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.

Microfiltration of raw whole milk to select fractions with different fat globule size distributions: process optimization and analysis.

We present an extensive description and analysis of a microfiltration process patented in our laboratory to separate different fractions of the initial milk fat globule population according to the size of the native milk fat globules (MFG). We used nominal membrane pore sizes of 2 to 12 microm and a specially designed pilot rig. Using this process with whole milk [whose MFG have a volume mean diameter (d43) = 4.2 +/- 0.2 microm] and appropriate membrane pore size and hydrodynamic conditions, we collected 2 extremes of the initial milk fat globule distribution consisting of 1) a retentate containing large MFG of d43 = 5 to 7.5 microm (with up to 250 g/kg of fat, up to 35% of initial milk fat, and up to 10% of initial milk volume), and 2) a permeate containing small MFG of d43 = 0.9 to 3.3 microm (with up to 16 g/kg of fat, up to 30% of initial milk fat, and up to 83% of initial milk volume and devoid of somatic cells). We checked that the process did not mechanically damage the MFG by measuring their zeta-potential. This new microfiltration process, avoiding milk aging, appears to be more efficient than gravity separation in selecting native MFG of different sizes. As we summarize from previous and new results showing that the physico-chemical and technological properties of native milk fat globules vary according to their size, the use of different fat globule fractions appears to be advantageous regarding the quality of cheeses and can lead to new dairy products with adapted properties (sensory, functional, and perhaps nutritional).

Influence of short-chain fatty acids on iron absorption by proximal colon.

BACKGROUND: Short-chain fatty acids produced by bacterial fermentation in the colon enhance the local absorption of cations, such as calcium, that could be used to improve the bioavailability of iron if a significant colonic absorption of iron were to occur. METHODS: Iron (iron gluconate, 100 microM) absorption by the caecum of the rat was compared with that in proximal sites of the small bowel using the Ussing chamber model; the influence of probiotic bacteria (Propionibacterium freudenreichii) on iron absorption was assessed and compared with that of two of their fermentation products (acetic and propionic acids) using the Ussing chamber and the ligated colon with gamma emitting iron as experimental models. RESULTS: The caecum absorbed less iron than the duodenum, but significantly more than the jejunum and ileum. This occurred mainly through an enhanced mucosal transfer of iron uptake. Propionibacteria enhanced iron absorption from the proximal colon; the same effect was observed in the presence of viable bacteria, or the culture medium free of viable bacteria, or acetate and propionate or propionate alone. CONCLUSIONS: The proximal colon could be a significant site available for iron absorption; this absorption can be enhanced by local production of short-chain fatty acids such as propionate.

Exopolysaccharide production by Propionibacterium acidi-propionici on milk microfiltrate.

AIMS: The aims of this work were to evaluate growth and exopolysaccharide (EPS) production properties of Propionibacterium acidi-propionici DSM 4900 on milk permeate. METHODS AND RESULTS: Anaerobic growth on milk permeate was only possible if supplemented with yeast extract (YE). Fermentation capacities of the strain were significantly improved by further increasing the supplemented YE. At 5 g l(-1) YE, consumption of 45 g l(-1) lactose to produce 9 g l(-1) biomass, 34 g l(-1) organic acids and 0.65 g l(-1) EPS was observed. From a kinetic point of view, EPS production occurred during the bacteria growth phase. At the excreted polysaccharide level, the medium showed shear-thinning behaviour with a relatively high apparent viscosity of up to 30 mPa.s (milli.Pascal.second) at a shear rate of 17 s(-1). CONCLUSION: EPS production by P. acidi-propionici DSM 4900 on milk permeate showed promising rheological behaviour of the milk-derived medium obtained, even at a low production level. SIGNIFICANCE AND IMPACT OF THE STUDY: A kinetic study on EPS production by a food-grade bacterium that could be used in situ in alimentation was carried out.

Study of the effects of temperature, pH and yeast extract on growth and exopolysaccharides production by Propionibacterium acidi-propionici on milk microfiltrate using a response surface methodology.

