Calcium citrate insolubilization drives the fouling of falling film evaporators during the concentration of hydrochloric acid whey.

When dairy powders are produced, the mineral fraction undergoes strong modifications during the vacuum concentration step, leading to the fouling of falling film evaporators. The objective of this study was to determine the nature of the deposits formed during the vacuum concentration of two fouling and highly mineralized products: hydrochloric acid whey and lactic acid whey. These products mainly differ in terms of their mineral composition: lactic acid whey contains a high level of lactic acid and traces of citrate, whereas hydrochloric acid whey contains citrate and no lactic acid. Concentrates at different concentration factors were produced using a pilot-scale falling film evaporator. The compositions of the fouling deposits as well as the precipitates present in the concentrates were deduced from the analytical determination of the composition of the concentrates and their respective diffusible phases. The behavior of the mineral fraction of both acid wheys during concentration was shown to be very different. In the case of hydrochloric acid whey, experimental results suggested a deposition of calcium and citrate ions in the evaporator as well as their precipitation in the highly concentrated products. On the contrary, neither mineral deposition nor precipitation occurred during the concentration of lactic acid whey. This study underlined the key role of citrate ions in the fouling of evaporators during the concentration of hydrochloric acid wheys.

Tritium levels in milk in the vicinity of chronic tritium releases.

Tritium is the radioactive isotope of hydrogen. It can be integrated into most biological molecules. Even though its radiotoxicity is weak, the effects of tritium can be increased following concentration in critical compartments of living organisms. For a better understanding of tritium circulation in the environment and to highlight transfer constants between compartments, we studied the tritiation of different agricultural matrices chronically exposed to tritium. Milk is one of the most frequently monitored foodstuffs in the vicinity of points known for chronic release of radionuclides firstly because dairy products find their way into most homes but also because it integrates deposition over large areas at a local scale. It is a food which contains all the main nutrients, especially proteins, carbohydrates and lipids. We thus studied the tritium levels of milk in chronic exposure conditions by comparing the tritiation of the main hydrogenated components of milk, first, component by component, then, sample by sample. Significant correlations were found between the specific activities of drinking water and free water of milk as well as between the tritium levels of cattle feed dry matter and of the main organic components of milk. Our findings stress the importance of the metabolism on the distribution of tritium in the different compartments. Overall, dilution of hydrogen in the environmental compartments was found to play an important role dimming possible isotopic effects even in a food chain chronically exposed to tritium.

Influence of pH on the heat-induced proteolysis of casein molecules.

Proteolysis of sodium caseinate solution (24.5 g/l) induced by heat treatment at 120 degrees C at different pH values was studied by measuring nitrogen content and relative fluorescence intensity in the 4% trichloroacetic acid filtrates. The low molar mass pepticles corresponding to the soluble nitrogen were identified using liquid chromatography/tandem mass spectrometry. Increase in proteolysis, deduced from the increase in soluble nitrogen content, was observed with heating time (10, 20 and 30 min) and pH (6.0, 7.0, 8.0 and 9.0). The fluorescence measurements showed that the release of peptides containing tryptophan was minimal at pH approximately 7.0. In parallel, eighteen low molar mass peptides were characterized, of which four came from kappa-casein, nine from beta-casein and five from alphas1-casein. Peptides were preferentially released under alkaline conditions.

Determination of ammonium in milk and dairy products by ion chromatography.

To control the quality and the biochemical evolution of milk and dairy products during their technological transformations, it is interesting to determine their ammonium concentrations. A chromatographic method for the determination of this compound is proposed. The method is based on the separation of ammonium by cation-exchange chromatography and its detection by suppressed conductivity. With an appropriate sample preparation, this method enabled identification and quantification of ammonium with good repeatability (relative standard deviation of about 5%). Moreover, good sensitivity (less than 0.5 mg/kg) and no interference between ammonium and other matrix components were determined. It was also shown that this method offers a very promising alternative for studying changes in ammonium concentration of milk or caseinate after their heat treatments and in different dairy products such as yoghurt and cheeses (hard cooked and mould ripened cheeses).

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).

Instability of the monofunctional adducts in cis-[Pt(NH3)2(N7-N-methyl-2-diazapyrenium)Cl](2+)-modified DNA: rates of cross-linking reactions in cis-platinum-modified DNA.

