Practical limitations in determining vegetable oil acid values by a novel pH-metric method

A novel pH-metric method is described for the determination of acid values (AV) in vegetable oils without titration. The method is based on a reagent containing triethanolamine, isopropanol, and water to which an oil sample is added before measuring pH. Oil samples with AV in the range 0.006–0.107 mg KOH/g oil were prepared from commercial soybean oil by treatment with a strong-base anion exchanger in OH⁻ form and addition of oleic acid. Compared to the standard titrimetric method, significantly greater AV were obtained at less then 0.02 mg KOH/g oil. This was due to the influence of triethanolamine hydrolysis on the acid-base equilibrium in the mixture “oil-reagent.” Thus, the AV 0.02 mg KOH/g oil is accepted as the limit of quantitation. Because refined oils usually have AV of 0.05 mg KOH/g oil or more, this method should be suitable for practical oil analyses.     

A Highly Efficient Automated Flow Injection Method for Rapid Determination of Free Fatty Acid Content in Corn Oils

In the present study, a new flow injection (FI) methodology for determination of free fatty acid (FFA) content in corn oil samples is proposed. The proposed method is based on monitoring the absorbance changes at 580 nm, λ_max, as a result of neutralization of FFA in oil samples by KOH in the reagent. The analyses were performed with a single‐line FI manifold system created by modification of high performance liquid chromatography. The main parameters, such as sample and reagent volumes, reaction coil, reagent concentration and temperature were all optimized. With the newly developed sampling strategy, the oil sample and reagent at micro level were directly injected together without any pre‐treatment. The proposed flow injection analysis method was validated statistically and it was found to be linear (between 0.09 and 1.50 FFA %), accurate (recovery 87.19–122.22 %), and precise (relative standard deviation <1 % for both intra‐day and inter‐day precision). The limit of detection and limit of quantification were found to be 7.41 × 10^?3 and 2.24 × 10^?2 oleic acid %, respectively. The results were also compared with those obtained by the American Oil Chemists Society (Ca‐5a‐40) method using statistical t and F tests, and a significant difference was not observed between the methods at the 95 % confidence level. These results strongly suggest that this method is suitable for automated routine analysis of FFA in edible oils due to its simplicity, reliability, speed, and economy of solvents and sample.     

Comparison of four analytical methods for the determination of peroxide value in oxidized soybean oils

Previous work in our laboratory demonstrated that soybean oil oxidation, expressed as PV, can be determined using NIR transmission spectroscopy as an alternative to the official AOCS iodometric titration method. In the present study, a comparison of four peroxide analytical methods was conducted using oxidized soybean oil. The methods included the official AOCS iodometric titration, the newly developed NIR method, the PeroxySafe^™ kit, and a ferrous xylenol orange (FOX) method, the latter two being colorimetric methods based on oxidation of iron. Five different commercially available soybean oils were exposed to fluorescent light to obtain PV levels of 0–20 meq/kg; periodic sampling was done to ensure having representative samples throughout the designated range. A total of 46 oil samples were analyzed. Statistical analysis of the data showed that the correlation coefficient (r) and standard deviation of differences (SDD) between the standard titration and NIR methods were r=0.991, SDD=0.72 meq/kg; between titration and the PeroxySafe^™ kit were r=0.993, SDD=0.56 meq/kg; and between the standard titration and FOX method were r=0.975, SDD=2.3 meq/kg. The high correlations between the titration, NIR, and PeroxySafe^™ kit data indicated that these methods were equivalent.     

