Thermal Generation of Aromas - American Chemical Society

Chapter 35

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Identification and Formation of Characteristic Volatile Compounds from Cooked Shrimp Kikue Kubota, Chinatsu Uchida, Keiko Kurosawa, Ayako Komuro, and Akio Kobayashi Ochanomizu University, Laboratory of Food Chemistry, 2-1-1, Ohtsuka, Bunkyo-ku, Tokyo 112, Japan

An investigation into the aroma of cooked small shrimp and k r i l l revealed more than forty sulfur- and/or nitrogen-containing heterocyclic substances. Among these eight were newly identified as contributors to food flavor. The main ring structures encountered were pyrazine, trithiolane, dithiin and dithiazine. It was a characteristic of small shrimp flavor that many varieties of cyclic polysulfides were formed during cooking. Some distinction in the composition of these heterocyclic substances were also observed among the shrimp species or from the treatment before heating, and led to the following conclusion: the content of free amino acids should give the initiative for the formation pathway of the heterocyclic substances in small shrimps. (All-Z)- and (5E,8Z,11Ζ)-5,8,11-tetradecatrien2-ones were determined as other important thermally generated constituents in shrimp. These two isomers had the most acceptable seafood aroma among the eight possible geometrical isomers that were synthesized. Seafood i s one of the world's main protein sources. Among seafood, shrimp i s consumed i n large quantity throughout the world because of i t s pleasant f l a v o r . Raw shrimp has l i t t l e odor, but develops a pleasant charac t e r i s t i c roast aroma upon cooking. Antarctic k r i l l i s becoming important protein source because of i t s abundance. Although taxonomically different, k r i l l i s much l i k e shrimp i n appearance and w i l l be treated as small shrimp i n this report. Cooked k r i l l , however, possesses a less acceptable aroma compared to shrimp. From our aroma research on boiled small shrimps, almost one hundred v o l a t i l e components were i d e n t i f i e d . Among them, more than forty components were determined as s u l f u r - and/or nitrogencontaining heterocyclic substances, together with various kinds of v o l a t i l e s that are well known to be thermally generated such as hydrocarbons, carbonyl compounds, alcohols and phenols. The shrimp 0097-6156/89/0409-0376$06.00/0 ο 1989 American Chemical Society Parliment et al.; Thermal Generation of Aromas ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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Characteristic VolatilesfromCooked Shrimp

377

samples evaluated include Sergia lucens (l-3) Acetas japonicus (4), Pleoticus muelleri (_5), Euphausia superba (Antarctic k r i l l ) (6-8) and Euphausia p a c i f i c a (9,10). It seems worthwhile to summarize the main c h a r a c t e r i s t i c heterocycles i n the v o l a t i l e s from cooked shrimps, because of the attention payed to heteroatomic substances as food f l a v o r s (11). In addition, two unknown polyunsaturated methylketones were isolated i n the v o l a t i l e s of boiled small shrimp. They drew our p a r t i c u l a r i n t e r e s t because t h e i r structure contained a l i n o l e n i c acid type component and because of t h e i r seafood-like odor. In the present paper, we describe the i d e n t i f i c a t i o n and formation of the previously-described heterocyclic compounds and polyunsaturated methylketones i n the v o l a t i l e s of boiled small shrimp.


