Novel Sulfur Compounds from Lipid-Maillard Interactions in Cooked

Chapter 5

Downloaded by UNIV OF TENNESSEE KNOXVILLE on October 14, 2015 | http://pubs.acs.org Publication Date: August 7, 2002 | doi: 10.1021/bk-2002-0826.ch005

Novel Sulfur Compounds from Lipid-Maillard Interactions in Cooked Meat Donald S. Mottram and J . Stephen Elmore School of Food Biosciences, The University of Reading, Whiteknights, Reading RG6 6AP, United Kingdom

A number of 3-thiazolines, thiazoles, thiapyrans, and thio -phenes with 2-alkyl substituents have been found in cooked beef and lamb. These compounds derive from the interaction of lipid autoxidation products, such as saturated and unsaturated aldehydes, with simple intermediates of the Maillard reaction, such as hydrogen sulfide, ammonia, and dicarbonyls. Although the aromas of these compounds are weak, they may influence flavor by modifying the formation of other compounds in the Maillard reaction or autoxidation of lipid.

The main reactions occurring during the cooking of meat, which are responsible for the characteristic flavor, are the Maillard reaction between amino acids and sugars, and the thermal oxidation of lipid. Sulfur-containing aroma compounds make particularly important contributions to meat flavor and the main source of sulfur for these compounds is the amino acid cysteine. In previous work using heated model systems containing ribose and cysteine, we demonstrated that the addition of phospholipid to these systems resulted in compounds formed by the interaction of lipid degradation products with intermediates of the Maillard reaction (1-3).

© 2002 American Chemical Society In Heteroatomic Aroma Compounds; Reineccius, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

93

94 A number of compounds that could be formed from the interaction of lipid with the Maillard reaction have also been found in the volatiles of cooked foods (4). These compounds include Ο-, N - or S-heterocycles containing long /?-alkyl substituents (C -C ). The alkyl groups usually derive from aliphatic aldehydes, obtained from lipid oxidation, while amino acids are the source of the nitrogen and sulfur. Meat volatiles have been found to contain a number of such compounds. Several thiazoles with C - C w-alkyl substituents in the 2-position have been reported in roast beef (5) and in fried chicken (6). Other alkylthiazoles with much longer 2-alkyl substituents ( C - C ) have been reported in the volatiles of heated beef and chicken, with the highest concentrations in beef heart muscle (7,8). Other heterocyclic compounds with long w-alkyl substituents found in meat include 2-alkylthiophenes with C - C alkyl substituents reported in pressure cooked beef (9,10), and 5-butyl-3-methyl-l,2,4-trithiolane and its 5pentyl homologue reported in fried chicken (11) and pork (12). This paper reviews some other sulfur heterocycles, which have recently been found in beef and lamb, and discusses the relationship to the fatty acid composition of the meat. 5

15


4

8

13

15

4

g

Effect Fatty Acid Composition on Meat Volatiles In recent years there has been increased interest in modifying the fatty acid composition in meat and milk to provide products that possess a fatty acid profile more appropriate to current human dietary guidelines (13,14). These recommend an increase in the polyunsaturated to saturated fatty acid ratio (P:S ratio), while decreasing the n-6 :n-3 ratio. The fatty acids deposited in beef tissues are relatively saturated, giving a low P:S ratio in the meat, but the ratio of n-6:n-3 fatty acids is beneficially low. Hence, strategies to improve the nutritional quality of beef need to increase the P:S ratio, while keeping the n6:n-3 ratio low. This may be achieved by altering the diet of cattle to increase the levels of components containing sources of long chain polyunsaturated fatty acids (PUFA) or by modifying the extent of the hydrogénation in the rumen. However, an increase in P U F A concentrations in meat may compromise oxidative stability resulting in flavor changes in the processed meat. Recently we examined the volatiles from cooked beef and lamb that had been fed diets containing linseed or fish oil. These diets gave enhanced levels of n-3 P U F A in the triglycerides (CI8:3 n-3) and in the phospholipids (CI8:3 n3, C20:5 n-3, C22:6 n-3). The volatiles from both cooked lamb and cooked beef with the increased n-3 P U F A showed increased levels of aliphatic aldehydes

In Heteroatomic Aroma Compounds; Reineccius, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

95 compared with control samples (15,16). This was due the enhanced autoxidation of the unsaturated fatty acids. However, sensory panels only found small differences in flavor in the cooked meat (17).


