[Application of the theory of oxygen transfer: determination of the Michaelis constant of glucose oxidase with respect to oxygen]. / Application de la théorie du transfert d'oxygène: détermination de la constante de Michaelis de la glucose oxydase par rapport à l'oxygène

The oxidation of beta-D-glucose with glucose oxidase generally requires oxygen, which, under normal conditions is present at low concentrations in the reaction medium. Experiments show that glucose oxidase is no longer saturated by oxygen at enzyme concentrations greater than 0.4 mg.ml1. This is due to the decrease in the oxygen concentration of the solution. The value of the oxygen mass transfer coefficients and dissolved oxygen concentrations are determined. These dissolved oxygen concentrations are found to correlate with direct measurements with an oxygen electrode. From this, the Michaelis constant of glucose oxidase for oxygen is calculated. These experiments also show that oxygen is a limiting factor for this reaction.

Continuous proteolysis with a stabilized protease. II. Continuous experiments.

Batch proteolysis experiments were performed in order to choose a protein-protease system to prepare a correct hydrolysate suitable for the enrichment of soft-drinks. The system eventually studied was casein-Alcalase. Comparative batch and continuous proteolysis of casein by Alcalase showed that the reaction, which does not exactly follow first order kinetics with respect to the substrate concentration, is inhibited by the reaction products. Furthermore, experiments were done in order to determine the reaction conditions (pH8.8 in the reactor, casein concentration 5%, 40 degrees C). Determining the molecular weight of Alcalase (43,000) suggested the choice of ultrafiltration membrane PM 30. Sutdies of continuous proteolysis with the chemically stabilized enzyme retained by the ultrafiltration reactor showed that protease reuse for seven days at 40 degrees C is possible and that the growth of microorganisms is practically inhibited under these conditions. Gel chromatography showed the molecular weight.range of the peptides to be less than 2,000. Triangular taste tests showed that the threshold identification concentration of the dry hydrolysate in orange juice is about 0.65%.

Continuous Proteolysis with a stabilized stabilized protease. I. Chemical stabilization of an alkaline protease.

Due to the loss of enzymatic activity as a function of time, an alkaline protease, selected for the continuous preparation of protein hydrolysates (J. Boudrant and C. Cheftel, Biotechnol. Bioeng., 18,1735, 1976), was chemically stabilized by a simple treatment with glutaraldehyde. Two fractions, soluble and insoluble, were obtained. The activities of these two fractions were measured with casein and N-benzoyl-L-arginine ethyl ester (BAEE) as a function of glutaraldehyde concentration used. It was noted that the insoluble fraction was practically inactive with the first substrate and that the heat stability of the soluble form was likewise enhanced. Molecular weights of these two forms were unchanged, but the uv-spectrum of the soluble form was modified. From amino acid analysis, it appears that this treatment mainly provokes a decrease in lysine content.

[Use of small peptides to study the loss of available methionine, lysine and tryptophan during technological processing]. / Utilisation de petits peptides pour l'étude des pertes de disponibilité de la méthionine, de la lysine et du tryptophane au cours de traitements technologiques.

Peptides such as glycyl-L-methionyl-glycine, glycyl-L-lysine, L-lysyl-glycine and glycine-L-tryptophyl-glycine are used to study side-chain reactivity of three essential amino acids during food processing. The treatment of glycyl-L-methionyl-glycine with sodium hypochlorite resulted in the two following types of reaction: 1. Methionyl residues are oxidised to the corresponding sulfoxide at sodium hypochlorite concentrations up to 0,1 p. 100 w/v; 2. Oxidation of methionine residues to methionine sulfone and deamination reactions also occur for sodium hypochlorite concentrations over 0,2 p. 100 w/v. Sodium hypochlorite treatment of glycyl-L-lysine and L-lysine-glycine causes, probably by deamination of the epsilon-NH2 groups, a loss of lysine of 20 and 30 p. 100, for sodium hypochlorite concentrations of 0,1 and 0,2 p. 100 w/v, respectively. Treatment of glycyl-L-tryptophyl-glycine with hydrogen peroxide (0,05 M) modifies tryptophan residues in such a way that it cannot be retrieved after hydrolysis of the tripeptide with methane sulfonic acid and subsequent chromatographic analysis; six new unidentified components appear on the chromatogram. Glycyl-L-methionyl-glycine was incubated with food constituents or additives such as reducing oses, acrolein, p-benzoquinone, methyl iodide, or dichloro I, I ethylene. Thin layer chromatography and RMN show that carbonyl compounds or quinones do not react with the thioether group. Alkylating agents sometimes used in food processing only gave traces of sulfonium compounds. Thus, it appears that the only reaction liable to render methionine residues unavailable in foods would be its oxidation to methionine sulfone.

[Intermediate moisture foods: polysaccharide and protein gels]. / Aliments a humidité intermédiaire: gels polysaccharidiques et protéiques.

The aim of this study was to prepare concentrated foods (20-40 p. 100 water), edible as such, chemically and microbiologically stable, nutritionally balanced, and which could be used as meal substitutes (travel, camping, snacks, etc.). With high methoxyl pectins, it was possible to obtain a pectic gel (pH 3,5), similar to a fruit jelly, but containing 20 p. 100 d.w. protein, and less sucrose. Water activity (Aw) was 0,75-0,78, for a 25 p. 100 water content, as a result of adding glucose syrup and sorbitol. After 4 months storage at 20 or 38 degrees C in aluminium pouches, no mold growth was detected (even following prior inoculation) nor practically any change in flavor, texture of Aw. With low methoxyl pectins, gel foods richer in water (35 p. 100), softer, less acid (pH 4,3) and containing even less sugars have been prepared (26 p. 100 d.w. protein, 35 p. 100 carbohydrates, 15 p. 100 lipids). Aw was lowered to 0,84 by adding humectants (sucrose, glycerol, sorbitol, citric acid, sodium citrate and chloride). Starch gels (40 p. 100 starch/d.w.), of pH less than 4,5, containing proteins and lipids, were flavored with vegetale powders. For 30 p. 100 water and with humectants, Aw was 0,84-0,88. The texture changes more or less favorably with time according to the nature of the starch used. Using the technology of processed cheeses, protein gels were made with either of the following characteristics: 1. A reduced Aw (0,86, for 38 p. 100 water) by adding humectants, but with a soft texture similar to that of a processed swiss cheese; 2. The same reduced Aw, with a starch content of 26 p. 100/d.w., and a harder texture, comparable to that of Emmenthal cheese.