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This chapter describes the chemical transformations of tryptophan induced by heat, acids, bases, carbonyl compounds, carbohydrates, halogens, lipids, nitrites, oxidizing agents, sulfites, other food ingredients, and some of the resulting nutritional and toxicological consequences. However, new data show that reactions of tryptophan during food processing, especially during thermal treatments, could be of nutritional and toxicological importance. Chemical transformations of tryptophan are essentially a function of the temperature and the duration of treatments. In the presence of hydrochloric acid and oxygen, tryptophan degradation increases with both time of heating and temperature. In the presence of hydrochloric acid and oxygen, tryptophan degradation increases with both time of heating and temperature. Organic acids, such as p-toluenesulfonic acid, mercaptoethanesulfonic acid, and methanesulfonic acid may be suitable for determining tryptophan under acid hydrolysis conditions. The effects of heating time and temperature on racemization of free tryptophan in strongly basic solutions were studied by Spies and Chambers.

Physicochemical properties of non-fat, low heat, reconstituted skim milk subjected to pH cycling using CO2 as an acidifying agent were studied. The pH was dropped to 5.5 at 4 °C for 15 min and temperature was subsequently kept at 4 °C or increased to 20 and 30 °C for further 45 min, before degassing under vacuum. Even though many overall properties of the micelles were not affected (calcium and protein content, size, zeta potential, acid gelling properties), it was shown that the internal organisation of the micelles did change. The structure of the micelle was dependent on carbonation temperature and evolved during chilled storage, as indicated by the rennet clotting time and micelle hydration variations. In contrast, modifications of the micellar calcium phosphate salt form, as reflected in the buffering capacity curves, did not depend on temperature of carbonation.

Les modifications chimiques des protéines alimentaires sont réalisées dans le but d’ améliorer leurs propriétés fonctionnelles. Elles sont également pratiquées pour des études fondamentales des relations structure/fonction des protéines. Les principales modifications chimiques des protéines alimentaires sont présentées ainsi que leurs incidences sur les propriétés fonctionnelles telles les propriétés émulsifiantes, moussantes, visqueuses, d’hydratation et de gélification.

The antimicrobial potential of CO 2 applied under 5 MPa pressure during heat treatment was investigated against Bacillus subtilis spores, Byssochlamys fulva ascospores and Aspergillus niger conidia. The rates of thermal inactivation were increased in the presence of pressurized CO 2 when treatment was carried out at a temperature higher than a threshold value depending on the tested microorganism (80 °C for B. subtilis and B. fulva , 50 °C for A. niger ). The antimicrobial effectiveness of CO 2 varied with the microorganisms, the temperature and the a w of the suspension medium. However, the increase in the destruction rate was relatively more extensive when the temperature was sublethal or low lethal. At elevated lethal temperatures, the antimicrobial effect of pressurized CO 2 disappeared. Moreover, a low a w antagonized the lethal effect of CO 2 under pressure.

β-Lactoglobulin (βLG) was submitted to various thermal treatments (75 and 90 °C), and its conformational modifications were assessed by competitive radioimmunoassay (RIA) with rabbit antisera against native βLG (βLG NAT ) or chemically modified βLG obtained by performic oxidation (βLG OX ). Heat-processed βLG reacted with both antisera. Molecule alteration was accompanied by an increased binding of polyclonal antibodies. Whatever the serum used, the RIA test revealed a thermal dependence that was influenced by pH (6.5 and 7.5). However, anti-βLG OX antiserum was shown to be the most useful tool for monitoring slight denaturation structural changes after heating. The test accurately detected local modifications of βLG epitopes that occurred after thermal processing of an industrial βLG isolate.

