity. Salmonella enterica serovar Enteritidis is the major pathogen responsible for egg-borne infection in humans, which is partly explained by its exceptional capacity for survival under the harsh conditions encountered within egg white. However, at temperatures up to 42 ˚C, egg white exerts a much stronger bactericidal effect on S. Enteritidis than at lower tempertaures, although the mechanism of egg-white-induced killing is only partly understood. Here, for the first time, the impact of exposure of S. Enteritidis to egg white under bactericidal conditions (45 ˚C) is explored by global-expression analysis. A large-scale (18.7% of genome) shift in transcription is revealed suggesting major changes in specific aspects of S. Enteritidis physiology: induction of egg-white related stress-responses (envelope damage, exposure to heat and alkalinity, and translation shutdown); shift in energy metabolism from respiration to fermentation; and enhanced micronutrient provision (due to iron and biotin restriction). Little evidence of DNA damage or redox stress was obtained. Instead, data are consistent with envelope damage resulting in cell death by lysis. A surprise was the high degree of induction of hexonate/hexuronate utilisation genes, despite no evidence indicating the presence of these substrates in egg white.

Egg white is a key ingredient in many food products as it combines high nutritional quality with excellent functional properties. However, it is also one of the leading causes of food allergy in childhood. Dehydrated egg white is a common form of industrial egg white. To increase the functional properties, egg white powders are heated which may change protein antigenicity and susceptibility to digestion. The present work highlighted the effect of a wide range of dry heating rates (from 1 to 10 days between 60 and 90 °C) simultaneously on the interfacial properties, antigenicity and susceptibility to in vitro digestion of egg white proteins. Thanks to a powerful statistical methodology, i.e. multiple factor analysis (MFA), that enables to consider all the data in a common space, intermediate dry heating treatments (2–5 days at 70 °C or 1–2 days at 80 or 90 °C) were found to be good compromise to improve egg white functionality without enhancing too much protein resistance to digestion or protein antigenicity.

A quoi reconnaît-on un oeuf frais ? L'oeuf est-il un aliment gras ? Comment la poule fait-elle son oeuf ? L'oeuf en poudre est-il encore de l'oeuf ? A quoi correspondent les codes sur les oeufs du commerce ? Cet ouvrage nous décrit l'ensemble de la filière oeufs, de l'élevage des poules au produit consommé dans nos assiettes. Il fait le point sur la biologie et la physiologie des oeufs, leurs qualités nutritionnelles, leur production industrielle et leur transformation en ovoproduits ou pour des usages non alimentaires. Tout en nous donnant des conseils de stockage, de consommation, et même d'élevage pour ceux qui rêvent d'un poulailler, il démystifie au passage certains croyances erronées sur l'oeuf.

Background: Ovalbumin-related protein X (OVAX) is an uncharacterized ovalbumin-serpin. Results: This egg white-specific serpin lacks protease inhibitory activity, but unlike its ovalbumin homolog, OVAX exhibits antibacterial properties, partly through its heparin-binding site(s). Conclusion: OVAX, a non-inhibitory serpin is a heparin-binding molecule with antibacterial activity. Significance: OVAX participates in egg defense and constitutes a natural agent against Listeria and Salmonella., Ovalbumin family contains three proteins with high sequence similarity: ovalbumin, ovalbumin-related protein Y (OVAY), and ovalbumin-related protein X (OVAX). Ovalbumin is the major egg white protein with still undefined function, whereas the biological activity of OVAX and OVAY has not yet been explored. Similar to ovalbumin and OVAY, OVAX belongs to the ovalbumin serine protease inhibitor family (ov-serpin). We show that OVAX is specifically expressed by the magnum tissue, which is responsible for egg white formation. OVAX is also the main heparin-binding protein of egg white. This glycoprotein with a predicted reactive site at Lys367-His368 is not able to inhibit trypsin, plasmin, or cathepsin G with or without heparin as a cofactor. Secondary structure of OVAX is similar to that of ovalbumin, but the three-dimensional model of OVAX reveals the presence of a cluster of exposed positive charges, which potentially explains the affinity of this ov-serpin for heparin, as opposed to ovalbumin. Interestingly, OVAX, unlike ovalbumin, displays antibacterial activities against both Listeria monocytogenes and Salmonella enterica sv. Enteritidis. These properties partly involve heparin-binding site(s) of the molecule as the presence of heparin reverses its anti-Salmonella but not its anti-Listeria potential. Altogether, these results suggest that OVAX and ovalbumin, although highly similar in sequence, have peculiar sequential and/or structural features that are likely to impact their respective biological functions.