A Greener Alternative Titration Method for Measuring Acid Values of Fats,Oils, and Grease

A greener alternative method is proposed for measuring acid values (AV) of fats, oils, and grease (FOG) based on visual titration. Compared with Official Method Cd 3d-63 of the American Oil Chemists' Society (AOCS), this greener alternative method can eliminate the use of toluene, which in turn reduces toxicity and cost. A total of 44 samples of yellow and brown grease with AV ranging from 0.13 to 170.37 (mg KOH) g⁻¹ were titrated using both methods. The alternative titration method can provide accurate and reliable results to determine the AV of FOG by various statistical analyses including repeatability, linear regression, f-test, t-test, and method accuracy calibration with AOCS Cd 3d-63. This low-cost method can be recommended for routine titration in research and development, and in biodiesel plants for most FOG samples.     

New FTIR method for the determination of FFA in oils

A rapid, practical, and accurate FTIR method for the determination of FFA in edible oils was developed. Analogous to the AOCS titration procedure, the FTIR FFA determination is effected by an acid/base reaction but directly measures the product formed rather than utilizing an end point based on an electrode potential or color change. A suspension of a weak base, potassium phthalimide (K-phthal) in 1-propanol (1-PrOH), is used to convert the FFA present in oils to their carboxylate salt without causing oil saponification, and differential spectroscopy is used to circumvent matrix effects. Samples are first diluted with 1-PrOH, then split, with one-half treated with the K-phthal reagent and the other half with 1-PrOH (blank reagent), their spectra collected, and differential spectra obtained to ratio out the invariant spectral contributions from the oil sample. Quantification of the percentage of FFA in the oil, expressed as %oleic acid, based on measurement of the peak height of the ν (COO⁻) absorption of the FFA salt formed, yielded a calibration with an SE of <0.020% FFA over the range of 0–4%. The method was validated by standard addition and the analysis of Smalley check samples, the results indicating that the analytical performance of the FTIR procedure is as good as or better than that of the standard titrimetric procedure. As structured, the FTIR procedure is a primary method, as calibration is not dependent on reference values provided by another method, and has performance criteria that could lead to its consideration as an instrumental AOCS procedure for FFA determination. The FTIR portion of the analysis is automatable, and a system capable of analyzing ∼60 samples/h was developed that could be of benefit to laboratories that carry out a large number of FFA analyses per day.     

Quantitative Fourier transform infrared analysis for anisidine value and aldehydes in thermally stressed oils

A Fourier transform infrared (FTIR) transmission-based spectroscopic method was investigated for the simultaneous monitoring of aldehyde formation and the determination of anisidine value (AV) in thermally stressed oils. Synthetic calibration standards were prepared by adding known amounts of hexanal,t-2-hexenal andt,t-2,4-decadienal to canola oil (these compounds considered representative of aldehydic compounds formed during oxidation) plus random amounts of other compounds representative of oxidation by-products. The standards were analyzed for their chemical AV. With the partial least squares (PLS) technique, an FTIR spectrometer was calibrated to predict both the concentrations of individual aldehyde types and AV, with the individual aldehyde contributions being related to the chemical AV by multiple linear regression to derive “apparent” AV values. The predictive capability of the PLS calibrations was assessed by analyzing canola oils that were thermally stressed at 120, 155, and 200°C. The apparent AV, predicted for these samples, matched the chemical AV values within ±1.65 AV units. A PLS calibration also was derived by using thermally stressed samples as calibration standards. This approach provided similar predictive accuracy as the use of synthetic calibration standards. As such, quantitative determination of AV by FTIR spectroscopy was shown to be feasible, and the synthetic calibration approach provided additional information on the aldehyde types present in a sample and allowed the use of a simple gravimetric approach for calibrating an FTIR spectrometer. This study provides the basis for the development of a rapid, automated FTIR method for the direct analysis for AV of thermally stressed fats and oils in their neat form without the use of chemical reagents. The implementation of such a method as a quality control tool would eliminate the use and disposal of hazardous solvents and reagents, required by the conventional chemical method, and drastically reduce analysis time (∼2 min/sample). Possible applications include monitoring of the oxidative state of frying oils or evaluation of oxidative stability of biodegradable lubricants.     