Single- and double-stranded oligonucleotides containing a single monofunctional cis-[Pt(NH3)2(dG)(N7-N-methyl-2-diazapyrenium)]3+ adduct have been studied at two NaCl concentrations. In 50 mM and 1 M NaCl, the adducts within the single-stranded oligonucleotides are stable. In contrast, they are unstable within the corresponding double-stranded oligonucleotides. In 50 mM NaCl, the bonds between platinum and guanine or N-methyl-2,7-diazapyrenium residues are cleaved and subsequently, intra- or interstrand cross-links are formed as in the reaction between DNA and cis-DDP. In 1 M NaCl, the main reaction is the replacement of N-methyl-2,7-diazapyrenium residues by chloride which generates double-stranded oligonucleotides containing a single monofunctional cis-[Pt(NH3)2(dG)Cl]+ adduct. The rates of closure of these monofunctional adducts to bifunctional cross-links have been studied in 60 mM NaClO4. Within d(TG.CT/AGCA), d(CG.CT/AGCG) and d(AG.CT/AGCT) (the symbol.indicates the location of the adducts in the central sequences of oligonucleotides), the half-lifes (t1/2) of the cis-[Pt(NH3)2(dG)Cl]+ adducts are respectively 12, 6 and 2.8 hr and the cross-linking reactions occur between guanine residues on the opposite strands. Within d(AG.TC/GACT), d(CG.AT/ATCG) and d(TGTG./CACA) or d(TG.TG/CACA) t1/2 are respectively 1.6, 8 and larger than 20 hr and the intrastrand cross-links are formed at the d(AG), d(GA) and d(GTG) sites, respectively. The conclusion is that the rates of conversion of cis-platinum-DNA monofunctional adducts to minor bifunctional cross-links are dependent on base sequence. The potential use of the instability of cis-[Pt(NH3)2(dG)(N7-N-methyl-2-diazapyrenium)]3+ adducts is discussed in the context of the antisense strategy.

Lability of monofunctional cis-platinum adducts: role of DNA double helix.

Recently, we have shown that the adduct formed in the reaction between the platinum-triamine complex cis-[Pt(NH3)2(N7-N-methyl-2-diazapyrenium)Cl]2+ and one single-stranded oligonucleotide was stable but became labile as soon as the platinated oligonucleotide was paired with its complementary strand (Gaucheron et al. Proc. Natl. Acad. Sci. USA 88, 3516-3519 (1991)). To generalize this finding we have now studied large DNA fragments containing several adducts. The stability of the adducts within single-stranded DNA is demonstrated by absorption spectrophotometry and by replication mapping experiments. Several approaches are used to prove the lability of the adducts within double-stranded DNA. Replication mapping experiments reveal that an unmodified single-stranded DNA when mixed with double-stranded DNA modified by the platinum-triamine complex behaves as a single-stranded DNA modified by the triamine complex. After double-stranded DNA is modified by the platinum-triamine complex, intrastrand and interstrand cross-links are progressively formed during subsequent incubation as revealed by transcription mapping experiments and gel electrophoresis under denaturing conditions. Finally, replication mapping experiments show that the lability of the adducts within a double-stranded DNA depends upon the nature of the flanking nucleotide residues. All these results support the proposal that the DNA double helix acts as a catalyst in the reaction between DNA, cis-diamminedichloroplatinum(II) and N-methyl-2,7-diazapyrenium.

Biophysical analysis of DNA modified by 1,2-diaminocyclohexane platinum(II) complexes.

Modification of DNA and double-stranded deoxyoligonucleotides with antitumour 1,2-diamino-cyclohexanedinitroplatinum(II) (Pt-dach) complexes was investigated with the aid of physico-chemical methods and chemical probes of nucleic acid conformation. The three Pt-dach complexes were used which differed in isomeric forms of the dach nonleaving ligand-Pt(1R,2R-dach), Pt(1S,2S-dach) and Pt(1R,2S-dach) complexes. The latter complex has lower antitumour activity than the other two Pt-dach complexes. Pt(1R,2S-dach) complex exhibits the slowest kinetics of its binding to DNA and of the conversion of monofunctional binding to bifunctional lesions. The anomalously slow electrophoretic mobility of multimers of the platinated and ligated oligomers suggests that bifunctional binding of Pt-dach complexes to a d(GG) site within double-stranded oligonucleotides induces bending of the oligomer. In addition, chemical probing of double-helical deoxyoligonucleotides modified by the Pt-dach complexes at the d(GG) sites reveals that Pt(1R,2S-dach) complex induces more extensive conformational changes in the oligomer than Pt(1R,2R-dach) and Pt(1S,2S-dach) complexes. It is proposed that different effects of the Pt-dach complexes on DNA observed in this work arise mainly from a steric crowding of the axially oriented cyclohexane ring in the DNA adduct of Pt(1R,2S-dach) complex.

Possible catalytic activity of DNA in the reaction between the antitumor drug cis-diamminedichloroplatinum(II) and the intercalator N-methyl-2,7-diazapyrenium.

The platinum(II) complex cis-[Pt(NH3)2(N7-N-methyl-2-diazapyrenium)Cl]2+ formed in the reaction between cis-diamminedichloroplatinum(II) and N-methyl-2,7-diazapyrenium reacts with N7 of guanine residues in DNA. The resulting adduct is kinetically inert within single-stranded DNA. Within double-stranded DNA, it is kinetically inert in 1 M NaClO4 and becomes labile as the salt concentration is decreased. Two products, cis-[Pt(NH3)2(N7-N-methyl-2-diazapyrenium)H2O]3+ and N-methyl-2,7-diazapyrenium, are released. The conformation of the platinated DNA is different in low- and high-salt conditions as shown by the chemical probe diethyl pyrocarbonate. These results are discussed in relation with a possible catalytic role played by the double-stranded DNA.