Flavor Compounds Formed from Lipids by Heat Treatment - ACS

Chapter 13

Flavor Compounds Formed from Lipids by Heat Treatment T. Shibamoto and H. Yeo

Downloaded by CORNELL UNIV on August 16, 2016 | http://pubs.acs.org Publication Date: May 13, 1992 | doi: 10.1021/bk-1992-0490.ch013

Department of Environmental Toxicology, University of California, Davis, CA 95616

Food components, including proteins, carbohydrates, amino acids, sugars, and lipids, degrade into smaller molecules which undergo secondary reactions to form many flavor chemicals in cooked foods. The best known reaction occurring during cooking foods is the Maillard reaction. Investigations of complex reactions occurring in heated foods most commonly use so-called Maillard model system consisting of a sugar and an amino acid. A sugar is used as a source of carbonyl compounds which react with aminesfromamino acid to produce flavor chemicals. Recently, lipids have begun to receive much attention as flavor precursors in cooked foods because lipids also produce many carbonyl compounds upon heat treatment. Aldehydes and ketones are precursors of many heterocyclic compounds which contribute roasted or toasted flavors to cooked foods. Lipids, upon exposure to heat and oxygen, are known to decompose into secondary products including alcohols, aldehydes, ketones, carboxylic acids, and hydrocarbons. Aldehydes and ketones produce heterocyclic flavor compounds reacting with amines, such as amino acids, via Maillard-type reactions in cooked foods (1). One of the major food constituents, lipids have been known to produce many aldehydes and ketones, which may undergo secondary reaction with amine compounds to yield flavor chemicals in cooked foods. In fact, lipid-rich foods (2) or deep-fat-fried foods (3) reportedly produce many flavor chemicals. Therefore, lipids may provide the aldehydes and ketones essential to theflavorformation. Volatile Compounds Formed in Beef Fat Beef fat was heated with or without glycine in a pressurized bottle at 200 °C for 4 h and the volatile chemicals formed were isolated using a simultaneous steam

0097-6156/92/0490-0175$06.00/0 © 1992 American Chemical Society

Teranishi et al.; Flavor Precursors ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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distillation/solvent extraction apparatus (SDE)(4,5). The extract was analyzed by gas chromatography/mass spectrometry (GC/MS). One hundred forty-three compounds were isolated and identified in the extracts. The major compounds identified in the extract of beef fat alone were n-alkanes, n-alkenes, n-alcohols, η-aldehydes, n-alkylcyclohexanes, and 2-ketones. Various isoacid derivatives such as isoalcohols were also found in this sample. Proposed formation mechanisms of the major compounds identified in this study are shown in Figure 1. Table I shows the number of each chemical group isolated from heated beef fat.

Table I. Number of volatile compounds in different chemical groups identified in heated beef fat Isolated by Compounds

SPE

SDE

n-Alkanes n-Alkenes n-Aldehydes 2-n-Ketones n-Alcohols

7 31 18 6 4

15 12 13 13 12

Many nitrogen-containing compounds were isolated from the beef fat heated with glycine. This extract gave an unpleasant odor possibly due to the presence of certain aldehydes and ketones. Alkylpyridines were also detected in this extract. The amount of 2-butyl- and 2-pentylpyridine was found to decrease as the amount of glycine added increased. The formation of n-nonanal in large quantity (7.4%) in the beef fat samples and its consumption in the presence of glycine suggest n-nonanal as a possible precursor of 2-butylpyridine. Similarly, 1-decanal is a possible precursor of 2-pentylpyridine. Homologues of 5-alkyldihydro-2(3H)-furanone were tentatively identified. As shown in Figure 1, the proposed formation mechanism of these compounds is from the RCOO* radical via ring closure between the carbonyl radical and γ-carbon atom followed by loss of γ-hydrogen as a radical. The formation of cyclohexane and alkylpyridine are well explained by RCOO*, RCH 0*, RO*, and R» radicals formed from triglyceride. The headspace volatiles from overheated beef fat were also isolated using a simultaneous purging extraction (SPE) apparatus and identified by GC/MS (6). The first column shows the numbers found in the extract from SPE and the second column shows the numbers identified in the extract from SDE. In the extract from SPE, 87 compounds were identified including 7 alkanes, 31 alkenes, 18 aldehydes, and 6 ketones. Aldehydes, which constituted 23.41% of the total volatiles isolated, were the major components identified in the extract from SPE. 2


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Volatile Aldehydes Formed in Heated Pork Fat Aldehydes and ketones formed in a headspace of heated pork fat were trapped in an aqueous cysteamine solution and recovered using an SPE (7). Table II shows aldehydes and ketones found in the extract.