f

I d e n t i f i c a t i o n of Sulfur- and/or Nitrogen-Containing Components

Heterocyclic

The v o l a t i l e s from cooked small shrimps were obtained using a simultaneous d i s t i l l a t i o n and extraction apparatus. These v o l a t i l e s were separated into neutral and basic fractions. The fractions were then analyzed by gas chromatography using a polar column and a FPD detector. Sulfur-containing compounds were found i n a segment of the high-boiling f r a c t i o n of j5^_ lucens v o l a t i l e s . The high b o i l i n g f r a c t i o n contained many i n t e r e s t i n g polysulfur-containing heteroc y c l i c compounds which were determined spectroscopically. On the other hand, the basic fraction, which had a strong roasted odor, contained the largest portion of the v o l a t i l e s from jS^ lucens. Many nitrogen-containing heterocyclic substances were found here. This pattern was also observed i n other shrimp samples. The structures of forty-four s u l f u r - and/or nitrogen-containing heterocyclic compounds were conclusively determined. The i d e n t i f i e d compounds and t h e i r occurrence i n small shrimps are l i s t e d i n Table I. Most classes of heterocycles found are also commonly observed in other cooked food flavors. Pyridines, pyrroles, thiophenes and a l k y l t h i a z o l e s were minor components. As main constituents, ten methyl- and ethyl-substituted alkylpyrazines were i d e n t i f i e d . The pyrazines occurred markedly i n a l l samples and seemed to be one of the main contributors to the roasted-nut l i k e f l a v o r of cooked small shrimp. Six kinds of a l k y l t r i t h i o l a n e s were found, and among them, dimethyl homologs were p r i n c i p a l i n the shrimps. 3,5-Dimethyl1,2,4-trithiolanes have been found i n the v o l a t i l e s from cooked beef (12), soy bean (13) and some s h e l l f i s h (14) and 3-ethyl-5-methyland 3,5-diethyl-l,2,4-trithiolanes were i d e n t i f i e d i n a commercial beef extract (15) and Allium plant (16), respectively. They have a strong Allium p l a n t - l i k e odor. 2,4,6-Trimethyl-l,3,5-dihydrodithiazine (thialdine), which had been i d e n t i f i e d as the f l a v o r c o n s t i tuent of cooked meats (12, 17-21) and beans (13,22), was also found in a l l samples and i t was the main constituent i n both £1^ lucens and A. japonicus. The odor of t h i a l d i n e at pH 8.2 has been evaluated as that of medium-roast shrimp by Kawai et a l . (23); therefore, i t i s thought that t h i a l d i n e influenced the flavor of our shrimp samples. On the other hand, dimethyl and methyl ethyl-1,3-dithiins, and ethyl-, propyl- and butyl-substituted homologs of dihydrodithiazines

Parliment et al.; Thermal Generation of Aromas ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

378

THERMAL GENERATION OF AROMAS

Table I. Heterocyclic Compounds Identified i n the V o l a t i l e s from Boiled Small Shrimps


Compound Pyridine Pyridine 2-Methyl3-Methyl3-Ethyl3-Ethyl-5-methylPyrazine Pyrazine 2-Methyl2,3-Dimethyl2,5-Dimethyl2,6-DimethylTrimethyl2-Ethy1-5-methyl2-Ethyl-3,5dimethyl2-Ethyl-3,δ α ime t h y l Tetramethyl-

Rem*

s,

s, s, s, s, s,

Indole Thiazole Acetyl2,4-Dimethyl4,5-Dimethyl2,4,5-Trimethyl2-Ethyl-4,5dimethyl2-Isopropyl-4,5dimethyl-

Thiophene 2-Formyl2-Formyl-6-methyl2-Acetyl2-Acetyl-6-methyl-

A,Ea,Ep Ea Ea Ea A

A, Ea

s,

A, Ea A, Ea

1,3-Dithiin 2,6-Dimethyl6-Ethyl-2-methy1-

A,P,Ea

s,

,A,Ea,Ep Ea Ea Ea

s,

Ea Ea Ea Ea

s,

P,Ea Ea

1,3,5-Dihydrodithiazine 2,4,6-TrimethylS,A, P,Ea, Ep (Thialdine) 2-Ethyl-4,6S,A, Ea dimethyl4-Ethyl-2,6dimethylS,A, Ea 2,4,6-TriethylΡ Ρ 4,6-Dimethyl4,6-DimethylEa 2-propyls, 4-Butyl-2,6Ea dimethyls,

Ea Ea s,

Rem

1,2,4-Trithiolane (cis; & trans) S,A, P,Ea 3,5-Dimethyl3-Ethyl-5-methyls, P,Ea Ea 3,5-Diethyl-

A, Ea,Ep A,Ea,Ep A,Ea,Ep A,Ea,Ep A,Ea,Ep A,Ea,Ep Ea, Ep

s,

Pyrrole Pyrrοle i-Methyl -

Compound

Bicyclo-1,3,5-dithiazine Pyrrolidino[1,2-e] 4H-2,4-dimethyl- s ,

Ea

Ea Ea

* Remark shows i n which samples each compound occurred. S: S. lucens (_1, 3^), A: A. japonicus (4), P: P. muelleri ( 5 h Ea: Antarctic k r i l l U ' P k r i l l (9^). E

:

P

a

c

i

f

i

c



35.

