Flavor and Lipid Chemistry of Seafoods - ACS Publications - American

Chapter 9

Character-Impact Aroma Compounds of Crustaceans 1

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Downloaded by UNIV OF TENNESSEE KNOXVILLE on August 23, 2014 | http://pubs.acs.org Publication Date: July 1, 1997 | doi: 10.1021/bk-1997-0674.ch009

H. H. Baek and Keith R. Cadwallader 1

Department of Food Engineering, Dankook University, Chunan 330-714, Korea Department of Food Science and Technology, Mississippi Agricultural and Forestry Experiment Station, Mississippi State University, Box 9805, Mississippi State, MS 39762-5953 2

Character-impact aroma compounds of cooked crustaceans, such as crab, crayfish, lobster, freshwater prawn, and shrimp, are reviewed as well as previous findings onfresharoma and off-flavor of crustaceans. Character-impact aroma compounds of cooked crustaceans include 2,3butanedione, 2-methyl-3-furanthiol, 2-acetyl-1-pyrroline, 3(methylthio)propanal, and 2-acetyl-2-thiazoline. These compounds are the most potent aroma compounds responsible for desirable meaty, nutty/popcorn, and salty aroma notes. 1-Octen-3-one, (Z)-4-heptenal, and 2-acetylthiazole also contribute to the aroma of crustaceans. These compounds were present in all crustaceans evaluated.

Crustaceans, such as crab, crayfish, lobster, prawn, and shrimp, are important and popular seafoods and have unique aromas and tastes. Cooked crustaceans have different aroma properties from other seafoods, such as fish and mollusks. It is well-known that cooked crustaceans have meaty and nutty/popcorn notes. Volatile and non-volatile flavor compounds responsible for crustacean aroma have been intensively studied (1-16). Volatile flavor compounds are usually generated by enzymatic reaction, lipid autoxidation, microbial action, environmental contamination, and thermal reaction. Of these, the lipoxygenase-mediated enzymatic reaction seems to play the most significant role in flavors of fresh seafoods (17). Fishy flavors of seafoods are associated with autoxidation of fish lipids and trimethylamine formation (18). Recently, it was found that bromophenols were responsible for sea-, brine-, and iodine-like aromas of saltwater fishes (19). The occurrence of off-flavor associated with bromophenols in prawns and lobsters has been reported (20). While fresh crustaceans possess mild fishy aromas, desirable and most familiar aromas of crustaceans are generated by thermal reaction during cooking. Thermally generated aroma compounds from crustaceans have been investigated (73

Corresponding author © 1997 American Chemical Society In Flavor and Lipid Chemistry of Seafoods; Shahidi, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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12, 15, 16); however, character-impact aroma compounds of cooked crustaceans are not well-understood as compared with the chemistry of fresh seafood flavors. Gas chromatography/olfactometry (GC/O), in particular aroma extract dilution analysis (AEDA), has been successfully applied to many foods (21, 22). The results from A E D A can be used to indicate potent aroma-active compounds. Also, A E D A allows for detection of important aroma-active compounds present at trace levels. Flavor dilution (FD) factors are used to compare potency of aromaactive compounds. A n FD factor is the last dilution at which an aroma-active compound is detected by GC/O. In this paper, previous studies on volatile aroma compounds from crustaceans are reviewed and recent findings on character-impact aroma compounds of cooked crustaceans will be discussed. Volatile Aroma Compounds of Crustaceans Lipoxygenase-Mediated Reaction of Polyunsaturated Fatty Acids. The mild aromas of freshly harvested seafoods are generated by lipoxygenase-mediated reaction. Volatile 6-, 8-, and 9-carbon carbonyls and alcohols formed by the reaction of 12- or 15-lipoxygenases and hydroperoxide lyase contribute to fresh aroma of seafoods (17, 23). Lipoxygenase has been found in fish (24, 25). The mechanism involves lipoxygenase action on eicosapentaenoic acid (17). 1,5Octadien-3-ol was formed by the action of 12-lipoxygenase and hydroperoxide lyase. l-Octen-3-ol and (5Z)-l,5-octadien-3-ol have been reported as mushroommetallic and metallic off-flavors in uncooked prawn and sand lobsters (3). Their ketones, l-octen-3-one and (5Z)-l,5-octadien-3-one have lower threshold values, contributing strongly to fresh crustacean aroma. Lipid Autoxidation. In addition to attack by lipoxygenase, polyunsaturated fatty acids of crustaceans may undergo autoxidation. After lipid autoxidation takes place, fresh aroma will be replaced by stale and oxidized aromas. Hexanal, 2,4heptadienal, 3,5-octadien-2-one, and 2,4-decadienal modify fresh aroma and contribute stale aromas. (E,Z)-2,4-Heptadienal and (E,Z)-3,5-octadien-2-one were formed via autoxidation of eicosapentaenoic acid (23). (Z)-4-Heptenal is responsible for the off-flavor of cold stored cod (26). This compound is formed from the retro-aldol condensation of (E,Z)-2,6-nonadienal (23). Highly oxidized fish aroma is related to 2,4,7-decatrienals (18). Thermally Generated Aroma Compounds. Pyrazines and sulfur-containing compounds have been shown to play important roles in both roasted and boiled shrimp (8). Dithiazine compounds were found to be important to the aroma of cooked krill and small shrimp (9, 10, 12). Both (5Z,8Z,llZ)-and (5E,8Z,11Z)5,8,1 l-tetradecatrien-2-one have been described as having a characteristic aroma of cooked shrimp (7, 11). These compounds were suggested to be formed through thermal decomposition of fl-keto acids produced by B-oxidation of long chain fatty acids (18).