Table II. Aldehydes and ketones determined in headspace of heated pork fat Compounds Acetaldehyde Formaldehyde Propanal 2-Pentanone Butanal 2-Hexanone Pentanal C H C H O (branched) 2-Heptanone Hexanal C H CHO (branched) 2-Octanone Heptanal C^H^CHO (branched) C^H^CHO (branched) Octanal CgH CHO (branched) C H CHO (branched) Nonanal 5

n

6

13

8

17

17

Concentration (mg/L) 49.1 7.99 71.3 1.25 224 6.51 1003 13.7 11.4 3090 40.1 15.2 1261 423 135 251 30.7 13.2 80.8

Nine aldehydes and four ketones were identified. The production of formaldehyde, which has never been reported prior to this study, was determined in appreciable amounts in the pork fat samples. A series of straight chain-aldehydes (n-C to n-C ) were identified. Hexanal was one of the major aldehydes produced. This suggests that linoleic acid esters are a major constituent of pork fat because it is generally recognized that the oxidative cleavage of double bonds produces alde hydes or ketones (8). Other major aldehydes recovered such as heptanal, nonanal, and butanal may correspond to the amount of possible fatty acid precursors in pork fat. x

9

Acrolein Formed in Heated Cooking Oils Acrolein, the simplest α,β-unsaturated aldehyde, has been found in various foods such as cooked horse mackerel (9). Acrolein produced from cooking oil heated at


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various temperatures in headspace was analyzed as a morpholine derivative (10). Table ΙΠ shows amounts of acrolein found in a headspace of various cooking oils.


Table III. Amount of acrolein recovered in headspace of 200 g of heated cooking oils using SPE with or Air stream

Cooking oil Corn oil Soybean oil Sunflower oil Olive oil Sesame oil

Iodine value 103 - 128 120 - 141 125 - 136 80- 88 103 - 195

Amount (mg) N 54.08 29.55 36.90 72.90 58.98 2

Air 81.05 76.11 57.61 103.63 85.51

The amount of acrolein formed appear to be inversely proportional to the iodine values which indicate the degree of unsaturation of fatty acid (77). Olive oil, which has the lowest iodine value among the oils used, produced the most acrolein while soybean oil, which has the highest iodine value, produced the least acrolein. The major pathway of acrolein formation in heated fats or oils is dehydration of glycerol which is formed from hydrolysis of triglycerides (72). Acrolein has been known as a lachrymator, and the vapor causes eye, nose, and throat irritation. However, an appropriate amount of acrolein stimulates the taste and olfactory organs and consequently plays an important role in physiological effects of cooked lipid-rich foods or deep-fat fried foods. Volatile Compounds Formed from a Corn Oil/Glycine Maillard Model System Headspace volatiles obtained from a corn oil heated with or without glycine were isolated using the SPE apparatus and identified by GC/MS (73). The major compounds identified in the corn oil samples were aldehydes, hydrocarbons, and ketones. With the addition of glycine, five of the unsaturated aldehydes formed in the corn oil samples were not detected. They were trans-2-butenal, trans-2pentenal, cis-2-hexenal, cis-2-heptenal, and 2-nonenal. The presence of glycine did not influence the amount of hydrocarbons, alcohols, esters, and aromatic compounds formed. Figure 2 shows the total amounts of unsaturated aldehydes formed with various amounts of glycine in corn oil. The addition of glycine to com oil decreased the amount of volatile unsaturated aldehydes by almost 100 times, suggesting that secondary reactions occurred between glycine and the aldehydes. Figure 3 shows the relative amounts of nitrogen- containing compounds produced in various corn oil/glycine mixtures. The amount of these compounds


13. SHIBAMOTO & YEO

Flavor Compounds Formed from Lipids

TRIGLYCERIDE _ (β-keto acid moiety)

R COOH

+NH , R O H 3

m

R -C(0)-NHR n

n

-C0

179

n

2

RnH |.», R COO*

-co.

n

R

+o

2

n

n

CH . 2

-OH*

+ RnH

R OO- — · - R OOH —

R„.

R O*

n

n

R -iCH 0.


n

2

-Η·

[R^CHO]

K

n-3

Ο

Ο

Figure 1. Proposed formation pathways of some major volatiles formed from lipids.

0.00

Glycine in Corn Oil (g)

Figure 2. Total amounts of unsaturated aldehydes formed with various amounts of glycine in corn oil. Teranishi et al.; Flavor Precursors ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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15η

Glycine in Corn Oil (g)

Figure 3. Relative amounts of nitrogen-containing compounds produced in various corn oil/glycine mixtures.


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Flavor Compounds Formed from Lipids

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increases with increasing amounts of glycine in corn oil. This observation is mainly due to the formation of nitriles, pyridines, and pyrroles. The majority of the monoalkylpyridines were 2- or 3-substituted. 1-Methy 1-2-propylpyrrole, 1-methy 1-2-butyl-pyrrole, and 1-methy 1-2-pentylpyrrole have never been reported prior to this study.


