Flavor reversion in soybean oil

Summary The ultraviolet spectral changes produced in soybean oil by treatment with heat and visible and ultraviolet light have been determined. Heat treatment appears to cause more deep-seated changes than does exposure to visible or ultraviolet irradiation. Distinct flavor changes occur in soybean oil under these different conditions; that produced by visible light of strong intensity being the easiest to recognize. A preliminary study is reported on the methods used to concentrate and isolate the substances responsible for the off-flavor of heat-treated soybean oil. The generous financial assistance of the National Association of Margarine Manufacturers is gratefully acknowledged. Contribution No. 592 from the Department of Chemistry, University of Pittsburgh.     

Flavor reversion in soybean oil

Summary It is evident that the oxidation rate of soybean oil may be varied over a considerable range without influencing the organoleptic evaluation of the degree of reversion. Even when the rate of oxidation is greatly reduced by the use of inert atmospheres, there is no diminution in the tendency to revert. On the contrary, with low oxygen concentrations, a type of reversion is produced that is more persistent in taste than that resulting in air or oxygen. The generous finacial assistance of the National Association of Margarine Manufacturers is gratefully acknowledged. Contribution No. 612 from the Department of Chemistry, University of Pittsburgh.     

Flavor reversion in soybean oil

Summary A simulated soybean oil has been synthesized from purified fatty acids. The flavor characteristics of the oil after heat and light treatment are described and compared to those of soybean and cottonseed oils. The generous financial assistance of the National Association of Margarine Manufactures is gratefully acknowledged. Contribution No. 613 from the Department of Chemistry, University of Pittsburgh.     

A review of soybean oil reversion flavor

Crude soybean oil has a characteristic “greenbeany” flavor, which during refining, bleaching and deodorization is eliminated to produce a bland tasting, light colored oil. However, flavor returns during storage and has been characteristically called the “reversion flavor” of soybean oil. This deleterious characteristics flavor has influenced the utilization of soybean oil and its fatty acids. Several theories for the cause of reversion flavor include: (a) oxidation of linolenic acid; (b) oxidation of isolinoleic acid of the 9,15-diene structure; (c) phosphatide reactions; (d) unsaponifiables; and (e) oxidative polymers. References are presented that support or contradict these theories. Recent publications concerning the isolation and characterization of the components of reversion flavor indicate slight oxidation of the fatty acids is the major cause. Techniques that are effective in increasing the flavor stability of soybean oil are presented.     

Practical aspects of hydrogenation and soybean salad oil manufacture

Most edible oils are hydrogenated in batch-type slurry converters similar in basic design to those employed when the process was first commercialized in 1911. One major company uses a proprietary continuous slurry process. Other novel batch and continuous slurry systems are available but have not enjoyed significant commercial success. Fixed bed hydrogenation has not been seriously investigated but offers intriguing possibilities. Energy economy is assuming ever greater importance in the design of hardening systems. The accelerated growth of hydrogenation since the 1940s parallels the rapid increase in soybean oil use. In part, it reflects the flavor instability of soybean oil caused by its linolenic ester. When this triunsaturate is lowered by hydrogenation to ca. 3%, a high-quality soybean salad oil can be produced. Standard hydrogenation and separation techniques work well. New separation equipment and processes are entering the marketplace.     

Characterization of the reversion flavor of soybean oil

The reversion flavor isolated from reverted-but-not-rancid soybean oil was separated into 14 fractions by gas-liquid chromatography. The gas chromatographic fractions were collected in specially designed gas cells and polyethylene capillary traps for the determination of their infrared spectra by micro techniques. Ethyl formate, ethyl acetate, ethyl alcohol, n-butyraldehyde, 2-heptanone, and 2-heptenal were positively identified. Presence of alcohol, ester, and possibly dimethyl amino compounds in the fractions with higher chromatographic retention times was indicated by infrared analyses. Ultimate analyses of the flavor compounds isolated from reverted-but-not-rancid soybean oil also indicated the presence of nitrogen compounds.     

Minor components responsible for flavor reversion of soybean oil

Edible refined, bleached and deodorized (RBD) soybean oil was fractionated by silicic acid column chromatography to identify minor components responsible for flavor reversion. Minor components from oil eluted with diethyl ether/n-hexane (1:1) were compared with those from corn and canola oils. All vegetable oils contain free fatty acids, diglycerides and sterols as major ingredients in this fraction. However, unusual triglycerides consisting of 10-oxo-8-octadecenoic acid and 10-and 9-hydroxy octadecanoic acids were detected in RBD and crude soybean oils.     

