Showing papers on "Citric acid published in 1996"

Citric Acid—A Dispersant for Aqueous Alumina Suspensions

Abstract: The interaction between citric acid and alumina in aqueous solution is characterized. Adsorption isotherms of the dispersant on the alumina surface, electrophoretic mobility of the alumina particles as a function of the citric acid concentration, and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy of the citratealumina surface complex have been used. The adsorption behavior of citric acid is dependent on the pH of the suspension and the concentration of the citric acid. The maximum amount of citric acid adsorbed on the alumina surface, 2.17 μ.mol/m2 at pH 3, decreases to 1.17 μmol/m2 at pH 8. The adsorption of citrate causes a highly negatively charged powder surface and a shift of the isoelectric point (IEP) to lower pH values. The IEP of alumina can be fixed at any pH value between 9 and 3 by proper adjustment of the citric acid concentration. In situ ATR-FTIR spectroscopy of the citrate-alumina surface complex gives evidence for a direct interaction between the carboxylate groups of the citrate and the surface aluminum(III) atoms. The rheological properties of alumina suspensions are studied as a function of the citric acid concentration. The data obtained from the viscosity and dynamic electrophoretic measurements correlate well and allow the construction of a stability map of alumina suspensions stabilized with citric acid. The influence of citric acid on the viscosity is discussed using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The interaction potential between the particles is determined by the citrate adsorbed on the surface, leading to a negative particle charge, and the citrate anions remaining in the solution, resulting in an increase of the ionic strength. The adsorption of citric acid also creates a steric barrier that inhibits the complete mutual approach of the individual alumina particles.

Intake of Ethanol, Sodium Chloride, Sucrose, Citric Acid, and Quinine Hydrochloride Solutions by Mice: A Genetic Analysis

Abstract: Mice of the 129/J (129) and C57BL/6ByJ (B6) strains and their reciprocal F1 and F2 hybrids were offered solutions of ethanol, sucrose, citric acid, quinine hydrochloride, and NaCl in two-bottle choice tests. Consistent with earlier work, the B6 mice drank more ethanol, sucrose, citric acid, and quinine hydrochloride solution and less NaCl solution than did 129 mice. Analyses of each generation's means and distributions showed that intakes of ethanol, quinine, sucrose, and NaCl were influenced by a few genes. The mode of inheritance was additive in the case of ethanol and quinine, for sucrose the genotype of the 129 strain was recessive, and for NaCl it was dominant. Citric acid intake appeared to be influenced by many genes with small effects, with the 129 genotype dominant. Correlations of sucrose consumption with ethanol and citric acid consumption were found among mice of the F2 generation, and the genetically determined component of these correlations was stronger than the component related to environmental factors. The genetically determined correlation between sucrose and ethanol intakes is consistent with the hypothesis that the higher ethanol intake by B6 mice depends, in part, on higher hedonic attractiveness of its sweet taste component.

Starch citrate : Preparation and ion exchange properties

Abstract: Starch was allowed to react thermochemically (oven versus drum drying-oven) with citric acid to potentially yield biodegradable products possessing high ion exchange capacity. Reaction variables studies were: starch type (amylose content 0-70%). pH (0.5-8.5), sodium level (0-3 moles), citric acid level (0.1-0.5 moles), reaction time, temperature (110-140°C) and sodium dihydrogen phosphate catalysis. Reaction efficiencies approaching 100% were achieved,while minimizing crosslinking and maximizing carboxyl content. Carboxyl content was determined and copper binding capacity at pH 4.5 was evaluated.

Proton motive force generation by citrolactic fermentation in Leuconostoc mesenteroides.

