Effect of water activity on secondary products formation in autoxidizing methyl linoleate

The role of water activity on the formation of peroxides and carbonyl compounds during lipid oxidation is important to know because there could be either beneficial or detrimental effects of water activity on lipid oxidation in stored foods. Therefore, methyl linoleate was chosen as a model lipid and was autoxidized to 1% at water activity ranging from 0.02 to 0.79 at 37°C. Oxygen uptake was monitored manometrically. The peroxide and carbonyl contents were determined upon termination of the autoxidation studies. Methyl linoleate autoxidation was characterized by three phases: i) an initial induction period of no oxygen absorption; ii) a slow rate of oxygen absorption, up to 0.15% oxidation; and iii) a relatively faster rate of oxygen absorption beyond 0.15% up to 1% oxidation. Water activity had considerable influence during the first phase. There was no induction period at or below water activity 0.22. The induction period begins at water activity 0.32 and could be extended to a limit with increase in water activity. Once the induction period was passed water activity had no influence on the rate of oxidation. However, during the second and third phases water activity becomes important in the stabilization of peroxides/hydroperoxides and decides the course of secondary reactions that follow peroxide decomposition. Higher water activity values, particularly water activity 0.67, tended to stabilize peroxides. Water activity had considerable influence on the formation of secondary products of autoxidation as evidenced by the variation in the type and quantity of carbonyl compounds at different water activity values.     

Gas chromatographic study on high-temperature thermal degradation products of methyl linoleate hydroperoxides

Chromatographic techniques were used to separate secondary products generated by thermal degradation of methyl linoleate hydroperoxides (MLHP). The MLHP were obtained by oxidation, selected, and concentrated by solid-phase extraction (SPE) and thin-layer chromatography (TLC). The purified MLHP were then thermo-degraded in the gas-chromatographic glass liner and analyzed on-line by gas chromatography-mass spectrometry (GC-MS). The MLHP were also thermodegraded and collected in a short silicic acid-packed column, eluted, separated by TLC, and then analyzed by GC. By considering the elution in TLC, the GC retention times and the GC-MS analyses, it was possible to characterize the mono- and the dioxygenated secondary products, particularly those having a boiling point higher than methyl linoleate. The peaks that corresponded to the mono-oxygenated products (epoxy, hydroxy, and keto) were identified, and, on the basis of their MS spectra, molecular structures were proposed. A specific elution order was suggested for keto derivatives: 9-keto,Δ^10,12- and 13-keto,Δ^9,11-octadecadienoate. The hydroxy derivatives, which show the typical fragmentations of 9-hydroxy,Δ^10,12- and 13-hydroxy,Δ^9,11-octadecadienoate, were also identified. On the other hand, identification of the di-oxygenated compounds was more difficult, and, therefore, it was not possible to indicate each positional isomer; however, their elution order could be epoxy-hydroxy and epoxy-keto derivatives.     

Catalytic epoxidation of methyl linoleate

The epoxidation of methyl linoleate was examined using transition metal complexes as catalysts. With a catalytic amount of methyltrioxorhenium (4 mol%) and pyridine, methyl linoleate was completely epoxidized by aqueous H₂O₂ within 4 h. Longer reaction times (6 h) were needed with 1 mol% catalyst loading. Manganese tetraphenylporphyrin chloride was found to catalyze the partial epoxidation of methyl linoleate. A monoepoxidized species was obtained as the major product (63%) after 20 h.     

The role of amino acids in the autoxidation of milk fat

The effects of amino acids and their analogs on milk fat oxidation were examined under various conditions by measuring oxygen consumption and total unsaturated fatty acids. All the amino acids tested acted as antioxidants, characteristically extending the induction period (IP). Not only primary amino groups are responsible for the antioxidative activities of amino acids, but also the side-chain groups contribute, at least partially, to the protective effects of L-cysteine, L-tryptophan and L-tyrosine. In aqueous and HCL solutions, the antioxidative effects of L-alanine were significantly reduced. The freeze-dried L-lysine-HCL and L-alanine-HCL accelerated, while the corresponding control amino acids inhibited, milk fat oxidation.     