Volatile Compounds Formed in Heated Lipid-Rich Foods, Eggs Eggs generally do not possess strong characteristics flavors. However, they contain amino acids, proteins, carbohydrates and lipids in large quantities, suggesting the possibility of flavor formation via the Maillard reaction. Headspace volatiles from heated whole egg, egg white, and egg yolk were isolated using an SPE apparatus and identified by GC/MS (14). A total of 141 volatile compounds were identified in cooked whole egg, egg yolk, and egg white. The percent yields of total volatiles from cooked whole egg, egg yolk, and egg white were 0.0083, 0.0030, and 0.0014%, respectively. The amount total volatiles produced from whole egg is considerably greater than did either egg yolk or egg white alone. This may be due to the presence of many precursors of the Maillard reaction products in whole egg. Table IV shows major volatile compounds found in heated eggs. Aldehydes were the major volatiles in cooked egg yolk. Whole egg and egg white, however, yielded only a few volatile aldehydes. 2-Methylpropanal, a major aldehyde in egg yolk and whole egg volatiles, may be formed from Strecker degradation of valine or oxidative decomposition of lipids (75), as egg yolk is rich in both valine and lipids (16). Low-molecular-weight ketones such as acetone were found in large amounts in whole egg and egg white, whereas higher molecular weight 2-methylketones (C -C ) were found only in egg yolk. Several alcohols, sulfides which posses a characteristic cooked egg flavor, and a large number of nitriles were found in the three egg samples. 7

Table IV.

10

Major volatile compounds identified in cooked egg samples

Compounds Aldehydes Ketones Nitriles Pyrazines Pyrroles Pyridines Alkylbenzenes Furans Thiazoles

Whole egg 10.73 12.43 20.73 6.65 6.64 2.81 20.07 0.52 a

Total GC peak area% Egg white Egg yolk 64.12 4.53 31.08 4.16 17.61 0.04 18.24 8.69 3.22 2.27 a 1.10 4.66 1.89 0.69 2.47 1.83 0.18

a Not detected


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Nitrogen-containing heterocyclic compounds such as pyridines, pyrazines, pyrroles, and thiazoles were the major flavor compounds identified in the egg samples. Many alkylpyrazines identified in the present study are known as the products of the Maillard reactions. They contribute roasted or toasted flavors to cooked foods. Some pyrroles reportedly contributed off-flavors to certain cooked food products. However, 2-acetylpyrrole, found predominantly in whole egg sample, possess a pleasant, caramel-like flavor. Egg white, which is rich in proteins, produced thiazoles in large quantities. Thiazoles are proposed to form from sugar and sulfur-containing amino acid. As these reports show, lipid must play an important role in the formation of flavor chemicals in cooked foods, in particular in lipid-rich foods.

Literature Cited 1. Shibamoto, T. Instrumental analysis of foods. Vol. I. G. Charalambous and G. Inglett Eds. Academic Press, New York, 1983, 229. 2. Mottram, D. S. Edwards, R. A. J. Sci. Food Agric. 1983, 34, 517. 3. Ho, C. T. Carlin, J. T. Huang, T. C. Flavor Science and Technology, Martens, M. Dalen, G. A. Russwum, H. Eds, Wiley & Sons, London, 1987, 35. 4. Ohnishi, S. Shibamoto, T. J. Agric. Food Chem. 1984, 32, 987. 5. Likens, S. T. Nickerson, G. B. J. Chromatogr. 1966, 21, 1. 6. Umano, K. Shibamoto, T. J. Agric. Food Chem. 1987, 35, 14. 7. Yasuhara, A. Shibamoto, T. J. Food Sci. 1989, 54, 1471. 8. Mottram, D. S. Edwards, R. A. MacFie, H. J. H. J. Sci. Food Agric. 1982, 33, 934. 9. Shinomura, M. Yoshimatsu, F. Matsumoto, F. Kaseigaku Zasshi 1971, 22, 106. 10. Umano, K., Shibamoto, T. J. Agric. Food Chem. 1987, 35, 909. 11. Codd, L. W. Dijkhoff, K. Fearon, J. H. van Oss, C.J. Roebersen, H. G. Stanford, E. G., Eds. "Oils and Fats". In Chemical Technology, an Encyclopedic Treatment, Barnes & Noble: New York, 1975, Vol 8. 12. Adkins, H. Hartung, W. H. "Acrolein". In Synthesis Organiques; Masson: Paris, 1935. 13. Macku, C. Shibamoto, T. J. Agric. Food Chem. 1991, 39, 1265. 13. Umano, K. Hagi, Y. Shoji, A. Shibamoto, T. J. Agric. Food Chem. 1990, 38, 461. 14. Beltz, H. D. Grosch, W. Eggs. In Food Chemistry, Springer-Verlag: Berlin, 1987. 15. Lundberg, W. O. Autoxidation and Autoxidants; Interscience: New York, 1962 Vol. I. RECEIVED December 18, 1991


Flavor Compounds Formed from Lipids by Heat Treatment - ACS

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