Oxidative Quality of Soybean Oil

Soybean oil is a polyunsaturated or linoleic type of oil that is highly susceptible to lipid oxidation. The rate of lipid oxidation depends primarily on the fatty acid composition and only secondarily on the stereospecific distribution of the fatty acyl groups, as described earlier. The mechanism of lipid oxidation and lipid hydroperoxide breakdown has been discussed thoroughly by Frankel (1998). 

Oxidative instability limits the use of soybean oil in certain applications, but hydrogenation and other means of composition modification have made soybean oils the most widely used of all vegetable oils. The following analytical methods are frequently used to quantify oxidation of soybean oil. 

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The quality changes, such as lipid oxidation and reduction of tocopherols and phytosteols during neutralization, are considerable compared with the other processing steps as shown by Wang and Johnson (174), and also as presented in Table 12. The further phospholipid removal (below 2 ppm phosphorus) also reduces the oxidative stability of soybean oil (175) due to the antioxidant property of these phospholipids. 

The last process that can change the flavor, odor, color, nutritional attributes, and the oxidative stability of soybean oil is deodorization. If the conditions for extraction and crude oil processing are satisfactory, the result is a tasteless, odorless, light-colored oil, free from peroxides and other contaminants. Storage, handling, and packaging of the purified oil must protect the achieved quality prior to use by the consumer or use in... 

Soybean oil has a high linoleic acid content of around 50.8% and 7% of linolenic acid and is thus susceptible to oxidation. Adverse effects caused by oxidation of vegetable oils include loss of essential fatty acids, production of off-flavors and toxic compounds (Sonntag, 1979). The oxidative stability becomes an important quality control parameter for the manufactures and users of vegetable oils. Although the oxidative stability of soybean oil has been greatly improved through the concerted efforts of universities, industries... 

Quality of soybean oil is affected by a number of factors such as quality of soybeans, content of moisture, storage temperature and processing conditions, exposure to air, prooxidants, light, minor compounds such as free fatty acids, mono- and diacylglycerols, phospholipids, oxidized triacylglycerols, tocopherols, triterpene alcohols, phytosterols, isoflavonones, and possibly some unknown compounds contained in soybean oil. Some of these minor compounds affect the oxidative stability of vegetable oils. 

The overall oil quality change during refining of soybean oil was examined by Jung et al. (178), and their results are shown in Table 13. A study of oxidative... 

Regardless of the official specifications for soybean oil and its products, the ultimate proof of the pudding is in the eating that is, sensory evaluation of the odors and flavors of soybean oil and its products is the ultimate method to assess oil quality and stability. Sensory evaluation cannot be replaced fully by any chemical or instrumental analysis, although some methods can correlate fairly well with sensory results. Sensory evaluation of oils usually is done by a panel of experts or a trained panel, and often the method recommended by the American Oil Chemists Society (300) is used. During the evaluation, the panel is asked to score the overall flavor quality, as well as the intensity of many individual off-fiavors. Although chemical and physical tests are more reproducible and less time consuming than sensory evaluations, oxidative rancidity and off-flavor evaluation of soybean oils are best done by sensory tests. Correlations established between sensory evaluation scores and... 

The non-TAG portion of soybean oils includes phospholipids, free fatty acid, chlorophyll pigment, oxidation products, and other unsaponifiable components such as tocopherols, sterols and hydrocarbons. Some of these minor components negatively affect oil quality, while some may play a positive role in nutrition and function. The goal of refining is therefore to remove the undesirable components and, at the same time, to maximize retention of the beneficial components. 

Soybean deodorizer distillate (SBDD) is the material collected from the steam distillation of soybean oil. It is a mixture of free fatty acids (particularly during physical refining), tocopherols, phytosterols and their esters, hydrocarbons, and secondary lipid oxidation products. The quality and composition of SBDD depends on feedstock oil composition and on processing conditions. Tocopherols and sterols are valuable components that can be further separated from the distillate and used in the nutrition-supplement and pharmaceutical industries (Pickard et al. 1996). Table 2.13 shows the composition of deodorizer distillates... 

