Dairy proteins and soy proteins in infant foods nitrogen-to-protein conversion factors.

Protein content of any source is classically determined through the analysis of its nitrogen content done for more 100 years by the Kjeldahl method, and the obtained result is multiplied by a number named nitrogen conversion factor (NCF). The value of NCF is related to the amino acid composition of the protein source and to the eventual presence of side groups covalently bound to some amino acids of the protein chain. Consequently, the value of NCF cannot be identical for all sources of food proteins. The aim of this paper is to review the available knowledge on the two allowed protein sources for infant food formulas, milk and soybean, in order to bring the right scientific basis which should be used for the revision of both European legislation and Codex Standard for Infant Formulas.

Improvement of rheological properties of firm acid gels by skim milk heating is conserved after stirring.

The enhancement of the strength of set acid gels by heating milk was related to rheological parameters (water retention capacity, storage modulus) of corresponding stirred gels. To obtain accurate rheological data from stirred gel it was necessary to maintain a constant granulometry of gel particles and to recognize time after stirring as a contributing factor. Two hours after stirring, the gel exhibited a higher storage modulus when milk was heated above 80 degrees C. A measurement of viscosity of just-stirred yoghurt was sufficient to predict correctly the quality of a stirred gel analysed by viscoelastic measurements. Increased resistance to syneresis of just-stirred gels was related to higher viscosity. The quantity of beta-lactoglobulin (beta-Ig) bound to casein micelles explains the improvement of these gel qualities. We have considered that the structure of the initial firm gel (mesostructure level) was conserved in fragments within the stirred gel. Consequently, the explanation given by various authors for the effect of heating milk on the properties of set gels can also be applied to stirred gels. The same mechanism, described in literature for structure formation of set gels from acidified milk is purposed to explain the role of heating milk on the recovery of gel structure after stirring. The beta-Ig association with casein micelles during heating favoured micelle connections during the acidification. It also favoured the association of gel fragments after stirring during the recovery in gel structure.

Study of mechanism of lipolysis inhibition by bovine milk proteose-peptone component 3.

Milk component 3 was an inhibitor of lipoprotein lipase activity responsible for spontaneous lipolysis occurring in milk stored at 4 degrees C. Experiments using a pH-stat apparatus and emulsified tributyrin showed that component 3 inhibited porcine pancreatic lipase. The lipolytic activity was fully restored by addition of sodium taurodeoxycholate and colipase to the emulsion containing component 3. Inhibition did not seem to be the result of a direct interaction between component 3 and the enzyme. Component 3 had a strong adsorption power superior to that of pancreatic lipase, as shown by tensiometric measurements at an n-tetradecane-water interface. Lipase inhibition by component 3 could be the consequence of a rapid diffusion and preferential adsorption of component 3 at the oil-water interface provoking an important decrease of interfacial tension and avoiding the adsorption of lipase.

[Influence of hydrothermic and enzymatic treatments on the nutritive value of Faba beans: in vitro study]. / Influence des traitements hydrothermiques et enzymatiques sur la valeur nutritionnelle des proteines de féveroles : étude par méthodes in vitro.

In order to improve the digestibility of the Faba bean flour an hydrothermic enzymic and fermentation treatment has been thought out (US patent 395 8015, Ets Ury, M. Gay). This study aims at stemming out the nutritionnal and structural repercusions of this treatment on the Faba proteins. The nitrogen distribution is deeply changed : total nitrogen increase (47%), water soluble nitrogen decrease (41%), water soluble non-protein nitrogen increase. The treatment can enrich the flour in nitrogen but the proteins supplement is insoluble in water. The treatment changed the electrophoretic behaviour of flour proteins letting disappear precipitable pH 4,5 proteins. Amino acid composition is slightly changed, however there is an increase of lysine and methionine (15 et 25%) and a decrease of cysteine (28%). The chemical score shows sulfur amino acids and tryptophan deficiency. Enzymatic (pepsine and pancreatine mixture) liberation of all amino acids is low and is not improved by the treatment but for the lysine. Cell proteins supplement, which appeared during the treatment (61%), water insoluble, could improve nutritive value of the flour if a more complete destruction of the cell walls permitted their liberation and their solubilization.