International audience; The present study investigated proteolysis combined with heat treatment to make hen Egg White (EW) an efficient emulsifier. EW was hydrolyzed by protease (Thermoase®) at various Enzyme Concentrations (EC) (w/w, 0.1%, 0.2%, 0.4%, 0.8%), followed by heating at 90°C for 8 min. Results showed that optimal emulsifying ability and stability, determined by measurement of emulsion turbidity, were obtained when EC was 0.4%, followed by heat treatment at 90°C. The hydrolysate thus prepared had higher emulsifying ability and stability than either native egg white (nEW) or small molecular weight EW peptides (Runpep®), close to the properties of Egg Yolk (EY) which was a reference as a food emulsifier. Surface hydrophobicity (H0) was found to be linearly related to the emulsifying activity and stability of hydrolyzed egg white proteins.

International audience; This study aimed to explore how food structure, in the form of different egg white gel matrices, impacts the peptide kinetics profiles using an in vitro model that simulated digestion in the adult gastrointestinal tract. Four different gel matrices were prepared by heating egg white gel solutions using different combinations of pH and ionic strength. Label-free quantitative peptidomics and statistical analysis of the resulting kinetics profiles were performed. The 3231 identified peptides were distributed in 7 clusters based on "Gel structure x Digestion time" interaction coefficients, indicating that peptide kinetics profiles were greatly influenced by the gel structure. Moreover, the protein within the gel from which the peptides were released was highly significantly correlated with the peptide clusters, demonstrating that the influence of the gel structure varied according to the protein in question. Such findings may have nutritional relevance in vivo as they imply that the peptides reaching the intestine are different according to the initial EW gel structure.

International audience; Egg white is an important contributor to the protection of eggs against bacterial contaminations during the first half of incubation (day zero to 12), prior to the egg white transfer into the amniotic fluid to be orally absorbed by the embryo. This protective system relies on an arsenal of antimicrobial proteins and on intrinsic physicochemical properties that are generally unfavorable for bacterial multiplication and dissemination. Some changes in these parameters can be observed in egg white during egg storage and incubation. The aim of this work was to characterize changes in the antibacterial potential of egg white in embryonated eggs (FE) during the first half of incubation using unfertilized eggs (UF) as controls. Egg white samples were collected at day zero, 4, 8, and 12 and analyzed for pH, protein concentration, and protein profile. Antibacterial properties of egg white proteins were evaluated against Listeria monocytogenes, Streptococcus uberis, Staphylococcus aureus, Escherichia coli, and Salmonella Enteritidis. During incubation, differential variations of egg white pH and protein concentrations were observed between UF and FE. At equal protein concentrations, similar activities against L. monocytogenes and S. uberis were observed for FE and UF egg white proteins. A progressive decline in these activities, however, was observed over incubation time, regardless of the egg group (UF or FE). SDS-PAGE analysis of egg white proteins during incubation revealed discrete changes in the profile of major proteins, whereas the stability of some less abundant antimicrobial proteins seemed more affected. To conclude, the antibacterial activity of egg white proteins progressively decreased during the first half of egg incubation, possibly resulting from the alteration of specific antimicrobial proteins. This apparent decline may be partly counterbalanced in embryonated eggs by the increase in egg white protein concentration. The antibacterial potential of egg white is very effective during early stages of embryonic development but its alteration during incubation suggests that extra-embryonic structures could then progressively ensure protective functions.