The relationship between the oxidative polymers of soybean oil and flavor reversion

Summary The polymers formed during the autoxidation of soybean oil at 60°C. were isolated by a solvent-extraction method, using diethyl ether and pentanehexane as solvents. The more polar polymer fraction was further oxidized by air at 30°C., and the volatile material obtained was fractionated and characterized. It contained the same carbonyl compounds as the volatile material which has previously been obtained from reverted soybean oil, namely acetaldehyde, propionaldehyde, n-hexanal, and 2-pentenal. The more polar polymer fraction contained 21.04% of oxygen and degraded to volatile carbonyl compounds under vacuum as well as under oxygen-free nitrogen. Depolymerization and ethanolysis of these polymers were carried out in ethyl alcohol which had been adjusted to a normality of 3.5 with anhydrous hydrogen chloride. The polymeric substance formed on the filter presses of a soybean oil refinery could be depolymerized in a similar manner. It is therefore evident that oxidative polymers could be unintentionally introduced into soybean oil in various ways and serve as precursors of reversion compounds. Presented at the 27th fall meeting of the American Oil Chemists’ Society, November 4, 1953, in Chicago, Ill. Funds for this study were furnished by a grant-in-aid from Armour and Company, Chicago, Ill.     

A systematic characterization of the reversion flavor of soybean oil

The volatile flavor compounds in a reverted soybean oil with a peroxide number of 4.3 meq/kg were isolated by a semicontinuous counter-current vacuum steam-distillation process, fractionated by repeated gas chromatography, and identified by infrared and mass spectrometry. A total of 71 compounds were identified, which included 19 acids, 39 nonacidic compounds, and 13 tentatively identified compounds. The acids consisted of eight normal saturated acids, nine α,β-unsaturated acids, a branch-chain acid, one hydroxy acid, two keto acids, three lactones, and one aromatic acid. The nonacidic compounds consisted of two esters, eight normal saturated aldehydes, two branched-chain aldehydes, five 2-enals, three dienals, eight ketones, eight alcohols, six hydrocarbons, and four aromatic compounds. The mechanism of formation of the identified compounds indicated that they were mostly primary or secondary autoxidation products of the hydroperoxides of the unsaturated fatty esters. Since many of the identified compounds were produced from oleic and linoleic acids, it is doubtful that linolenic acid was solely responsible for the reversion flavor. Of the compounds identified two are of unusual interest. They are 1-decyne and 2-pentyl furan. The former is the first acetylenic compound reported as the autoxidation products of unsaturated fatty esters which contained only double bonds. The latter imparts to an oil at concentrations of 5–10 ppm a beany and grassy flavor reminiscent of that of a reverted soybean oil. Since this compound is postulated as being produced by the autoxidation of linolenic acid, it is suggested that the presence of linolenic acid catalyzes the autoxidation of linoleic acid and possibly alters the decomposition pattern of its hydroperoxides.     

Reduction of green color from soybean oil

The green color in a refined bleached soybean oil extracted from green soybeans was removed substantially by partially hydrogenating the oil with 1% copper chromite catalyst at 175 C and 30 psig. Hydrogenating the same oil to the identical IV (110) with 0.1% nickel at 150 C and 15 psig was ineffective.     

Method for measuring color of soybean oil

Summary 1. The specific light absorption coefficients of concentrations of iodine (100 to 2,000 mgs. per liter) in two per cent potassium iodide solution in water at 560 milli mu are given. These absorption coefficients are proportional to the concentration. 2. The molecular extinction coefficients of two solutions of iodine (0.00788 and 0.0394 molar I₂) in two per cent potassium iodide solution in water at nine wave lengths from 460 milli mu to 640 milli mu are given. These values for both concentrations are in close agreement. 3. Within the limits studied solutions of iodine in two per cent potassium iodide solution in water obey Beer’s law. 4. A rapid, convenient and reliable colorimetric method for determining the color number of soybean oil is described. The color number is expressed as the number of milligrams of free iodine per 100 c.c. dissolved in a two per cent solution of potassium iodide in water, of which a 50 millimeter column just matches a 50 millimeter column of oil. 5. The spectral transmission curves of a solution of iodine in a two per cent solution of potassium iodide which matches the color of soybean oil, and of the oil are very similar. A paper presented at the eighth fall meeting of the American Oil Chemists’ Society in Chicago, October 11, 1934 Part of these data are from a thesis submitted by G. E. Halliday to the Faculty of the Graduate School of Purdue University in partial fulfillment of the requirements for the degree of Master of Science, August, 1934.     