A novel method for the determination of acid value of vegetable oils

The voltammetric behavior of naphthoquinone in the presence of free fatty acids (FFA) at the polypyrrole (PPy)‐modified electrode was investigated in an ethanol/1,2‐dichloroethane (3 : 1) solution containing 0.1 M LiClO₄. A well‐defined new reduction peak appeared at a more positive potential and was higher than that obtained at the bare Pt electrode. Based on the fact that the new reduction peak current showed a good correlation with the concentration of fatty acids, an electroanalytical method for the acid value (AV) of vegetable oils was developed using the PPy‐modified electrode in linear potential sweep voltammetry. The experimental parameters were optimized to obtain a sensitive voltammetric response in this work. A linear calibration graph was obtained in the concentration range of 5.0×10^–6–6×10^–3 M for FFA (R = 0.993), with a sensitivity of 2.41×10^–2 A L/mol and a detection limit of 1.2×10^–6 M (S/N = 3). Each assay of vegetable oil sample took about 80 s. The developed method is applied to the AV determination of six commercial vegetable oils. The results well agreed with those obtained by the titration method. Compared to the conventional titration method, the proposed method is superior in sensitivity and accuracy and requires a small amount of vegetable oil sample, with no pretreatment.     

Development of a flow injection chemiluminescent assay for the quantification of lipid hydroperoxides

An automated flow injection chemiluminescence (FICL) system for measuring lipid hydroperoxide (LOOH) concentrations in oils was developed. Initially, a crude oil-in-water emulsion (formed by mixing solvent-diluted oil with the aqueous-based CL compound, luminol, and the catalyst for the reaction, cytochrome c) was tested. The assay was rapid (60 samples per hour), reproducible (CV no greater than 10%, n=3) and had a low sample requirement (1 mg of oil) because of its high sensitivity (0.5 nmol LOOH). CL intensity was influenced by the amount and type of oil under analysis. Owing to these factors, quantitative data were attainable only with a uniform oil concentration and with a calibrant derived from an oil equivalent to that under analysis. This method yielded quantitative data in good agreement with an iodometric titration assay for LOOH (r=0.9204). A refinement of the first method consisted of replacing the luminol and cytochrome c CL compounds with lucigenin, resulting in an assay insensitive to α-tocopherol. A monophasic reaction solution was devised to remove the effect of turbidity; however, the CL signal was still influenced by oil type. Therefore, quantitative data were still attainable only when the same type of oil was used for calibration.     

Application of asymmetric Si3N4 hollow fiber membrane for cross-flow microfiltration of oily waste water

The asymmetric morphology of silicon nitride (Si₃N₄) ceramic hollow fiber membrane with a selective spongiform outer layer was optimized by the air gap distance and the internal rate of coagulate for oil/water emulsion microfiltration. The effect of trans-membrane pressure (TMP), feed flow rate (FFR), and pH of the feeding emulsion on the separation performance were determined experimentally. Membrane fouling has increased by dissociation of oil droplets during filtration at high TMP and FFR values. Fouling phenomena were studied based on standard pore blocking model. The pH by affecting the surface charge of the Si₃N₄ hollow fibers and zeta potential of the feed emulsion has also been introduced as a prominent influential factor on separation efficiency. The highest values of permeate flux (390 Lm^?2h^-1) and oil rejection (95%) were recorded in alkaline pH. The fabricated Si₃N₄ ceramic membranes were completely recovered (≤99%) by simple thermal treatment at 400 °C.     

