CHEMICAL CHANGES DURING STORAGE OF TEA

CHEMICAL CHANGES DURING STORAGE OF TEA

CHAPTER 12 CHEMICAL CHANGES DURING STORAGE OF TEA Tei Yamanishi Ochanomizu University Tokyo, Japan I. II. I. Introduction Chemical Changes Respon...

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CHAPTER 12

CHEMICAL CHANGES DURING STORAGE OF TEA

Tei Yamanishi Ochanomizu University Tokyo, Japan

I. II.

I.

Introduction Chemical Changes Responsible for Deterioration in Quality of Tea A. Black Tea B. Green Tea Tables Figures References

665 666 666 668 671 680 683

INTRODUCTION

The characteristics of tea as a beverage are its taste, aroma, and color. These are quite different from coffee, fruit juices, and other soft drinks. Polyphenols such as catechins and amino acids such as theanine are the main contributors to the unique taste and color of tea. The components of essential oil in fresh tea leaves and volatile compounds developed during the manufac­ turing process form the characteristic tea flavor.

Handbook of Food and Beverage Stability: Chemical, Biochemical, Microbiological, and Nutritional Aspects

Copyright © 1986 by Academic Press, Inc. All rights of reproduction in any form reserved. 665

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Although many types of tea are available around the world, teas may be classified into three general categories according to the manufacturing process used: fermented (black tea), semifermented (oolong and pouchong), and non­ fermented (green tea). The different manufacturing conditions cause the differ­ ences in taste, aroma, and color as well as in storage stability or shelf-life of tea.

II.

CHEMICAL CHANGES RESPONSIBLE FOR DETERIORATION IN QUALITY OF TEA

The important factors affecting tea quality are flavor (aroma and taste) and color of tea liquor. Deterioration in quality of tea is caused principally by chemical changes in the components that contribute to these factors.

A.

Black Tea

Freshly made black tea has a raw or "green" flavor, but this rawness is replaced by a balanced astringency and flavor. However, prolonged storage leads to a deterioration in the quality of the tea. Deterioration in black tea is caused by (1) losses of volatile components, (2) changes in catechins, amino acids, theaflavins, and other pigments, and (3) increases in unde­ sirable "taints" such as oxidative reaction products from fatty acids, and oxidation and condensation products from soluble polyphenols such as catechins and theaflavins. These reactions are accelerated by tea moisture content, elevated temperature, and exposure to light. 1.

Volatile Components

Stagg (9) reported that the volatile fraction of black tea showed an overall decline that was accelerated with moisture uptake and, to some extent, by storage at elevated temperature, as shown in Table I. The decreases are found among aliphatic aldehydes and alcohols with b.p. 108-157°C, while other components show little change. Table II shows the change in aroma pattern of black tea during storage at different temperatures (6). An elevated temperature accelerates the variation in aroma compositions. The proportions of (E)-2-hexenal, benzaldehyde, pentanol.

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12. Tea

hexanol, and (Z)-3-hexen-l-ol, which give the fresh green note to the aroma of black tea, decrease, while those of {E,E)-2,4-heptadienal and (E ,E)-2,4-decadienal increase dur­ ing storage. (E,E)-2,4-Heptadienal is one of the main components of deteriorated odor of tea and is well known to be an oxidative degradation product from linolenic acid. 3-Ionone and 5,6-epoxy-3-ionone are degradation products from 3-carotene that are increased by storage. All the above-mentioned changes, both decreases and increases, are more pronounced at 20°C than at 5°C in Table II. 2.