SummaryIn order to predict the oozing susceptibility of heat-induced milk protein gels such as processed cheeses during storage, ultracentrifugation forces are required to accelerate the expulsion of water from the gel structure. Two predictive methods have been tested. Direct ultracentrifugation of processed cheese was used to study effects of centrifugal compression on the water-holding ability. The water release with optimal parameters (time, temperature and ultracentrifugation force) from a freshly manufactured processed cheese was correlated with visual assessment of oozing after 6 months storage. A second method was based on the susceptibility of gelled proteins to resist the dissociating action of solutions of chemical agents such as SDS, urea, EDTA and 2-mercaptoethanol. Most of the protein sedimented by ultracentrifugation in the presence of SDS represented that still in complexed form. This correlated with optimal water binding by the processed cheese. Response-surface methods, used to optimize the ultracentrifugation variables and the composition of the dissociation solutions, showed that the dissociation test was best with a dispersion of processed cheese in SDS solution (10 g/l) at a ratio of 1:6 (w/v) at 20°C, ultracentrifuged at 86000 g and 20°C for 25 min. Analysis of processed cheese cooked at 115°C and then dissociated in different solutions showed various protein–protein interactions in the gel network, whereas hydrophobic interactions were the most important stabilizers of the protein matrix of cheese cooked at higher temperature.

Two methods were studied in order to estimate the degradability of packaging materials, using cellulosic materials—such as cellophane or paraffin waxed paper—as models. A preliminary orientation test was carried out, based on the visual observation of microbial development on the materials used. This qualitative test permitted an initial classification of materials according to their potential degradability, and especially allowed the identification of microbial species, particularly fungi, most likely to grow on the materials being studied. The biofragmentation of materials was evaluated from their dry weight loss. For a fixed time of incubation, the classification of these materials was based either on the percentage or on the absolute value of this weight loss. For the method applied to a solid medium, we found that an inoculum deposited on an agar surface was more suitable than direct inoculation of the samples, in fact, pure fungi strains, especially Trichoderma viride or Aspergillus niger, were found to be more suitable for cellulosic materials than the bacterial strains (Pseudomonas fluorescens) or bacterial mixture used.

An edible wheat gluten film was developed and effects of gluten concentration, ethanol concentration (ET) and pH of the film-forming solution on various film properties were evaluated using Response Surface Methodology. pH and ethanol concentration had strong interactive effects on film opacity, water solubility and water vapor permeability. A simultaneous variation of ethanol concentration and pH between 32.5% ET, pH 4 and 45% ET, pH 2 resulted in homogeneous and transparent film with relatively low water solubility. The lowest water vapor permeability would be expected with 20% ethanol concentration and pH 6. Mechanical properties were mainly affected by gluten concentration and pH. The most resistant film was obtained at high gluten concentration (12.5%) and pH 5.

International audience; Reconstituted skim milk was fortified at 2, 5, 10 and 20 mmol Fe kg(-1) with ferric and ferrous chloride iron with or without reversible acidification by injection of CO2 under pressure. Carbonation improved transfer of iron from the soluble to the colloidal phase and accelerated ferrous iron oxidation. Fe-57 Mossbauer spectra of the freeze-dried casein micelles collected by centrifugation demonstrated that iron is present in a distorted octahedral coordination and is chelated by phosphate rather than citrate. Milk iron fortification induced several changes in the mineral component of the casein micelles, which are explained by the formation of a tertiary complex: inorganic phosphate-iron-organic phosphate.

Correspondance: sraouche@univ-montp2.fr; International audience; Casein micelle stability and reactivity were assessed on milk subjected to reversible acidification by carbonation. Pressurised CO2 was injected at 4 degrees C, leading to controlled acidification from 6.63 +/- 0.02 to a target pH (5.5 or 5.2). After holding the pressurised milk under these conditions for 15 or 60 min, the pressure was released and the milk pH returned to its initial value under stirring and vacuum degassing. Upon CO, treatment, calcium and protein partition, zeta potential and size of casein micelles were restored directly after nentralisation. The rheological properties of the gel obtained by acid coagulation Of CO2-treated milk did not change as a result of carbonation. Micelle hydration increased after neutralisation and during storage. Milk buffering capacity in the pH range of 4.5-5.5 decreased after neutralisation of milk acidified by carbonation, but increased during chilled storage of this milk. Holding time of carbonated milk at low pH was found to have no impact on the physicochemical characteristics of casein micelles and the rheological properties of the gel obtained by acid coagulation of this milk

Thermal inactivation of Escherichia coli was studied under CO2 pressures of 1.2, 2.5, and 5 MPa at 25, 35, and 45°C. Two phases were observed in the destruction curves. The earlier stage was characterized by a slow rate of inactivation, which increased sharply at the later stage. An increase of pressure and/or temperature enhanced the antimicrobial effects of CO2 under pressure. The effects on cell structure were studied by scanning electron microscopy and the specific mechanism of action appeared to be related to enzyme inactivation.

