Flavor and oxidative stability of oil processed from null lipoxygenase-1 soybeans

The role played by lipoxygenase in the flavor quality of soybean oil was investigated by comparing the oil processed from special soybeans lacking lipoxygenase-1 (Forrest x P.I. 408251) with the oil from normal (Century) beans. Quality assessment was based on sensory evaluations and on capillary gas chromatographic (GC) analyses of volatiles of the extracted crude, partially processed, and refined, bleached and deodorized oils. In direct comparisons of oil products from the two types of beans, no significant differences were found in either flavor quality or in flavor stability based on total volatiles, and in analyses for 2,4-decadienal. Although thermal tempering did not significantly affect the initial flavor scores of crude and degummed oils from Century and low L-1 soybeans, the initial scores of refined and bleached oils from Century soybeans were significantly improved by this treatment. Similarly, thermal tempering was just as important in producing good quality flour from the special beans lacking lipoxygenase-1 as the flour from normal beans. Therefore, factors other than lipoxygenase-1 appear to affect the food quality of soybean oils and meals.     

Factors affecting the content of tocopherol in soybean oil

The state of soybeans prior to extraction affected the tocopherol content of crude soybean oils. Soybean flakes with a thickness of 0.16–0.33 mm had higher extracted oil yield but a slightly lower tocopherol content of the oils than did cracked beans and thicker bean flakes. Highmoisture content and long storage of soybeans resulted in lower tocopherol content in the crude oils, with moisture content being more important than storage time at decreasing the tocopherol content of oils. Soybean oil from stored beans with 15±1% moisture content led to a more significant decrease in the tocopherol content than did oil from stored beans with low (12%) or high (18%) moisture contents. Soybean flakes contaminated with oxidized oil had a significant effect on the decrease of the tocopherol content in crude oils. The high amount of phospholipids in crude soybean oil might result in a smaller decrease in the tocopherol content of oil during heating.     

Oxidative stabilities of soybean oils that lack lipoxygenases

Lipoxygenase (LOX)-null soybean lines that lack LOX 2, or LOX 2 and 3, and contain normal (8.0–8.6%) or low (2.0–2.8%) linolenate (18∶3) amounts were evaluated for their oil qualities and storage stabilities. Soybean oils of six genotypes were extracted by both laboratory-scale and pilot-plant systems and were refined, bleached, and deodorized in the laboratory. Citric acid was added to oils during the cool-down stage of deodorization. Two replications, separated at the point of conditioning, were evaluated for each genotype. Under storage conditions of 55–60°C in the dark, soybean oils with low 18∶3 contents were significantly (P<-0.05) more stable as measured by peroxide values than were oils with normal 18∶3 contents, regardless of the LOX content of the beans. The volatile analysis showed few differences between oils with low and high 18∶3 contents or among oils from beans that lack different LOX enzymes. After 16 d of storage, the amount of 1-octen-3-ol was significantly greater in oils with low 18∶3 content, and soybean oils from beans with normal LOX content had a significantly (P<-0.05) lower amount of 1-octen-3-ol than did the oils that lacked LOX enzymes. Storage at 35°C under light showed no differences in volatile amounts or sensory evaluations after 14 d of storage. During storage, peroxide values tended to be lower in oils from beans with normal 18∶3 content and in oils from beans with normal LOX content. Generally, the abscence of LOX 2 or LOX 2 and 3, although having a small effect on lipid oxidation, was not as important to oil quality as was the 18∶3 content.     