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Characteristic Volatiles from Cooked Shrimp

379

were newly i d e n t i f i e d i n the v o l a t i l e s from shrimp, which have only previously been reported as products of the model or synthetic reactions from corresponding aldehydes, hydrogen sulfide and ammonia or aldehydes and ammonium s u l f i d e (10, 23-26). To date, no report concerning these as products from foodstuffs has been available. The concentration of these compounds were not as high as that of thialdine; however, as each compound had a c h a r a c t e r i s t i c odor l i k e fuel gas or Allium plant, and the threshold values were low, they must also have had a s i g n i f i c a n t effect on shrimp flavor. It i s concluded that the formation of various c y c l i c polysulfides i s important to obtain an c h a r a c t e r i s t i c cooked shrimp f l a v o r . Recently, we i d e n t i f i e d a new type of b i c y c l i c derivative of a dithiazine compound, i . e . pyrrolidino[l,2-e]4H-2,4-dimethyl-l,3,5dithiazine (10) i n the cooked v o l a t i l e s from S^ lucens and precooked E. superba. Such a b i c y c l i c compound i s not common i n the v o l a t i l e of food, and no b i c y c l i c compound has previously been found under the formation system for thialdine homologs; i n t h i s case, a nitrogen source other than NHg should be considered. Further d e t a i l s on the precursor are under investigation. The odor of this b i c y c l i c dithiazine was s i m i l a r to that of thialdine and the concentration was almost as large as that of thialdine among the dithiazine contents i n lucens and E^ superba. Composition of the Main Heterocyclic Compounds As previously mentioned, the main components of the v o l a t i l e s from small boiled shrimp were pyrazines and c y c l i c polysulfides. The composition of the v o l a t i l e s was roughly divided into two types, those with a higher content of pyrazines or with a higher content of c y c l i c polysulfides. The compositions of pyrazines, t r i t h i o l a n e s and dithiazine derivatives i n the v o l a t i l e s from boiled shrimps were calculated from the GC peak areas (1,4,7,27) and are shown i n Figure 1. The contents of t r i t h i o l a n e s and dithiazines were larger than that of pyrazines i n raw jS^ lucens and A^_ japonicus, both of which belong to the same taxonomical group. Conversely, i n raw Antarctic k r i l l , alkylpyrazines comprised the predominant part of the volat i l e s with few c y c l i c polysulfides also present. The composition of precooked k r i l l corresponded to that of lucens, and that of fermented A. japonicus to raw k r i l l . Some reports (23-25) proved that t r i t h i o l a n e s , d i t h i i n s and dithiazines are e a s i l y produced from aldehydes, ammonia and hydrogen sulfide, which are universally generated from the thermal cleavage of sugar, l i p i d and amino acid i n the cooked foodstuffs. As shown in Table II, i t seems that the main reaction was due to the content of t o t a l free amino acids. As the protease a c t i v i t y of Antarctic k r i l l i s high (28), raw k r i l l i n the market contains more free amino acids than S^_ lucens or A^ japonicus. Precooked k r i l l was produced on a commercial basis by b o i l l i n g on board for 5 minutes i n 90°C sea water just after harvest to deactivate protease (28), the elution of free amino acids into sea water lowering the amino acid content i n precooked k r i l l . On the other hand, the content of free amino acids are greatly increased during the formation of raw fermented A. japonicus by i t s own protease. In conclusion, the concentration of free amino acids as the



380

(GC P e a k A r e a Pyrazine

%)

Polysulfide

Material


Η

Η

Η

24J6

S. l u c e n s

Raw E, s u p e r b a 13.3

Raw 0.3

trace

0.1

12.8

Figure 1. Compositions of Main Heterocycles i n the V o l a t i l e s from Cooked Shrimps.

precursor should give the i n i t i a t i v e for the formation pathway of the cooked v o l a t i l e i n small shrimps; i.e. a larger quantity of free amino acids would result i n the predominant formation of pyrazines and, on the contrary, the formation of such c y c l i c polysulfides as t r i t h i o l a n e s and dithiazines would precede the pyrazine formation when the concentration of free amino acids i s less i n shrimps.


35.

KUBOTAETAL.

381


Table I I . pH, Ammonia Ν and Free Amino Acid Content of S. lucens, E. superba (7) and A^_ japonicus (4^)

Amino acid Sample

pH

Ammonia Ν

Cys

**


mol/g s. lucens (raw) E. superba (raw) (precooked)

8.0-8.8 7.7-8.4 8.0-8.7

A. japonicus (raw) (fermented)

7.4 6.9

* **

41.3 50.1 10.4

Total

5.8 11.9 1.8

mol/g 701.7 1113.4 479.6

12.9 39.4

mol/N 100g** 4794.5 8190.4

pH shows b e f o r e c o o k i n g - a f t e r c o o k i n g i n d r y w e i g h t o f sample