AIMS: To study the effects of temperature, pH and yeast extract (YE) concentration on growth and exopolysaccharide (EPS) production by Propionibacterium acidi-propionici DSM 4900 cultivated on milk microfiltrate. METHODS AND RESULTS: A multifactorial approach using a Response Surface Methodology (RSM) was followed. The results indicated that both growth, and EPS and organic acids production, were influenced by pH, temperature and YE concentration. Biomass and organic acids production occurred in all the tested domains, whereas EPS production was only possible in a narrow pH range (5.3-6.5). The results clearly showed that the optimal conditions for EPS production were different to those for optimal growth. The effect of YE on EPS production was not only due to an increase in growth but also to a direct effect on the production of EPS. The temperature played a major role. A decrease of temperature induced a slowing down of both growth and organic acids production, making the essential factors of the medium and the precursors of EPS biosynthesis more available and hence, leading to an increase in EPS production. CONCLUSION: The effects of pH, temperature and YE were determined, allowing the definition of favourable, though non-optimal, conditions for EPS production: 23 degrees C, pH 6 and 3 g l(-1) YE concentration. SIGNIFICANCE AND IMPACT OF THE STUDY: The use of a multifactorial approach for investigating the effect of fermentation conditions on EPS production has been demonstrated.

New genetic variants identified in donkey's milk whey proteins.

Novel genetic variants for donkey milk lysozyme and beta-lactoglobulins I and II have been identified by the combined use of peptide mass mapping and sequencing by tandem mass spectrometry in association with database searching. The novel donkey lysozyme variant designated as lysozyme B (Mr 14,631 Da) differed in three amino acid exchanges, N49 --> D, Y52 --> S, and S61 --> N, from the previously published sequence. Three novel genetic variants for donkey beta-lactoglobulins were identified. One of them is a type beta-lactoglobulin I with three amino acid exchanges at E36 --> S, S97 --> T, and V150 --> I (beta-lactoglobulin I B, Mr 18,510 Da). The two others are type beta-lactoglobulins II with two amino acid exchanges at C110 --> P and M118--> T (beta-lactoglobulin II B, Mr 18,227 Da) and with three amino acid exchanges at D96 --> E, C110 --> P, and M118 -->T (beta-lactoglobulin II C, Mr 18,241 Da). All these primary structures are closely related to those of homologous proteins in horse milk (percent identity >96%).

Iron tissue storage and hemoglobin levels of deficient rats repleted with iron bound to the caseinophosphopeptide 1-25 of beta-casein.

Caseinophosphopeptides (CPP) issued from enzyme digestion of caseins bind cations and keep them soluble in the digestive tract. They could be used as ligands to improve iron (Fe) bioavailability. Fe-deficient young rats were repleted with Fe (40 or 200 mg/kg of diet) bound either to the beta-CN (1-25) of beta-casein or to whole beta-casein or as FeSO(4). A control pair-fed group was given 200 mg of Fe (FeSO(4))/kg of diet for 6 weeks. After repletion, hemoglobin concentration of the control group was reached only by the ) animals fed 200 mg of Fe/kg; beta-CN (1-25) bound Fe (40 and 200 mg) produced higher Fe liver and spleen stores than FeSO(4). Binding Fe to the whole, nonhydrolyzed beta-casein gave results intermediate between the other experimental groups. Binding Fe to phosphoserine residues of low molecular weight CPP improved its ability to cure anemia and to restore iron tissue stores, as compared to Fe bound to the whole casein and to inorganic salts.

Unfolding and refolding of bovine beta-lactoglobulin monitored by hydrogen exchange measurements.

Bovine beta-lactoglobulin (beta-LG) is a widely studied protein belonging to the lipocalin family, whose structural characterisation has been reported by X-ray crystallography and NMR studies at physiological and acidic pH, respectively. Bovine beta-LG consists of nine antiparallel beta-sheets and a terminal alpha-helix segment. The beta-sheets form a calyx structure with a hydrophobic buried cluster conferring stability to the protein while a hydrophobic surface patch provides stabilising interactions between the barrel and the flanking terminal helix. Here, the stability and the folding properties of bovine beta-LG in the presence of a chemical denaturant is probed. The analysis of the NMR spectra recorded in aqueous solution with increasing amounts of urea revealed that the intensities of the backbone cross-peaks decrease upon increasing urea concentration, while their secondary shifts do not change significantly on going from 0 to 5 M urea, thus suggesting the presence of slow exchange between native and unfolded protein. Hydrogen exchange measurements at different urea concentrations were performed in order to evaluate the exchange rates of individual backbone amide protons. The opening reactions that determine protein exchange can be computed for the most slowly exchanging hydrogen atoms, and the measured exchange rates and the corresponding free energies can be expressed in terms of the equilibrium energetic for the global transition and the local fluctuations. Most of the residues converge to define a common isotherm identifying a unique cooperative folding unit, encompassing all the strands, except strand betaI, and the terminal region of the helix. The amides that do not join the same global unfolding isotherm are characterised by low DeltaGH20op and especially by low m values, indicating that they are already substantially exposed in the native state. A two-state unfolding model N <==> U is therefore proposed for this rather big protein, in agreement with CD data. Renaturation studies show that the unfolding mechanism is reversible up to 6 M urea and suggest a similar unfolding and refolding pathway. Present results are discussed in light of the hypothesis of an alpha-->beta transition proposed for bovine beta-LG refolding.