Products of the surface oxidation of methyl linoleate

Methyl linoleate with 5% by weight of methyl palmitate as an internal standard was deposited as a monolayer (20% by weight) on Silica gel H and oxidized at 7,25, and 40°C. Oxidation was followed by iodometric PV practiced directly on aliquots of the silica gel. Lower temperatures gave higher PV maxima but after longer times. Oxidation of methyl linoleate at all three temperatures followed first-order kinetics, and the energy of activation was 15.0 kcal/mol. The products recovered from the chloroform-acetic acid layer of the peroxide determination were analyzed by GC and identified by El- and CI-MS. Calculation based on methyl palmitate as an internal standard showed that the total peak area decreased to about 40% that of the original methyl linoleate when the residual methyl linoleate was reduced to less than 2%. The chief nonscission products (NSP) of linoleate oxidation were epoxy, hydroxy, hydroxy-epoxy, dihydroxy, and trihydroxy methyl esters. The greatest NSP concentrations were obtained about the time of the greatest PV, and the yield at 40°C was greater than those at 7 and 25°C. Scission products (SP) increased rapidly until the greatest PV was reached. After this time, SP declined slightly and plateaued at 40°C, but at 7 and 25°C, SP continued to increase slowly with further oxidation.     

Antioxidant constituents of peanut oil

The components responsible for increased stability of raw peanut oil at a high relative humidity (RH) of 91% were examined in peanut oil and methyl linoleate systems. Of the constituents, the native gums, which were mostly phospholipids and glycolipids, showed increased antioxidant activity at 91% RH. The isolated components of the gums, namely phospholipids and glycolipids, were prooxidant individually or in combination. Tocopherols did not show increased antioxidant activity at 91% RH.     

Moderate antioxidative efficiencies of flavonoids during peroxidation of methyl linoleate in homogeneous and micellar solutions

The relative reactivities as well as the stoichiometric coefficients for a number of flavonoids, catechols, and—for comparison—standard phenolic antioxidants were determined by analyzing the kinetics of oxygen consumption in organic and micellar systems, with peroxidation initiated by lipid- and water-soluble azo initiators. The results demonstrated that the flavonoids did not behave as classic phenolic antioxidants such as α-tocopherol, but showed only moderate chain-breaking activities. The results were in line with other structure-activity relationship studies on the importance of the B-ring catechol structure, the 2,3-double bond, and the 3,5-hydroxy groups. The data are discussed in view of possible explanations of the deviations flavonoids reveal in their behavior compared with regular phenolic antioxidants.     

Oxidation of methyl linoleate in the presence of lignin

The overall aim of this study is to develop new wood modifications using vegetable oils to obtain improved durability of wood materials in an environmentally friendly way. Nuclear magnetic resonance (NMR) and Fourier-transform infrared (FT-IR) spectroscopies were used to study oxidation and possible chemical coupling reactions between polyunsaturated fatty acids and model lignin compounds in order to better understand the interactions between oxidatively drying systems such as vegetable oils or alkyds with the lignin part in wood. This was done by studying mixtures of different model lignin compounds and methyl linoleate. The oxidation process was analyzed at 70 °C both in methyl linoleate alone and in combination with 20 wt% of lignin model compounds. The effects of those compounds on the oil polymerization processes were monitored by NMR (both ¹³C and ¹H experiments) and the domain specific reactivity and patterning were then combined with FT-IR data. No covalent bonds having formed between the oil and the model compounds were detected by combination of several ¹³C/¹H 2D NMR methods. From the spectra, the oxidation degrees of model compounds were calculated, and for some lignin model compounds alcohols were oxidized to carbonyls during the process. Those results were in excellent agreement with FT-IR data and oxidation mechanisms were proposed. The combination of both analytical techniques was necessary to have a better understanding of these systems: NMR demonstrated the absence of chemical bond and quantified oxidation degree of model lignin molecules while FT-IR focused on oil oxidation.     

