Volatile flavor compounds from lipid oxidation

FIGURE 7.3 Mechanisms for the formation of volatile flavor compounds via lipid oxidation. (A) R is saturated. (B) R contains an cue system. (From Grosch, W., Food Flavours, Part A Introduction, I.D. Morton, A.J. MacLeod, Eds., Elsevier, New York, 1982, p. 325. With permission.)... 

The studies published in this promising area of flavor chemistry and physiology have been limited to a few volatile compounds. They need to be extended to the multitude of other volatile compounds derived from lipid oxidation. Many of these volatiles are known to have a significant impact on quality and acceptability of lipid-containing foods. The complex volatiles produced by food lipids containing n-3 polyunsaturated fatty acids reported to impart nutritional and health benefits are especially important because they develop undesirable fishy odors and flavors at extremely low levels of oxidation (Chapter 4.D Chapter 5.F Chapter ll.E). 

Not all of the potent volatile compounds are derived from lipid oxidation, including a number of lactones that come from naturally occurring hydroxy fatty acids, diacetyl and vanillin in butter oil (from melted butter). The concentrations of the mixtures of carbonyl compounds exceed the flavor threshold values for individual aldehydes, and the oxidized flavor results from a combination of volatile compounds. 

Volatile flavor compounds may be formed from lipids via several different pathways. The primary pathways include a- and P-oxidation, and oxidation via lipoxygenase enzymes [8,9]. Some of the earliest work in this area was on the development of banana aroma by Tressl and Drawert [10]. Recent work showing pathways for the formation of several key volatiles derived from apple lipids are illustrated in Figure 4.2. They demonstrated the conversion of labeled acetate to acetate esters and labeled butanoate to butanoate esters by postchmacteric banana slices. They have further shown the conversion of hexanoic acid to hexanol by these tissues. 

Marsili compared SPME and dynamic headspace (DH) GC/MS techniques for the analysis of light-induced lipid oxidation products in milk (4). In the SPME method, 3 g of milk (2% milkfat) and 4-methyl-2-pentanone internal standard (10 jL of a 20 ppm solution in methanol) were placed in a 9-mL vial and capped. A 75- j,m Carboxen-1006/PDMS fiber was inserted into the headspace above the milk sample. Hie Carboxen/PDMS fiber has a combination of micro-, meso-, and macro-pores ranging from 6 to 50A. The volatile flavor compounds that are the best indicators of light-induced oxidation in milk are pentanal, hexanal, and dimethyl disulfide. The Carboxen/PDMS fiber was selected for this study because it is well suited for the analysis of low-molecular-weight volatiles. Adsorption of volatiles from the milk onto the SPME fiber was conducted at 45 C for 15 min with stirring. The sealed vial was allowed to equilibrate for 2 min at 45 C before the SPME fiber was inserted. 

Flavor is one of the major characteristics that restricts the use of legume flours and proteins in foods. Processing of soybeans, peas and other legumes often results in a wide variety of volatile compounds that contribute flavor notes, such as grassy, beany and rancid flavors. Many of the objectionable flavors come from oxidative deterioration of the unsaturated lipids. The lipoxygenase-catalyzed conversion of unsaturated fatty acids to hydroperoxides, followed by their degradation to volatile and non-volatile compounds, has been identified as one of the important sources of flavor and aroma components of fruits and vegetables. An enzyme-active system, such as raw pea flour, may have most of the necessary enzymes to produce short chain carbonyl compounds. 

The volatiles from cooked meat contain large numbers of aliphatic compounds including aldehydes, alcohols, ketones, hydrocarbons and acids. These are derived from lipids by thermal degradation and oxidation (J7) and many may contribute to desirable flavor. In addition, the aldehydes, unsaturated alcohols and ketones produced in these reactions, as well as the parent unsaturated fatty acids, are reactive species and under cooking conditions could be expected to interact with intermediates of the Maillard reaction to produce other flavor compounds. 