Wang and Johnson (2001) reported on test measurement methods that were major indicators of soybean oil quality. These tests included peroxide value, anisidine value, FFA content, phospholipid content, total tocopherol content, oxidative stability index, color, and moisture content. For soybean meal, they reported on urease activity, protein dispersibility index (PDI), rumen bypass or rumen undegradable protein, trypsin inhibitor activity, moisture content, residual oil content, protein content, fiber content, color, amino acid profiles, and protein solubility under alkaline (KOH) conditions. 

Toasting of soybeans by heating at 100-1 lO C to inactivate lipoxygenase and antinutritional factors (e.g. trypsin inhibitor) is important to improve the quality of soybean protein products used for either animal feed or human consumption. However, this thermal enzyme inactivation is not carried out in the conventional processing of soybean oil. Therefore, the soybean flakes must be solvent extracted without delay to minimize free fatty acid and peroxide formation in the extracted crude oil and to produce a finished oil of improved oxidative and flavor stability. 

Studies evaluated the correlation between sensory parameters and the oxidative stability indices of soybean oils [4]. Furthermore were carried out comparative studies on the oxidative stability of edible oils evaluating their stability by DSC and OSI [14]. On the other hand investigated the thermal stability of high oleic acid vegetable oils with antioxidants [6]. Was evaluated further the influence of the methods of extraction on the yield and quality of Brazil nut oil [23]. 

Polyunsaturated fatty acids in vegetable oils, particularly finolenic esters in soybean oil, are especially sensitive to oxidation. Even a slight degree of oxidation, commonly referred to as flavor reversion, results in undesirable flavors, eg, beany, grassy, painty, or fishy. Oxidation is controlled by the exclusion of metal contaminants, eg, iron and copper addition of metal inactivators such as citric acid minimum exposure to air, protection from light, and selective hydrogenation to decrease the finolenate content to ca 3% (74). Careful quality control is essential for the production of acceptable edible soybean oil products (75). 

The raw materials for the manufacture of soap, the alkali salts of saturated and unsaturated C10-C20 carboxylic acids, are natural fats and fatty oils, especially tallow oil and other animal fats (lard), coconut oil, palm kernel oil, peanut oil, and even olive oil. In addition, the tall oil fatty acids, which are obtained in the kraft pulping process, are used for soap production. A typical formulation of fats for the manufacture of soap contains 80-90% tallow oil and 10-20% coconut oil [2]. For the manufacture of soft soaps, the potassium salts of fatty acids are used, as are linseed oil, soybean oil, and cottonseed oil acids. High-quality soap can only be produced by high-quality fats, independent of the soap being produced by saponification of the natural fat with caustic soda solution or by neutralization of distilled fatty acids, obtained by hydrolysis of fats, with soda or caustic soda solutions. Fatty acids produced by paraffin wax oxidation are of inferior quality due to a high content of unwanted byproducts. Therefore in industrially developed countries these fatty acids are not used for the manufacture of soap. This now seems to be true as well for the developing countries. 

Cottonseed oil has long sold at a slight premium over soybean oil because of greater stability to oxidation, and desirable flavor in fried snack foods such as potato chips. However, gossypol content, and lower protein quality put the meal at a price disadvantage. Feeding whole cottonseed to dairy cattle, whose rumen microorganisms can detoxify... 

Peroxide value, expressed as milliequivalents of peroxide per kilogram of oil, measures the primary oxidation products of oils— the hydroperoxides. The peroxide value has shown a particularly good correlation with sensory flavor scores of soybean oU, and its use during storage is quite common. The peroxide value is an index to the oxidative state of an oU. Soybean oU is considered fresh with a peroxide value <1.0 mEq/kg, to have low oxidation with 1.0-5.0 mEq/kg, to have moderate oxidation at 5.0-10.0 mEq/kg, to have high oxidation at > 10.0 mEq/kg, and to have poor flavor quality at >20 mEq/kg (6). Several methods (300-303) can be used to measure the peroxide value of an oil depending on the specific circumstance. 

As aldehydes and some ketones have long been identified as oxidation and breakdown products of fats, their determination also has been common in soybean oil quality control. The p-anisidine value (300) measures light absorbance of aldehydes, primarily 2-alkenals, and 2,4-dienals at 350 nm. However, this measure is not entirely specific, because the color intensity developed depends not only on the concentration but also on the structure of the aldehyde. Therefore, the results are comparable only within oils of similar type and treatment (304). 

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