I d e n t i f i c a t i o n of 5,8,1l-Tetradecatrien-2-one The cooked aroma of E^_ p a c i f i c a (Pacific k r i l l ) i s fishy and r e l a t i v e l y unacceptable. Its GC p r o f i l e i s quite d i f f e r e n t than that of other samples, because N,N-dimethyl-2-phenylethyl amine was a p r i n c i p a l v o l a t i l e component while only small quantity of pyrazine and thialdine were observed (see Table I). This amine was not produced by cooking, i t occurs i n vivo (27). V o l a t i l e s from both raw and boiled (85°C, 90 min) E^ pacif i c a were prepared by steam d i s t i l l a t i o n under reduced pressure (20 Torr). Gas chromatograms of these v o l a t i l e s are shown i n Figure 2. The GC p r o f i l e s show that two compounds (peaks A and B) were newly produced by cooking. By s n i f f i n g the effluents from a gas chroma tograph, i t was determined that these two components have a charac t e r i s t i c seafood-like aroma, although not intense. The spectroscopic and synthetic investigations suggested that these two compounds A and Β were novel unsaturated methylketones determined as (5Z,8Z, 11Z)- and (5E,8jZ, 11Ζ)-5,8,1 l - t e t r a d e c a t r i e n - 2 one, respectively (10). They are stereoisomers to each other, compound A having the same p a r t i a l structure and configuration as that of a ω-3 f a t t y acid. Both these methylketones were detected together i n the v o l a t i l e s from a l l cooked samples of Su_ lucens, A. japonicus, P. muelleri and E^ superba. It appears that these novel methylketones are important constituents i n addition to the above-described heteroatomic substances of the flavors of cooked shrimps. Sensory Evaluation and Occurrence of Eight Isomers As these methylketones have three double bonds i n t h e i r structures, there should exist eight geometrical isomers. We synthesized a l l eight isomers (29) and checked the occurrence of the isomers i n the v o l a t i l e s of shrimps by mass chromatography. Only the ( a l l - Z ) - and (5E,8Z, 11Z)-isomers could be found i n shrimp v o l a t i l e s .



382

CH-CH-N' 3 CH 3

Heated

Sample

I.S.


90 m i n

O^CH-OH

IliLi 20

ι

40

60

Non-Heated I.S.

11 ι 20

40

U

Sample

JI>*LLILJL 60

mxn

Figure 2. Gas Chromatograms of the V o l a t i l e s from E. p a c i f i c a Prepared by Steam D i s t i l l a t i o n under the Reduced Pressure. Conditions; Column, PEG 20M 0.25mm i.d. X 50m FS-WC0T, Oven Temp., 60 — » 180°C, 2 °C/min I.S.: Internal standard, Undecanol. Peaks A and B: 5,8,ll-Tetradecatrien-2-one

In addition, t h e i r aroma c h a r a c t e r i s t i c s were evaluated by a sensory test, the results being summarized i n Table III (29). I t i s intere sting that two ketones found i n pacif ica (I and II) had a t y p i c a l seafood aroma reminiscent of cooked small shrimps and s h e l l f i s h . Since compounds III and V also had an aroma l i k e seafood products, the presence of the C - l l (Z) double bond seems to be a key factor i n the aroma of seafood products.


35.

KUBOTA ET AL. Table I I I .

Characteristic Volatiles from Cooked Shrimp Kovat's Index Data and Aroma C h a r a c t e r i s t i c s o f Isomers o f 5 , 8 , l l - T e t r a d e c a t r i e n - 2 - o n e


**

KI Value*

Isomer

Aroma Characteristics

I

{5Z_ QZ_ 11Z)***

2014

shrimp, c r a b , s h e l l

II

(5JE,8Z^ 11Z)

2033

shrimp, c r a b , sea cucumber

III

(5^,812,11Z)

2037

fruity,

oily,

seafood

IV

(5JZ, 8Z_,

HE)

2011

fruity,

oily,

milk

V

(5E,8E,

HZ)

2041

s h o r t - l e g g e d clam, s e a f o o d

VI

(5^,8^, 11E)

2029

s h o r t - l e g g e d clam,

VII

(5£, 81^,1 IE)

2030

fruity,

VIII

(5IS,8IS, 11JE)

2033

oily,

r

383

t

f

fish

oily

cucumber, white-meat

fishly,

fash

dry bonito

* **

KI v a l u e was measured by u s i n g a PEG 20M FS-WCOT column. Sensory t e s t was done by s m e l l i n g paper d i p p e d i n t o a 3-4% e t h a n o l s o l u t i o n o f each sample. *** Numbers a r e the c a r b o n numbers, and Z_ and JE a r e t h e a r r a n g e ments o f t h e i r atoms i n s p a c e . Reproduced w i t h p e r m i s s i o n from Ref. 29. C o p y r i g h t 1989 A m e r i c a n Chemical S o c i e t y .