Volatile compounds in oils after deep frying or stir frying and subsequent storage

Soybean oil (900 g) was heated by deep frying at 200°C for 1 h with the addition of 0, 50, 100, 150 and 200 mL water, and then stored at 55°C for 26 weeks. Soybean oil, corn oil and lard were heated by stir frying and then stored at 55°C for 30 weeks. The volatiles and peroxide values of these samples were monitored. All samples contained aldehydes as major volatiles. During heating and storage, total volatiles increased 260-1100-fold. However, aldehyde content decreased from 62–87% to 47–67%, while volatile acid content increased from 1–6% to 12–33%; especially hexanoic acid which increased to 26–350 ppm in the oils after the storage period was completed. Water addition to the oils heated by deep frying tended to retard the formation of volatile compounds. The total amount of volatile constituents of lard heated by stir frying increased more during storage than that of corn oil or soybean oil. Peroxide values did not reflect the changes of volatile content in the samples.     

Effect of water content on volatile compounds derived from soybean oils in cans

Soybean oil-water cans with different soybean oil/water ratios were prepared and stored. The volatiles and POV values of these samples were monitored. Water in cans caused the production of larger amounts of volatiles just after sterilization, however, during the storage period, cans without water had a higher rate of production of volatile compounds. It was postulated that POV values do not reflect the changes of volatile compounds in cans.     

Analysis of vegetable oil volatiles by multiple headspace extraction

Quantitative determination of the volatiles produced from oxidized vegetable oils is an important indicator of oil quality. Five vegetable oils, low-erucic acid rapeseed, corn, soybean, sunflower and high oleic sunflower, were stored at 60°C for four and eight days to yield oils with several levels of oxidation. Peroxide values of the fresh oils ranged from 0.6 to 1.8 while those of the oxidized oils were from 1.6 to 42. Volatile analysis by the multiple headspace extraction (MHE) technique, which includes a pressure and time controlled injection onto the gas chromatography (GC) column (a chemically bonded capillary column), was compared with that obtained by static headspace gas chromatography (SHS-GC). Several volatile compounds indicative of the oxidation of polyunsaturated fatty acids from the vegetable oils were identified and measured by MHE; pure compounds of twelve major volatiles also were measured by MHE, and peak area was determined. Multiple extractions of the oil headspace provided a more reproducible measure of volatile compounds than was obtained by SHS-GC. Concentration of all volatiles increased with increased oxidation as measured by peroxide value of the oil. Presented at the Annual American Oil Chemists' Society Meeting, May 8–12, 1988, Phoenix, AZ.     

An evaluation of the oxidative and flavor stability of stored soybean oils

Results of 4-year storage tests are reported for crude and refined soybean oils held in 50-gal drums under conditions simulating field tank operations. Once-refined oils stored in filled drums without breathers showed lower peroxide values and lower dimer contents than oil stored in full drums with breathers. Refined oils in half-filled drums exhibited higher storage temperatures and, consequently, higher peroxide values and dimer contents than any other storage condition. Nondegummed and degummed crude oils held in drum storage had lower peroxide values and lower dimer contents than refined oils stored under similar conditions. Relationships are significant not only between storage peroxide values and dimer contents, but also each of these with flavor scores. Evidently, stored crude or stored refined soybean oils with peroxide values under 60 could be deodorized to produce salad-grade oils with initial flavor quality equal to that of oils processed from stocks having considerably lower initial peroxide values. The relative rate of peroxide increase for field tank storage can be estimated from linear regression analysis on data from stor-age of soybean oil in drums. Once-refined soybean oil held under large field tank storage conditions would not be expected to reach peroxide levels of 60 until after 3-4 years, even in warm areas. ARS, USDA. No. Utiliz. Res. Dev. Div.. ARS, USDA.     

Headspace volatile analysis to evaluate oxidative and thermal stability of soybean oil. Effect of hydrogenation and additives