Thiazoles and Thiazolines in Beef and Lamb A series of alkylthiazoles and alky 1-3-thiazolines were found in the volatiles of cooked beef and lamb from these studies. The number and concentration of these heterocycles were higher in the meat from the linseed and fish oil diets than in controls. A total of 46 alkyl-3-thiazolines were found in beef and 12 in lamb (15,16,18). Most contained methyl or ethyl groups in the 4 and 5 positions with long alkyl chains (C -C ) in the 2-position (Figure 1). It was the first time any of these compounds had been reported in a food product, except for 2isobutyl-4,5-dimethyl-3-thiazoline, which had been found in a yeast extract (19). It was shown that these thiazolines were the main products in the reaction between ot-hydroxycarbonyls, such as l-hydroxy-2-butanone or 3-hydroxy-2butanone, ammonium sulfide and an n-alkanal or a Strecker aldehyde, such as 2methylbutanal (20). In these thiazolines the alkyl chain of the alkanal occupied the 2-position on the ring. In the cooking of meat, the Maillard reaction could give the hydroxycarbonyls, ammonia and hydrogen sulfide would be produced from cysteine, while the alkanals derive from lipid oxidation products or the Strecker degradation of amino acids. A smaller number of alkylthiazoles were also found in the beef and lamb, but generally at lower concentrations than the corresponding 3-thiazolines. 4

9

RJ = H , C H or C H 3

2

2

R = C H or C H 3

2

5

1

5

2

R and R = C H o r C H n = 4 - 8 , 15 3

2

5

n = 3-9,15 Figure 1. Alkyl-3-thiazolines and alkylthiazoles found in cooked beef and lamb. The odors of thiazolines and thiazoles with 2-«-alkyl substitution were described as slightly fatty, but they did appear to have low odor thresholds and could not be detected by GC-olfactometry in any of the volatile extractions of the cooked meat. Thiazolines and thiazoles with methyl or acetyl substituents


96 possess thresholds in the low μg/kg range, but it appears that the larger molecules with long alkyl substituents are not such potent odorants.

Thiophenes and Thiapyrans Until recently only one alkylthiapyran had been reported in food: 2pentylthiapyran in cooked beef heart (7). However, 2-alkyl-(2//)-thiapyrans with C - C alkyl chains, along with their 2-alkylthiophenes isomers with C - C alkyl chains, were isolated from cysteine/ribose/lecithin reaction mixtures (21). 2-Pentylthiapyran and 2-hexyithiophene were identified as major products when 2,4-decadienal and hydrogen sulfide were reacted in aqueous solution at p H 8 (22). In our recent work examining meat with modified P U F A content, we found six 2-alkylthiophenes and six 2-alkyl-(2//)-thiapyrans in volatiles of both cooked beef and lamb (Figure 2) (23).


2

7

3

n= 1

8

6

Figure 2. n-Alkylthiophenes and n-alkyl-(2H)-thiapyrans found in the volatiles of cooked beef and lamb. Although these compounds were found in all samples of meat examined, both diet and species affected the quantities obtained. Quantities of both thiophenes and thiapyrans were higher in the meat of animals fed P U F A supplements, corresponding to higher quantities of n-3 P U F A found in the muscles of these animals. These compounds may be formed by the reaction of 2,4-alkadienals with hydrogen sulfide (Figure 3). The higher levels of lipid breakdown products, such as 2,4-dienals, in the cooked meat with increased levels of PUFA were considered to be responsible for the larger quantities of thiophenes and thiapyrans. In order to confirm the identities of these thiapyrans, 2,4-alkadienals were reacted with hydrogen sulfide in aqueous solution. 2-Alkylthiophenes and 2alkyl-(2//)-thiapyrans were produced in all of these reactions. However, the quantities of alkylthiapyrans recovered were up to 100 times greater than those of the equivalent isomeric thiophenes. The relative quantities of thiophenes and thiapyrans in meat were similar, suggesting that a mechanism other than that given in Figure 3 may be required to explain the formation of alkylthiophenes.



97

Figure 3. Pathways for the formation of 2-alkylthiophenes and 2-alkyl-(2H)thiapyrans from 2,4-alkadienals and hydrogen sulfide. The aromas of the alkylthiapyrans were weak and thiophene-like, and the lower molecular weight thiapyrans possessed unpleasant garlic-like notes. The weak odor intensities suggest that it is unlikely that any of these compounds contribute directly to cooked meat aroma. However, the presence in the meat confirms that lipid-Maillard interactions do take place during the cooking of meat. Such interactions will modify the profile of aroma compounds produced by the Maillard reaction and by lipid degradation and, therefore, have an indirect affect on the aroma profile. For example, the reaction of 2,4alkadienals with hydrogen sulfide, to form 2-alkyl-(2#)-thiapyrans, leads to decreases in the concentrations of the dienals, which are potent aroma compounds, whilst forming compounds with low aroma significance.