In Flavor and Lipid Chemistry of Seafoods; Shahidi, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

9. BAEK & CADWALLADER

Aroma Compounds of Crustaceans

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Aroma Compounds from Environment. It has been shown that iodoform-like flavor defects exist in the flavor of fresh prawns and saltwater fishes (2, 19). The compounds responsible for this off-flavor were identified as 2-and 4-bromophenol, 2,4- and 2,6-dibromophenol, and 2,4,6-tribromophenol (2). Algae and bryozoa, from diet of prawns, were considered to be the possible origin of bromophenols (2). It is known that these compounds are found primarily in saltwater seafoods (19). Microbial Action. Garlic-like off-flavors have been detected in deep-water prawn and sand lobster (2). The causative agents were identified as bis-(methylthio) methane and trimethylarsine. These compounds are believed to be produced by microbial action in the gut of these crustaceans. Trimethylamine, which is responsible for the fish-house like odor of seafoods, is a product of microbial reduction of trimethylamine oxide (18). Cooked Crustacean Aroma: Character-Impact Aroma Compounds Compared with fresh seafood aroma and off-flavors associated with the environment via diet and microbial action, cooked aroma of crustaceans is of recent research interest. Recently, various cooked crustaceans, such as crab, lobster, and freshwater prawn have been evaluated using A E D A to identify the potent characterimpact aroma compounds (4-6, 27). Also presented in this paper will be some recent findings on A E D A of crayfish and shrimp, in which we employed previously described procedures (4). Aroma-active compounds in cooked meats from crustaceans are listed in Table I. Lipid-derived compounds contributed to the aroma of both cooked and fresh crustaceans. Among these, (Z)-4-heptenal (Fig. 1,1), l-octen-3-one (Fig. 1, H), and (E,Z)-2,6-nonadienal were detected with relatively high F D factors (4). These compounds are considered undesirable to the cooked aroma of crustaceans. Pyrrolidine (alkaline/raw egg aroma note) and (E)-4-decenal (potato aroma note) were detected in only crab (5). Trimethylamine was detected in lobster, crab, and freshwater prawn (4, 5, 27). Thermally generated aromas, such as pyrazines and thiazoles, were important for the nutty/popcorn aroma note of lobster (4). A large number of unknowns having this type of aroma note, as well as meaty aroma note, were detected in other crustaceans evaluated (5, 27). Some compounds were commonly detected in cooked crustaceans, strongly suggesting a common precursor system. The commonly detected aroma-active compounds with high F D factors were 2,3-butanedione (Fig. 1, HI), (Z)-4-heptenal, l-octen-3-one, 2-methyl-3furanthiol (2-MF, Fig. 1, IV), 2-acetyl-1-pyrroline (2-AP, Fig. 1, V), 3(methylthio)propanal (3-MP, Fig. 1, VI), 2-acetylthiazole (Fig. 1, VII), and 2acetyl-2-thiazoline (2-AT, Fig. 1, VIET). Most of these character-impact compounds were thermally generated. Due to their high F D factors and aroma properties, 2,3-butanedione, 2-MF, 2-AP, 3-MP, and 2-AT are considered the most important character-impact aroma compounds in cooked crustaceans.



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Vv=v A A A / /Y o

o (II)

(I)

o (III)

SH

(IV)

(V)

I

I

N

o (VII)

(VI)

N

o (VIII)

Figure 1. Commonly detected aroma-active compounds in cooked crustaceans



f

e

b

8

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d

a

Flavor and Lipid Chemistry of Seafoods - ACS Publications - American

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