Polyphenolic Compounds and Amino Acids

The chief contributors to the unique taste and color of black tea are polyphenols and amino acids. Black tea con­ tains important polyphenolic compounds in the following pro­ portions: catechins 5%, theaflavins 1-2%, thearubigins 15%, and complex tannins (formed from polyphenols and proteins) 5%. Theaflavins are bright orange in color and their chemical structure consists of a benzotropolone nucleus, formed by oxidation of catechins by polyphenol oxidase during the fermentation stage of black tea manufacture. Thearubigins are a complex mixture of polymeric polyphenols and are dark reddish in color. The mode of formation and the nature of thearubigins has not as yet been resolved satisfactorily. Theaflavin content (or theaflavin:thearubigin ratio) shows a high positive correlation with evaluations of color and taste (7). Theaflavins contribute to the quality and the brightness of the color of black tea liquor, but much of the color, strength (powerful tea character), and mouth-feel is due to a heterogeneous group of compounds known as the thearubigins. Theaflavins and thearubigins constitute as much as 30 to 60% of solids in a black tea infusion (12). Deterioration in the quality of black tea is caused by losses of theaflavins; loss of its astringency follows changes in catechins, resulting in a dull, dark color and flat taste. The loss of theaflavin upon storage is ac­ companied by losses of amino acids, sugars, pigments (chlorophylls, carotenoids, and flavonoids), and some vola­ tile compounds. Figure 1 shows the changes in moisture and theaflavin content during storage of black tea in (A) a tightly closed clear glass bottle and (B) a loosely covered wooden box (13). During a period of 22 weeks, moisture content of the tea rises from 4.2 to only 5% in an airtight glass bottle, whereas it increases to 9.9% in a wooden box from which air is not excluded. The rate of reduction in theaflavins is

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668

more gradual in the airtight bottle than in the wooden box, and the final value attained after 20 weeks in the former is higher than in the latter. The changes in the quantities of epigallocatechin gallate (EGCG) and epicatechin gallate (ECG) in the same experiment are shown in Table III (13). Both gallates change irregularly during storage, and the final level of EGCG after 16 weeks of storage is not reduced, whereas that of ECG is approximately half the original value. Therefore, reduction of ECG seems to be one of the factors in the change in tea taste to "flat". Figure 2 shows a decrease of theaflavins and increase in nondialyzable pigments, which contain highly polymerized polyphenols, during storage under different conditions. Both trends of variation are acceler­ ated by moisture uptake and temperature elevation (9). Table IV shows the changes in free amino acid contents in black tea that is packed in different types of sachets (9). The rate of reduction of amino acids is also influenced by moisture content. Jayaratnam and Kirtisinghe (4) reported that for humidities in the range of 32% at a temperature of 20°C under light-excluding conditions, black tea could be stored for a period of 300 days without loss of tea character. Wickremasinghe (13) suggested that light ac­ celerates photooxidation of lipid and nonenzymic browning reaction, and causes heavier deterioration in the quality of black tea.

B.

Green Tea

Steaming (Japanese type) or panning (Chinese type) is the first step of green tea manufacture, in which the poly­ phenol oxidase and other enzymes are inactivated and the green color of tea leaves is maintained in the finished product. Also, ascorbic acid (vitamin C) in fresh tea leaves is retained in green tea at an average level of about 250 mg/100 g sen-cha (Japanese type) and 200 mg/100 g kamairi-cha (Chinese type). Deterioration in quality of green tea during storage is recognized by the following phenomena: 1. 2. 3. 4.

Vitamin C content is reduced. Bright green color of tea changes to olive green and then brownish green and dull color. Color of tea liquor varies from bright yellow or slight greenish tone to brownish yellow. Characteristic leafy and refreshing aroma of green tea changes to dull and heavy odor.

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12. T e a

5.

The well-balanced, complex taste, which consisted of umami, astringency and bitterness, changes to flat and loses its characteristic briskness.

These deteriorations in quality of green tea are accelerated by moisture, oxygen, elevated temperature, and exposure to light, much as in the case of black tea. 1.