Alkylthiophenes and Alkyl-(2/J)-thiapyrans Formed in Model Systems Containing Linoleic and Linolenic Acids The two main series of naturally occurring polyunsaturated fatty acids, n-3 and n-6, will give different profiles of volatile aldehydes and other products during autoxidation. Aldehydes from the former will generally be unsaturated because of the double bonds at the ω3 and ω6 positions on the alkyl chain. However, the n-6 series, will give both saturated and unsaturated aldehydes, and the latter may have up to seven saturated carbon atoms in the alkyl chain. Figure 4 shows some of the saturated and unsaturated aldehydes which may form during the autoxidation of 18:2 n-6 and 18:3 n-3. If 2-alkylthiophenes and 2-


98 alkyl-(2//)-thiapyrans are formed in meat from the reaction of dienals with hydrogen sulfide (Figure 3), then the relative amounts of different alkylthiophenes and alkylthiopyrans formed will depend on the proportions of 18:2 n-6 and 18:3 n-3 fatty acids in the meat.

linolenic (18:3 n-3) C H - C H - CH=C H - C H - C H =C H - CH — CH=C H - (CH^COOH


3

2

2

2

J autoxidation C H - C H = C H - CHO

C H - C H - C H = C H - C H - CHO

3

3

C H - C H - C H = C H - CHO 3

2

2

2

C H - C H - C H - C H = C H - CHO 3

2

2

CH -CH2- C H = C H - C H = C H - CHO 3

C H - C H 2 - C H = C H - C H - C H = C H - CH= C H - C H O 3

2

linoleic acid (18:2 n-6) C H ( C H ) - CH=CH- C H - CH=CH- (CH ) COOH 3

2

4

2

2

4

J autoxidation CH (CH2) - CHO 3

N

(n = 3-5)

C H ( C H ) _ C H = C H - CHO 3

2

n

(n = 3-6)

C H ( C H ) _ CHrr C H - CE=z C H - C H O 3

2

4

Figure 4. Some aldehydes formed in the autoxidation of linolenic and linoleic acids. We have examined the formation of 2-alkylthiophenes and 2-alkyl-(2//)thiapyrans in two aqueous model systems, both containing cysteine and ribose as a source of hydrogen sulfide. One contained linoleic acid (18:2 n-6) methyl ester and the other contained ot-linolenic acid (18:3 n-3) methyl ester. The systems were heated at 140°C in a sealed container for 30 min, then the headspace volatiles were extracted using solid phase microextraction and were analyzed by G C - M S . The profiles of thiapyrans and thiophenes produced were compared with those in the headspace of cooked beef and lamb from animals fed on diets containing either linseed or fish oil supplements (Table I). In the model systems containing 18:3 n-3 methyl ester, the main thiapyran was 2-ethyl-(2//)-thiapyran, which could be formed from 2,4-heptadienal, a major autoxidation product of 18:3 n-3. Small amounts of the methyl and propyl homologues were also found, but no thiapyrans with longer alkyl chains. In the 18:2 n-6 reaction systems these thiapyrans containing shorter alkyl chains were absent and the reaction favored alkylthiophenes, with 2-pentylthiophene


99 the most abundant. In the meat, both beef and lamb fed with linseed or fish oil had raised levels of n-3 fatty acids. The cooked meat from these diets had higher levels of 2-ethyl , 2-methyl- and 2-propylthiapyrans compared with the control meats. T


Table I. Quantities of Alkylthiophenes and Alkylthiapyrans Found in Beef and Lamb Fed on PUFA Rich Diets, Compared with Quantities Produced in Model Reaction Systems Lamb Compound

Beef

Control Linseed Fish

2-alkylthiophenes ethyl 13 propyl 2 butyl 2 pentyl 4 4 hexyl

14 3 4 3 4

32 4 4 4 5

!-alkyl-(2H)-thiapyrans methyl 3 3 ethyl 22 59 2 propyl 4 butyl tr tr pentyl 5 4

6 131 15 2 8

Control Linseed 5 tr

Model Fish

18:2

18:3

15 tr tr tr tr

3 7 8 21 9

99 9 tr tr tr

-

19 399 3

tr tr

-

tr tr

6 tr tr tr tr

_

-

-

3

6 tr

14 2

-

_

-

-

-

tr

tr

Quantities in headspace above meat are ng/100g meat, quantities in headspace above model reaction mixture are ng per 0.5 mmol fatty acid; - not detected, tr = trace. Model systems: cysteine + ribose + fatty acid methyl ester (18:2 n-6 or 18: n-3); 0.5 mmol each.