Rapid Determination of Free Fatty Acid in Extra Virgin Olive Oil by Raman Spectroscopy and Multivariate Analysis

We introduce a visible Raman spectroscopic method for determining the free fatty acid (FFA) content of extra virgin olive oil with the aid of multivariate analysis. Oleic acid was used to increase the FFA content in extra virgin olive oil up to 0.80% in order to extend the calibration span. For calibration purposes, titration was carried out to determine the concentration of FFA for the investigated oil samples. As calibration model for the FFA content (FFA%), a partial least squares (PLS) regression was applied. The accuracy of the Raman calibration model was estimated using the root mean square error (RMSE) of calibration and validation and the correlation coefficient (R ²) between actual and predicted values. The calibration curve of actual FFA% obtained by titration versus predicted values based on Raman spectra was established for different spectral regions. The spectral window (945–1600 cm⁻¹), which includes carotenoid bands, was found to be a useful fingerprint region being statistically significant for the prediction of the FFA%. High R ² and small RMSE values for calibration and validation could be obtained, respectively.     

NMR Spectroscopy: Determination of Peroxide Value in Vegetable and Krill Oil by Using Triphenylphosphine as Tagging Reagent

Hydroperoxides are formed as the primary product during lipid oxidation, being analyzed as the peroxide value to detect the degradation level of oils and fats. As an alternative to the classical titration method according to Wheeler, a ¹H‐{³¹P} decoupled NMR method is developed using triphenylphosphine (TPP) as a tagging agent. TPP reacts with peroxides to form TPP oxides. The quantification of the peroxide value is performed by comparing the amount of reacted TPP oxide and non‐reacted TPP. This approach eliminates the requirement for an additional internal standard. Low‐oxidized oils (peroxide value < 3 meq/kg) and high‐oxidized oils with peroxide values of 150 meq/kg are precisely quantified with an relative standard deviation (RSD) of 4.90% and 0.16%, respectively. A total number of 108 oil samples are examined using the newly‐developed ¹H‐{³¹P} decoupled NMR method, indicating the applicability for vegetable oils and krill oils. Practical Applications: The developed NMR method is applicable for the determination of the peroxide value in vegetable, marine and krill oils presenting a powerful alternative for the Wheeler titration method.     

Stoichiometric determination of hydroperoxides in fats and oils by fourier transform infrared spectroscopy

A primary Fourier transform infrared (FTIR) spectroscopic method for the determination of peroxide value (PV) in edible oils was developed based on the stoichiometric reaction of triphenylphosphine (TPP) with hydroperoxides to produce triphenylphosphine oxide (TPPO). Accurate quantitation of the TPPO formed in this reaction by measurement of its intense absorption band at 542 cm⁻¹ provides a simple means of determining PV. A calibration was developed with TPPO as the standard; its concentration, expressed in terms of PV, covered a range of 0–15 PV. The resulting calibration was linear over the analytical range and had a standard deviation of ±0.05 PV. A standardized analytical protocol was developed, consisting of adding ∼0.2 g of a 33% (w/w) stock solution of TPP in hexanol to ∼30 g of melted fat or oil, shaking the sample, and scanning it in a 100-μm KCI IR transmission cell maintained at 80°C. The FTIR spectrometer was programmed in Visual Basic to automate scanning and quantitation, with the reaction/FTIR analysis taking about 2 min per sample. The method was validated by comparing the analytical results of the AOCS PV method to those of the automated FTIR procedure by using both oxidized oils and oils spiked with tert-butyl hydroperoxide. The two methods correlated well. The reproducibility of the FTIR method was superior (±0.18) to that of the standard chemical method (±0.89 PV). The FTIR method is a significant improvement over the standard AOCS method in terms of analytical time and effort and avoids solvent and reagent disposal problems. Based on its simple stoichiometry, rapid and complete reaction, and the singular band that characterizes the end product, the TPP/TPPO reaction coupled with a programmable FTIR spectrometer provides a rapid and efficient means of determining PV that is especially suited for routine quality control applications in the fats and oils industry.     

