Thermal Generation of Aroma Compounds from Tea and Tea

Chapter 29

Thermal Generation of Aromas Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 02/12/16. For personal use only.

Thermal Generation of Aroma Compounds from Tea and Tea Constituents 1

1

1

1

Tei Yamanishi , Michiko Kawakami , Akio Kobayashi , Tsuyoko Hamada , and Yulina Musalam 2

1

Ochanomizu University, Ohtsuka, Bunkyo-ku, Tokyo 112, Japan Research Institute for Tea and Cinchona, Gambung, Bandung 40001 Indonesia

2

The heat processing of tea leads to many complex chemical changes in tea. Tea's taste and aroma is affected by heat in at least three ways: by reducing the content of bitter soluble catechins, by the development of roast aromas and by the thermal degradation of β-carotene. Studies pertaining to the heat-induced changes in tea and appropriate model systems are reviewed. Additional investigations are required to more fully understand the thermal generation of aroma compounds from tea. Thermal processing i s a part of tea manufacture, whether for green, oolong, black or other types of tea. The aroma of tea i s greatly influenced by the type of heat treatment the tea receives. The chemical changes occuring i n tea during heat processing are very complex and not f u l l y understood. In addition, tea aroma may be formed through more than one route. This paper describes some of the chemical changes resulting from heating tea and two model systems that may be important i n the generation of tea aroma. Comparison of Chinese vs Japanese Tea Processing Tea i s made from the tender young leaves of Camellia sinensis. The young leaves are c a l l e d "tea flush" or "tea shoot tips". The production of tea from the tea flush i s outlined i n Figure 1. In the processing of green tea, the tea flush i s f i r s t steamed in the case of Japanese "sen-cha" or pan-fired to produce Chinese "kamairi-cha". This heat treatment inactivates enzymes i n the tea leaves. Steaming produces fewer chemical changes than pan-firing. The heating conditions i n the f i n a l drying and r e f i n i n g stages influence the flavor of the finished green tea product. As the moisture content of the tea leaves decrease, more s i g n i f i c a n t chemical changes, both q u a l i t a t i v e l y and quantitatively, occur. No pyrazines or pyrroles are found i n the aroma concentrate of 0097-6156/89/0409-0310$06.00/0 c 1989 American Chemical Society

Withered j—-^Panning Tea Leaves 150-160°C 8-10 min

Figure 1.

85°C 20 min

Crude Oolong Tea

J

Fermented Tea Leaves

BLACK TEA

Hot Wind Type o r Drum H e a t i n g Type

-->Ref i n i n g

Final Heating^Rolling

^ B l a c k Tea 80-90°C 15^20 min

-•^Drying

Refined Green Tea

Heat Treatment during Tea Manufacturing.

Rm. Temp.

-^Rolling—->Drying

Crude Geen Teal

Secondary Primary HeatingHeating->S t e a m i n g — ^ R o l l i n g — * R o l l i n g - • ^ R o l l i n g

Crude ^ D r y i n g — i j G r e e n Tea (Sen-cha) 150-100 70 Rm. Temp. 45-50 70-v75 Temp. (°C) 100 20^5 30^40 20^30 5-10 30-40 Time (min) 0.5-4.0 35 40 10 12 3 5 60 60 Moisture(%) 80 Crude >Pan F i r i n g — - ^ R o l l i n g — * P a n F i r i n g — * D r y i n g - - - ^ S e c o n d a r y — ^ F i n a l - i(Green Tea (Kamairi-cha)l Drying Drying (Parehing) 100 80 Temp.(°C) 230 Rm. Temp. 150 110 25-35 40-60 Time (min) 8-10 10 10-15 15 20 5-10 Moisture(%) 60 60 55 40