The effect of amino acids and amino acid analogues on growth of an obligate methanotroph,Methylosinus trichosporium OB3b

The sensitivity of the obligate methanotroph, Methylosinus trichosporium OB3b to a number of amino acid analogues and amino acids has been examined. Sulphaguanidine (5 μg ml^?1) norleucine (100 μg ml^?1) m-fluorophenylalanine (50 μg ml^?1), p-fluorophenylalanine (50 μg ml^?1) and S2-aminoethyl-C-cysteine (1000 μg ml^?1) inhibited growth. Proline, threonine, methionine and lysine (2–4 μg ml^?1) also inhibited growth, but arginine and leucine at similar concentrations had no effect. Attempts were made to isolate amino acid analogue resistant mutants, which would be expected to overproduce amino acids. Two approaches were made, selection of spontaneous and induced mutants in plate culture, and selection of a spontaneous mutant during growth in continuous culture under selective conditions, but no mutants were obtained. Possible reasons for failure to obtain such mutants are discussed.     

Optical characteristics of responsive biopolymers; co-polycondensation of tri-functional amino acids and Cy-3 bis-amine with diacylchlorides

The synthesis and fluorescence behaviour of a range of amphiphilic poly(l-lysine co-bis-amine-Cy-3 iso-phthalamide) polymers that incorporate low levels of Cy-3 dye co-monomer (0.5-2.1% (w/w)) in their backbone have been investigated in aqueous solution over a range of pH values. The desired functionality of these biocompatible, hydrophobically modified polyelectrolytes was achieved by incorporating pendant hydrophilic carboxyl groups along the polymer backbone, via the l-lysine moiety, balanced by a degree of hydrophobicity introduced via the iso-phthaloyl moiety. Thus, the polymer changes from an extended conformation at high degrees of ionisation to a compact conformation stabilised by hydrophobic association at low degrees of ionisation and eventually precipitates from solution. At intermediate degrees of ionisation, such polymers exhibit amphiphilic properties. A bis-functional cyanine fluorophore derivative, co-polymerised within the polymer backbone, is demonstrated to act as a fluorescent reporter on the conformational state of the polymer. The materials have higher intrinsic fluorescence intensity per fluorophore monomer over a broad range of concentrations than bis-amine Cy-3 and their maximum fluorescence intensity is up to eight fold higher than the maximum intensity of bis-amine Cy-3, which is limited by quenching. It is also shown that the free fluorophore can be used to probe the conformation of unlabelled poly(l-lysine iso-phthalamide) through electrostatic interaction with the polymer. The technique allows rapid spectrophotometric determination of polymer conformation and offers the potential of an environmentally sensitive molecular pH probe for in vivo use.     

Corrosion control of Cu-Ni alloys in neutral chloride solutions by amino acids

The corrosion inhibition of Cu-Ni alloys was investigated in aqueous chloride solutions using amino acids as environmentally safe materials. The corrosion rate was calculated in absence and presence of the corrosion inhibitor using polarization and impedance techniques. The inhibition efficiency of the different amino acids was also calculated.The experimental results have shown that a simple amino acid like glycine can be used as efficient corrosion inhibitor for the Cu-Ni alloys in neutral chloride solutions. An inhibition efficiency of about 85% could be achieved at very low concentrations of the amino acid (0.1 mM). For low Ni content alloy (Cu-5Ni), 2.0 mM cysteine shows a remarkable high (∼96%) corrosion inhibition efficiency. The experimental impedance data were fitted to theoretical data according to a proposed equivalent circuit model for the electrode/electrolyte interface, and the mechanism of the corrosion inhibition process was suggested. Different adsorption isotherms were tested and the corrosion inhibition process was found to depend on the adsorption of the amino acid molecules and/or the deposition of corrosion products on the alloy surface. The adsorption free energy of cysteine on Cu-5Ni (−37.81 kJ mol⁻¹) reveals a strong physical adsorption of the inhibitor on the alloy surface.     

