Flavor reversion

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 high oxidation rates of EPA and DHA and the instability of their hydroperoxides caused the rapid formation of secondary products such as volatile aldehydes and other compounds, which, in turn, impart flavor reversion in fish oils (56). The hydroperoxides produced from autoxidation of EPA (73) and DHA (74) have been identified but not quantified. They form eight and ten isomers, respectively. Noble and Nawar (75) analyzed the volatile compounds in autoxidized DHA and identified a number of aldehydes. Most of the aldehydes identified could be explained by the p-scission of alkoxy radicals generated by the homolytic cleavage of each isomer of the hydroperoxides as shown in Figure 9. 

T. H. Smouse, Flavor Reversion in Soybean Oil, American Oil Chemists Society Publication, Peoria, Illinois, 1985. 

At low levels of oxidation, the degree and type of change known as flavor reversion is characteristic for each oil. For example, soybean oil develops a flavor that is described as beany or grassy. This flavor has been attributed to the formation of 2-pentylfuran (18) and 3-c -hexenal (19). Canola oil (LEAR) develops flavors similar to those of soybean oU. The flavor reversion of sunflower seed and safflower seed oils are described as seedy. Similarly, com and pahn oils develop flavors of distinct type. These reversion flavors are observed long before other objectionable oxidative off-flavors are formed. With many of these oils, the cause of the reversion flavors is not well understood. In the case of most well-processed oils, the reversion flavor is not a frequent problem. 

The purpose of degumming is to remove almost all phospholipids, which could settle out during storage or cause flavor reversion of bottled oil or darkening during... 

Ullrich F. and Grosch W. (1988b) Flavor deterioration of soybean oil identification of intense odor compounds formed during flavor reversion. Fett Wiss. Techn. (Fat Sci. Technol.) 90 (9), 332-6. 

Sensory evaluation is a specialized discipline, using trained panels to measure and analyse the characteristics of food lipids evoked by the senses of taste, smell, sight and mouth feel. Sensory analyses are those most closely associated with the quality of food lipids, but their usefiilness is limited because they are costly and require a well-trained taste and odor panel and the proper facilities. However, sensory analyses provide sometimes ausefiil approach to identifying flavor or odor defects in the processing of food lipids that caimot be detected by other more objective chemical or instrumental analyses. For example, certain flavor defects characterized as grassy or fishy in linolenate-containing oils such as soybean and low-erucic rapeseed (canola) oils (Chapter 1) occur at such low levels of oxidation that they can only be detected by sensory analyses. The old term flavor reversion for soybean oil is based on the characteristic of this oil undergoing flavor deterioration at unusually low levels of oxidation that cannot be measured by peroxide value determination. Oils derived from fish... 

An overview of die free radicals and reactions thereof is presented. Free radicals are atoms or groups having an unpaired electron and hence are paramagnetic. Electron paramagnetic resonance spectroscopy (EPR) and trapping methods are used to analyze radicals. In lipids, radical reactions lead to autoxidation and hence flavor reversion. Reactive oxygen species are key components involved in such reactions. Finally, descriptions for phenolic, sequesterant and enzymatic antioxidants and their mode of action are provided. 

---