Headspace gas chromatographic analysis of heated soybean oil was investigated as a tool to determine what effect hydrogenation and additives have on the formation of total and individual volatile components. Soybean oil was hydrogenated to varying linolenate (Ln) contents with either nickel (Ni) or copper catalysts. Oils were stabilized with citric acid (CA) or a combination of CA with tertiary butyl hydroquinone (TBHQ) and/or methyl silicone (MS). Volatiles were analyzed with a capillary gas chromatography equipped with a headspace sampler positioned on the injector. Oxidative stability was determined after storage of the oils at 60 C. To study thermal abuse and frying performance of oils, samples were heated for several, hours with prolonged bread frying. The deterioration of the oil was accelerated further by static heating in air within the headspace sampler. All hydrogenated oils produced less total volatiles than the unhydrogenated control oil after prolonged heating and bread frying. Static heating at 190 C for one hr showed that the oil hydrogenated with Ni to 0.4% Ln was the most stable. MS decreased the formation of volatiles in all samples and was particularly effective, in stabilizing the hydrogenated oils. However, MS had little effect on volatiles in the oil hydrogenated to 0.4% with Ni. Unique volatile compounds identified included 2,4-heptadiental in nonhydrogenated soybean oil and 2-nonenal in most hydrogenated oils. On heating, the amount of 2-heptanal decreased significantly in the Ni hydrogenated oils compared to the control. Hexanal, on the other hand, decreased in all hydrogenated oils compared to the control.     

Flavor evaluation of crude oil to predict the quality of soybean oil

Reliable methods for evaluation of crude oils are needed to assist processing and to improve flavor quality of finished products. The quality of crude oils from soybeans of different sources and treatments was determined by sensory evaluation and by capillary gas chromatographic (GC) analyses of volatiles. Taste panelists were specially trained in using a new technique to evaluate crude oils by dilution and comparison with freshly deodorized oils. The flavor quality of crude oils from untempered soybeans was significantly poorer than that of oils from soybeans steam-tempered at 104 C for four min. Capillary GC analyses of total volatiles and hexanal correlated well with differences in flavor quality and stability. Crude oils extracted from soybeans damaged by storage at 45 C and 13% moisture received decreasing flavor scores with prolonged storage time. Similarlly, hexanal and total volatile contents increased with storage times. Commercial crude oils from several geographic locations showed a wide range in flavor scores. However, flavor scores of crude oils showed good agreement with flavor stabilities (decrease in flavor scores after storage at 60 C) of the corresponding oils after refining, bleaching, and deodorization. Therefore, the combined use, of sensory evaluations and GC-volatile analyses of crude oils can provide convenient, rapid, sensitive and reliable screening methods to assist in improving the quality of finished soybean oils by controlling soybean storage and processing.     

Quantification of carbonyl compounds in oxidized low-linolenate,high-stearate and common soybean oils

The carbonyl compounds in five oxidized soybean oils (SBO) of various fatty acid compositions were determined. Three were from common normal soybean varieties, and two were from lines developed from new mutant varieties. One mutant line had a linolenate (18:3) content of 3.5% (A5), and one had a stearate (18:0) content of 24% (A6). SBO were stored at 28 C and 60 C. Trichlorophenylhydrazones (TCPH) of carbonyls formed during oxidation were quantified and tentatively identified by gas chromatography. The storage temperature and the composition of the oils affected the types and amounts of volatiles produced. Hexanal was the major volatile in the oils in both storage tests. After 60 C storage, 2- and/or 3-hexenal was present only in the oil with the highest 18:3 content (BSR 101, 18:3=9%). The amounts of the carbonyls formed in A5 were 2 to 5 times less than the amounts formed in BSR 101. The amounts of many of the carbonyls were converted into relative flavor potency by using reported data. Hexanal was the major contributor to flavor. After storage at 28 C, 2- and/or 3-hexenal was the second most intense flavor compound regardless of the 18:3 content of the oil. The amount of a compound and the threshold value did not always predict its flavor importance according to the flavor potency data.     