Volatile Constituents

High-quality early spring green tea is recognized by its typical fresh natural aroma. (Z)-3-Hexenyl hexanoate, the major contributor to this characteristic aroma (10), decreases upon prolonged storage. Instead, there is an increase in 2,4-heptadienal as shown in Table V (2). Decrease of (Z)-3-hexenyl hexanoate is accelerated by higher temperature. More detailed analytical data on the variations of aroma composition in high-grade and low-grade green teas during storage at 25°C are shown in Table VI (3). Here again, a decrease of (Z)-3-hexenyl hexanoate and an increase of 2,4-heptadienal are evident, as well as increases of several unsaturated carbonyl compounds. As can be seen in Table VI, carotenoid degradation products (5) such as 2,6,6-trimethyl2-hydroxycyclohexanone, β-cyclocitral, a-ionone, β-ionone, 5,6-epoxy-B-ionone, and 1dihydroactinidiolide increase markedly after 4 months storage. The aroma of early-spring green tea is perfectly pre­ served only by low-temperature storage at -70°C by comparison of sensory scores for green tea stored at 5°C or at room temperature in the case of nitrogen packing. 2.

Color of Green Tea and Its Liquor

The discoloration of green tea during storage is caused by changes of chlorophyll a and b into pheophytin a and b, respectively. The change in chlorophyll a is greater than that in chlorophyll b (11). Principally, chlorophylls are not soluble in water, but a very minor amount is found in the liquor from high-quality green tea (8a). The color of green tea liquor contains various flavonoid yellowish pigments and also oxidation and condensation products of catechins. Flavonoid pigments are said to con­ tribute ^24% to the total color of the tea infusion (8). The oxidation and condensation products from catechins act on the color of the green tea brew to make it brownish. Since cate­ chins are the most important components contributing to tea taste, it is obvious that the discoloration of green tea positively correlates with the change in tea taste.

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At the same time, there are high correlations between the rate of conversion of chlorophyll to pheophytin and the deterioration in quality of green tea (11). 3.

Moisture and Vitamin C

The content of vitamin C in green tea decreases during storage, and the rate of decrease correlates with the moisture content. Changes in moisture and vitamin C content of green tea during storage in pouches made of various materials are shown in Table VII (1). Packaged tea in the aluminum-film combination pouch shows the best quality among five different packaged tea samples. The shelf-life of the packaged green tea can be expected to be at least 3 months if green tea is packaged in a plastic film pouch, in which moisture content of <5.5% and a vitamin C residual ratio of >70% are retained in green tea (1). There are positive correlations between decrease of vitamin C and the quality deterioration, as shown in Fig. 3 (1). When the green tea is stored in a nitrogen-packed can 1 C is retained and kept at low temperature (5°C), vitamin almost perfectly even after 12 months storage. If the green tea is stored in a can from which air is not excluded, and is kept at room temperature, vitamin C content decreases markedly within 3 months, as can be seen in Fig. 3. Deterioration in quality of green tea is greatly af­ fected by oxygen as well as moisture and elevated temperature. The storage stability of green tea is the lowest among various teas including black tea, oolong tea, and pouchong tea. To protect the quality of green tea during storage most effec­ tively, it is necessary to use nitrogen packing or vacuum packaging.

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12. T e a

TABLE I Relative Composition of Volatile Fraction of Black Tea after Storage for 6 Weeks Expressed in Arbitrary Unit/Unit of Dry Matter

N 2

Atmosphere Relative Humidity

Amb* 54% 35°C

4°C

Storage Temp.

Air

58%

Amb* 54%

17°C

2.91 11.49

Moisture content %

Air

Air

Amb*

35°C

17°C

7.60

Air

35 °C

3.05 11.68

2.67

•" D G I O r S

Storage 21

23

16

8

11

14

11

36

39

2

40

42

2

57

278

174

12

143

129

0

110

2

45

56

55

42

41

29

Z-2-Penten-l-ol

57

36

0

32

37

0

35

Z-3-Hexen-3-ol

62

49

73

53

51

29

44

4

5

6

3

3

0

3

Linalool oxide(trans)103

105

112

114

101

80

95

Linalool & Octanol

319

328

220

279

243

155

225

Phenylacetaldehyde

51

40

10

29

45

8

31

Methyl salicylate

10

30

55

28

25

43

30

Nerol & Geraniol

2

0

0

0

0

0

0

Phenethyl alcohol

2

0

0

0

0

0

0

Other 17 Compounds

2179

1985

1883

1798

1972

2096

1957

Total

3126

2859

2445

2582

2701

2468

2627

Hexanal l-Penten-3-ol E-2-Hexenal Pentanol

Hexanol

* Amb =

ambient.