Drying characteristics and modeling of yam slices under different relative humidity conditions

The drying characteristics of yam slices under different constant relative humidity (RH) and step-down RH levels were studied. A mass transfer model was developed based on Bi-Di correlations containing a drying coefficient and a lag factor to describe the drying process. It was validated using experimental data. Results showed that the drying air with constant RH levels of 20, 30, and 40%, temperature of 60°C, and air velocity of 1.5 m/s had an insignificant effect on drying time. This phenomenon was likely attributed to the fact that higher RH led to a rapid increase in sample’s temperature. The higher sample temperature could provide an additional driving force to water diffusion and thereby promote the moisture movement, which could minimize the negative effect of lower the drying rate in the initial drying stage. Applying air with 40% RH for 15 min in the initial stage achieved the desired color and reduced the drying time by 25% compared to the drying time under continuous dehumidification from an initial RH of 40%. Using the developed Bi-Di correlation, the estimated Biot number, effective moisture diffusivity, and mass transfer coefficient ranged from 0.1024 to 0.1182, 1.1133 × 10^?10 to 8.8144 × 10^?9 m²/s, and 1.8992 × 10^?9 to 1.7364 × 10^?7 m/s, respectively. A rather high correlation coefficient of determination (R² between 0.9871 and 0.9971) was determined between the experimental and predicted moisture contents. The present findings contribute to a better understanding of the effect of relative humidity on drying characteristics. The developed Bi-Di correlation provided a new method to determine the effective diffusivity of moisture in drying.     

Nonionic adsorption of aromatic amino acids on a cation-exchange resin

Nonionic adsorption of aromatic amino acids on a cation-exchange resin (Dowex 50W-X8) was studied. At low concentrations, phenylalanine and tyrosine were adsorbed on the resin by monovalent ion-exchange process. Deviation from stoichiometric ion exchange became large as phenylalanine and tyrosine concentrations increased. On the other hand, aliphatic amino acids, glycine and alanine, were adsorbed on the resin by monovalent ion-exchange process. In column breakthrough experiments, the bed capacities for phenylalanine and tyrosine were higher than those for metal ions (Na⁺, Pb²⁺, and Ni²⁺) and the aliphatic amino acids. The increased capacities may be due to nonionic interaction between the aromatic amino acids and the polystyrene resin matrix. The nonionic interaction could be suppressed in 25% ethanol solution.     

Enantioselective recognition of amino acids based on molecularly imprinted polyaniline electrode column

A novel molecularly imprinted polyaniline (PAn) electrode column is introduced as a new technique for the enantioselective recognition of amino acids. The principle of the molecularly imprinted electrode column is based on the reversible doping/de-doping property of PAn. PAn is an organic semiconductor and thus from an electrochemical point of view the column packed with PAn can be regarded as a packed-bed electrode of a three-electrode arrangement. The electric potential of this column can be easily controlled by a potentiostat. Various important factors influencing the performance of the molecularly imprinted PAn column have been investigated using fluorescence spectrometry in conjugation with the electrochemical quartz crystal microbalance (EQCM) technique. The advantage of this method proposed over previously reported is that the enantio-recognition can be performed in a high efficiency without requiring an expensive chiral column. Especially, the ejection and re-binding of amino acids can be accomplished by adjusting the potential applied on the column packed with molecularly imprinted PAn.     

Preparation, characterization, and surface and biological properties of N-stearoyl amino acids

Several lipoamino acids were synthesized, in which n-octadecanoic acid (stearic acid) was coupled with the α-amino group of an amino acid. The products were characterized and their identities confirmed by advanced analytical techniques like Fourier transform infrared ¹H nuclear magnentic resonance spectroscopy, and differential scanning, calorimetry. Their surface properties, such as critical micelle concentration (CMC) and foaming properties, biodegradability, and antimicrobial activity were also evaluated. The N-stearoyl amino acids (NSA) had low CMC values, and some of them showed good foaming properties. They were screened for antimicrobial activity against the gram-positive bacteria Staphylococcus aureus, Micrococcus luteus, and Bacillus cerceus, the gram-negative bacteria Escherichia coli and Pseudomonas aeruginosa, and the yeast Candida albicans. All the compounds inhibited at least one of these organisms. N-Stearoyl proline was the most effective, the order of antimicrobial activity being aromatic NSA> acidic NSA>basic NSA. However, the effective inhibition by all the compounds indicates the desirability of more thorough investigation and suggests that some of these compounds may have potential utility as biostatic additives in commercial products. All NSA are highly biodegradable and can readily be removed under conditions of normal secondary sewage treatment.     