Optical microscopic observation of the coagulation of rennet-treated milk was performed using a conventional light microscope under standardized conditions. The microscopic images, i.e., number and average area of milk aggregates, were analyzed using Optiscan software. The evolution of the latter parameters was followed as a function of the rennet coagulation time of milk. The curves presented two inflection points which correlated with the rennet flocculation time and the rennet clotting time values, as determined by reference methods. These relationships were validated on milk supplemented with different concentrations of calcium (5.0–7.5 m m ) and subjected to varying temperatures for rennet coagulation (22.5–40°C). This optical microscopic method with associated image analysis software allowed (i) the observation of structural changes occurring during gel formation, even at low magnification (200X) and (ii) the determination of the rennet flocculation and clotting times.

3 tables 3 graph.; International audience; Packaging films based on fish myofibrillar proteins have been developed by two processes. The “solvent process” based on the casting of a film‐forming solution is affected by protein concentration and pH. The “thermoplastic process” is based on thermoplastic properties of myofibrillar proteins. The glass transition was observed between 215 and 250 °C for the dry protein; addition of water or hydrophilic plasticizers (sucrose and sorbitol) induced large decreases in the glass transition temperature (Tg). Packaging fabrication by a thermomoulding technique was possible at temperature higher than the Tg for a given moisture content. The functional properties of the packaging films were close to those determined for known protein‐based films, with tensile strength close to those of low density polyethylene films. Hydration and temperature effects on functional properties could be explained by the glass transition theory.

Myofibrillar protein-based films were developed from a film-forming solution based on fish mince. Glycerol, sorbitol, or sucrose was incorporated as plasticizer at various concentrations. The increase in film solubility in water with plasticizer content reflect plasticizer solubility in water. Solubility in water of protein network is not significantly affected by plasticizer concentration whatever the plasticizer. On the other hand, plasticization of myofibrillar protein-based films induced large decreases in film strength and elasticity and increases in deformation properties and water vapor permeability. No significant difference was observed in variations of functional properties as a function of plasticizer type when plasticizers were introduced at the same molecular contents, due to the structural similarities between glycerol, sorbitol, and sucrose. Keywords: Biopackaging; myofibrillar proteins; plasticizer; functional properties

A phenomenological rheological model was developed to describe the molecular interactions and predict the mechanical properties of fish myofibrillar protein-based films, irrespective of their thickness. The model was built of a parallel array of Maxwell and elastic elements in which a fracture element was introduced. The total number of parallel branches was directly correlated with the most characteristic dimension of the samples, i.e., the film thickness. The rheological parameters (α, elastic modulus, and viscosity coefficient) were determined from numerous experimental data sets and appeared to be independent of thickness. They could be used to characterize the molecular interactions in the films. These parameters were only sensitive to the type of data used for calculations (force−deformation, relaxation, or creep) and to the range of experimental conditions applied to obtain these data. The model successfully predicted force−deformation−time relationships for these biopolymer-based films. Keywords: ...

Fish myofibrillar protein-based films formulated with saccharose as a plasticizing agent were stored for several weeks under regulated temperatures and relative humidities. Solubility in water, water vapour permeability and mechanical properties remained constant for 8 weeks at 20°C and 58.7% relative humidity. A slight yellow colour was the only modification observed. This colour modification, which could be associated with nonenzymatic browning reactions, was characterized as a function of time, temperature, relative humidity and plasticizer type. A regression model correlating the pertinent parameters was defined.

Fish myofibrillar protein-based biopackagings were developed from a film-forming solution cast on plates. Film thickness was modulated by variations in “apparent thickness” of the cast solution to evaluate effects on the main functional properties. The thickness variations seemed to have no influence on optical properties (apparent opacity) or on percent solubility in water. Study of mechanical properties revealed that elongation at break and relaxation coefficient were not dependent on film thickness, while force at break was directly proportional to film thickness. This could be interpreted by variations in the number of potential intermolecular interactions in the films. Analysis of water vapor transport showed that the biopackaging did not behave as an ideal material according to Fick's and Henry's laws.