Effect of soybean pretreatment on the color quality of soybean oil

Color reversion in soybean oil can be prevented by reducting the enzyme activity of soybeans before cracking and flaking. Soybean oil extracted from steamed, intact soybeans (18% moisture) had lower Rm (max. red) values in RBD oil, higher amounts of γ-tocopherol, plus its isomers, in both crude and RBD oil, and also higher amounts of hydratable phosphatides in crude oil than those in the oils from the same beans without steam treatment. For soybean pretreatments, a toasting process is less effective than the steaming process for the inhibition of color reversion of soybean oil. To prevent the occurrence of color reversion in RBD soybean oil, the amount of γ-tocopherol and γ-TED (5-[tocopheryloxy]-γ-tocopherol) should be above 550 ppm in crude oil.     

Isolation and identification of 2-pentenylfurans in the reversion flavor of soybean oil

The volatile flavor compounds in a reverted soybean oil with a peroxide number of 6.0 meq/kg were isolated by semicontinuous countercurrent vacuum steam distillation. Based upon the gas Chromatographic retention times and mass spectra of the four synthesized 2-pentenylfurans, it was found that cis- and trans-2-(l-pentenyl)furans and a mixture of cis- and trans-2-(2-pentenyl)-furans are present in reverted soybean oil. At least the greater portion of 2-(2-pentenyl)furans identified was cis-isomer. These compounds may contribute to the reversion flavor of soybean oil. Presented at the 73rd AOCS annual meeting, Toronto, 1982.     

A technique for testing the reversion properties of hydrogenated soybean oil shortenings

Summary A technique has been described which was found to yield specific and reproducible evaluations of the reversion properties of shortening samples containing hydrogenated soybean oil. It was shown to be advantagenous to conduct the test by smelling rather than by tasting. The use of a control rendered the test specific and by expanding to a series of controls of varying concentrations, it was found possible to increase the discrimination of the test. The statistical method reported by Dr. Bliss (3) was used to analyze the data obtained.     

Sunflower oil processing from crude to salad oil

World-wide use of sunflower oil is second only to soybean oil. Interest in domestic use as a premium salad oil is very recent. The high ratio of polyunsaturated-to-saturated fatty acids makes sunflower oil a premium salad oil. Sunflower oil, however, contains a small amount of high melting wax which must be removed to avoid settling problems. It is possible to produce a brilliant, dewaxed, deodorized sunflower oil with over a 100-hr cold test at 0 C. This quality oil can be produced by conventional caustic refining, dewaxing, bleaching and deodorization. A quality finished oil may also be produced by dewaxing and steam refining. This paper reviews various methods for processing sunflower oil from the crude state through the finished, dewaxed, deodorized salad oil. Presented at the ISF/AOCS Meeting, New York, April, 1980.     

The production of salad oil by fractional crystallization with solvents

A fractional crystallization method for the winterization of cottonseed oil with solvents was developed in our laboratories and pilot plant. Small amounts of solvent (e.g., 10% by weight of acetone) were mixed with the oil. This mixture was rapidly chilled to 0°C. and kept at this temperature for 3–4 hrs.; the liquid portion was separated from the solids. After solvent evaporation a salad oil of good quality was obtained. The yield is equal to or better than that obtained with conventional methods. The method is suitable for a continuous operation since rapid chilling can be used, and only a short over-all time is necessary. Great advantages of the process are the use of low amounts of solvent and of a refrigeration system not requiring temperatures below 0°C.     

Long term storage of soybean and cottonseed salad oils

Commercially prepared and packaged soybean and cottonseed salad oils from several different processors were evaluated periodically during storage for 12 months. Partially hydrogenated and winterized soybean oils, as well as unhydrogenated soybean salad oils, were stored in bottles and cans at 78 and 100 F. Control samples of all oils were held at 0 F during the entire test. Some lots in bottles and cans were packaged under nitrogen to improve storage stability. Agreement was good between organoleptic and oxidative evaluation of aged oils. After 26 weeks of storage at 100 F, the flavor of partially hydrogenated-winterized oils packaged under nitrogen showed a minimum loss. These same oils did not exhibit much, if any, reduction in their oxidative stability as indicated by storage peroxide values (active oxygen method). Soybean oil not protected with nitrogen demonstrated progressive flavor deterioration at 100 F. After 10 weeks of storage, the deterioration became marked and the flavor score was below 5. From limited observations, bottled oils appear to have a better stability than oils packaged in screw-cap tin cans. Hydrogenated oils packaged under nitrogen in cans had good oxidative stability, but some lowering of the flavor score was observed. Nonhydrogenated soybean oils packaged in tin cans not under nitrogen exhibited the most rapid flavor deterioration of all lots of oil investigated. Presented in part at the AOCS meeting, New York, October 1968 ARS, USDA