33

E. T. Contis et al. (Editors) Food Flavors: Formation, Analysis and Packaging Influences © 1998 Elsevier Science B.V. All rights reserved 3 5 3

The effect of drying treatment on the flavor and quality of Longan fruit

C. Y. Chang", C. H. Chang", T. H. Yu", L. Y. Lin^ and Y. H. Yen"

^Department of Food Engineering, Da-Yeh University, Chang-Hwa, Taiwan, R.O.C.

* Department of Food Nutrition, Hungkung Institute of Technology, Taichung, Taiwan, R.O.C.

Abstract The volatile compounds of the Longan fruit (Euphoria longana, Lamarck) were isolated

using vacuum distillation and dichloromethane extraction. Samples were analyzed by GC and GC-MS to study the changes of the flavor of the fruit during the drying process. A total of 102 different volatile compounds were identified from the fruit samples. Based on the fiinctional groups, these compounds can be grouped into 9 classes: hydrocarbons, alcohols, acids, phenols, ketones, aldehydes, furans, esters, and miscellaneous compounds. Ocimenes were found to be the predominant and characteristic volatile compounds in the raw and dried fruits. After drying, the amounts of ketones and alcohols decreased whereas the amounts of furans and esters increased. Overall, the total amount of the volatile compounds decreased slightly after drying. The Aw of the fruit decreased after 18-hrs drying. During drying, the Hunter L and b values of the flesh of the fruits decreased while the a value increased in the early stage and then decreased. The amount of total reducing sugars and total free amino acids decreased after drying. Based on the results of the volatile and quality changes of the fruit during drying process, it can be deduced that the ft)rmation of the volatile compounds of the fruit was a result of reactions between reducing sugars and amino acids in the flesh of the fruits.

1. INTRODUCTION

Longan (Euphoria longana, Lamarck) is a sub-tropical fruit, which grows in the southern area of Mainland China. It is widely cultivated in Kuangtung, Kuanghsi, Shichuan, and Fuchien provinces. The total area in Taiwan devoted to the growth of Longan fiuit is around 12,000 hectares. The fruit is mainly cultivated in Tainan, Nanto, Taichung, Changhwa, Kaoshiung, and Chiayi counties (Yen, 1993). Taiwan's climate permits Longan fruit to be one of the major fruits produced in summer.

Longan fruit is rich in sugar and very nutritious. It has been used as a dietary supplement in China since ancient time, and is widely applied in herb medication for benefiting the mind and spleen. Longan fruit can be served fresh or processed as dried fruit, jam, and wine. The dried

354

fruit is highly desired by consumers because of its unique flavor (Hwang, 1987). There are significant differences in flavor and eating quality of fresh and dried Longan

fruits. These differences begin to develop from fertilization through harvesting of Longan fruit. The growth of fruit flesh results in a reduction in acidity and an increase in sugar content. A mature Longan fruit is plump and juicy with a brownish fruit shell and a yellowish flesh (Yen and Chang, 1991). Ocimene has been identified as the predominant and characteristic volatile compound in the raw fruit (Kuo et al, 1985) whereas other volatile compounds contribute to the flowery and sweet odor of the fruit (Liu, 1993). The flavor of dried Longan fruit might come from the products of Maillard reaction and/or caramelization during the drying process. Changes of flavor and quality of Longan fruit during the drying process have not been studied yet. In this research, a convection oven was employed to dry fresh Longan fruits to simulate the process of commercial production. The raw fruits were dried in batch and the products were sampled during the drying process. The volatile compounds and proximate composition of each sample were analyzed to understand the development of changes of quality due to changes in volatile compound profiles. The relationship between the development of the volatiles and the changes of quality of dried Longan fruit was also investigated.

2. EXPRIMENTAL PROCEDURES

2.1. Materials The raw Longan fruits used in this study (Figure 1) were cultivated and harvested from

the same Longan tree in a mountainous area of Changhwa county. The Longan fruit stems were cut off and the shells were kept intact.

Figure 1. The fresh Longan fruits used in this study.