High-temperature stabilities of oils from soybeans that lack lipoxygenases

Oils from normal or low-linolenic acid (18:3) soybeans that lack lipoxygenase (LOX) 2 or LOX 2 plus LOX 3 activities were evaluated for their stability during frying and for oxidative stability in bread cubes stored after frying. Soybean oils were extracted by a pilot-plant system and were refined, bleached, and deodorized in the laboratory. Citric acid was added to oils during the cool-down stage of deodorization. Two replications, separated at the point of conditioning, were evaluated for each genotype. Each sample (250 g) was heated to 180±5°C in a minifryer. Bread cubes were fried at the beginning of heating and after 20 h of heating. Heating of the oils was continued for 10 h each day for three consecutive days. Soybean oils with low 18:3 contents were significantly (P ≤ 0.05) more stable, as measured by conjugated dienoic acids and polymer values, than were oils with normal 18:3 contents. Low-LOX 2 or low-LOX 2 + 3 activity had no effect on peroxide values of soybean oils extracted from bread cubes. Sensory evaluation did not differentiate between oils that contained low or high 18:3 amounts or among oils from beans that lacked different LOX enzymes.     

Detection of adulteration of sesame and peanut oils via volatiles by GC × GC–TOF/MS coupled with principal components analysis and cluster analysis

The method of headspace coupled with comprehensive two‐dimensional GC–time‐of‐flight MS (HS‐GC × GC–TOF/MS) was applied to differentiate the volatile flavor compounds of three types of pure vegetable oils (sesame oils, peanut oils, and soybean oils) and two types of adulterated oils (sesame oils and peanut oils adulterated with soybean oils). Thirty common volatiles, 14 particular flavors and two particular flavors were identified from the three types of pure oils, from the sesame oils, and from the soybean oils, respectively. Thirty‐one potential markers (variables), which are crucial to the forming of different vegetable oil flavors, were selected from volatiles in different pure and adulterated oils, and they were analyzed using the principal component analysis (PCA) and cluster analysis (CA) approaches. The samples of three types of pure vegetable oil were completely classified using the PCA and CA. In addition, minimum adulteration levels of 5 and 10% can be differentiated in the adulteration of peanut oils and sesame oils with soybean oils, respectively. Practical applications: The objective was to develop one kind of potential differentiated method to distinguish high cost vegetable oils from lower grade and cheaper oils of poorer quality such as soybean oils. The test result in this article is satisfactory in discriminating adulterated oils from pure vegetable oils, and the test method is proved to be effective in analyzing different compounds. Furthermore, the method can also be used to detect other adulterants such as hazelnut oil and rapeseed oil. The method is an important technical support for public health against profit‐driven illegal activities.     

Oxidative stabilities of soybean oils with elevated palmitate and reduced linolenate contents

Oils from soybean lines, developed to contain different amounts of palmitate (16:0) and linolenate (18:3), were evaluated for oxidative stability. Oils were extracted in the laboratory from the soybean seeds and refined, bleached, and deodorized. Two replications, separated at the point of conditioning, were evaluated for each genotype, including Hardin 91 (normal beans), P9322 (10.6% 16:0 and <2.6% 18:3), A91-282036 (26.3% 16:0 and 9.8% 18:3), and HPLL (23.2% 16:0 and 3.5 % 18:3). Elevating 16:0 and/or lowering 18:3 increased the oxidative stability of soybean oils as measured by peroxide values. Soybean oils with elevated 16:0 had higher solidification temperatures than did oils with normal 16:0 content, and soybean oils with low 18:3 content had higher solidification temperatures than did oils with normal 18:3 contents.     

A methodology study to evaluate quality of soybeans stored at different moisture levels

The quality of soybeans and oil extracted from seeds stored at different moisture contents was evaluated by static headspace gas chromatography, near-infrared spectrometry, fluorescence measurements, and silicic acid chromatography. Headspace gas chromatographic analysis of both ground beans and crude oils provided a sensitive measure of oxidative deterioration based on hexanal and total volatiles. Near-infrared analyses at 2260 nm showed a correlation coefficient of 0.864 with titratable free fatty acids. Fluorescence measurements on chloroform-methanol extracts were much less sensitive and showed an increase only in the most damaged samples. Silicic acid chromatography of crude oils showed a significant decrease of polar lipids and increase of less polar lipids with storage at high moisture levels, in agreement with the decrease in phosphorus observed. Among the methods tested, headspace gas chromatography is most sensitive to evaluate oxidative deterioration, and near-infrared analysis is most suitable and rapid to evaluate hydrolytic deterioration in stored soybeans. This methodology can be used to evaluate factors affecting the food quality of soybeans for domestic and foreign markets.     