The impact of egg white gel (EWG) structure on the process of digestion was investigated using an in vitro digestion model and a multi-scale characterization of the EWG structure. Four different structures of EWG were prepared combining various pH and ionic strength conditions before heating. The extent of digestion, estimated by the appearance of soluble peptides, was greater for the particulate gel comprising spherical aggregates as compared to the filamentous gel comprising linear aggregates. This result may be explained by an alteration of enzyme diffusion due to the microstructural characteristics of the gel, or an alteration of digestive enzyme adsorption or catalytic activity due to local differences in pH within the gel matrices. Peptide identification using LC-MS/MS highlighted that the aggregate morphology modulated ovotransferrin hydrolysis which resulted in the release of specific peptides depending on the EWG structure. This work illustrates the links existing between two multi-step processes, protein structuration into gel matrices and matrix destructuration by digestive enzymes. Such quantitative and qualitative differences reinforce the importance of the food matrix on the digestibility of food and its subsequent nutritional quality.

International audience; Proteins started being used as pharmaceuticals in the 1920s with insulin extracted from pig pancreas. In the early 1980s, human insulin was prepared in recombinant bacteria and it is now used by all patients suffering from diabetes. Several other proteins and particularly human growth hormone are also prepared from bacteria. This success was limited by the fact that bacteria cannot synthesize complex proteins such as monoclonal antibodies or coagulation blood factors which must be matured by post-translational modifications to be active or stable in vivo. These modifications include mainly folding, cleavage, subunit association, gamma-carboxylation and glycosylation. They can be fully achieved only in mammalian cells which can be cultured in fermentors at an industrial scale or used in living animals. Several transgenic animal species can produce recombinant proteins but presently two systems started being implemented. The first is milk from farm transgenic mammals which has been studied for 20 years and which allowed a protein, human antithrombin III, to receive the agreement from EMEA (European Agency for the Evaluation of Medicinal Products) to be put on the market in 2006. The second system is chicken egg white which recently became more attractive after essential improvement of the methods used to generate transgenic birds. Two monoclonal antibodies and human interferon-beta 1a could be recovered from chicken egg white. A broad variety of recombinant proteins were produced experimentally by these systems and a few others. This includes monoclonal antibodies, vaccines, blood factors, hormones, growth factors, cytokines, enzymes, milk proteins, collagen, fibrinogen and others. Although these tools have not yet been optimized and are still being improved, a new era in the production of recombinant pharmaceutical proteins was initiated in 1987 and became a reality in 2006. In the present review, the efficiency of the different animal systems to produce pharmaceutical proteins are described and compared to others including plants and micro-organisms.

Comité d'organisation:Prof Perla RELKIN, AgroParisTech-Centre de Massy (President)SPAB, Department of Engineering and Science of Food and Bioproducts1 Avenue des Olympiades, 91300 MASSY, FranceDr Monique AXELOS, INRA, France (Vice-President)Head of the division for Science and Process Engineering of Agricultural ProductsRue de la Géraudière BP 71627INRA_logo44316 NANTES, France; The ability of egg white proteins to form gels with very different structures means that egg white has considerable potential as a functional food with controlled delivery properties. This work aimed to investigate the links between the gastric digestion process and the structural properties of three different egg white matrices.The heat-induced egg white gels had the same composition, but different physicochemical properties; a smooth-rigid gel at pH 9, a granular-spongy gel at pH 5 and an intermediate gel at pH 7. Each gel was fed to pigs (n=33) as a single meal and gastric chyme samples were collected from 10 different stomach locations over a 6h-postprandial period in order to study digestion as a function of stomach location and postprandial time.At 20min postprandial, the pH of the chyme was similar to the pH of the gel ingested (Fig.1). Acidification of the chyme commenced near the pylorus, and 4h to 6h postprandial was required to uniformly acidify (pH < 3) the whole stomach. Up to 2h postprandial time, the pH 9 gel acidified more rapidly than the pH 7 gel even though the gastric emptying rate was not significantly different between these two gels. Thus, the acidification kinetics of the stomach contents was driven not only by the initial pH of the diet but also by the gels structural properties. Degree of hydrolysis measurements, as well as microscopic, rheological and particle size characterization of the gastric contents are currently in progress and will enable a better understanding of the gastric digestion process.This work gives insights into the heterogeneous nature of gastric chyme and the role food structure and pH may have on gastric pH behavior. These results question the relevance of static in vitro gastric digestion simulation models in which the incubation pH is typically set around 2.5-3.5, the optimum for pepsin activity. It also implies that coordinated delivery of nutrients to the gastrointestinal tract can be achieved through knowledge of the food matrix characteristics.

