Rancidity of oils

Propyl gallate has become widely used as an antioxidant in cosmetics, perfumes, foods, and pharmaceuticals since its use in preventing autoxidation of oils was first described in 1943. It is primarily used, in concentrations up to 0.1% w/v, to prevent the rancidity of oils and fats it may also be used at concentrations of 0.002% w/v to prevent peroxide formation in ether, and at 0.01% w/v to prevent the oxidation of paraldehyde. Synergistic effects with other antioxidants such as butylated hydroxyanisole and butylated hydroxytoluene have been reported. Propyl gallate is also said to possess some antimicrobial properties see Section 10. 

In addition to the numerous beneficial effects of n-3 fatty acids on several chronic diseases, effects on SLE patients are also encouraging, but progress in this area is slow due to several uncertainties (109-121). Although many earlier studies do show some benefit on SLE, some of the reasons favorable results have not been seen consistently include variability in the quantity and quality of fish oil used, variations in the oils used as controls, variability in patient selection, variabihty in the amount of antioxidants used to prevent rancidity of oils. 

Maleic acid is used in the pharmaceutical industry as a pH modifier and a buffering agent. It is also used to prevent rancidity of oils and fats. 

Nitrobenzene, CgH5N02, also known as oil of mirbane, is an artificially prepared benzene derivative, having a coarse, but powerful odour resembling that of oil of almonds. Its coarse odour renders it quite unsuitable for fine perfumery, but it finds considerable employment in the manufacture of cheap soaps, polishes, and other articles where more or less rancid fats are used, as it covers the bad odour of the fat and gives the product a coarse almond perfume. 

Powdered aluminium had been added to oleic acid. The mixture detonated after being prepared. Such an accident could not be repeated and it was thought that it was caused by the presence of a peroxide formed by the effect of air on oleic acid. In fact, the acid functional group has obviously nothing to do with the peroxidation. It is more likely that the chain s double bond that activates p hydrogen atoms (ally position) was involved in it. This is a well-known phenomenon since it is responsible for the rancidity of some oils and greases. 

When oils are used as vehicles in ophthalmic fluids, they must be of the highest purity. Vegetable oils such as olive oil, castor oil, and sesame oil have been used for extemporaneous compounding. These oils are subject to rancidity and, therefore, must be used carefully. Some commercial oils, such as peanut oil, contain stabilizers that could be irritating. The purest grade of oil, such as that used for parenteral products, would be advisable for ophthalmics. 

The assay of fixed oils provides a special case of ester hydrolysis since they are triesters of glycerol. The saponification value for a fixed oil is the number of mg of KOH equivalent to 1 g of oil. A high value means rancidity, a low value possible adulteration with mineral oil. Almost all edible oils have a saponification value between 188 and 196. Hydrolysis of the fixed oil is carried out with ethanolic KOH. 

Malonaldehyde has been detected in the leaves of pea and cotton plants. It is found in many foodstuffs and can be present at high levels in rancid foods. It has been detected in fish meat, fish oil, rancid salmon oil, rancid nuts, rancid flour, orange juice essence, vegetable oils, fats, fresh frozen green beans, milk, milk fat, lye bread and in raw, cured and cooked meats (United States National Library of Medicine, 1997). 

Storage of fats and oils leads to rancidity, a largely oxidative deterioration that causes development of unpleasant tastes, odors, and toxic compounds.239 Similar chemical changes account for the "drying" of oil-based paints and varnishes. These reactions occur... 

Rancidity in food, measurement of oil stability index, 541 -545 secondary lipid oxidation, 531-539 Rapeseed oil, properties and composition of, 474 (table)... 

Normally, low levels of oil can be added to diets, otherwise the diets become soft, difficult to pellet and subject to rancidity. All of the oils are unstable and subject to rancidity therefore, they should be used rapidly after delivery and not stored for extended periods. Purchased oils may be stabilized by the addition of an antioxidant which should meet organic standards. 