Sensitivity of beta-casein phosphopeptide-iron complex to digestive enzymes in ligated segment of rat duodenum.

Binding iron to the phosphorylated beta(1-25) peptide derived from beta-casein improves iron bioavailability in the rat. The aim of the present work was to learn how injected beta(1-25) and iron-beta(1-25) complex behave in the duodenum of rats using the technique of intestinal ligation in situ and reversed-phase (RP)-high performance liquid chromatography-electrospray mass spectrometry analysis of the lumen contents. The results demonstrate that beta(1-25) is sensitive to digestive enzymes including proteases/peptidases and phosphatases during duodenal transit. The lumen contents of rats perfused with iron free beta(1-25) contained all peptidic sequences derived from beta(1-25). In contrast, the phosphorylated part of beta(1-25) [i.e., beta(15-24)] was not detected in lumen of rats perfused with iron-beta(1-25) complex.

Mechanisms of absorption of caseinophosphopeptide bound iron.

Binding iron (Fe) to the 1-25 caseinophosphopeptide obtained from enzyme hydrolysis of beta casein (beta CPP) improves Fe bioavailability in the rat. To assess the mechanisms involved in its absorption, a perfused, vascularized duodenal rat loop model was used in controls and in Fe-deficient (bleeding of 25% blood volume) rats. Inhibitors of oxidative phosphorylation [2-4 dinitrophenol (DNP)] and/or of endocytosis [phenylarsine oxide (PAO)] were added to the perfusion solution containing 50 microM Fe as beta CPP bound Fe (Fe-beta CPP) or gluconate (Fe Gluc). Fe-beta CPP enhanced Fe uptake, reduced mucosal storage, and improved net absorption both in controls and in deficient animals. DNP reduced uptake, mucosal storage, and net absorption by the same percentage in Fe-beta CPP and Fe Gluc perfused rats in both control and Fe-deficient animals. PAO decreased uptake, mucosal storage, and net absorption of Fe-beta CPP but not of Fe Gluc. At the end of the experiment Fe serum levels were increased only in Fe Gluc animals. These results confirm the improved bioavailability of beta CPP bound Fe. They suggest that at least part of its absorption can occur by a different pathway than usual Fe salts. Fe-beta CPP can be taken up by endocytosis and absorbed bound to amino acids or peptides.

Improvement of zinc intestinal absorption and reduction of zinc/iron interaction using metal bound to the caseinophosphopeptide 1-25 of beta-casein.

Binding zinc (Zn) to soluble caseinophosphopeptides (CN), produced by the hydrolysis of caseins, improves its absorption and could prevent inhibition by other nutrients such as iron (Fe). The absorption of Zn (100 mumol/L) bound to the 1-25 CN (beta-CN(1-25)) of beta-casein, or as ZnSO4 was studied using the isolated, perfused rat intestinal loop system. Fe (Fe-CN or Fe gluconate (Fe Gluc)) was added at Zn/Fe ratios of 2:1, 1:5 and 1:10. Disappearance from the lumen (Q1) and net absorption (ZnAbs) of Zn-CN were statistically greater than for ZnSO4; Zn retention by the mucosa (Q2) did not significantly differ. Fe Gluc reduced Q1, Q2 and ZnAbs for ZnSO4 at ratios of 1:5 and 1:10 and for Zn-CN at a ratio of 1:10. Fe-CN reduced Q1 and ZnAbs of both forms of Zn at a ratio of 1:10; Q2 remained unchanged. Binding Zn to beta-CN(1-25) improved Zn absorption and prevented Fe from inhibiting its absorption.

Characterization by ionization mass spectrometry of lactosyl beta-lactoglobulin conjugates formed during heat treatment of milk and whey and identification of one lactose-binding site.