Egg white is a common gelling ingredient that can produce gels with varying macro- and microstructures depending on the conditions (pH and/or ionic strength) employed, without the need to change the chemical composition. Consequently, egg white gels provide interesting models for investigating the food matrix effect on food digestion. In the present study, 4 gels were prepared by heating egg white at 80°C for 1h with pH varying from 2 to 9. Multi-scale characterization of the gels was performed using infra-red analysis (FTIR), electronic microscopy (SEM), and texturometry techniques. Each gel was subjected to static in vitro digestion using a two-step model (gastric step for 60 min, followed by an intestinal step for 30 min). Digest samples were taken throughout the digestion period and analyzed for the extent and rate of proteolysis using SDS-PAGE; the concentration of soluble peptides was determined by RP-HPLC. The amino acid sequence (for identification) and relative amount of each peptide was then determined using nano-LC-MS/MS. The macroscopic differences between the gels resulting from the different gelation conditions were related to textural differences. For example, gel hardness was maximum for the pH 5 gel and minimal for the pH 2 gel, cohesiveness and resilience decreased when gel pH decreased from 9 to 2. Furthermore, SEM observations revealed smooth microstructures for the pH 9 and pH 2 gels, whereas the pH 5 and 7 gels were comprised of aggregates. Concurrently, different in vitro digestion kinetics were observed: the pH 7 gel was digested more quickly and more extensively than the other gels. Moreover, quantitative peptidomics and multivariate data analysis of the in vitro digests enabled peptide clustering based on the nature and relative amount of the peptides generated; this highlighted the impact of gel structural and textural characteristics on the digestion process. Combining a food structure characterization and peptidomic approach, the present study provides original results that contribute to the understanding of the relationship between food matrix structure and digestion.

The ability of egg white proteins to form gels with very different structures means that egg white has considerable potential as a functional food with controlled delivery properties. This work aimed to investigate the links between the gastric digestion process and the structural properties of three different egg white matrices.The heat-induced egg white gels had the same composition, but different physicochemical properties; a smooth-rigid gel at pH 9, a granular-spongy gel at pH 5 and an intermediate gel at pH 7. Each gel was fed to pigs (n=33) as a single meal and gastric chyme samples were collected from 10 different stomach locations over a 6h-postprandial period in order to study digestion as a function of stomach location and postprandial time.At 20min postprandial, the pH of the chyme was similar to the pH of the gel ingested (Fig.1). Acidification of the chyme commenced near the pylorus, and 4h to 6h postprandial was required to uniformly acidify (pH < 3) the whole stomach. Up to 2h postprandial time, the pH 9 gel acidified more rapidly than the pH 7 gel even though the gastric emptying rate was not significantly different between these two gels. Thus, the acidification kinetics of the stomach contents was driven not only by the initial pH of the diet but also by the gels structural properties. Degree of hydrolysis measurements, as well as microscopic, rheological and particle size characterization of the gastric contents are currently in progress and will enable a better understanding of the gastric digestion process.This work gives insights into the heterogeneous nature of gastric chyme and the role food structure and pH may have on gastric pH behavior. These results question the relevance of static in vitro gastric digestion simulation models in which the incubation pH is typically set around 3.5-2.5, the optimum of pepsin activity.