Effects of Deuterium Oxide on the Oxidative Stability and Change of Headspace Volatiles of Corn Oil

Effects of deuterium oxide and deuterium oxide-free water on the oxidative stability and formation of headspace volatiles were determined for corn oils to evaluate the role of moisture as an active influential factors during lipid oxidation. Mixtures of corn oil and water with different ratios of deuterium oxide were prepared, and the mixtures were stored at 60 °C for 2 days. Headspace oxygen contents, conjugated dienoic acid (CDA) values, and p-anisidine values (p-AV) were analyzed as a measure of oxidative stability, and headspace volatiles were analyzed by solid phase microextraction and a gas chromatography mass selective detector to determine the involvement of deuterium in volatiles. Deuterium oxide accelerated the rate of lipid oxidation in corn oil compared to oils with deuterium-free water based on the results of headspace oxygen content, CDA, p-AV, and total volatile content. Fragmented mass to charge ratios (m/z) of 73.1/72.1 for d ₁-pentane/pentane and 57.0/56.0 for d ₁-2-propenal/2-propenal from samples containing deuterium oxide were significantly higher than those from deuterium oxide-free water, implying that moisture participated to form volatiles in corn oil oxidation under air-tight condition. Deuterium oxide appeared to accelerate the rate of lipid oxidation in corn oils and participated to form volatiles from oils during oxidation.     

Iron and phosphorus contents of soybean oil from normal and damaged beans

Analyses of commercial crude soybean oils showed a highly significant correlation of 0.74 between free fatty acid and iron content. Poor flavor characteristics exhibited by finished oils extracted from damaged beans may be caused in part by a higher free fatty acid and related higher iron content in crude oils. Source of the increased iron appears to be both damaged beams and steel processing equipment. Crude oil from damaged beans is 2–10 times higher in iron than crude oil extracted from sound beans. Iron appears loosely bound in soybeans, since autoclaving, spontaneous heating in storage, or treating with alcohol increased the level of iron in laboratory extracted crude oil from 0.2 to more than 1 ppm. Present data do not indicate that iron and phosphorus contents are associated statistically in extracted oils.     

Dynamic headspace capillary gas chromatographic analysis of soybean oil volatiles

To develop new knowledge on preventing or eliminating the formation of undesirable flavors in soybean oil, we analyzed quantitatively the major volatile products in samples that were oxidized during storage in the dark at ambient conditions. The volatiles formed were recovered and separated by dynamic headspace capillary gas chromatography. The effect of sampling temperatures was investigated by heating the sample, sweeping the volatiles with helium and trapping and desorbing them from a porous polymer Tenax trap. The volatiles were flushed from the trap onto a fused silica capillary column with a bonded mixed dimethyldiphenyl siloxane phase. At peroxide values between 2 and 13, the major volatile products found were acrolein, pentene, pentane, 1-penten-3-ol, pentanal, hexanal, 2-hexenal, 2-heptenal, 2,4-heptadienal, 2-octenal and 2,4-decadienal. The profile of volatiles was significantly affected by the sampling temperature used and by the presence or absence of citric acid in the oils before storage. The relative amounts of volatile thermal decomposition products of linolenate and linoleate hydroperoxides, such as 2,4-heptadienal and 2,4-decadienal, increased significantly when samples were heated above 90 C. Dynamic headspace gas chromatography made it possible to analyze the volatiles in samples heated to 60 and 90 C. These volatiles may be representative of those present in oils at time of tasting.     