The extent of the early stage of the Maillard-type reaction that impaired functional properties of whey proteins was evaluated by electrospray ionization mass spectrometry. Under conditions of mild heat treatment (63 degrees C for 20 s) applied to milk before whey separation at room temperature 23 degrees C), a modification of the relative molecular mass of beta-lactoglobulin (beta-LG) was observed that differed from that of the native form by 324. This specific modification of beta-LG occurred in acidified whey as well as in sweet whey and increased with the extent of the heat treatment. Incubation of purified beta-LG dissolved in milk ultrafiltration permeate or in lactose solution at 50 to 80 degrees C demonstrated the presence of a lactosyl residue that was covalently bound to beta-LG; beta-casein, used as a control, showed no mass modification. Studies of kinetics showed that a maximum of 35% of the beta-LG was lactosyl-beta-LG conjugate after heat treatment at 70 degrees C for 1 h. This study provides the first direct evidence of specific lactosylation of beta-LG during the initial stage of the Maillard reaction. One of the first lactose-binding sites was identified as a Lys47 by protease mapping and analysis by means of on-line liquid chromatography combined with mass spectrometry. In addition, collision-activated dissociation performed on the lactosylated peptide beta-LG (f 46-51) showed the rearrangement reactions occurring during the fragmentation process by electrospray. A mechanism is proposed.

Electrophoretic pattern of peptidoglycan hydrolases, a new tool for bacterial species identification: application to 10 Lactobacillus species.

Lactobacilli have been used as industrial starters for a long time, but in many cases their phenotypic identification is still neither easy nor reliable. Previously we observed that the cell wall peptidoglycan hydrolases of Lactobacillus helveticus were highly conserved enzymes; the aim of the present work was to determine whether peptidoglycan hydrolase patterns obtained by renaturing SDS-PAGE could be of interest in the identification of lactobacilli species. For that purpose, the peptidoglycan hydrolase patterns of 94 strains of lactobacilli belonging to 10 different species were determined; most of the species studied are used either in dairy, meat, bakery or vegetable fermentations: L. helveticus, L. acidophilus, L. delbrueckii, L. brevis, L. fermentum, L. jensenii, L. plantarum, L. sake, L. curvatus and L. reuteri. Within a species, the strains exhibited highly similar patterns: the apparent molecular weights of the lytic bands were identical, with only slight variations of intensity. Moreover, each species, including phylogenetically close species such as L. sake and L. curvatus, or L. acidophilus and L. helveticus, gave a different pattern. Interestingly, the closer the species were phylogenetically, the more related were their patterns. The sensitivity of the method was checked using various quantities of L. acidophilus cells: a peptidoglycan hydrolase extract of 5 x 10(6) cells was sufficient to obtain an informative pattern, as was a single colony. Finally, the method was also successfully applied to distinguish two Carnobacterium species. In conclusion, the electrophoretic pattern of peptidoglycan hydrolases is proposed as a new tool for lactobacilli identification: it is rapid, sensitive and effective even for phylogenetically close species. Furthermore, this work provides the first evidence of the potential overall taxonomic value of bacterial peptidoglycan hydrolases.

Selective separation of amino acids with a charged inorganic nanofiltration membrane: effect of physicochemical parameters on selectivity.

A charged organic-inorganic nanofiltration (NF) membrane prototype was used to separate a mixture of nine amino acids (AA) on the basis of differential electrostatic interactions with the membrane because, for a given pH, some of them were positively charged, some were negative, and some were zwitterions. Effect of pH, amino acid concentration (C(r)), and added ionic strength ([NaCI]) on the process selectivity was studied. A global statistical study revealed that pH was the dominant parameter regarding fractionation. C(r) and [NaCI] had a weaker effect, but the ratio C(r)/[NaCI] demonstrated a pronounced effect on system selectivity. Two split-ups of the mixture were obtained at pH 2 and at pH 12, for a 1-g/L total AA concentration and a C(r)/[NaCI] ratio of 0.16. Under these conditions, the differences in transmissions between basic and acid AA were higher than 70%. Interpretation of the results according to the Donnan theory allows us to foresee the potentialities of charged nanofiltration membranes for the fractionation of a complex mixture, such as peptidic hydrolysate to streams containing peptides and amino acids having different isoelectric points. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 54: 291-302, 1997.

Slow and fast dietary proteins differently modulate postprandial protein accretion.

The speed of absorption of dietary amino acids by the gut varies according to the type of ingested dietary protein. This could affect postprandial protein synthesis, breakdown, and deposition. To test this hypothesis, two intrinsically 13C-leucine-labeled milk proteins, casein (CAS) and whey protein (WP), of different physicochemical properties were ingested as one single meal by healthy adults. Postprandial whole body leucine kinetics were assessed by using a dual tracer methodology. WP induced a dramatic but short increase of plasma amino acids. CAS induced a prolonged plateau of moderate hyperaminoacidemia, probably because of a slow gastric emptying. Whole body protein breakdown was inhibited by 34% after CAS ingestion but not after WP ingestion. Postprandial protein synthesis was stimulated by 68% with the WP meal and to a lesser extent (+31%) with the CAS meal. Postprandial whole body leucine oxidation over 7 h was lower with CAS (272 +/- 91 micromol.kg-1) than with WP (373 +/- 56 micromol.kg-1). Leucine intake was identical in both meals (380 micromol.kg-1). Therefore, net leucine balance over the 7 h after the meal was more positive with CAS than with WP (P < 0.05, WP vs. CAS). In conclusion, the speed of protein digestion and amino acid absorption from the gut has a major effect on whole body protein anabolism after one single meal. By analogy with carbohydrate metabolism, slow and fast proteins modulate the postprandial metabolic response, a concept to be applied to wasting situations.