Salmonella enterica serovar Enteritidis is the prevalent egg-product-related food-borne pathogen. The egg-contamination capacity of S. Enteritidis includes its exceptional survival capability within the harsh conditions provided by egg white. Egg white proteins, such as lysozyme and ovotransferrin, are well known to play important roles in defence against bacterial invaders. Indeed, several additional minor proteins and peptides have recently been found to play known or potential roles in protection against bacterial contamination. However, although such antibacterial proteins are well studied, little is known about their efficacy under the environmental conditions prevalent in egg white. Thus, the influence of factors such as temperature, alkalinity, nutrient restriction, viscosity and cooperative interactions on the activities of antibacterial proteins in egg white remains unclear. This review critically assesses the available evidence on the antimicrobial components of egg white. In addition, mechanisms employed by S. Enteritidis to resist egg white exposure are also considered along with various genetic studies that have shed light upon egg white resistance systems. We also consider how multiple, antibacterial proteins operate in association with specific environmental factors within egg white to generate a lethal protective cocktail that preserves sterility.

Maison de la Science en Bretagne Située sur le campus de Beaulieu de l'Université de Rennes 1, la Maison pour la science en Bretagne a ouvert ses portes en septembre 2014. Initiée par la Fondation "La main à la pâte" et par l’Académie des sciences, cette Maison est le fruit d’une collaboration exemplaire entre les universités, les grandes écoles bretonnes (dont AGROCAMPUS OUEST) et le rectorat d’académie. Elle propose à l’ensemble des professeurs des écoles et des collèges de l’académie de Rennes de participer à des actions de développement professionnel dans les domaines scientifiques. La 4 ième action de développement personnel, à destination des enseignants du collège, a eu lieu les 19 et 21 février avec l’intervention de plusieurs enseignants-chercheurs du dépt Productions animales, agroalimentaire, nutrition (PSAN). Romain Jeantet, Amélie Deglaire, Valérie Lechevalier, Françoise Nau et Catherine Guérin ont animé le module : « Les pieds dans le plat : transformer et conserver les aliments ». Infos : www.maisons-pour-la-science.orgContact : romain.jeantet@agrocampus-ouest.fr; L’OEuf dans tous ses états, modèle d’ingrédient culinaire. 4 iéme action de développement personnel, à destination des enseignants du collège: module : « Les pieds dans le plat : transformer et conserver les aliments ».

As a 1st step, this study aimed at investigating the microbial quality of liquid egg white in a French eggprocessing company. Thirty raw and 33 pasteurized liquid egg white samples were analyzed. Pasteurization was globally found efficient on mesophilic contaminants (1.7 ± 1.6 and 0.8 ± 0.9 log CFU/mL in raw and pasteurized samples, respectively), including for the control of Salmonella. However,Gram-positive enterococci were still detected in the pasteurized samples. As a 2nd step, a representative bacterial collection was built for exploring the spoilage issue in egg-based chilled desserts. Custard cream was chosen as growth medium since this food is widely used for the production of French chilled desserts. All of the 166 isolates of the bacterial collection were shown to be able to grow and to induce spoilage of the custard cream at refrigeration temperature (10 °C). Several spoilage types were highlighted in the custard cream, on the basis of changes regarding pH, consistency, production of holes or gas. As a 3rd step, bacterial enzymatic activities were explored on custard cream-based agar media. The bacterial collection was reduced to 43 isolates, based on further selection regarding the genera and the spoilage types previously highlighted. Albeit to different degrees, all these isolates were able to produce proteases. A large part of these isolates also expressed lipolytic and amylolytic activities. This studyemphasizes the need to control egg white contamination and especially with Gram-positive heat-resistant Enterococi, in order to guarantee the shelf life of egg-based chilled desserts.