Routine procedures to assay the extent of oxidation in lipids and lipid-containing foods should be simple, reliable and sensitive. Results from routine procedures should ideally correlate well with results obtained from sensory taste panels. St. Angelo (1996) has described volatile compound profiles formed during lipid oxidation in different groups of food products. However, because of the complexity of lipid oxidation, no single test can be equally useful at all stages of the oxidative process. The methods should be capable of detecting autoxidation before the onset of off-flavor. This is particularly true in the case of milk products where a low level of oxidation can lead to off-flavor. 

We originally believed that the short chain saturated fatty acids In oxidizing fish lipids contributed to burnt/flshy flavors. Saturated fatty acid concentrations (C4 - Cg) measured by volatile headspace analysis (32, 59) reached levels as high as 3 ppm In highly oxidized fish oils (32). Flavor threshholds for these short n-chaln fatty acids In oil systems In the literature (>.66 ppm, 63) Indicate that they could contribute notes to oxidizing fish oils. However, studies designed to document the role of short chain acids as flavor compounds detracting from the flavor quality of fish oils did not confirm earlier beliefs. 

Our own work has shown a great decrease In the concentration of the volatiles In the cured, as compared to uncured, meats (Figure 5) (Ifi). The concentration of aldehydes originally present in cooked pork was reduced to < 12 of their original quantities (Table III, unpublished results). However, we did not Identify any new flavor active compound which could have been responsible for the cured flavor. Lipid oxidation, as measured by TBA number, was almost eliminated In cooked pork by adding nitrite at a level of 150 ppm (13.) Furthermore in preliminary evaluations, our untrained panelists were unable to differentiate amongst the flavor of nitrite-cured meats prepared from beef, chicken, mutton and pork (unpublished results). 

Sensory evaluation of lipid oxidation has been conducted by many researchers (98-100). However, as a subjective method, the reproducibility of sensory analysis is generally considered worse than that of chemical or instrumental methods. More recently, use of an electronic nose to monitor the formation of volatile compounds associated with off-flavors from hpid oxidation has been proposed to supplement information from human sensory panels (101). 

Flavors and aromas commonly associated with seafoods have been intensively investigated in the past forty years ( l-7), but the chemical basis of these flavors has proven elusive and difficult to establish. Oxidized fish oils can be described as painty, rancid or cod-liver-oil like (j ), and certain volatile carbonyls arising from the autoxidation of polyunsaturated fatty acids have emerged as the principal contributors to this type of fish-like aroma ( 3, 5, 9-10). Since oxidized butterfat (9, 11-12) and oxidized soybean and linseed oils (13) also can develop similar painty, fish-like aromas, confusion has arisen over the compounds and processes that lead to fish-like aromas. Some have believed that the aromas of fish simply result from the random autoxidation of the polyunsaturated fatty acids of fish lipids (14-17). This view has often been retained because no single compound appears to exhibit an unmistakable fish aroma. Still, evidence has been developed which indicates that a relatively complex mixture of autoxidatively-derived volatiles, including the 2,4-heptadienals, the 2,4-decadienals, and the 2,4,7-decatrienals together elicit unmistakable, oxidized fish-oil aromas (3, 9, 18). Additionally, reports also suggest that contributions from (Z -4-heptenal may add characteristic notes to the cold-store flavor of certain fish, especially cod (4-5). 

Lipid-derived volatile compoimds dominate the flavor profile of pork cooked at temperatures below 100°C. The large numbers of heterocyclic compounds reported in the aroma volatiles of pork are associated with roasted meat rather than boiled meat where the temperature does not exceed 100 C (34,35). Of flie volatiles produced by lipid oxidation, aldehydes are the most significant flavor compounds (35,36). Octanal, nonanal, and 2-undecenal are oxidation products from oleic acid, and hexanal, 2-nonenal, and 2,4-decadienal are major volatile oxidation products of linoleic acid. 