Summary Various formulations and methods of fabricating edible bilayer films consisting of wheat gluten as a structural layer and a thin lipid layer as a moisture barrier were investigated and examined for water vapour permeability. Solid lipids such as beeswax or paraffin wax deposited in a molten state onto the base film were the most effective water vapour barriers. A film consisting of wheat gluten, glycerol and diacetyl tartaric ester of monoglyceride as one layer, and beeswax as the other yielded a water vapour permeability of 0.0048 g mm m−2 mmHg−1 24h−1, which was less than that obtained with low density polyethylene.

Biopackaging materials based on fish myofibrillar proteins have been developed. The effects of protein concentration, pH, temperature and storage time before casting on the apparent viscosity of the film forming solution (FFS) were evaluated using experimental design methodology. The first objective was to determine a feasible experimental range for film-forming. The pH and protein concentration had strong interactive effects on FFS viscosity. During FFS storage before casting, partial degradation of high molecular weight protein components led to decreased viscosity, allowing thin layer casting. In the experimental range for film-forming, none of the conditions affected film functional properties. Standard conditions were determined at: pH 3.0, 2.0g protein/100g FFS, 25°C and 6 hr storage. The functional properties of the standard biopackaging were slightly better than those that determined for known protein-based films, with tensile strength close to those of low density polyethylene films.

To estimate either the susceptibility of packaging materials to enzymes or their potential biodegradability, experiments were conducted using cellulosic materials as models with Trichoderma vinde cellulases. A classification of materials was carried out using two kinds of experiments. First, experiments were conducted to determine the percentage of biofragmentation after 5 h of incubation. Results were obtained on the global kinetics of the reducing sugars produced and on the percentage of hydrolysed glycosidic bonding (HGB) at the end of the experiment. This latter parameter allowed the study of the effect of material shape: pulverized or in film form. The classification from the decreasing values of the percentage of HGB obtained with pulverized substrates was: native cellulose > filter paper > noncoated cellophane # crystalline cellulose # paraffin-waxed paper > coated cellophane. Except for the experiment with the filter paper, the available surface area of a material had a significant effect, especially with the paraffin-waxed paper. The other experiments, based on shorter incubation times and entailing a rigorous kinetic methodology, allowed access to apparent kinetic parameters (denoted K'm and V'max). After a hydration of pulverized materials, the classification according to catalytic efficiency was the same as that based on the percentage of HGB, except for the paraffin-waxed paper. For the latter, the catalytic efficiency was as high as that on the filter paper. (Resume d'auteur)

Un procede de preparation de films proteiques et lipoproteiques a partir de farine de graines de coton avec ou sans gossypol est decrit. Des modifications chimiques des proteines des graines de coton avec le gossypol, le formaldehyde et le glutaraldehyde sont effectuees pour augmenter la force de rupture et diminuer la solubilite des films dans une optique d'emballage biodegradable. Des resultats prometteurs ont ete obtenus avec la farine de varietes de coton glandless. Le formaldehyde et le glutaraldehyde se sont reveles efficaces. Le gossypol peut egalement etre un agent utile

To measure the biodegradability of packaging materials, a scientific and methodological approach is proposed, using the results obtained with cellulosic materials as models. Their degradation was evaluated using a respirometric method based on the measurement of CO2 released by microorganisms (Pseudomonas fluorescens and a mixture of cellulolytic bacteria). Our study was conducted in confined conditions, and the CO2 released was detected by gas chromatography (GC). Glucose was used as a reference compound because of its complete bioassimilation by the strains studied. The percentage of carbon converted into carbon dioxide was the parameter used to classify materials according to their potential biodegradability. With both inocula, the percentage of absolute conversion of glucose to CO2 reached about 36% compared with 24% and 13% for the hydrated crystalline cellulose and the paraffin waxed paper, respectively. Measurements of the percentage of absolute conversion of glucose to CO2, of the dissolved carbon in the culture medium, of the biomass carbon and of the residual glucose permitted the calculation of an approximate carbon balance with Pseudomonas fluorescens. The percentages of absolute and relative conversion of packaging materials into CO2 were calculated and the growth rates evaluated. It was shown that to evaluate the biodegradation of insoluble substrates, the biomass produced and the dissolved carbon must be taken into account.

International audience; An edible wheat gluten film was developed and relationships between film-formation conditions and properties were studied using Response Surface Methodology. pH and ethanol concentration of film-forming solution had strong interactive effects on film water solubility and moisture permeability. Mechanical properties were affected by gluten concentration and pH. Various formulations and methods of fabricating films consisting of gluten and lipids were investigated.