Analysis of volatile fraction ; 500 mg Black tea—> steam distillation—>Distillate-^GC. (10

JC1)

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Tei Yamanishi

TABLE II Changes of Aroma Composition

of Black

b Tea

after Storage for 11 Weeks Before

Components

Storage

Storage Temp. 5°C

20°C

Carbonyl compounds %

%

%

Hexanal

0.37

0.17

0.11

Nonanal

0.40

0.49

0.52

E-2-Hexenal

3.22

1.15

0.75

E-2-0ctenal

0.33

0.13

0.17

Ε, Ε-2,4-Heptadienal

0.60

1.23

1.56

Ε,Ε-2,4-Decadienal

0.57

0.75

0.99

2,4,6-Decatrienal

0.58

0.51

0.91

Benzaldehyde

0.37

0.18

0.19

Phenylacetaldehyde

0.79

0.84

1.31

fr-Ionone

0.65

0.92

1.64

5,6-Epoxy- β-ionone

0.19

0 o24

0.33

Pentanol

0.31

0.10

0.05

Hexanol

0.70

0.60

0.51

Octanol

0.31

0.44

0.55

l-Penten-3-ol

0.05

0.03

0.02

Z-2-Penten-l-ol

0.59

0.85

0.74

Z-3-Hexen-l-ol

8.87

7.41

6.10

E-2-Hexen-l-ol

1.70

1.56

1.54

28.24

31 028

28.91

3.46

3.18

3.21

Linalool oxide II(trans , furanoid) 12.94

11.53

12.21

0.27

0.25

0.30

Linalool oxide IV(trans , pyranoid) .1.10

0.61

0.92

Alcohols

Linalool Linalool oxide I (cis. furanoid)

Linalool oxide III (cis. pyranoid)

12. T e a

673

TABLE II (Continued) Before Components

e

Storage Temp, ^ Q

3,7-Dimethyl-l,5,7-octatrien-3-ol

0.36

0.42

0.42

a-Terpineol

1.00

1.15

1.20

Nerol

0.25

0.37

0.46

Geraniol

3.60

4.10

4.36

Nerolidol

1.42

2.00

2.30

Benzyl alcohol

0.39

0.27

0.81

Phenethyl alcohol

0.39

0.25

0.40

Hexanoic acid

0.16

0.16

0.42

Octanoic acid

2.00

0.46

0.91

Nonanoic acid

0.19

0.41

0.50

Decanoic acid

0,17

0.16

0.31

Dodecanoic acid

0.49

0.41

0.66

trans-Geranic acid

0.39

0.30

0.58

1.34

1.35

1.30

19 032

21.70

19 085

0.28

0.14

0.18

Benzyl cyanide

0.25

0 o22

0.33

Indole

0.63

0.45

0.68

Acids

Esters Z-3-Hexenyl hexanoate Methyl salicylate Methyl palmitate Nitrogenous Compounds

a Presented by peak area percentages of gas chromatogram. GC instrument; Shimadzu GC-7A (FID) connected with a computing integrator C-RIB, Column 30mX0.25mm fused silica WCOT, PEG 20M. ^Uva quality tea(Sri Lanka), manufactured on middle July, 1983, packed in the Al-laminated paper pouch, sent to Tokyo on Sept. 12, 1983 by air-freight. Storage experiment started on the next day.