Abstract: In Leuconostoc mesenteroides subsp. mesenteroides 19D, citrate is transported by a secondary citrate carrier (CitP). Previous studies of the kinetics and mechanism of CitP performed in membrane vesicles of L. mesenteroides showed that CitP catalyzes divalent citrate HCit2-/H+ symport, indicative of metabolic energy generation by citrate metabolism via a secondary mechanism (C. Marty-Teysset, J. S. Lolkema, P. Schmitt, C. Divies, and W. N. Konings, J. Biol. Chem. 270:25370-25376, 1995). This study also revealed an efficient exchange of citrate and D-lactate, a product of citrate/carbohydrate cometabolism, suggesting that under physiological conditions, CitP may function as a precursor/product exchanger rather than a symporter. In this paper, the energetic consequences of citrate metabolism were investigated in resting cells of L. mesenteroides. The generation of metabolic energy in the form of a pH gradient (delta pH) and a membrane potential (delta psi) by citrate metabolism was found to be largely dependent on cometabolism with glucose. Furthermore, in the presence of glucose, the rates of citrate utilization and of pyruvate and lactate production were strongly increased, indicating an enhancement of citrate metabolism by glucose metabolism. The rate of citrate metabolism under these conditions was slowed down by the presence of a membrane potential across the cytoplasmic membrane. The production of D-lactate inside the cell during cometabolism was shown to be responsible for the enhancement of the electrogenic uptake of citrate. Cells loaded with D-lactate generated a delta psi upon dilution in buffer containing citrate, and cells incubated with citrate built up a pH gradient upon addition of D-lactate. The results are consistent with an electrogenic citrate/D-lactate exchange generating in vivo metabolic energy in the form of a proton electrochemical gradient across the membrane. The generation of metabolic energy from citrate metabolism in L. mesenteroides may contribute significantly to the growth advantage observed during cometabolism of citrate and glucose.

Reactive species produced on irradiation at 365 nm of aqueous solutions of humic acids

Abstract: In order to characterize the reactive species produced on irradiation at 365 nm of humic acid (HA) solutions, the photoinduced transformation of aromatic compounds was studied as a function of several parameters (concentrations of substrates, HA and oxygen, pH of the solution) and in the presence of additives such as isopropanol, ethylenediaminetetraacetic acid (EDTA), H2O2 and Fe2+. No consumption of the substrates was observed in deoxygenated medium. In oxygenated solutions containing commerical HA, the rates of phenol disappearance were significant; the rate laws were dependent on the substrate and HA concentrations. In weakly oxygenated medium, the consumption rates were lower and other reactive species were involved. EDTA and citric acid led to partial inhibition of the reactions, indicating that the reactive species were related to metallic cations contained in HA. In acidic solution, hydroxyl radicals were produced by a photo-Fenton reaction. HA synthesized from phenol was incapable of photoinducing the transformation of the substrates, whereas HA synthesized from phenol-Fe3+ produced transformation of the substrates. The formation of singlet oxygen on irradiation of commercial HA was characterized previously. Unexpectedly, no consumption of phenolic substrates through singlet oxygen reaction was observed.

Minimally Processed Pomegranate Seeds

Abstract: Ready-to-eat pomegranate seeds were prepared and their quality attributes and appearance was followed during storage for 7 d under different conditions Quality attributes, titratable acidity, total soluble solids, L* values, absorbance at 510 nm (anthocyanin content) and absorbance at 446 nm (brown metabolites) were analysed Respiratory activity, appearance of the seeds (shriveling and browning), and weight loss were also evaluated The influence of different washing treatments, storage temperatures (8, 4 and 1 °C) and actively or passively modified atmosphere packaging on the quality of the minimally processed pomegranate seeds was studied Washing the seeds with chlorine and chlorine followed by ascorbic and citric acid were selected, and storage at 1 °C led to the best quality preservation The best overall results were obtained for seeds washed with chlorine (100 mg/kg) plus antioxidants (5 g/L ascorbic acid and 5 g/L citric acid), and sealed in oriented polypropylene film using an initial atmosphere actively modified to 20 mL/L O 2 and 0 mL/L CO 2 Under these conditions, minimally processed pomegranate seeds maintained good quality and appearance for 7 d of storage at 1 °C without fungal attacks or off-flavour development

Interaction of Citric Acid with Hydroxyapatite: Surface Exchange of Ions and Precipitation of Calcium Citrate

