Effects of free fatty acids on oxidative stability of vegetable oil

The effect of free fatty acid (FFA) content on the susceptibility to thermooxidative degeneration of vegetable oils was determined by Rancimat analysis. A prooxidant effect of FFA was observed in all filtered oils, independently of lipidic substrate and of its state of hydrolytic and oxidative alteration. The intensity of this effect was related to FFA concentration, but regression analysis of the experimental data did not show a general correlation law between FFA concentration and induction time (I _t). Different results were obtained for freshly processed virgin olive oils, characterized by postpressing natural suspension-dispersion: opposite behavior was observed of FFA content as regards oxidative stability, depending on the presence of suspended-dispersed material. This fact is of interest because the dispersed particles play a double stabilizing effect on both oxidative and hydrolytic degradation. These results showed that avoidance of oil filtration is highly desirable to extend olive oil’s shelf life.     

Study on the composition of rice bran oil and its higher free fatty acids value

The compositions of rice bran oils (RBO) and three commercial vegetable oils were investigated. For refined groundnut oil, refined sunflower oil, and refined safflower oil, color values were 1.5–2.0 Lovibond units, unsaponifiable matter contents were 0.15–1.40%, tocopherol contents were 30–60 mg%, and FFA levels were 0.05–0.10%, whereas refined RBO samples showed higher values of 7.6–15.5 Lovibond units for color, 2.5–3.2% for unsaponifiable matter, 48–70 mg% for tocopherols content, and 0.14–0.55% for FFA levels. Of the four oils, only RBO contained oryzanol, ranging from 0.14 to 1.39%. Highoryzanol RBO also showed higher FFA values compared with the other vegetable oils studied. The analyses of FA and glyceride compositions showed higher palmitic, oleic, and linoleic acid contents than reported values in some cases and higher partial glycerides content in RBO than the commonly used vegetable oils. Consequently, the TG level was 79.9–92% in RBO whereas it was >95% in the other oils studied. Thus, refined RBO showed higher FFA values, variable oryzanol contents, and higher partial acylglycerol contents than commercial vegetable oils having lower FFA values and higher TG levels. The higher oryzanol levels in RBO may contribute to the higher FFA values in this oil.     

Rapid colorimetric determination of free fatty acids

In 1964, a method was described for the determination of free fatty acids (FFAs) in vegetable oil. This paper describes an expansion of that work, improving the sensitivity and reproducibility of the method, as well as examination of solubilities of the copper soaps as a function of chain length and unsaturation. Involvement of the micellar structure was reviewed. Finally, a procedure is described that permits very rapid determination of FFA at the 2.0–14.0 μmol (0.5–4.0 mg oleic acid) level, and the results with several oils are given. Particular attention was given to evaluation of solvent systems which would extract the copper complexes. Presented in part at the AOCS Meeting in San Francisco, April 1969. Technical Paper No. 4036, Oregon Agricultural Experiment Station.     

Effect of fatty acids and tocopherols on the oxidative stability of vegetable oils

The oxidative stability of vegetable oils is determined by their fatty acid composition and antioxidants, mainly tocopherols but also other non‐saponifiable constituents. The effect of fatty acids on stability depends mainly on their degree of unsaturation and, to a lesser degree, on the position of the unsaturated functions within the triacylglycerol molecule. Vegetable oils contain tocopherols and tocotrienols, especially α‐ and γ‐tocopherols, as their main antioxidants. The antioxidant behavior of tocopherols represents a complex phenomenon as they are efficient antioxidants at low concentrations but they gradually lose efficacy as their concentrations in the vegetable oils increase. The “loss of efficacy” of tocopherols, sometimes referred to as a “pro‐oxidant effect”, is witnessed by an increase in the rate of oxidation during the induction period, despite elongation of this phase. The phenomenon is much obvious for α‐tocopherol, but is also evident for other tocopherols. In agreement with nature's wisdom, the tocopherol levels in vegetable oils seem to be close to the optimal levels needed for the stabilization of these oils. The presence of other antioxidants in the oils, e.g. carotenoids, phenolic compounds, and Maillard reaction products, may synergize with tocopherols and minimize this loss of efficacy.     

Complex formation and reversible oxygenation of free fatty acids

Combined manometric and spectrophotometric studies reveal that prior to true oxidation unsaturated fatty acids undergo an oxygenation which is reversible in response to changes in oxygen pressure. The oxygenation seems to result from the oxygen molecule having a specific affinity for pairs of olefinic bonds.     