2

Q

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674

TABLE III Changes in Quantities of Epicatechin

Gallate

(ECG) and Epigallocatechin Gallate(EGCG) during Storage of Black Tea

No. of Weeks

mg/g dry weight of black tea

a

_ b BOX

Bottles EGCG

ECG

EGCG

ECG

0

1.04

2.04

1.04

2.04

1

1.30

1.04

1.90

1.70

2

0.60

1.00

0.90

1.44

5

1.30

0.90

1.40

2.00

8

0.84

0.90

0.86

1.34

12

0.99

0.87

0.92

1.60

16

1.02

0.99

1,02

1.00

a Tightly-covered, clear glass bottle. ^Loosely covered wooden box. Taste of both EGCG and ECG is sharp and strong astringency with bitterness, whereas the free catechins (EGC and EC) have mild astringency and pleasant after taste.

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12. Tea

TABLE IV Free Amino Acid

Contents in Black Tea after

Storage for 31 Weeks under the Different

b _ a Can

Package

(°C)

Moisture Content(%) after 31 Weeks

PP

WP-1

WP-2

f 90

Amb

90

90

90

Amb

30

Amb

30

30

30

5.28

5.16

5.11

Humidity(%) A m b

Storage Temp.

C

Al/PE

e Relative

Conditions

Amino Acids

^

13.21

13.55

20.82

mol/g dry wt of tea

376

326

342

149

206

0

Glutamic acid

18

16

17

7

9

0

Aspartic acid

18

16

16

7

10

0

Serine

9

8

11

3

4

0

Arginine

6

6

6

3

4

0

Phenylalanine

3

4

3

2

3

0

Threonine

2

3

2

1

1

0

2

2

2

1

2

0

Valine

2

3

2

1

2

0

Isoleucine

2

1

1

1

1

0

Leucine

2

3

2

1

1

0

Tyrosine

2

3

3

2

2

0

Theanine

Glycine

-\

Alanine

J

"Unlined seamed can.

^Ambient,

b Aluminum/Polyethylene laminate sachet. Paper/Plastic laminate d

Wax paper sachet.

sachet. f

Stored in a humidity

cabinet.

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676

TABLE V

a Changes of Aroma Pattern of Green T e a during Storage at Different Temperatures

Ratio of Peak Height Components

5 °C

Before Storage

l-Penten-3-ol

-

unknown

59

Z-2-Penten-l-ol

-

Z-3-Hexen-l-ol

2 Mon

25°C

4 Mon

2 Mon

4 Mon

-

55

32

94

33

33

16

13

-

26

15

45

16

17

29

26

60

104

69

51

24

22

-

-

17

-

16

-

-

14

12

17

100

100

100

100

100

Octanol

95

83

86

86

85

Z-3-Hexenyl hexanoate

85

68

65

46

36

130

123

125

133

130

Nonanal 2,4-Heptadienal 3,5-0ctadienone Benzaldehyde

J

Linalool

Nerolidol

Packed in

aluminum laminate film pouch.

Expressed by relative peak height to Linalool = 1 0 0 .

— λ

0.18

Linalool oxide(cis, furanoid)

Benzaldehyde

E-3,Z-5-Octadien-2-one

E-2 ,Ε-4-Heptadienal — ν

Linalool oxide (trans, furanoid)—J

0.20

0.18

0.37

0.73

Nonanal

E-2,Z-4-Heptadienal

0.22

Z-3-Hexen-l-ol

Hexanol

0.29

0.19

Z-2-Penten-l-ol

6-Methyl-5-hepten-2-one

0.47

Pentanol

0.38

0.46

0.65

0.18

0.51

0.78

0.58

0.53

0.54

1.08

0.90

0.36

0.62

1.47

1.08

1.21

0.67

1.36

0.72

0.14

l-Penten-3-ol 0.65

4

2

J

of

High grade of spring tea

Ratio

0

Components

area

0.40

0.34

0.84

1.73

0.66

0.32

0.29

0.38

0.47

0.27

0

1.75

6.26

10.13

1.73

1.06

0.83

0.58

5.02

0.99

5.45

2

2.21

11.60

9 o03

1.93

1.35

1.04

1.08

8.46

1.12

9.78

4 Months

Low grade of summer tea

peak

Variation in Aroma Composition of Green Tea during Storage

TABLE VI

678

0.41 0.61

0.37 0.65

0.25 0.70

Dihydroactinidiolide

Indole

1.03

0.13

0.78

0o35

1.45

1.24

^atio of peak area of compound/peak area of internal standard(ethyl decanoate).