Le glycérol améliore l'extensibilité mais réduit la résistance à la rupture, l'élasticité, les propriétés barrière à la vapeur d'eau. L'effet plastifiant est très dépendant de la teneur en eau du film. Au cours de la réhydratation du film de gluten on observe une diminution importante de la résistance à la rupture, de l'élasticité et une augmentation de l'extensibilité et de la transmission de vapeur d'eau, pour des températures 5, 30 et 50¼C avec respectivement des teneurs en eau de 30 (0,8 aw), 15 (0,7 aw) et 5% (0,4 aw). Ceci peut-être correllé à la rupture de liaison polymère-eau ainsi qu'au phénomène de transition vitreuse

The extent of oxidation of methionine residues has been determined in food proteins submitted to various oxidative treatments. The nutritional value of oxidised casein, in which 98% of the methionine residues were converted to the corresponding sulphoxide, was assessed both by proteolysis in vitro and on rats. The nutritional availability of residues of methionine sulphoxide appears to be only slightly inferior to that of methionyl residues.

Revue bibliographique concernant les proprietes de cet acide amine essentiel, sa stabilite et les effets toxiques eventuels de ses produits de degradation

This paper examines the influence of Thelohania maenadis (Protozoa: Microsporida), a muscle microsporidian, on certain features of the metabolism of Carcinus mediterraneus (Crustacea: Decapoda). Comparison of biochemical parameters reveals that only hemolymph total protein and glucose levels are reduced significantly by this parasitic infection. Muscular effort results in a diminution of muscular glycogen and hemolymph glucose levels, while hepatopancreatic glycogen remains unaffected; lactate accumulates in the muscle tissue. Experimental variations in the environmental water temperature and salinity affect protein levels and metabolic indicators of carbohydrate metabolism. Proteinemia varies with temperature and salinity, but parasitic infection accentuates these variations. Glucose and lactate concentrations in the hemolymph are increased in healthy specimens after muscular effort, under conditions of increased temperature or hypoosmotic shock; microsporidiosis provokes the reverse of these tendencies, on a less pronounced scale. Hyperosmotic shock at high temperature leads to a slight reduction in glucose and lactate concentrations. The paper concludes with a discussion of the physiopathological significance of the results obtained as an expression of the effects of parasitism infestation.

Resume L'influence d'une microsporidie, Thelohania maenadis Perez, 1904 (Protozoa: Microspora) sur les teneurs en acides amines libres de l'hemolymphe et du muscle chez Carcinus mediterraneus Czerniavsky, 1884 (Crustacea: Decapoda: Brachyura) est etudice. La technique d'analyse des acides amines libres permet de mettre en evidence chez les crabes dont la musculature est detruite a 70–80%, quatre composes supplementaires non identifies dans l'hemolymphe et onze dans le muscle. Le parasitisme n'induit pas de variation importante des acides amines libres totaux de l'hemolymphe et du muscle. On note dans ce dernier une augmentation tres importante en methionine sulfoxyde, en methionine et en serine; la glycine, la proline et l'arginine diminuent alors que l'alanine et taurine augmentent. La diminution des acides amines libres totaux de l'hemolymphe, chez les crabes sains et parasites soumis a dessalure a temperature moyenne, est surtout due a la baisse des concentrations de la proline, la glycine et l'alanine. La sursalure n'entraine qu'une legere diminution de la teneur totale des acides amines libres chez les animaux sains et n'a aucune action chez les parasites. En effet, la nette augmentation de la proline et de l'alanine est compensee par la diminution d'autres acides amines, notamment l'arginine, l'acide glutamique, la glycine, le tryptophane et la taurine chez les crabes sains et l'arginine, le tryptophane, la taurine et la serine chez les parasites. L'influence de la combinaison des facteurs salinite et temperature a ete etudiee dans des conditions comparables a celles regnant l'hiver et l'ete dans l'etang d'ou sont originaires les crabes. Aux temperatures tres basses et tres hautes, l'action de la salinite s'inverse chez les crabes sains: la dessalure entraine une augmentation des acides amines libres. Le parasitisme diminue cet effet des temperatures extremes sur l'influence de la salinite et peut meme l'annuler completement a temperature elevee.