Abstract: The use of citric acid is efficacious and distinctive in the demineralization of dentinal root surfaces for periodontal regeneration and in the etching and conditioning of enamel or dentin for bonding restorative resins. To decipher the role of citric acid in these applications, it is important that one have a basic understanding of its interaction with synthetic hydroxyapatite. The uptake or removal of citrate ions from aqueous solutions of citric acid (4 to 100 mmol/L, 10 mL) by hydroxyapatite (1 g) was studied at 22 degrees C after a given reaction period (from 3 hr to 11 days) by immediate spectrophotometric monitoring of the concentrations of the filtrates (214 nm). The concentrations of calcium, phosphate, and hydrogen ions were also determined in the same solutions. The interaction: (i) is a time-independent ionic-exchange process with the substrate when the initial acid concentrations are dilute (4 to 12.5 mmol/L), and (ii) is a reactive process that is time-dependent for higher acid concentrations. The exchange process shows an adsorption of about one citrate ion per (100) face of the unit cell of hydroxyapatite for a maximally exchanged surface. The curves representing the reactive process may be quantitatively or qualitatively explained on the basis of the supersaturation of the solutions with respect to calcium citrate and its slow precipitation. The physico-chemical analysis of the needle-shaped birefringent crystals of the precipitate from the supersaturated solutions confirms the precipitate to be Ca3(citrate)2.4H2O.

Glucose transport by Aspergillus niger: the low-affinity carrier is only formed during growth on high glucose concentrations

Abstract: The filamentous fungusAspergillus niger accumulates large levels of citric acid in the medium when grown under conditions favouring a high rate of sugar catabolism. With the aim of understanding the mechanisms involved in this process we investigated glucose transport in this fungus. To this end a medium was designed that enables growth of the fungus into a fine, hairy filamentous mycelium, suitable for transport studies. It was found thatA. niger contains a single, high-affinity glucose transporter when grown on a low (1% w/v) glucose concentration, but forms an additional low-affinity transporter when grown on a high (15% w/v) glucose concentration. Both glucose transporters exhibit decreased activities at low pH and are inhibited by citric acid. However, the activity of the low-affinity transporter is much less affected by these conditions. Two 2-deoxyglucose-resistant (dgr) mutants ofA. niger, which produce citric acid at a much lower rate than the parent strain, are impaired in the formation of the low-affinity transporter, but form the high-affinity transporter with higher activities. We conclude that the low-affinity glucose transporter takes part in the mechanism by whichA. niger responds to high extracellular glucose concentrations leading to citric acid accumulation.

The bioavailability of 26Al-labelled aluminium citrate and aluminium hydroxide in volunteers.

Abstract: A study was undertaken to determine the fraction of ingested aluminium taken up by two male volunteers, following their ingestion of either aluminium citrate or aluminium hydroxide. In addition, the effects of simultaneous citrate ingestion on the gastrointestinal absorption of aluminium from its hydroxide was studied. Volunteers received three oral doses of 26Al-labelled aluminium compound in water. The doses were administered directly into the stomach using a paediatric feeding tube. Blood samples were collected from the volunteers at 1, 4 and 24 h after administration, and their daily output of urine and faeces was collected for 6 days. These samples were analysed for their 26Al content using either coincidence gamma-counting or accelerator mass spectrometry. The uptake of aluminium was greatest following its administration in the citrate form and was least following intake as the aluminium hydroxide suspension. The co-administration of citrate, with the aluminium hydroxide suspension, was found to enhance the levels of 26Al uptake in both volunteers. Using a urinary excretion factor based on the results of previous studies, the fractional aluminium uptake from each of the species was calculated: aluminium citrate, 5.23 x 10(-3); aluminium hydroxide, 1.04 x 10(-4); aluminium hydroxide with citrate, 1.36 x 10(-3).

Evidence for electrostatic channeling in a fusion protein of malate dehydrogenase and citrate synthase.