355

2.2. Sample Preparation Raw Longan fruits (6,000 g) were put into a convection oven and dried at 70 °C. During

drying, the fruits were turned over and over for one min every one hour to let the temperature of the center point of each fruit attain around 55-60°C. Six batches of Longan fruit with different degrees of drying (0, 6, 12, 18, 24, and 36 hrs) were prepared. The volatile compounds, chemical compositions, and physical properties of these samples were analyzed.

2.3. Analysis of the Volatile Compounds of Longan Fruits Isolation of the Volatile Compounds The flesh (350 g, on a dry basis) of Longan fruit

sample was placed into a 1000 mL flask and 500 mL of distilled water was added. The sample flask was sealed with aluminum foil and stirred for 1 min at 30-min intervals for a total of 2 hrs. The material in the flask was transferred to a blender and homogenized for 1 min. During blending, 5 mL of heptanone stock solution [0.886 mg/mL of dichloromethane (CH2CI2)] was added as the internal standard. The solution was vacuum-distilled in a water bath at 30-40 mbar and 45 °C.

After distillation, 100 mL of CH2CI2 and a small amount of NaCl were added to the distillate. The mixture was stirred for 2 hrs with a magnetic stirrer. The CH2CI2 layer was collected from a separatory funnel and dried over anhydrous sodium sulfate. The CH2CI2 extraction procedure was repeated and the extractants pooled. A final concentrated volume of 0.05 mL was accomplished by purging the sample with a stream of nitrogen.

Gas Chromatography Analysis and Quantification of Volatile Compounds A Hitachi G-3000 gas chromatograph equipped with a fused silica capillary column (50 m x 0.32 mm id.; 1 |im, DB-Wax, J&W Inc.) and a flame ionization detector was used to analyze the volatile compounds. The operating conditions were as follows: injector temperature, 250 °C; detector temperature, 270 °C; nitrogen carrier flow rate, 1.2 mL/min; temperature program, 40 °C (5 min), 2 °C/min, 240 °C (99 min). A split ratio of 50:1 was used. The content of volatile compounds, expressed as ppb, was estimated by computing the GC area against that of the area of the internal standard. Peaks of different volatile compounds with the same area were assumed to have the same quantity. Quantitation of the volatile compounds were based on one determination.

Gas Chromatography-Mass Spectrometry (GC-MS) Analysis The concentrated sample was analyzed by GC-MS using a Hewlett-Packard 5890 gas chromatograph coupled to a Hewlett-Packard 5971 MSD containing the same column as that for the gas chromatography. Operating conditions were as follows: injector temperature, 250 °C; detector temperature, 270 °C; helium carrier flow rate, 3 mL/min; temperature program, 50 °C (1.5 min), 2 °C/min, 200 °C (99 min); the temperature of interface, 200 °C. Mass spectra were obtained by electron ionization at 70 eV and an ion source temperature at 200 °C.

Tentative Identification of the Volatile Compounds Tentative identification of the volatile compounds in the Longan fiiiit isolate was based mostly on GC-MS. The structural assignment

356

of volatile compounds was accomplished by comparing the mass spectral data with those of compounds available from the Browser-Wiley computer library, the EPA/NIH data base (Heller and Milne, 1978; 1980), the NBS computer library, TNO (1988), or previously published literature (Yu et al., 1989; 1993; 1994). The retention indices were used for the confirmation of structural assignments.

2.4. Analysis of the Physical Properties of Dried Longan Fruits Relative Weight and Moisture Content The relative weight of the dried fi\iits was

expressed by assigning the weight of the fresh finit as 100. The flesh (2-3 g) of the fruit sample was dried to a constant weight in an oven at 105 °C. The moisture content of Longan finit sample was measured using the AOAC oven-heating method (1983).

Water Activity (Aw) Aw of the flesh (5.0 g) of Longan fixiit sample was measured using a Rotronic-Hygroskop DT water activity measuring instrument (Rotronic Instrument Co., Switzerland) at 25 °C. The Aw measurements were triplicated for each fiuit sample.

