Vegetable products peroxide value

The thiocyanate method involves measurement of the peroxide value using linoleic acid as substrate and has also been widely used to measure the antioxidant activity in plant-based foods such as ginger extracts (Kikuzaki and Nakatani 1993), fruit peels (Larrauri and others 1996 1997), extracts from vegetable by-products (Larrosa and others 2002 Llorach and others 2003 Abas and others 2006 Peschel and others 2006), blueberry juice, wines, and vinegars (Su and Chien 2007). 

Active Oxygen Method for Fat Stability (AOM) (Cd 12-57) determines the time (in hours) for a sample of fat or oil to attain a predetermined peroxide value (PV) under the conditions of the test. The method is used to estimate the comparative oxidative stability of fats and oils. The method has been placed in surplus, in favor of Cd 12b-92 (Oil Stability Index), but retains official status and is still used in domestic industry. p-Anisidine Value (AV) (Cd 18-90) determines the amount of aldehydes (principally 2-alkenals and 2,4-dienals) in animal and vegetable fats and oils. These are degradation products of peroxides, which are not removed by bleaching. Some fats and oils chemists propose increased use of this method in purchase specifications. Bleaching Test for Soybean Oil (Cc 8e-s63) determines the color of a sample of soybean oil after treatment with a specified bleaching earth. Specific methods exist for other oil species. 

Peroxide Value, Fats and Oils (PV) (Cd 8-53) determines all substances, in terms of milliequivalents of peroxide per 1000 g of sample, that oxidize potassium iodide (KI). These substances generally are assumed to be peroxides or products of fat oxidation. Phosphorus in Oils (Ca 13-55) estimates the phospholipid content of crude, degummed, and refined vegetable oils in terms of phosphorus. Refineries often use induction coupled plasma (ICP) spectrographs to analyze divalent cations rapidly in aspirated crude oil. The calcium and magnesium measured are mainly responsible for nonhydratable phosphatides (NHP) and are determined directly. An AOCS method for analysis by ICP is being developed. 

In addition to the USDA standards, The Named Vegetable Oil Standard from the Codex Alimentarius Committee on Fats and Oils (Codex Alimentarius, 2006) identifies quality characteristics that oils must meet for international trade. For example, the standard states that an oil should be characteristic of the designated product and be free of foreign and rancid odor and taste. Maximum levels of matter volatile at 105°C (ISO 662 1998) (ISO, 2005) should be 0.2% m/m, insoluble purities (ISO 663 1998) (ISO, 2005) 0.05% m/m, and 0.005% m/m soap content [AOCS Cel7-95 (97)] (AOCS, 2005). Limits for metals are 1.5 mg/kg iron (Fe) in refined oils, whereas only 0.1 mg/kg of copper (Cu) is allowed in refined oils. For methods to measure Fe and Cu, use ISO 8294 1994 (ISO, 2005) or AOCS Ca 18b-91 (97) (AOCS, 2005). Limits for oil deterioration include peroxide value and acid value. Codex allows up to 10 meq/kg oil for refined oils. Acid value limits range from a low of 0.6 mg/KOH/g oil for refined oils. 

Studies of autoxidation and rancidity in relation to off-flavour production in foods cannot be overemphasized. Rancidity development in food products such as milk, meat, dehydrated food, potato chips, fruits and vegetables is a prime concern of food chemists. The process of autoxidation and rancidity not only involves cis to trans isomerization but also production of many other compounds, such as peroxides and hydroperoxides. IR spectroscopy has been used sucessfully to follow the autoxidation process qualitatively and quantitatively. Bands at 2.93, 3.2, 5.72 and 10 to 11 have been associated quantitatively with hydroperoxides, free acid, C=0, and cis to trans isomerization, respectively. Analysis by IR absorption was found to be more sensitive to changes than a qualified taste panel and considerably more sensitive than determination of peroxide values (O Connor, 1956). 

A large proportion of the volatiles identified in vegetable oils are derived from the cleavage reactions of the hydroperoxides of oleate, linoleate, and linolenate (Section D). A wide range of hydrocarbons (ethane, propane, pentane and hexane) appears to be formed in soybean oil oxidized to low peroxide values. A number of volatiles identified in vegetable oils that are not expected as primary cleavage products of monohydroperoxides include dialdehydes, ketones, ethyl esters, nonane, decane, undecane, 2-pentylfuran, lactone, benzene, benzaldehyde and acetophenone. Some of these volatiles may be derived from secondary oxidation products, but the origin of many volatiles still remains obscure. However, studies of volatile decomposition products should be interpreted with caution, because the conditions used for isolation and identification may cause artifacts, especially when fats are subjected to elevated temperatures. 

---