Conclusions The interaction of products of the Maillard reaction with saturated and unsaturated aldehydes from the autoxidation of lipids results in the formation of heterocyclic sulfur compounds, such as thiazolines, thiazoles, thiapyrans, and thiophenes, all with alkyl substituents derived from the aldehyde. A l l these classes of compounds have recently been found in cooked meat. Although these compounds may not have significant aromas, the reactions by which they are formed may influence the relative quantities of other odor compounds in the aroma profiles of foods.


100

References 1. 2.


3. 4. 5. 6. 7. 8.

9. 10. 11. 12.

13. 14. 15. 16. 17. 18. 19. 20.

Whitfield, F. Β.; Mottram, D. S.; Brock, S.; Puckey, D. J.; Salter, L . J. J. Sci. Food Agric. 1988, 42, 261-272. Farmer, L . J.; Mottram, D . S.; Whitfield, F. B . J. Sci. Food Agric. 1989, 49, 347-368. Farmer, L . J.; Mottram, D . S. J. Sci. Food Agric. 1992, 60, 489-497. Whitfield, F. B . Crit. Rev. Food Sci. Nutr. 1992, 31, 1-58. Hartman, G . J.; Jin, Q. Z.; Collins, G . J.; Lee, K. N.; Ho, C.-T.; Chang, S. S. J. Agric. Food Chem. 1983, 31, 1030-1033. Tang, J.; Jin, Q. Z.; Shen, G . H . ; Ho, C.-T.; Chang, S. S. J. Agric. Food Chem. 1983, 31, 1287-1292. Farmer, L . J.; Mottram, D . S. In Trends in FlavourResearch;Maarse, H.; van der Heij, D. G., Eds; Elsevier: Amsterdam, 1994; pp 313-326. Elmore, J. S.; Mottram, D. S. In Flavor Analysis: Developments in Isolation and Determination; Mussinan, C. J.; Morello, M . J., Eds; American Chemical Society: Washington DC, 1998; pp 69-77. Wilson, R. Α.; Mussinan, C. J.; Katz, I.; Sanderson, A . J. Agric. Food Chem. 1973, 21, 873-876. Min, D . B.; Ina, K . ; Peterson, R. J.; Chang, S. S. J. Food Sci. 1979, 44, 639642. Hwang, S. S.; Carlin, J. T.; Bao, Y . ; Hartma, G . J.; Ho, C.-T. J. Agric. Food Chem. 1986, 34, 538-542. Werkhoff, P.; Brüning, J.; Emberger, R.; Güntert, M . ; Hopp, R. In Recent Developments in Flavor and Fragrance Chemistry; Hopp, R.; Mori, K . , Eds; V C H : Weinheim, 1993; pp 183-213. Enser, M.; Hallett, K . G.; Hewett, B . ; Fursey, G . A . J.; Wood, J. D.; Harrington, G . Meat Sci. 1998, 49, 329-341. Wood, J. D.; Enser, M.; Fisher, Α. V . ; Nute, G . R.; Richardson, R. I.; Sheard, P. R. Proceedings of the Nutrition Society 1999, 58, 363-370. Elmore, J. S.; Mottram, D. S.; Enser, M. B . ; Wood, J. D. J. Agric. Food Chem. 1999, 47, 1619-1625. Elmore, J. S.; Mottram, D. S.; Enser, M.; Wood, J. D. Meat Sci. 2000, 55, 149-159. Vatansever, L . ; Kurt, E.; Enser, M.; Nute, G . R.; Scollan, N. D.; Wood, J. D.; Richardson, R. I. Animal Science 2000, 71, 471-482. Elmore, J. S.; Mottram, D. S.; Enser, M.; Wood, J. D. J. Agric. Food Chem. 1997, 45, 3603-3607. Werkhoff, P.; Bretschneider, W.; Emberger, R.; Guentert, M . ; Hopp, R.; Koepsel, M. Chem. Mikrobiol. Technol. Lebensm. 1991, 13, 30-57. Elmore, J. S.; Mottram, D. S. J. Agric. Food Chem. 1997, 45, 3595-3602.


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21. Farmer, L . J.; Mottram, D. S. J. Sci. Food Agric. 1990, 53, 505-525. 22. van den Ouweland, G. A . M.; Demole, E . P.; Engisst, P. In Thermal Generation of Aromas; Parliment, T. H . ; McGorrin, R. J.; Ho, C.-T., Eds; American Chemical Society: Washington D C , 1989; pp 433-441. 23. Elmore, J. S.; Mottram, D. S. J. Agric. Food Chem. 2000, 48, 2420-2424.


Novel Sulfur Compounds from Lipid-Maillard Interactions in Cooked

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