OOLONG TEA

Tea Flush

GREEN TEA


Ï

I

S

ι

S3

ν©


312

THERMAL GENERATION OF AROMAS

the steam processed sen-cha (1)· In pan-fired kamairi-cha, however, several of these roasty flavored compounds have been detected. Figure 2 compares gas chromatograms of the aroma concentrates from pan-fired Chinese green vs Japanese tea. The Japanese tea was from Kumamoto Prefecture i n southern Japan while the Chinese was a Longjing style tea. As seen from the chromatograms, more v o l a t i l e s are formed i n the Chinese Longjing tea. The Longjing tea was browner i n color indicating that more strenuous heating conditions are used for the Chinese tea compared to those used for Japanese kamairi-cha. As seen i n Table I, the concentration of pyrazines, pyrroles and ionone related compounds (from β-carotene) were greater in Longjing than i n Japanese kamairi-cha (2).

Figure 2. Gas Chromatograms of the Aroma Concentrates from Chinese Longjing Tea and Japanese Tea.

Green tea i s usually made from Camellia sinensis var. sinensis (small leaf type). In China, jasmine tea i s made from common kamairi-cha (from var. sinensis) while var. assamica (large leaf type) i s used for Indonesian jasmine tea. The Indonesian jasmine tea has a stronger roast aroma than Chinese jasmine tea (3). The var. assamica tea leaf also contains a much higher l e v e l of p o l y p h e n o l i c catechins than var. sinensis. Catechins are b i t t e r and astringent and thus a high content i s unacceptable. To reduce the concentration of soluble catechins, the pan-fired green tea i s subjected to a r e - f i r i n g p r i o r to scenting with jasmine flowers i.e. Jasminum sambac. As a result, the ref i r e d green tea has additional heat generated v o l a t i l e s . Table II shows some of the heat generated aroma compounds that have been i d e n t i f i e d (_3).

29. YAMANISHI ET AL.

Thermal Generation of Aroma Compounds from Tea

Table I. The Composition of Heat Generated Aroma Compounds from Chinese Longjing and Japanese Kamairi Tea

Peak Area %

Peak No.


in F i g .2

Compound

Longjing

i n Aroma Japanese

11 16 18 23

Pyrazines 2,5-Dimethylpyrazine 2-Methyl-5-ethylpyrazine Trimethylpyrazine 2,5-Dimethyl-3-ethylpyrazine

0.3 0.1 0.1 0.3

-

37 40 76

Pyrroles l-Ethyl-2-formylpyrrole l-Ethyl-2-acetylpyrrole 2-Acetylpyrrole

1.8 0.5 3.0

1.8

Ionone Related Compounds 2,6,6-Trimethyl-2-OH-cyclohexanone β-Cyclocitral 2,6,6-Trimethylcyclohex-2-l,4-dione β-Ionone & (cis-jasmone) 5,6-Epoxy-fi-ionone Theaspirone D ihydroac t i n i d io1ide

1.9 1.1 0.2 3.5(1 .2) 2.2 0.5 1.7

0.8 0.5

36 38 47 74 77 92 106

—

-

1.1

-

2.2(3.8) 2.1

-

1.4

Adapted from Kawakami, M. and Yamanishi, T. (1983)

Table I I . Components of Indonesian Pan-fired Green Tea

Pyrazines 2-Methyl2-Ethyl-

2,5-Dimethyl2,3-Dimethy12,3,5-Trimethyl-

2,6-Dimethyl2-Methyl-5-ethyl2,3,5,6-Tetramethyl-

2-Acetyl-

l-Ethyl-2-formyl(isomer)

5-Methylfurfural

Furfuryl alcohol

2-Methyl-6-ethylPyrroles l-Ethyl-2-formylFurfural Furans Acids Hexanoic

Octanoic

Ionone related Compounds a-Ionone β-Ionone Dihydroactinidiolide 2,6,6-Trimethylcyclohex-2-enone trans-Geranic acid