Covalent bonds formed in proteins during milk sterilization: studies on caseins and casein peptides.

Formation of covalent bonds at milk sterilization temperatures was studied using caseins and casein peptides. At 120 degrees C lysinoalanyl residues produced even at pH 7.0 were derived from intra-molecular interactions between phosphoserine and lysine; the conditions of formation were determined. It was also found that the formation fo isopeptidic cross-links was significant with conditions more severe than those used for milk sterilization.

[Ultra-flash sterilization of fluid food by friction]. / La stérilisation ultra-rapide par friction des aliments liquides.

The ATAD process without heat exchange nor steam mixing allows the sterilization in a very short time. With milk, the sterility has been attained regularly at 140 degrees C/0,54 seconds. Only small changes occurred in nitrogen distribution. An homogeneizing effect upon fat emulsion is noticeable. The amino-acid composition of isoelectric fraction is not that of casein owing to the coprecipitation of a part of whey protein at pH 4,6, but they do not reveal any degradation. The enzymatic digestion of proteins (pepsin + Pancreatin) is a little improved by this treatment; the availability of lysine, measured by dinitrophenylation or guanidination, is hardly modified. All the blocked forms of lysine (Maillard reaction, lysino-alanine reaction, peptidoïd boundings) occurs only under more drastic conditions. The beta-lactoglobulin-kappa-casein complex formation slightly hampers the enzymatic clotting of sterilized milk. The forms of calcium are not seriously disturbed and such a milk is suitable for cheesemaking; we can obtain cheese with very few non-lactic bacteria.

[Utilization of pure proteins and peptides for the study of chemical and nutritional changes suffered by milk during heating]. / Utilisation de protéines pures et de peptides pour l'étude des modifications chimiques et nutritionnelles subies par le lait au cours du chauffage.

Milk proteins, and in particular the caseins, undergo during heat treatments (120 degrees C, 20-30 min) physico chemical and nutritional modifications. Using pure proteins (alphas and beta caseins) and peptides, it has been possible to dissociate the effects of heat treatments which, in milk can hide each other. Physico chemical properties of caseins (the ability to bind Ca++ and anionic dyes, acid-basic titration curves, electrophoretic behavior) are strongly altered, while the constitutive aminoacids are less modified. The accessibility of the aminogroups of lysine to the fluorodinitrobenzen and to the O-méthylisourea is lowered with a small rate. Lysinoalanyle interactions between serine and lysine are weak at a neutral pH while the isopeptide bonds epsilonN (gamma glutamyl)lysyle are only produced with more severe conditions as those of sterilization. The digestibility to the proteases is enhanced with low heat treatments then decreases with more severe treatments (120 degrees C-80 min.). Many peptides, which are released from the caseins during heating are issued from low specific split peptidic bonds; these peptides have often stimulating properties on the growth of lactic bacteria.

Dephosphorylation of bovine casein by milk alkaline phosphatase.

The pH of optimum activity of alkaline phosphatase from cow's milk depended on the substrate, being 10-1 for rho-nitrophenylphosphate, 8-6 for phosphoserine, 8-0 for phosvitin and 6-8 for casein. Individual casein components were dephosphorylated more rapidly than mixtures of alphas- and beta-caseins or of alphas-, beta-and kappa-caseins and micellar casein. Mixtures of 2 components involving kappa-casein were more readily dephosphorylated than alphas- and beta-casein mixtures. At pH 6-8, lactose, whey proteins and phosphate ions had an inhibitory effect. beta-Lactoglobulin had an inhibitory effect only when the pH of the reaction was lower than the optimum pH value of the enzyme. Mg2+ and Zn2+ were not inhibitory. The optimum conditions for dephosphorylation of casein are described.