Precision and accuracy in the color specification of virgin olive oils from the bromthymol blue method

Twenty experienced observers with nondefective color vision judged 27 virgin olive oil samples within an acceptable color range, using the bromthymol blue (BTB) method, under controlled observation conditions (daylight source with a correlated color temperature of 6500 K, and standard gray back-ground). On the average, 44.8% of the observers agreed in their selections of the BTB standard solution matching a given oil sample, and this percentage increased to 88.2% considering ±one step in the two dimensions (pH and concentration) of the BTB scale. On the average, the lowest color difference between oil samples and available BTB solutions was 6.6 Commission Internationale de l'éclairage 1976-(L^*a^*b^*) (CIELAB) units, but this color difference was approximately two times greater for the color difference between oil samples and BTB solutions selected by our observers. The colors of the BTB standard solutions in the CIELAB space are not uniformly distributed, and thus one step in pH or concentration is equivalent to CIELAB color differences varying in a wide range (1.7–13.5 and 1.7–26.3 CIELAB units, respectively). From these values, indicating low precision, accuracy, and uniformity, some suggestions are made for future improvements of the current BTB method.     

Comparative study of the extraction and measurement of cottonseed free fatty acids

Crude oil was extracted from cottonseed by three different methods to study the influence of extraction technique on the free fatty acid (FFA) concentration. Extraction procedures that recovered more oil had higher levels of FFA. In addition, the highest concentration of FFA was found in oil recovered by Soxhlet reextraction of a meal initially defatted by a room-temperature extraction process. The FFA concentrations of oils recovered by Soxhlet extraction were highly correlated with the FFA concentration of oils recovered by the other extraction methods studied (R ²>0.96). Titration of oil and gas chromatography of silylated oil were compared as methods to determine FFA concentration. The methods compared well (R ²=0.998) with the titration method, giving ∼5% higher values for FFA than the chromatography method. Half of this difference appeared to be due to the oleic acid approximation used in the titration approach. The other half of the difference is likely due to the detection of other acidic components in crude oil.     

Determination of phosphorus in edible oils by inductively coupled plasma—Atomic emission spectrometry and oil-in-water emulsion of sample introduction

The determination of phosphous was carried out using emulsion sample preparation followed by analysis by inductively coupled plasma-atomic emission spectrometry. The optimal oil-in-water (o/w) emulsion and surfactant concentrations were found by applying response surface methodology. Two phosphorus emission lines were tested. Ethoxynonylphenol was a good choice for the emulsion preparation. The optimal concentration for the oil in the o/w emulsion and the surfactant concentrations were ca. 8.0–37% w/w and ca. 1.0–10% w/w ethoxynonylphenol, respectively, for the 213.620 nm line; and an oil phase emulsion concentration between ca. 18–32% w/w and ca. 3–8% w/w ethoxynonylphenol for the 214.911 nm emission line. Good agreement was found between calibration curves for emulsified aqueous standards solutions and o/w emulsions. The use of aqueous standards for calibration purposes is a good addition in the analysis of this type of oil sample where certified standards are not available. Recoveries ranged from 98 to 105% and from 102 to 116% for the 213 and 214 nm lines, respectively with relative standard deviations lower than 7%.     

The Role of Oil Phase in the Stability and Physicochemical Properties of Oil-in-Water Emulsions in the Presence of Gum Tragacanth

Different emulsions based on six types of vegetable oils (sunflower, canola, sesame, olive, coconut, and palm olein) were studied to investigate the role of the oil phase in the stability and physicochemical characteristics of oil-in-water emulsions prepared with gum tragacanth. The results indicated that the stability, rheological parameters, and size distribution of emulsions were dependent on the oil type. Based on the interfacial tension value, the type of oil did not have a significant effect on the gum tragacanth-emulsifying properties. The formulation based on sunflower and coconut oil led to producing more stable emulsion and a sample containing palm olein resulted in an unstable emulsion. Rheological analysis revealed that the sample based on palm olein showed the lowest consistency coefficient (2.10 ± 0.05 Pas ⁿ), elastic modulus (3.90 ± 0.21 Pa), and energy of cohesion (80.87 ± 1.1 J m⁻³). This study revealed that using oils with lower viscosity and higher density led to the higher stability of the emulsion samples.     