Abstract: Brownian dynamics simulations were performed to investigate a possible role for electrostatic channeling in transferring substrate between two of the enzymes of the citric acid cycle. The diffusion of oxaloacetate from one of the active sites of malate dehydrogenase (MDH) to the active sites of citrate synthase (CS) was simulated in the presence and absence of electrostatic forces using a modeled structure for a MDH-CS fusion protein. In the absence of electrostatic forces, fewer than 1% of substrate molecules leaving the MDH active site are transferred to CS. When electrostatic forces are present at zero ionic strength however, around 45% of substrate molecules are successfully channeled. As expected for an electrostatic mechanism of transfer, increasing the ionic strength in the simulations reduces the calculated transfer efficiency. Even at 150 mM however, the inclusion of electrostatic forces results in an increase in transfer efficiency of more than 1 order of magnitude. The simulations therefore provide evidence for the involvement of electrostatic channeling in guiding substrate transfer between two of the enzymes of the citric acid cycle. Similar effects may operate between other members of the citric acid metabolon.

Acid-catalyzed reductive amination of aldoses with 8-aminopyrene-1,3,6-trisulfonate

Abstract: The reductive amination of monosaccharides with 8-aminopyrene-1,3,6-trisulfonate (APTS) in seven different organic acids including the commonly used acetic acid was investigated by capillary electrophoresis (CE) with laser-induced fluorescence (LIF) detection. The correlation between the yields of the saccharide-APTS adducts and pKa of the organic acid catalyst is consistent with general acid catalysis of the rate-determining step of the reductive amination reaction. Derivatization in the presence of organic acids of higher strength than acetic acid produced substantially higher yields of APTS-sugar adducts, an effect which is more pronounced for N-acetylamino sugars. Optimum yields were obtained using citric acid as a catalyst. Conversion of a few nanomoles of neutral saccharides to the APTS derivatives is achieved at 75°C in less than 60 min.

Inhibition of Browning of Harvested Head Lettuce

Abstract: Treatments to control browning of lettuce stem discs (cv Iceberg) were evaluated by measuring changes in the L*, a* and b* parameters as well as Hue angle values Cysteine, resorcinol, EDTA and citric acid prevented discoloration Different concentrations of organic acids (citric, acetic and gluconic), vinegar and lemon juice were also tested The best results were obtained when the discs were treated with 10 mL/L and 50 mL/L acetic acid solutions and vinegar These treatments were then applied to head lettuces and browning was measured during 7 days storage at 2°C plus an additional commercialization of 3 days at 13°C Vinegar and 50 mL/L acetic acid solution inhibited browning on lettuce stem cuttings and could be used to prevent lettuce butt discoloration during cold storage and commercial handling

Color and shelf stable beverage compositions containing tea extract

Abstract: The present invention relates to color and shelf stable beverage compositions comprising: a) from about 0.4% to about 1.5% tea extract wherein the level of caffeine is from about 0.85% to about 3.2%; and the tea solids content is from about 12% to about 60%; b) from about 10 ppm to about 40 ppm colorant; c) from about 300 ppm to about 1500 ppm erythorbic acid or ascorbic acid; d) from about 0.5% to about 8.0% citric acid; e) an effective amount of sweetener; and f) water; and wherein the tea extract contains unoxidized flavanols.

Influence of Citric Acid and Glycine on the Adsorption of Mercury (II) by Kaolinite Under Various pH Conditions

Abstract: This investigation was carried out to study the effect of different concentrations of citric acid and glycine, which are common in freshwaters, on the kinetics of the adsorption of Hg by kaolinite under various pH conditions. The data indicate that Hg adsorption by kaolinite at different concentrations of citric acid and glycine obeyed multiple first order kinetics. In the absence of the organic acids, the rate constants of the initial fast process were 46 to 75 times faster than those of the slow adsorption process in the pH range of 4.00 to 8.00. Citric acid had a significant retarding effect on both the fast and slow adsorption process at pHs of 6.0 and 8.0. It had a significant promoting effect on the fast and slow adsorption process at pH 4.00. Glycine had a pronounced enhancing effect on the rate of Hg adsorption by kaolinite during the fast process. The rise in pH of the system further increased the effect of glycine on Hg adsorption. The magnitude of the retarding/promoting effect upon the rate of Hg adsorption was evidently dependent upon the pH, structure and functionality of organic acids, and molar ratio of the organic acid/Hg. The data obtained suggest that low-molecular-weight organic acids merit close attention in studying the kinetics and mechanisms of the binding of Hg by sediment particulates and the subsequent food chain contamination.