Aldehydes are by far the most numerous compounds identified as dry-cured ham odorants, with different odors (green, rancid, toasted) and thresholds in air ranging from 0.09 to 480 ng/L (Table 1). Most of them were identified in the first works focused on dry-cured ham volatile compounds (7,2). Aldehydes are essential for meat flavor (70), but large quantities in meat and meat products have been related to lipid oxidation and deterioration (77). The effect of several quality factors has been researched and it was found that the rearing system of pigs (S) and ripening conditions (7) influence on the contribution to odor and the content of some aldehydes. 

The most popular method involves 2-thiobarbituric acid (TBA) two molecules of 2-thiobarbituric acid are condensed with malonaldehyde. The emergent chromogen — the two tautomeric structures of the red TBA-malonaldehyde adduct — is determined at 532 nm, and also often at 450 nm, to determine aUcenals and aUcanals, respectively. The qualitative Kreis test was based on a similar principle it involved detection of the epihydrine aldehyde — a tautomeric malondialdehyde — in a color reaction with resorcine or phloroglucinol. The popularity of the TBA test stems from a correlation between the results and sensory evaluations. Paradoxically, this is related to the most important drawback of the TBA technique — its lack of specificity. In addition to the reaction with malonaldehyde, TBA forms compounds of identical color with other aldehydes and ketones, products of aldehyde interaction with nitrogen compounds, and also with saccharides, ascorbic acid, creatine, creatinine, trimethylamine oxide, trimethylamine, proteins, and amino acids. For this reason, the TBA test may even be treated as a proteolysis indicator (Kolakowska and Deutry, 1983). Recently, TBA-reactive substances (TEARS) were introduced, primarily to stress that the reaction involves hydroperoxides in addition to aldehydes. Due to the nonspecificity of the TEARS test, its results reflect the rancidity of food better than other conventional methods, especially off-flavor, which is caused by volatiles from lipids as well as being affected by products of lipids interaction with nitrogenous compounds. 

The sensitivity of taste or odor panels can be measured by the ability of individuals to detect sensory characteristics. Threshold values are measures of the least concentrations of volatile compounds detected in a food matrix (oil or water) or minimum detectable level by at least 50% of the panelists. This definition is, however, now commonly used to refer to detection by 100% of the panelists. There is a considerable difference in the flavor significance of volatile decomposition products formed in oxidized or rancid lipids. Hydrocarbons have the highest threshold values ranging from 90 to 2150 ppm, and the least impact on flavor. Substituted furans with threshold values of 2-27 ppm, vinyl alcohols with threshold values of 0.5-0.3 ppm, and 1-alkenes... 

The structure of the food matrix is also known to affect the release of volatile compounds having an impact on flavors and aroma. Changes in flavor result from the interactions of lipid-derived carbonyl compounds by aldolization with the amino groups of proteins. Undesirable flavors are produced when beef or chicken are fried in oxidized fats by the interaction of secondary lipid oxidation... 

Carbonyl compounds in oxidized lipids are the secondary oxidation products resulting from the decomposition of the hydroperoxides. They can be quantified by the reaction with 2,4-dinitrophenylhydrazine and the resulting colored hydrazones are measured spectrophotometrically at 430-460 nm. The carbonyl value is directly related to sensory evaluation, because many of the carbonyl molecules are those responsible for off-flavor in oxidized oil. The anisidine value is a measure of carbonyl compounds that have medium molecular weight and are less volatile (Frankel 1998). It can be used to discover something about the prior oxidation or processing history of an oil. 

Oxidation of unsaturated acyl chains of lipids is a major route to volatile compounds during cooking of fat-containing food of either animal or vegetable origin. The unsaturated fatty acids, readily susceptible to the attack by oxygen, form hydroperoxides which in themselves are odorless and tasteless. The compounds that influence the flavor of the product result from a further breakdown of these hydroperoxides, and, normally, include saturated and unsaturated aldehydes, alcohols, and ketones. The carbonyl compounds resulting from autoxidation impart specific flavors that are normally detrimental to food products (Table 9.3). It should be pointed out, however, that they may also contribute to the desirable characteristic flavor of foods [48]. 

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