Iodometry on peroxidized olive oils gives the peroxide value (PV), which is expressed as the milliequivalent of active oxygen in the peroxides present in 1 kg of olive oil. Experienced oil analysts can relate PV values with rancidity and oil stability. Typical reports are that a fresh genuine oil has a PV of 2-5, a fresh deodorized oil a PV near to zero and that PV values of 10 or more are for oils prone to produce off-flavour volatiles. 

Uitrasound irradiation of an edibie oii (particuiariy direct irradiation of the sample) causes fast oxidation of biophenois present and produces a rancid smell as a result, mainly with easiiy oxidized oiis such as those from sunfiower [73]. Ultrasound energy allows Rancimat times from 129 h to be reduced to 50 min therefore, the overall time required for the determination of oil stability, even for highly stable virgin olive oils, is less than 1 h. 

Peroxide value. The oxidation of oils and fats leads to the formation of hydroperoxides. The hydroperoxides readily decompose to produce aldehydes, ketones, and other volatile products, which are characteristic of oxidation rancidity. The method for determination of peroxide concentration is based on the reduction of the hydroperoxide group with HI (or KI) to liberate free iodine, which may be titrated. The... 

The development of a characteristic, objectionable, beany, grassy, and hay-like flavor in soybean oil, commonly known as reversion flavor, is a classic problem of the food industry. Soybean oil tends to develop this objectionable flavor when its peroxide value is still as low as a few meq/kg, whereas other vegetable oils, such as cottonseed, com, and sunflower, do not (15, 51). Smouse and Chang (52) identified 71 compounds in the volatiles of a typical reverted-but-not-rancid soybean oil. They reported that 2-pentylfuran formed from the autoxidation of linoleic acid, which is the major fatty acid of soybean oil, and contributes significantly to the beany and grassy flavor of soybean oil. Other compounds identified in the reverted soybean oil also have fatty acids as their precursors. For example, the green bean flavor is caused by c/i-3-hexenal, which is formed by the autoxidation of linolenic acid that usually constitutes 2-11% in soybean oil. Linoleic acid oxidized to l-octen-3-ol, which is characterized by its mushroom-like flavor (53). 

Recent advances in the technology of multisensor arrays and neural computing have made the development of the electronic nose of great interest to the food industry for discrimination between odors (26). Provided the instrument has been calibrated properly, the technique is rapid, nondestructive, and objective. Shen et al. (27) found the electronic nose was capable of measuring changes in volatile compounds associated with lipid oxidation in canola, com, and soybean oils stored under accelerated conditions and Aparicio et al. (28) found the electronic nose could be calibrated to detect rancidity levels in good quahty ohve oil spiked with rancid olive oil. 

Peroxide Value. Oxidation of oils is a major cause of their deterioration. Hydroperoxides are the primary products formed by the reaction between oxygen and the unsaturated fatty acids. Hydroperoxides have no flavor or odor but break down rapidly to form aldehydes, which have a strong, disagreeable flavor and odor. The peroxide concentration, usually expressed as peroxide value (PV), is a measure of oxidation or rancidity in its early stages. PV measures the concentration of substances, in terms of milliequivalents of peroxide per 1000 grams of sample, that oxidize potassium iodide to iodine. AOCS Method Cd 8-53 (103) is the official method for peroxide value determinations. 

Oxidation. Oxidation of oils and fats is due to prolonged exposure to air. By virtue of the low polyunsaturated fatty acid content, palm oil is relatively more stable to oxidative deterioration than the polyunsaturated vegetable oils. However, in the presence of trace metals such as iron and copper, excessive oxidation at the olefin bonds of the oleic and linoleic acids can occur, resulting in rancidity. Highly oxidized crude palm oil is known to have poor bleachability and thus requires more bleaching earth and more severe refining conditions, and the final product will likely be of poor stability (44, 45, 68). 

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... 

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