Acute postprandial changes in leucine metabolism as assessed with an intrinsically labeled milk protein.

Mechanisms of protein gain during protein feeding have been investigated using a combination of oral and intravenous labeled leucine in healthy young men. The oral labeled leucine was administered as a free oral tracer ([13C]- or [2H3]leucine) added to unlabeled whey protein or as whey protein intrinsically labeled with L-[1-13C]leucine. When the oral tracer was free leucine, it appeared in the plasma more rapidly than the unlabeled leucine derived from the whey protein, and this resulted in an artifactual 88% decrease of protein breakdown. When the oral tracer was protein bound, protein breakdown did not change significantly after the meal. In contrast, nonoxidative leucine disposal (i.e., protein synthesis) was stimulated by 63% by the meal. In conclusion, 1) an intrinsically labeled protein is more appropriate than an oral free tracer to study postprandial leucine kinetics under non-steady-state conditions and 2) protein gain after a single whey protein meal results solely from an increased protein synthesis with no modification of protein breakdown.

Selective determination of phosphopeptide beta-CN(1-25) in a beta-casein digest by adding iron: characterization by liquid chromatography with on-line electrospray-ionization mass spectrometric detection.

A beta-casein tryptic digest has been analysed by reversed-phase high-performance liquid chromatography (RP-HPLC) with on-line electrospray-ionization mass spectrometry (ESI-MS). Analyses of peptides were carried out before and after addition of iron(II) to the peptides in solution. In both cases, the majority of peptides were identified by the determination of molecular masses by ESI-MS and by prior knowledge of the amino acid sequence of beta-casein, and thus of its corresponding tryptic peptides. In the presence of iron(II), only phosphopeptide beta-CN(1-25) was able to bind iron to form different complexes that have increased retention times on the RP-HPLC column and that also absorbed at 280 nm. The method presented here appears to be selective for peptides containing phosphoseryl cluster(s).

Production of large amounts of [13C]leucine-enriched milk proteins by lactating cows.

During protein metabolism kinetic studies, oral tracers are administered as labeled free amino acids and do not necessarily represent the metabolic fate of amino acids ingested as proteins. However, sufficient quantities of 13C-labeled proteins are not currently available. We here present a new methodology for producing large amounts of milk proteins intrinsically labeled with [13C]leucine. After surgical preparation, two lactating cows were infused with 80-90 g of L-[1-13C]leucine for 24-32 h, and milk was collected during and after the infusion. Casein and whey protein fractions were purified by membrane separation techniques. Arteriovenous balance across the udder indicated a very efficient extraction of leucine by the mammary gland. Five batches of pure casein and whey proteins, totaling 3854 g of protein of excellent bacteriological quality, were obtained. Two thirds of these proteins had [13C]leucine enrichments ranging from 10.5 to 19.4% ([13C]leucine atom percent excess). The overall tracer recoveries were 22 and 27% (cows 1 and 2, respectively). Thus, pure milk proteins were produced in large amounts with sufficient 13C enrichment to be used in human protein metabolism studies.

Two-step chromatographic procedure for the purification of hen egg white ovomucin, lysozyme, ovotransferrin and ovalbumin and characterization of purified proteins.

An improved procedure is described involving gel permeation and anion-exchange chromatography for the purification of four major hen egg white proteins. The procedure involves a first-step purification of ovomucin and lysozyme by gel permeation on a Superose 6 Prep Grade column. In the second step, anion-exchange chromatography on Q Sepharose Fast Flow led to the isolation of ovotransferrin and ovalbumin from a gel permeation chromatographic peak. The purities were estimated as ca. 80, 100, 80 and 100% for ovomucin, lysozyme, ovotransferrin and ovalbumin, respectively. The purification yield was over 60% for each protein. Further characterization of purified lysozyme revealed that it was fully active and homogeneous in relation to the electrospray ionization mass spectrum. The electrospray ionization mass spectrum showed different ovotransferrin species. The amino acid composition of purified ovomucin was compared to those published previously.