313

314



Products of Heated Tea In Japan, a lower grade of green tea, "ban-cha" i s roasted to make i t s flavor more acceptable. Roasted ban-cha i s c a l l e d h o j i cha. The optimum temperature for roasting i s about 180°C. The aroma concentrate from hoji-cha has a strong c h a r a c t e r i s t i c roast aroma. Hoji-chà produced about 3 times the aroma concentrate than the o r i g i n a l ban-cha. From the basic f r a c t i o n which comprised 29% of the aroma concentrate, 19 d i f f e r e n t pyrazines were i d e n t i f i e d . The neutral f r a c t i o n which was 47% of the aroma concentrate contained furans and pyrroles along with the o r i g i n a l tea aroma. In addition, ionone related compounds such as theaspirone, dihydroactinidiolide and a large amount of β-ionone were found i n the neutral fraction. Table III shows the increase of furan and pyrrole content due to roasting of the ban-cha (4 ). Pyrazines and pyrroles are generated from amino acids and sugars by heating. β-carotene, another important component of tea leaf i s the precursor of pleasant aromatic compounds. I t i s present the var. s i n e n s i s leaves at about 21.7 mg/100 g dry weight. To c l a r i f y the role of β-carotene to the aroma of roasted green tea, β-carotene was heated i n a pyrolyzer at 180°C for 6 minutes. The reaction was carried out under a i r with and without catechin gallates, a component of tea leaves. The v o l a t i l e products were trapped i n a precolumn cooled by dry ice/acetone. The precolumn was then connected to a GC c a p i l l a r y column and the v o l a t i l e s then analyzed by GC-MS.

Table I I I .

Increase of Furans and Pyrroles during Roasting of Ban-cha to Produce Hoji-cha

Area % i n Aroma Concentrate Ban-cha

Hoji-cha

Furans Furfuryl alcohol Furfural 2-Acetylfuran 5-Methylfurfural

0.5 0.2

13.7 9.3 4.7 2.9

Pyrroles 2-Acetylpyrrole 2-Formylpyrrole l-Ethyl-2-formylpyrrole a Pyrrole derivative

0.9 0.9 0.9

5.4 2.6 4.7 9.1

Ten v o l a t i l e compounds were produced from the pyrolysis of Bcarotene. Among them, toluene, xylene, β-cyclocitral, ionene, βionone, 5,6-epoxy-β-ionone and dihydroactinidiolide were i d e n t i f i e d . The addition of catechin gallates reduced the quantity of the ten


29. YAMANISHI ET AL.

Thermal Generation ofAroma Compounds from Tea

v o l a t i l e s by about two thirds (5). Ionone related compounds such as β-ionone, 5,6-epoxy-ionone and dihydroactinidiolide were also i d e n t i f i e d i n the aroma concentrate from sen-cha (1) . In another study, β-carotene was heated i n aqueous medium at 90°C, 120°C and 150°C. More than 40 d i f f e r e n t compounds were found i n the ether extracts by GC-MS as shown i n Figure 3. Dihydroactini d i o l i d e (sweet peachy aroma) was found i n highest concentration at a l l temperatures studied. At 90°C, 5-6-epoxy-B-ionone (sweet, v i o l e t - l i k e ) was found i n second highest quantity, while at 150°C, 2,6,6-trimethyl-2-hydroxy-cyclohexanone (green, citrusy) and 2,6,6trimethyl-2-hydroxy-cyclohexan-l-aldehyde ( f l o r a l , geraniol-like) were found i n large quantity. At 120°C, these compounds were more evenly balanced than at 90°C or 150°C. A balance of ionone related compounds seem to contribute to an a t t r a c t i v e green tea flavor. This data i s outlined i n Table IV (6).

90 ° C

10

20

1

1

i

30

r

1

40 min

150 ° C

10

^

20

&

a

b

h

i

M

L

N

6 x 5f°

c

d

e

^

t^^^^y 9

40

30

j

k

ι OH

fie- » 4

0 ,

Figure 3. Products of the thermal degradation of ^-carotene in aqueous medium. (Data are from ref. 6.)