One o f the n a t u r a l l y o c c u r r i n g m e t h y l k e t o n e s had an E_ d o u b l e bond i n i t s s t r u c t u r e ; however, i t r e t a i n e d two Z_ c o n f i g u r a t i o n s , and no o t h e r IS i s o m e r s c o u l d be found t h r o u g h our s t u d y . We assume t h a t b o t h p a r t i a l s t r u c t u r e s o f t r i d o u b l e bonds were o r i g i n a l l y p r e s e n t i n raw shrimps, and were n o t i s o m e r i z e d i n t o each o t h e r d u r i n g the h e a t i n g and e x t r a c t i n g p r o c e s s . Formation of

5,8,ll-Tetradecatrien-2-one

C o n s i d e r i n g t h a t u n c o n j u g a t e d (Z)-double bonds s e p a r a t e d by methylene i s a t y p i c a l p a r t i a l s t r u c t u r e of a n a t u r a l u n s a t u r a t e d f a t t y a c i d , i t was i n v e s t i g a t e d whether the l i p i d f r a c t i o n o f s h r i m p are i n v o l v e d i n the f o r m a t i o n o f t h e s e m e t h y l k e t o n e s o r not. N e i t h e r a l i p i d e x t r a c t w i t h c h l o r o f o r m - m e t h a n o l (2:1) n o r t h a t w i t h i s o p r o p a n o l f r o m s m a l l s h r i m p produced any i s o m e r s o f 5 , 8 , 1 1 - t e t r a d e c a t r i e n - 2 - o n e on h e a t i n g . In c o n t r a s t , b o t h t h e ( a l l - Z ) - and (5E,8Z, 11Z)-isomers o f t h e m e t h y l k e t o n e were d e t e c t e d i n v o l a t i l e s f r o m the d e f a t t e d r e s i d u e h e a t e d w i t h water. F u r t h e r m o r e , the q u a n t i t y formed was i n f l u e n c e d by t h e h e a t i n g t e m p e r a t u r e and t i m e . As shown i n F i g u r e 3, when t h e h e a t i n g t e m p e r a t u r e was r a i s e d t o 80°C, t h e amount o f m e t h y l k e t o n e s formed was i n c r e a s e d markedly; and the l o n g e r the h e a t i n g t i m e , the g r e a t e r t h e q u a n t i t y o f k e t o n e s produced. In b o t h c a s e s , the i n c r e a s e o f (Ε,Ζ,Ζ) i s o m e r s was



384


Ketone/I.S, Peak A r e a % 60 H

•

ΑΑΛΑΛ

ο

ΑΛΑΛ/y

40-4

20 H

H e a t i n g Time Temp.

ο Room Temp.

90 60

30

60 90

90

(min)

80

95 (°C)

Figure 3. Effect of Heating Temperature and Heating Time on the Formation of 5,8,ll-Tetradecatrien-2-ones i n the V o l a t i l e of E^_ pacif i c a .

greater. Denaturation of the protein i n the sample was observed at c. 80°C. These results show that the precursor existed i n the defatted f r a c t i o n and that a temperature higher than 80°C was needed for the formation of methylketones. We are now assuming that the protein f r a c t i o n should have some relationship to the methylketone formation. Conclusion Various kinds of heterocycles and two unsaturated methylketones were i d e n t i f i e d as c h a r a c t e r i s t i c components i n the v o l a t i l e s from cooked small shrimps. Without exception, they were a l l thermally generated compounds. Some v o l a t i l e components from cooked small shrimps were in common with those of other animal protein foodstuffs l i k e meat; however, various types of compounds found i n another foodstuffs were composed of the v o l a t i l e s from s p e c i f i c shrimp species. Both the precursors and the formation pathways for the t y p i c a l aroma com pounds have already been elucidated, even though i t i s d i f f i c u l t to explain the d i f f e r e n t constituents of the v o l a t i l e components among shrimp species. In future, i t w i l l be necessary to investigate the key factors which define the possible pathway to form c h a r a c t e r i s t i c v o l a t i l e s i n each foodstuff.

Literature Cited 1. 2.

Kubota, K.; Kobayashi, Α.; Yamanishi, T. Nippon Nogeikagaku Kaishi 1982, 56, 1049. Choi, S. H.; Kato, H. Agric. Biol. Chem. 1984, 48, 1479.


35. KUBOTA ET AL. 3. 4. 5. 6.


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