Zinc(II) complex of terpyridine-crown macrocycle: A new motif in fluorescence sensing of zwitterionic amino acids

A new fluorescence macrocyclic receptor based on the Zn(II) complex of a C₂-terpyridine and a crown ether has been developed for molecular recognition of zwitterionic amino acids in water/DMF solution with remarkable selectivity towards l-aspartate (K = 4.5 × 10⁴ M⁻¹) and l-cysteine (K = 2.5 × 10⁴ M⁻¹).     

Remarks on official methods employing boron trifluoride in the preparation of methyl esters of the fatty acids of fish oils

Some “official methods” for preparing methyl esters of the fatty acids from oils or fats may be referred to by users as the boron trifluoride (BF₃) method and invariably have two stages. The first stage, brief treatment with alkali [commonly NaOH in methanol (MeOH), sometimes NaOCH₃] and heat has been popularly described as a saponification step for over 30 yr. In fact, the disappearance of visible fat or oil is mostly transesterification, which can be accomplished in a few minutes under mild conditions. Free fatty acids (FFA) originally present, or produced by saponification, are not converted to methyl esters at this stage. The second stage, heating in BF₃-MeOH, has in practice been as short as 2 min. It can convert all FFA to methyl esters, but this step requires at least 30 min. Examples from the recent literature illustrate the necessity of extending the time for BF₃-MeOH transesterification of lipids or oils and methylation of FFA. No alkali transesterification is needed. Presented in part at the 88th Annual Meeting of the American Oil Chemists’ Society, Seattle, WA, May 1997.     

Wang-Landau Monte Carlo simulation of capillary forces at low relative humidity in atomic force microscopy

A lattice gas model is used with Wang-Landau Monte Carlo sampling to predict the capillary force between a model of an atomic force microscopy (AFM) probe and a smooth surface as a function of separation, relative humidity (RH), and tip hydrophilicity. Completely wetting AFM tips exhibit a maximum in the capillary force as the RH increases, while the magnitude of the capillary force in the presence of partially wetting and partially drying tips is relatively independent of the RH. Capillary forces can also be significant in low RH environments and should not be discounted in AFM studies involving hydrophilic surfaces.     

Thermoacoustic properties of methyl esters ofn-alkanoic acids

A number of thermoacoustic parameters, including the Sharma constant S₀, the isochoric temperature coefficient of volume expansivity (d Inα/d InT)_V, the isochoric temperature coefficient of internal pressure (d InP _i/d InT)_V, reduced volume V, reduced compressibility β and the Huggins parameter F, are estimated by using only the volume expansivity α from the density temperature data for a number of methyl esters ofn-alkanoic acids. The methyl esters give excellent separation of saturated fatty acid mixtures. The Sharma constant, which relates the molecular constantn and other thermoacoustic parameters, is a constant with a characteristic value of 1.11+0.01 for all these esters.     

Corrosion inhibition of mild steel in sulfamic acid solution by S-containing amino acids

R_p, potentiodynamic polarization curves and EIS techniques were applied to study the effect of five S-containing amino acids on the corrosion of mild steel in 5% sulfamic acid solution at 40 °C. The compounds are effective inhibitors and the inhibition efficiency follow the order: N-acetylcysteine (ACC) > cysteine (RSH) > S-benzylcysteine (BzC) > cystine (RSSR) ≅ methionine (CH₃SR). The inhibitors affect the anodic dissolution of steel by blocking the anodic sites of the surface. EIS measurements indicated that charge transfer is the rate determining step in the absence and presence of the inhibitors and the steel/solution interface can be represented by the equivalent circuit R_s(R_ctQ_dl). Adsorption of RSH, CH₃SR and RSSR follows the Langmuir model while the Temkin isotherm describes the adsorption of ACC and BzC. From the application of the Flory–Huggins isotherm, the number of water molecules displaced by the adsorbing inhibitor molecules was estimated. The potential of zero charge pzc of mild steel without and with the inhibitors is calculated and the mechanism of corrosion inhibition is discussed in the light of the molecular structure.