315

316

THERMAL GENERATION OF AROMAS Table IV.

Ionone Related Compounds Identified i n the Thermal Degradation of β-Carotene

Peak


90°C Dihydroactinidiolide* 5,6-Epoxy-B-ionone 2,6,6-Trimethyl-2-hydroxycyclohexane-1-aldehyde (tentative) 2,6,6-Trimethyl-2-hydroxycyclohexanone B-Ionone 2,6,6-Trimethyl-2 3-epoxycyclohexliden-l-acetaldehyde 4-0χο-β-ionone β-Cyclocitral Ionene β-Damascone 2,6,6-Trimethyl-cyclohex-2-enone 2,6,6-Trimethyl-cyclohexanone f

*

Area 120°C

% 150°C

35.5 24.3 4.6

42.2 17.1 2.2

45.4 9.0 14.9

3.8

3.5

9.2

2.5 2.5

7.9 2.0

1.5 2.0

1.4 0.8 0.2 0.2 0.4 0.2

1.5 2.9

0.2 1.0 0.5 0.5 0.5 0.5

-0.1 0.1

Found i n pyrolized β-Carotene (_5) Data are from ref. 6.

SOURCE

The Role of Catechins Catechins are the most abundant components i n tea flush. As previously mentioned, catechins have a b i t t e r , astringent taste. The concentration of individual catechins i n tea flush are shown i n Table V. The most abundant catechin i s (-)-epigallocatechin gallate.

Table V.

(-)-Epigallocatechin gallate (-)-Epicatechin gallate (-)-Epigallocatechin (-)-Epicatechin

Catechins i n Tea Flush

(i) (ii) (iii) (iv)

10.7 3.3 3.2 1.2

14.4 4.3 2.9 1.0

% Dry Wt.

Table VI shows the decrease of catechin by heating as reported by Anan and Kato 1984 (7). The loss of catechins at 70°C i s greatly influenced by the addition of amino acids.

29.

YAMANISHI ET AL. Table VI.

Sample

Changes i n Catechin Content During Heating of Catechin-Amino Acid Blend and Catechin Alone

Time (min) at 150°C

A Thermal Generation of Aromas Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 02/12/16. For personal use only.


30 70 30 70

Β

Loss of Catechin (%) (iii) (ii)

(i) 11.8 61.2 4.6 6.5

36.5 90.4 19.2 26.9

11.7 52.4 3.8 2.8

(iv) 28.3 80.0 14.5 38.4

Sample A:

Mixture of crude catechin, theanine, glutamic acid, arginine and serine (20:3:1:1:1) Sample B: Crude catechin alone Adapted from Anan, T. and Kato, H. (1984) (7)

L-Theanine i s the most abundant amino acid i n tea flush. V o l a t i l e s produced by pyrolysis at 180°C of (A) L-theanine, (B) (-)-epigallocatechin gallate and (C) a mixture of (A) and (B) were examined. The procedure was the same as that reported e a r l i e r for the pyrolysis of β-carotene. The results of the GC-MS analysis are shown i n Figure 4. From L-theanine alone, a large amount of Nethyl-formamide was formed, along with ethyl amine, propyl amine, 2pyrrolidone, N-ethyl-succinimide and l-ethyl-3,4-dehydropyrrolidone.

Peak a E t h y l amine b P r o p y l amine c 2-Pyrrolidone d Water e N - E t h y l formamide f N-Ethyl succinimide & l-Ethyl-3,4-dehydropyrrolidone g Unknown

0

10

20

30

40

50

60

70

80

min

Figure 4. Gas Chromatograms of Thermal Degradation Products from (A) Theanine, (B) Theanine and Epigallocatechin Gallate, and (C) E p i g a l l o c a t e c h i n Gallate. GC C o n d i t i o n s ; Column SE 30 SCOT, 15 m X 0.5 mm i.d., Column Temp., 30°C(10 min hold) 170°C(3°C/min), Carrier Gas, Helium 3.8 ml/min