0.94

0.93

0.70 1.03

0.73

1.28

1.78

3.41

1.23

0.61

0.32

2.83

1.26

3.76

0.74

1.07

1.58

2.46

1.08

0.56

0.33

2.07

1.26

3.78

Packed in the aluminum laminate pouch and stored in a dark room at 25°C .

0.76

0.75

0.98

Nerolidol

1.19 0.70

0.85 0.59

0 o36

0.66

5,6-Epoxy-g-ionone

β-Ionone

—J

—'

Benzyl alcohol

cis-Jasmone

—\

α-Ionone 1.43

0 o96

0.72

0 o6 8

0 o67

Geraniol 0.96

0.53

0.75

0 o67

0.83

Linalool oxide(trans, pyranoid)

0.88

0.36

0.93

1.04

1.20

Z-3-Hexenyl hexanoate

1.54

0.66

0.43

-

3,7-Dimethyl-l,5,7-octatrien-3-ol

-

0.60

0.21

0.95

0.52

-

3.01

β-Cyclocitral

2,6,6-Trimethyl-2-hydroxy-cyclohexanone

0.20

1.35

1.12

0.99

Octanol

E-3,E-5-Octadien-2-one

1.10

1.05

1.02

Linalool

TABLE VI ( Continued )

3

CD

8.8

6.9

10.8

6.0

3.2

3.5

4.0

3

42.1

20.5

5.6

5.0

6.7

2

Polyvinylidene chloride.

h Paper.

Aluminum.

» 'Polyethylene

g

Polypropylene.

f

are 3.1 % and 372.3 mg/100g tea.respectively.

'Cellophane.

vitamin C

High grade spring green tea, original contents of moisture and

27.9

8.1

4.6

1.4

7.4

1

69.4

32.1

8.7

8.6

8.8

3 Months

Percent loss of Vitamin C

Packed in the pouch and stored at 25°C, RH 80%.

a

5.4

4.4

3.2

3.2

PT #300.Al 15Λ.ΡΕ40Λ f g

coating.PE 60* OPP 2Q^.PVDC 2A h PT 30Λ.ΡΕ 13y^.P 40g.PE 20Λ

3.2

3.6

2

PT #300.Al 112*.PE4Q*

1 3 09

Θ 3.5

d

Moisture content(%)

in Various Packages

PT #300. Al 9Jt. ΡΕ 40Λ

C

Material of pouch

of Green Tea

Changes of Moisture and Vitamin C Contents during Storage'

TABLE VII

680

Fig. 1.

Tei Yamanishi

Variation in theaflavin ( ) and moisture contents ( ) during storage of black tea (A) in an air­ tight glass bottle (·), and (B) in a loosely covered wooden box (o).

12. T e a

Fig. 2.

681

Changes in theaflavins and nondialyzable pigments during storage of black tea (arbitrary units/unit dry weight).

Conditions of storage Temperature (°C) • Δ • Ο • ο

35 17 35 17 35 17 x 35 » 17

Moisture contents after

RH (%)

7 weeks

14 weeks

75 76 61 56 45 44 a o 0

11.10 11.27 8.43 7.75 5.11 5.55 2.02 2.51

11.88 11.51 8.62 7.94 5.19 5.98 1.85 3.31

a Stored in a desiccator with silica gel.

682

Fig. 3.

Tei Yamanishi

Correlation between rate of unchanged vitamin C and grade of deterioration in quality of green tea. - 1 , Scarcely recognizable deterioration; - 2 , slight­ ly; - 3 , considerably; - 4 , extremely. Storage temperature: ambient ( ) ; 5°C ( ) .

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683

REFERENCES

1. 2.

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