Application of a computer-controlled flow analyser to the determination of glucose and to the study of β -galactosidase activity

Glucose was determined using an automated flow analyser and a commercial reagent preparation based on the glucose oxidaseperoxidase reaction. The coupling system for the hydrogen peroxide liberated consisted of 4-aminophenazone and 2,4-dichlorophenol. The analyser demonstrated linearity of>0.9997 for five consecutive sets of standards. The average RSD for individual standards is 0.76%, and a sample throughput rate of >80 h^-1 was achieved. The study of β-galactosidase, using a novel substrate, was carried out in a simple single-line manifold. The enzyme-substrate reaction mixture was injected into a pH 10 buffer carrier to stop the reaction, and at the same time, propel the reaction zone to the flow cell. K_m and V_max values were calculated.     

An automated gas-liquid chromatographic method of measuring free fatty acids in canola

An automated method for measuring free fatty acids (FFA) in canola seed was developed by using gas-liquid chromatography (GLC). The results from the GLC method were linearly related (r²=+0.98) to those obtained with the traditional method involving Soxhlet extraction followed by a titration. In a nested experiment of over 11 different seedlots, the extraction and injection errors were 0.11 and 0.018%, respectively, of the total variation. The variation attributed to sampling within a seedlot was twice the variation attributed to extraction and injection errors combined. A seed sample size of at least 10 mL was needed to prevent the standard deviation from increasing as the FFA mean increased. The GLC method was precise and rapid, and also identified which fatty acids were being cleaved from the oil, but a linear adjustment improved accuracy.     

Rapid quantitative determination of free fatty acids in fats and oils by fourier transform infrared spectroscopy

Rapid direct and indirect Fourier transform infrared (FTIR) spectroscopic methods were developed for the determination of free fatty acids (FFA) in fats and oils based on both transmission and attenuated total reflectance approaches, covering an analytical range of 0.2–8% FFA. Calibration curves were prepared by adding oleic acid to the oil chosen for analysis and measuring the C=O band @ 1711 cm^–1 after ratioing the sample spectrum against that of the same oil free of fatty acids. For fats and oils that may have undergone significant thermal stress or extensive oxidation, an indirect method was developed in which 1% KOH/methanol is used to extract the FFAs and convert them to their potassium salts. The carboxylate anion absorbs @ 1570 cm^–1, well away from interfering absorptions of carbonyl-containing oxidation end products that are commonly present in oxidized oils. Both approaches gave results comparable in precision and accuracy to that of the American Oil Chemists’ Society reference titration method. Through macroprogramming, the FFA analysis procedure was completely automated, making it suitable for routine quality control applications. As such, the method requires no knowledge of FTIR spectroscopy on the part of the operator, and an analysis takes less than 2 min.     

Geopolymer with Hierarchically Meso‐/Macroporous Structures from Reactive Emulsion Templating

We present a simple synthetic route to hierarchically porous geopolymers using triglyceride oil for a reactive emulsion template. In the new synthetic method, highly alkaline geopolymer resin was first mixed with canola oil to form a homogeneous viscous emulsion which was then cured at 60°C to give a hard monolithic material. During the process, the oil in the alkaline emulsion undergoes a saponification reaction to be decomposed to water‐soluble soap and glycerol molecules which were then extracted with hot water to finally yield porous geopolymers. Nitrogen adsorption studies indicated the presence of mesopores, whereas the SEM studies revealed that the mesoporous geopolymer matrix are dotted with spherical macropores (10–50 μm) which are due to oil droplet template in the emulsion. Various synthetic parameters including the precursor compositions were examined to control the porosity. BET surface area and BJH pore volume of the materials were up to 124 m²/g and 0.7 cm³/g, respectively, and the total pore volumes up to 2.1 cm³/g from pycnometry.