317


318


The addition of catechins (sample B) greatly reduced these products. Thus catechins show a s i m i l a r e f f e c t on the pyrolysis of β-carotene and L-theanine. (Yamanishi, T. & Hamada, T. unpublished.) Hara i n 1981 (8) reported on v o l a t i l e s produced by roasting Ltheanine and glucose at about 150°C for one hour. l-Ethyl-3,4dehydropyrrolidone was the main product. Five pyrroles, three a l k y l pyrazines and four furans which were i d e n t i f i e d by GC-MS and NMR are shown i n Table VII. With the exception of l-ethyl-3,4-dehydropyrrolidone these products are quite d i f f e r e n t than found from Lthreonine alone. Surprisingly, l-ethyl-3,4-dehydropyrrolidone has never been found i n tea aroma.

Table VII.

V o l a t i l e Compounds Identified from Roasting L-Theanine and D-Glucose

Compounds

Peak Area

l-Ethyl-3,4-dehydropyrrolidone 1-Εthy1-5-methylpyrrο1e-2-a1dehyde 5-Methyl-2-furfuryl alcohol 2,3-Dihydro-3,5-dihydroxy-6-methyl4H-pyran-4-one a Pyrrole derivative 1-Ethylpyrrole a Pyrrole derivative a Pyrrolidone derivative a Pyrrole derivative Methylpyrrole 1- Ethyl-2-acetylpyrrole 2- Acetylfuran 5-Methyl-2-furaldehyde 2,5- (or 2,6)-Dimethylpyrazine Trimethylpyrazine 2-Acetylpyrrole 2-Furaldehyde 2-Methylpyrazine Adapted from Hara, T.

(1981)

%

41.1 9.8 9.0 8.7 5.1 4.0 3.6 3.4 2.1 1·5 1.1 1.0 1.0 0.8 0.4 0.3 0.3 0.2

(8)

CONCLUSIONS It has been shown that the aroma of tea i s affected by the heat treatment received. Tea aroma and flavor are greatly influenced by catechins and proceeds by more than one pathway. The catechins influence tea aroma and flavor i n three ways. F i r s t , catechins have a b i t t e r , astringent taste. Second i n order to reduce the l e v e l of soluble catechins (i.e. i n var. assamica), a second heat treatment i s required. This r e f i r i n g produces a stronger roast aroma. Third catechins strongly influence the pyrolysis of β-carotene and Ltheanine. A tea of good quality possesses a balance of tea aroma compounds. This requires control of the heating process. Because

29. YAMANISHI ET A L


of i t s complicated nature, further detailed research on the thermally generated aroma of tea i s necessary.


Literature Cited 1. Takei, Y.; Ishiwata, K.; Yamanishi, T. Agric. Biol. Chem. 1976, 40, 2151-2157. 2. Kawakami, M.; Yamanishi, T. Agric. Biol. Chem. 1983, 47, 2077-2083. 3. Mussalam, Y.; Kobayshi, Α.; Yamanishi, T. Proceesings of the 10th International Congress of Essential Oils, Fragrances and Flavors, Washington, DC U.S.A., 1986, 659-668. 4. Yamanishi, T.; Shimojo, S.; Ukita, M.; Kawashima, K.; Nakatani, Y. Agric. Biol. Chem. 1973, 37, 2147-2153. 5. Kawashima, K.; Yamanishi, T. Nippon Nogeikagaku Kaishi 1973, 47, 79-81. 6. Kawakami, M. Nippon Nogeikagaku Kaishi 1982, 56, 917-921. 7. Anan, T.; Kato, H. Nippon Shokuhin Kogyo Gakkaishi 1984, 31, 321-326. 8. Hara, T. Nippon Nogeikagaku Kaishi 1981, 55, 1069-1072. RECEIVED May 11, 1989

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Thermal Generation of Aroma Compounds from Tea and Tea

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