Trehalose-6-phosphate synthase A affects citrate accumulation by Aspergillus niger under conditions of high glycolytic flux

Abstract: Accumulation of citric acid by Aspergillus niger depends on a high flux through glycolysis. We have investigated the possibility of control of this flux by trehalose 6-phosphate, an inhibitor of hexokinase of Saccharomyces cerevisiae and other eukaryotes (Blasquez et al., FEBS Lett. (1993) 329, 51-54). Hexokinase of A. niger was shown in vitro to be only weakly inhibited by trehalose 6-phosphate (KI 1.5-2 mM). To investigate the in vivo relevance of this inhibition, we used isogenic strains of A. niger, carrying either a disruption or an amplification of the trehalose-6-phosphate synthase A (T6PSA)-encoding gene (ggsA) and exhibiting corresponding differences in T6PSA activity. These strains produced citric acid at comparable rates and with similar yields on 1 or 2.5% (w/v) sucrose. At 5-14% (w/v) sucrose, the ggsA disrupted strain initiated citric acid accumulation earlier, whereas the multicopy strain showed the reverse effect. When sucrose was replaced by lactose, which enabled only low rates of catabolism irrespective of its concentration (1-8%), no differences in the initiation or rate of citric acid accumulation were observed between the three strains. The possible mechanisms by which ggsA controls glycolytic flux in A. niger in the presence of high sugar concentrations are discussed.

Mechanical and Water Vapor Transmission Properties of Polysaccharide Films

Abstract: The film-forming properties of chitosan, chitosan glutamate, sodium alginate, and hydroxypropyl methylcellulose (HPMC) were investigated. Films were produced by a casting/solvent evaporation method from plasticizer-free and plasticizer-containing aqueous solutions. The water vapor transmission and mechanical properties (puncture strength and % elongation) of the films were investigated as a function of the polymer type and viscosity, plasticizer type (glycerin, propylene glycol, polyethylene glycol, triethyl citrate), plasticizer concentration, and type and concentration of acid used to dissolve chitosan. The effect of storage humidity was also examined. Glycerin and water were good plasticizers for chitosan glutamate. The chitosan film properties were dependent on the type and concentration of acid used to dissolve it, citric acid being a good plasticizer. The mechanical and water vapor transmission properties of alginate and HPMC films were less influenced by the investigated variables.

Optimizing the concentrations of carbon, nitrogen and phosphorus in a citric acid fermentation with response surface method

Abstract: Response surface designs and analyses were used to optimize the concentrations of carbon, nitrogen and phosphorus for citric acid production by Aspergillus foetidus CCRC 30206. The citric acid production was adequately approximated with a full quadratic equation obtained from a three‐factor‐five‐level central composite design. Analyses of the quadratic surfaces showed that in a 7‐day‐fermentation at 30 °C the maximum citric acid concentration of 43.92 g/1 appeared at sucrose, NH4NO3 and KH2PO4 concentrations of 109.6, 1.83 and 0.40 g/l, respectively. The concentrations of nitrogen and phosphorus were more important than that of carbon in determining the citric acid concentration and fungal growth. Furthermore, citric acid production was more sensitive to changes in phosphorus concentrations than to changes in nitrogen concentrations in the vicinity of the optimum point of citric acid production.

Synthesis of metastable tetragonal ( t′ ) zirconia-calcia solid solution by pyrolysis of organic precursors and coprecipitation route

Abstract: Three kinds of chemical processes, citrate gel process, acetate gel process, and coprecipitation route, have been applied to the synthesis of homogeneous metastable tetragonal (t′) and cubic solid solutions of ZrO2−X mol% CaO (X = 4–20). From a Raman scattering study, the citrate gel process based on the gelation of the aqueous solution of citric acid containing Zr and Ca ions was found to produce compositionally homogeneous samples in comparison with the other two methods. The axial ratio c/a decreases with increasing concentration of CaO and becomes unity around 8–10 mol% CaO composition.