Kinetic studies on the esterification of free fatty acids in jatropha oil

Kinetic studies have been carried out on the esterification of free fatty acids (FFAs) in jatropha oil with methanol in the presence of sulphuric acid catalyst at 5 and 10 wt% concentrations relative to free fatty acids (0.4–0.8 wt% relative to oil) and methanol–FFA mole ratios ranging from 20:1 to 80:1. It has been found that a 60:1 methanol–FFA mole ratio and 5 wt% catalyst at 60°C and 500 rpm or above provided a final acid value lower than 1 mg KOH/g oil within 60 min. A kinetic model has been proposed with second‐order kinetics for both the forward and backward reactions. The effect of temperature on the reaction rate constants and equilibrium constant has been determined using Arrhenius and von't Hoff equations, respectively. The heat of reaction was found to be ?11.102 kJ/mol.     

Bactericidal activity of certain fatty acids

Fatty acids in the C₉ to C₁₂ range are bactericidal to a number of different organisms, and this activity is markedly enhanced with increasing acidity. Optimum bactericidal activity toStaphylococcus aureus was exhibited by undecylic acid. Since the activity is of the same order as that of the quaternary ammonium germicides in alkaline solution, an explanation for both types of germicidal activity is presented in which the bacteria or some protein essential to the bacteria are considered to be “suffocated” by a “coating” of fatty groups in chemical combination with the protein. The application of these findings to bactericidal activity encountered in rancid fats is discussed briefly in connection with the oxidative degradation of oleic acid derivatives to pelargonic acid. In addition, the possible usefulness of the bactericidal activity as a microbiological assay method for certain fatty acids is pointed out.     

Peroxidation of free and esterified fatty acids by horseradish peroxidase

Linoleic and arachidonic acids and unsaturated esterified fatty acids of soybean lecithin liposomes were oxidized by horseradish peroxidase in the presence of hydrogen peroxide. The major products formed in the presence of oxygen were fatty acid hydroperoxides. In the absence of oxygen, other unidentified products were formed. Diene conjugation was about 5 times faster in oxygen than in nitrogen. Malondialdehyde was formed only in the presence of oxygen. Superoxide, singlet oxygen and hydroxyl radical may have been involved in the free fatty acid oxidation system but not in the liposome system. Replacement of hydrogen peroxide with the hydrogen peroxide (and superoxide) generators xanthine oxidase or galactose oxidase caused a more efficient oxidation in the presence of peroxidase than in its absence, suggesting that the in vivo toxicity of hydrogen peroxide and superoxide may be greatly increased in the presence of peroxidase.     

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.     

Rapid methods for determination of free fatty acids contents in fatty oils

Rapid qualitative and quantitative methods for determining the free fatty acid (FFA) contents in common oils and fats are reported. Qualitative method is based on the type of color developed in the presence of BDH indicators (Universal and “678”) when a known excess of alkali is added to an alcoholic solution of oil or fat. By this method, low (0.0–0.25), medium (0.26–0.99) and high (1.0 and above) FFA levels in fatty oils may be distinguished. Quantitative method is a simplified modification of the usual procedure of determining the FFA contents of oils and fats by titration against standard alkali solution in the presence of BDH Universal or “678” indicator. The results of the rapid methods agree well with those of the standard AOCS method.     

An automatized method for determination of free fatty acids

An automated colorimetric method is described for determining free fatty acids (FFA) in vegetable oils using the flow injection analysis (FIA) technique. In this procedure, an almost linear relationship exists between the peak height and the FFA concentration. Liquid samples can be poured directly into the sample cups on the sampler for an automatic analysis of the FFA content. The dynamic range of this method is from 0.01 to almost 5%. Samples with higher FFA content must be diluted before analysis. The sample capacity is 12–20 injections/hr. No evidence of the existence of the earlier proposed cage-like complex (Cu(II)(FFA)₂)₂ in the organic phase was observed in this study.     

Inhibitors of the prooxidant activity of α-tocopherol

The prooxidant activity ofα-tocopherol (α-T) can be reduced or inverted into antioxidant activity by various compounds. An important antioxidant activity was obtained by the association ofα-T with cysteine, BHT, hydro-quinone and ascorbyl palmitate. EDTA 10^-4M, phos-phoric, malonic and citric acids can partially decrease the prooxidant effect ofα-T, although the strong acids exhibited no antioxidant activity withoutα-T. Other amino acids such as glycine and alanine do little to reduce the prooxidant behavior of α-T while they showed a strong antioxidant activity without α-T. The distribution of hydroperoxide isomers of linoleic acid formed with and withoutα-T was not modified by the addition of the various compounds except BHT, which led to the production of only 13c,t and 9c,t isomers as observed withα-T. The oxidation ofα-T in the presence of linoleic acid was completely inhibited by the addition of cysteine, phosphoric, malonic and citric acids, BHT, hydro-quinone, ascorbyl palmitate and EDTA 10^-4M. The inhibitors of the prooxidant activity ofα-T could act in two different ways: by chelating the prooxidant metals traces (i.e., amino acids, EDTA) and/or by regeneratingα-T, thus reducing the concentration of chromanoxy radical which would be involved in the prooxidant activity (i.e. phosphoric, malonic, citric acids, cysteine and the common antioxidants).     