Storage stability of potato chips fried in genetically modified canola oils

The storage stability of potato chips fried in regular (RCO), hydrogenated (HYCO), low-linolenic (LLCO), and high-oleic (HOCO) canola oils was compared. Potato chips were fried in each oil over a 5-d period for a total of 40 h of frying. Chips from frying day 1 and 5 were packaged and stored at 60°C for 0, 1, 2, 4, 8, and 16 d. Lipids were extracted from the stored chips and analyzed for peroxide values, free fatty acids (FFA), conjugated dienoic acids (CDA), and polar components. A trained sensory panel evaluated the stored chips for odors characteristic of oxidation. Chips were also analyzed for volatile components. Potato chips fried in RCO, LLCO and HOCO developed an intense painty odor, whereas chips fried in HYCO developed an intense stale/musty odor by the end of the 16 d of storage. Chips fried in RCO had greater rates of accumulation of peroxides, FFA, CDA, and polar components and developed higher levels of total volatiles over the 16 d of storage than chips fried in the other three oils. Chips fried in HYCO had lower rates of accumulation of peroxides and CDA than chips fried in LLCO and HOCO, and lower rates of FFA accumulation than chips fried in LLCO. Chips fried in HYCO and HOCO had the lowest amounts of total volatiles during storage. The effect of oil degradation products on potato chip storage stability was not shown in this study since only the chips fried in HYCO from frying day 5 exhibited a significantly greater rate of off-odor development than chips from frying day 1, and only the chips fried in LLCO from frying day 5 had a greater rate of accumulation of volatiles than chips from frying day 1.     

Formation of headspace volatiles by thermal decomposition of oxidized fish oilsvs. oxidized vegetable oils

To understand the reasons for differences in oxidative stability among edible oils, the temperature dependence was investigated for the development of volatile lipid oxidation products in fish oils and in vegetable oils. A rapid headspace capillary gas chromatographic method was developed to determine volatile oxidation products of omega-6 (n-6) polyunsaturated fats (pentane and hexanal) and omega-3 (n-3) polyunsaturated fats (propanal) at different decomposition temperatures. Headspace gas chromatographic analyses of partially oxidized menhaden, bonita and sardine oils could be performed at 40°C, whereas soybean, canola, safflower, high-oleic sunflower and high-oleic safflower oils required temperatures greater than 100°C. Volatile formation by thermal decomposition of oxidized oils had lower apparent activation energies in fish oils than in vegetable oils, and significantly higher apparent activation energies in high-oleic oils than in polyunsaturated oils. The activation energy data on headspace volatiles provided another dimension toward a better understanding of the thermal stability of flavor precursors in unsaturated fish and vegetable oils. Presented at the ISF/AOCS joint meeting, Toronto, Canada, May 10–14, 1992.     

Oxidative stabilities of low-linolenate,high-stearate and common soybean oils

It is generally agreed that the high linolenate (18:3) content of soybean oil (SBO) contributes to its flavor instability. In this study, the oxidative stability of five SBO of various fatty acid (FA) compositions was compared by using peroxide values, conjugated dienoic acid values and sensory panel scores. Three of the oils were from common commercial varieties representing the range of 18:3 content normally found in SBO. The other two oils were from seed developed in a mutation breeding program. One of these oils from the line A5 had an 18:3 content of 3.5%, and the other from the line A6 had a stearate (18:0) content of 24%. Seed from the five soybean varieties was cold pressed, refined and deodorized without additives under laboratory conditions. Two oxidation experiments were conducted. In the first, the oils were stored at 28 C for 67 days. In the second, the oils were stored at 60 C for eight days. Sensory comparisons were done by using the AOCS Flavor Intensity Scale. The A5 and A6 oils were more stable than the commercial varieties as measured by chemical tests, but the sensory data were inconclusive. Oils with similar 18:3 contents did not have similar rates of oxidation. The differences between the oils were not as distinct in the 60 C test as in the 28 C test.