There has been extensive research in the last decade trying to understand the fundamentals of dissolution of whey protein gels in order to lay the foundations of proteinaceous cleaning processes. To date, experimental data has been limited to whey proteins, raising questions of the applicability of the theoretical frameworks developed for whey proteins to other protein systems. In this work the dissolution of crude egg white gels is studied, which are rich in ovalbumin protein, and which have recently shown a different cleaning behavior to whey deposits. Egg white gels made in well-defined conditions were dissolved in a wide range of alkali and sodium chloride concentrations, and temperature. The dissolution rate was found to increase linearly with the alkali concentration up to 1 M, hence confirming the absence of an optimum cleaning concentration observed in whey protein cleaning studies. The apparent activation energy of dissolution for particulate egg white gels was 70–80 kJ mol −1 , suggesting that the dissolution of egg white gels is controlled by the β-elimination of disulfide bonds. Dissolution experiments with salts at different temperatures confirmed that swelling is very important in the dissolution of whey protein gels, but not in egg white gels.

Although egg white is known as expressing efficient antimicrobial properties under physiological conditions, little is known on the microbial quality of industrial egg white. This issue is of great importance when egg white enters into the composition of highly perishable products, such as chilled egg-based desserts. Due to the high thermo-sensitivity of its constitutive proteins, liquid egg white is stabilized at low heat-treating times and temperatures (2 to 6 min at 55°C to 57°C in France). The remaining of heat-resisting bacteria may shorten the shelf-life of the desserts, with damaging health and economical consequences. The objective of this study was to investigate the microbial quality of raw and pasteurized liquid egg white products collected in a French company at two consecutive warm and cold seasons. A total of 63 samples were analyzed, including 30 raw and 33 pasteurized (at 57°C for 6 min) liquid egg white products. The level and type of bacterial contamination was evaluated by pour-plating in PCA (incubation for 24h at 30°C under aerobic conditions) and 16SrDNA sequencing of the colonies. The average counts were fairly low and a broad range of population was highlighted from one sample to another, namely 1.7 ± 1.6 log CFU/mL

Des scientifiques rennais étudient les propriétés antibactériennes d’une protéine du blanc d’œuf.Battu en neige, il fait gonfler les gâteaux, et mousser le chocolat ! Mais le blanc d’œuf renferme d’autres secrets. Il contient notamment du lysozyme, une protéine qui lutte naturellement contre les bactéries. Cette action est connue depuis longtemps, et est déjà utilisée dans les domaines pharmaceutique et agroalimentaire. « Le lysozyme est connu pour son action sur un certain type de bactéries, dites à Gram positif, explique Françoise Nau, de l’unité Science et technologie du lait et de l’œuf à l’Inra de Rennes.Nous avons voulu savoir ce qu’il en était sur les bactéries à Gram négatif. » En thèse au laboratoire, Mélanie Derde a pu confirmer que la protéine était bien active contre ces dernières, et explorer les mécanismes de cette action. « Le problème des bactéries dites à Gram négatif, c’est qu’elles ont une membrane extérieure supplémentaire, qui les protège, détaille Françoise Nau. Nous avons montré que, dans certains cas, le lysozyme pouvait créer des pores dans cette membrane, affaiblissant la bactérie. » Pour y parvenir, les biologistes ont mis la protéine en présence d’Escherichia coli, une bactérie modèle à Gram négatif. Puis en travaillant sur des membranes artificielles, ils ont confirmé que la protéine désorganisait les couches lipidiques qui composent la membrane. « Nous cherchons également à comprendre pourquoi le lysozyme agit plus efficacement lorsqu’il est chauffé. » Ces résultats, publiés en 2013 et cette année, seront présentés par Mélanie Derde lors de sa soutenance de thèse en octobre prochain. Ils permettront peut-être, à terme, de lutter plus efficacement contre les bactéries et de satisfaire les consommateurs en demande de conservateurs naturels.