Effect of epinephrine on the oxidative desaturation of fatty acids in the rat adrenal gland

Delta-6 and Δ5 desaturation activity of rat adrenal gland microsomes was studied to determine the effect of microsomal protein and the substrate saturation curves. This tissue has a very active Δ6 desaturase for linoleic and α-linoleic acids and a Δ5 desaturase for eicosa-8,11,14-trienoic acid. The administration of epinephrine (1 mg/kg body weight) 12 hr before killing, produced approximately a 50% decrease in desaturation of [1-¹⁴C]linoleic acid to γ-linolenic acid, [1-¹⁴C]α-linolenic acid to octadeca-6,9,12,15-tetraenoic acid and [1-¹⁴C]eicosa-8,11,14-trienoic acid to arachidonic acid. A 30% decrease in Δ5 desaturation activity was also shown after 7 hr of epinephrine treatment. The changes on the oxidative desaturation of the same fatty acids in liver microsomes were similar. No changes were observed in the total fatty acid composition of adrenal microsomes 12 hr after epinephrine treatment. Mechanisms of action of the hormone on the biosynthesis of polyunsaturated fatty acids in the adrenal gland are discussed.     

Reactions of conjugated fatty acids. VIII. Dibasic acids by hydrogenation and oxidative cleavage

Summary Conjugated linoleic acid can be hydrogenated as sodium soap in an aqueous or ethylene glycol solution with commercial nickel catalysts. Under suitable conditions the acid is reduced predominantly to monounsaturated acids with only a slight increase in saturated acids. An alkali-conjugation reaction mixture may be hydrogenated without isolating the conjugated acids. One set of conditions found suitable for hydrogenation is as follows: 10 g. of conjugated linoleic acid, 7 g. of sodium hydroxide, 250 ml. of water, and 0.05 g. of nickel placed under 40 p.s.i. hydrogen pressure and heated at 140°C. for 1 hour. Acids prepared from this reaction mixture have an iodine value of about 90. Oxidation and chromatographic analyses of the resultant dibasic acids indicate that with alkali-conjugated linoleic acid, 1,2, 1,4, and 3,4 addition of hydrogen take place with equal ease. The reduced acids contain 66%trans acids. Withtrans,trans conjugated linoleic acid, 1,4 addition takes place to a greater extent that 1,2 and 3,4 addition, and the reduced acids are allrans. Presented at the fall meeting, American Oil Chemists' Society, Cincinnati, O., September 30–October 2, 1957. Part VII is in press, Journal of Organic Chemistry.     

Germination and free fatty acids in seed stock lots of cottonseed

Summary The relationship of the free fatty acid content in the oil to the percentage germination for 254 samples of cottonseed of different varieties indicates that the free fatty acid content may be used as a practical screening index for use in selecting lots of cottonseed to be reserved for seeding purposes and subsequent testing for germination. The percentage germination decreases in general with increasing free fatty acid content. The mathematical probability that a given lot of seed will exceed a specified minimum germination value decreases markedly as the free fatty acid content of the oil increases. Insofar as practical, it is suggested that cottonseed reserved for seeding have a low free fatty acid content, less than 0.75% in the oil if at all possible. One of the laboratories of the Bureau of Agricultural and Industrial Chemistry, Agricultural Research Administration, U. S. Department of Agriculture. In cooperation with the Production and Marketing Administration, U. S. Department of Agriculture, under Marketing Act of 1946.     

Determination of free fatty acids and diacylglycerols in native vegetable oils

The analysis of free fatty acids (FFA) and diacylglycerols (DG) by GLC may be used to detect the deacidification of oils. Free fatty acids are removed from the oil during neutralization or physical refining, while the corresponding DG remain in the oil. This will change the ratio of FFA to DG. For analysis, oils with tricaprin as internal standard are silylated and injected on-column onto a short high-temperature capillary column. Extracted oils showed higher amounts of FFA and DG than pressed oils from the same batch of seeds. There are only minor changes in the ratio of FFA to DG according to the yield of pressing or due to washing the oil.