The processes of degumming, alkali refining, bleaching and deodorization removed 99.8% phospholipids, 90.7% iron, 100% chlorophyll, 97.3% free fatty acids and 31.8% tocopherols from crude soybean oil. The correlation coefficient between the removals of phosphorus and iron in soybean oil during processing was r = 0.99. The relative ratios of α-, β -, γ- and δ-tocopherols in crude oil, degummed oil, refined oil, bleached oil and deodorized soybean oil were almost constant, γ- and δ -tocopherols represented more than 94% of tocopherols in soybean oil. The order of oxidation stability of oil is crude > deodorized > degummed > refined > bleached oil.

Effects of processing steps on the contents of minor compounds and oxidation of soybean oil

Three samples each of soybean, sunflower and low erucic acid rapeseed (LEAR) oils were evaluated for flavor and oxidative stability. The commercially refined and bleached oils were deodorized under identical conditions. No significant differences were noted in initial flavor quality. After storage at 25°C or 60°C in the dark, soybean oils—with or without citric acid—were more stable than either sunflower or LEAR oils. However, in the presence of citric acid, soybean oils were significantly less stable to light exposure than either LEAR or sunflower oils. In contrast, in the absence of citric acid, soybean oils were significantly more light stable than LEAR oils. In either the presence or absence of citric acid, sunflower oil was significantly more stable to light than soybean oil. Analyses by static headspace gas chromatography showed no significant differences in formation of total volatile compounds between soybean and LEAR oils. However, both oils developed significantly less total volatiles than the sunflower oils. Each oil type varied in flavor and oxidative stability depending on the oxidation method (light vs dark storage, absence vs presence of citric acid, 100°C vs 60°C).

Flavor and oxidative stability of soybean, sunflower and low erucic acid rapeseed oils

Dry-milled corn germ was extracted with supercritical carbon dioxide (SC-CO2) at 5,000-8,000 psi and 50°C. CO2-extracted oil was lower in free fatty acids and refining loss, and was lighter in color when compared with a commercial expeller-milled crude oil. Total unsaponifiable and tocopherol contents were similar for both oil types. The defatted, highly friable flour has a shelf-stable moisture content of 2-3% and good flavor quality. The flour contains 20% protein with good amino acid balance, meeting FAO specifications for food protein supplements. High pressure SC-CO2 extraction also denatures the proteins, including oxidative enzymes. Peroxidase activity is reduced tenfold in SC-CO2-extracted flour when compared with hexane-extracted flours. Storage tests for 5 wk at 38°C and for 2 months at 25°C show that flavor quality of untoasted SC-CO2-defatted germ flour is maintained even under these extreme conditions.

Supercritical Fluid Extraction of Dry‐Milled Corn Germ with Carbon Dioxide

Animal fats and partially hydrogenated vegetable oils (PHVO) have preferentially been used for deepfrying of food because of their relatively high oxidative stability compared to natural vegetable oils. However, animal fats and PHVO are abundant sources of saturated fatty acids and trans fatty acids, respectively, both of which are detrimental to human health. Canola (Brassica napus) is the primary oilseed crop currently grown in Australia. Canola quality Indian mustard (Brassica juncea) is also being developed for cultivation in hot and low-rainfall areas of the country where canola does not perform well. A major impediment to using these oils for deep-frying is their relatively high susceptibility to oxidation, and so any processing interventions that would improve the oxidative stability would increase their prospects of use in commercial deep-frying. The oxidative stability of both B. napus and B. juncea crude oils can be improved dramatically by roasting the seeds (165 7C, 5 min) prior to oil extraction. Roasting did not alter the fatty acid composition or the tocopherol content of the oils. The enhanced oxidative stability of the oil, solventextracted from roasted seeds, is probably due to 2,6-dimethoxy-4-vinylphenol produced by thermal decarboxylation of the sinapic acid naturally occurring in the canola seed.

Seed roasting improves the oxidative stability of canola (B. napus) and mustard (B. juncea) seed oils

Dry-milled corn germ, soybean and cottonseed flakes were extracted (at 70-90 C and 12,000 psi) with supercritical carbon dioxide (SC-CO2) to yield crude oils. Oxidative stability of the crude oils was determined and compared to similar products obtained by conventional expeller and/or prepress solvent extraction. Under Schall oven storage conditions (60 C), SC-CO2-extracted oils undergo rapid deterioration and fail to show the normal induction period observed with conventional expeller and solvent-extracted crude oils. The levels of tocopherols found in SC-CO2-extracted oils are comparable to those obtained by expeller or solvent extraction, while phospholipids present in significant amounts in conventional crude oils are essentially absent from SC-CO2-processed crudes. The addition of phosphatides to SC-CO2-extracted crude oils improves oxidative stability, which suggests that both tocopherols and phospholipids are required to stabilize crude oils against autoxidation. Heating of SC-CO2-extracted crude oils to deodorization temperatures improves oxidative stability. The destruction of fat hydroperoxides under these conditions probably accounts for improved oxidative stability. A combination of heat and the addition of citric acid and phenolic antioxidants resulted in further improvement of oxidative stability.

Oxidative stability of seed oils extracted with supercritical carbon dioxide.

The oxidative stability of partially processed soybean oil has been investigated in laboratory-and plant-scale tests. Oxidation, as measured by peroxide formation, has been related to storage time and temperature, and availability of oxygen. The effects of this oxidation on finished product quality have been measured. Soybean oil stores best as crude oil. After refining and bleaching, the oil is more susceptible to oxidation than at other stages in processing. Increase in peroxide content is both time and temperature dependent. Substantial improvement in product quality can be obtained by minimizing exposure to oxygen.

L. H. Going

Papers

Oxidative deterioration of partially processed soybean oil