This work has been supported by the grant Reducing Egg Susceptibility to Contamination in Avian Production in Europe (RESCAPE) from the European Commission (FOOD-CT-2006-036018).; Bacillus cereus group bacteria are opportunistically pathogenic spore-forming microorganisms well known in the sector of pasteurized food products because of their involvement in spoilage events. In the sector of egg product processing, these bacteria may lead to important economic losses. It seemed then relevant to study their behavior in egg white, a widely used egg product usually recognized as developing different levels of antimicrobial activities depending on the environmental conditions. A strong bactericidal effect (decrease in the bacterial population of 6.1 ± 0.2 log CFU/ml) was observed for 68 B. cereus group isolates, independently incubated at 30°C in egg white at pH 9.3 (natural egg white pH). To determine which components could explain such a strong bactericidal effect, an experimental strategy was carried out, based on egg white fractionation by ultrafiltration and by anion-exchange liquid chromatography. The role of the protein fraction was thus demonstrated, and subsequent nano-liquid chromatography-tandem mass spectrometry analyses allowed identification of ovotransferrin as a major protein involved. The strong bactericidal effect was confirmed in the presence of commercial ovotransferrin. Such a bactericidal effect (i.e., a decrease in the bacterial population through cell death) had never been described because ovotransferrin is known for its bacteriostatic effect (i.e., inhibition of growth) due to its ability to chelate iron. Surprisingly, the addition of iron did not reverse the bactericidal effect of ovotransferrin under alkaline conditions (pH 9.3), whereas it completely reversed this effect at pH 7.3. Ovotransferrin was shown to provoke a perturbation of the electrochemical potential of the cytoplasmic membrane. A membrane disturbance mechanism could, hence, be involved, leading to the lysis of B. cereus group bacteria incubated in egg white.

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International audience; Bacterial membranes are often thought to be the main targets of the antimicrobial activity of egg white. In order to test this hypothesis, the state of the membranes of Escherichia coli K-12 cells during either bactericidal (45°C) or bacteriostatic (30°C) incubation in egg white at natural alkaline pH was studied by biochemical methods. Namely, the permeability of the outer membrane was evaluated through its ability to incorporate a hydrophobic fluorescent probe (1-N-phenylnaphthylamine), and the permeability of the cytoplasmic membrane was evaluated through the release of a specific intracellular enzyme (β-galactosidase). The bacteria were observed by atomic force microscopy in order to support the biochemical results. At 45°C, the outer membrane of E. coli K-12 incorporated the hydrophobic probe, suggesting that it was disrupted. In addition, the cytoplasmic β-galactosidase was released at this temperature. The atomic force microscopy analysis revealed the formation of spheroplasts, which provided further evidence of the cell wall disruption and a progressive release of cellular contents. At 30°C, biochemical and micrographic experiments confirmed that membrane integrity was preserved. These techniques provide a useful approach for studying the mechanisms of bacterial cell death in egg white.

Ce travail porte sur l’analyse de 63 échantillons de blanc d’oeuf dont 30 de blanc cru et 33 de blanc pasteurisé. Les analyses réalisées sont le dénombrement de la flore total à 30°C, l’étude des capacités d’altération des isolats après inoculation (de 3 à 5 log UFC/mL) dans de la crème anglaise et incubation à 10 °C pendant 21 jours. Les isolats altérants la crème anglaise sont ensuite identifiés par séquençage de l’ADN 16S puis caractériser selon leurs capacités à produire des protéases, lipases et amylases sur des milieux gélosés à base de crème anglaise. La révélation de ces activités enzymatiques sont basés sur la présence et la mesure d’un halo d’éclaircissement, la fluorescence due à la rhodamine B, ou la présence d’un halo après coloration au lugol pour les activités protase, lipase et amylase respectivement. Les résultats montrent que près de 80% des échantillons de blanc analysés étaient contaminés avec des niveaux de populations autour de 2±1,6 et 0,94±0,8 log UFC/mL du blanc cru et pasteurisé respectivement. L’ensemble des isolats étaient capables d’altérer la crème anglaise à 10 °C. Cette altération se caractérise par des niveaux de populations autour de 9,3 log UFC/ml, une diminution du pH, un épaississement, une apparition d’un déphasage, et une présence de bulles ou trous ainsi que des bruits à l’ouverture du tube dus à la production de gaz. La présence d’un de ces critères d’altération, variant selon les genres identifiés, a permis de distinguer 17 profils d’altération. L’identification de bactéries d’altération, a montré une prépondérance des genres Enterobacter et Escherichia dans le blanc d’oeuf cru et d’Enterococcus dans le blanc d’oeuf pasteurisé. Les autres genres identifiés sont Yersinia, Rahnella, Salmonella, Pantoae, Pseudomonas et Bacillus dans le cru et des genres tels que Enterococcus, Enterobacter, Escherichia, Serratia, Shigella et Klebsiella communs aux 2 types de blanc d’oeuf. L’ensemble des isolats testés est capable de produire des protéases mais à des niveaux variables (épaisseurs des halos de 1 à 6,2 mm). Soixante pourcent, cent pourcent et quatre-vingt pourcent des isolats présentent des activités lipase, protéase et amylase respectivement. Ce travail souligne l’importance de la maitrise des flores bactériennes issues des ovoproduits qui sont capable d’altérer des produits finis tels que des desserts réfrigérés., This work focuses on the analysis of 63 liquid egg white samples, including 30 raw and 33 pasteurized samples. The performed analysis are the total enumeration flora at 30 °C, the spoilage abilities of the isolates after inoculation (3 to 5 log CFU/mL) and incubation in the custard cream at 10 °C for 21 days. The spoilage isolates were then identified by DNA 16S sequencing and characterized according to their ability to produce proteases, lipases and amylases on agar custard based media. The revelation of these enzymatic activities are based on the presence and translucent halo extent, fluorescence due to rhodamine B, or the presence of a halo after Lugol staining for the protease, lipase and amylase activities respectively. The results show that about 80 % of egg white samples were contaminated with population levels around 2±1,6 and 0,94±0,8 log CFU/mL in raw and pasteurized white respectively. All isolates were able to spoil custard cream at 10 °C. The custard cream spoilage is characterized by a bacterial growth reaching level as high as 9,3 log CFU/mL, a decrease in the pH, thickening, an appearance of a phase shift and the presence of bubbles or holes as well as noises opening tube due to gas production. The presence of these alteration criteria, varying genera identified, allowed to distinguish 17 alteration profiles. The identification of spoilage bacteria showed a preponderance of Escherichia and Enterobacter in raw egg white and Enterococcus in the pasteurized one. Other identified types are Yersinia, Rahnella, Salmonella, Pantoae, Pseudomonas and Bacillus exclusively found in raw egg white and Enterococcus, Enterobacter, Escherichia, Serratia, Shigella and Klebsiella common to the two types of egg white. All isolates tested can produce proteases but at varying levels (thicknesses halos 1 to 6,2 mm). Sixty percent, one hundred percent and eighty percent of the isolates have lipase, protease and amylase activities, respectively. This work highlights the importance of controlling the bacterial flora from egg products which are able to spoil finished products such as chilled desserts.

Egg powders are used in many sectors of the food industry since they are easy to handle in a safe manner, are not susceptible to bacterial growth, and can utilize precise water dosing in their formulation. Egg powders provide consumers with advanced characteristics as well as technological advantages that are not found in liquid egg products. To compete with other functional ingredients, egg powder products are often specifically designed for customers’ formulations, a technique greatly enhanced by the ingredient’s diverse technical possibilities.

Etude du comportement aux interfaces du lysozyme natif de blanc d'oeuf de poule en vue d'une étude comparée avec du lysozyme traité thermiquement. 5. Rencontres de Biologie Physique du Grand Ouest