Color The Hunter color L, a, and h values of the flesh of the sample were directly measured using a Hunter colorimeter (Color Analyzer, Color Mate OEM, Milton Roy Co., USA). Three determinations were conducted randomly on the surface of each sample. Measurements for each sample were triplicated.

2.5. Determination of the Free Sugars and the Free Amino Acids of Longan Fruits

Free Sugars Ten grams of Longan finit flesh was added with 200 mL of 80 % ethyl alcohol. The material was then refluxed in a water bath at 95 °C for 1 hr. At which time, the sample was filtered. The fi-ee sugar content of the filtrate was analyzed using HPLC (Jasco PU-980, Japan). The operating conditions for HPLC were as follows: mobile phase, acetonitrile/H20=75/25 (VA^); column, Sphereclone 5|i NH2 250 X 4.6 mm; flow rate, 0.5 mL/min; detector, RI (Jasco 830, Japan) (Lo, 1987).

Free Amino Acids Twenty-mg of the Longan fiaiit flesh was added with 500 \xL of citrate buffer. The mixture was centrifiiged and the clear supernatant solution was collected. The residue was extracted five times with citrate buffer to recover its free amino acids. The clear supernatant solution were combined and filtered afl;er centrifiigation. Free amino acid content of the filtrate was determined using an amino acid analyzer (Beckman 6300, USA).

3. RESULTS AND DISCUSSION

3.1. Changes in the physical properties of Longan fruits during the drying process

The change in the relative weight of Longan finits during drying process is shown in Figure 2. The relative weight of the finits dropped rapidly during drying. The relative weight of

357

110

100 H

b 90

I 80

« 70 > 5 60 H Pi

50 -I

40 -I

30 _l ^ ^ ^ 6 12 18 24

Drying Time (hr)

Figure 2. Change in the relative weight of Longan fruit dried at 70 °C.

The rapid reduction in fruit weight during the drying process was caused by the evaporation of water from the fruit shell, flesh, and seed. The change in the moisture content of the fruit flesh during drying process is shown in Figure 3 which reveals that the weight reduction of the fruit during drying was due mainly to the loss of water from the fruit flesh.

1 6

1 1 1 12 18 24 Drying Time (hr)

1 30

1 36

Figure 3. Change in the moisture content of Longan fruit flesh dried at 70 °C.

358

Water activity (Aw) is recognized as a reliable indicator of food perishability. A high Aw value in fresh Longan fruits (0.99, Figure 4) suggests that they would easily deteriorate during storage. Figure 4 depicts the effect of drying time on the Aw of the fruits. It shows that the value of the Aw dropped dramatically after drying for 24 hrs and remained 0.61 after the process. At the Aw of 0.61, most microorganisms cannot grow, even the osmophilic yeasts (Beauchat, 1981). Because of its low Aw, dried Longan fiaiits can be stored at room temperature for a long period of time.

1.0 H

0.9 H

^ 0.8 <

^ 0.7 A

0.6 H

0.5 I I I 6 12 18 24 30 36

Drying Time (hr)

Figure 4. Change in the water activity of Longan fiiiit flesh dried at 70 °C.

The Hunter L, a, and b values of Longan fiiiit flesh after drying for different period of time are shown in Figure 5. The onset of the decrease of Hunter L value of the fiaiit flesh apparently was about at the drying time of 6 hrs. The Hunter h value also had a decreasing trend during drying process; the Hunter a value increased only slightly from the beginning period of drying and then decreased after 30 hours. The visual browning of the fruit flesh suggested that there were Maillard reactions that may have occured between the sugars and amino acids in the fiuit flesh during the drying process.

359

18 24

Drying Time (hr)

36

Figure 5. Change in the Hunter L, a, and b values of Longan fruit flesh dried at 70 °C.

3.2. Changes of the content of the free sugars and the free amino acids in Longan fruits during drying process

Changes in free sugar content of Longan fruits during drying are shown in Table 1. The amount of free sugars in the flesh of raw fruit is presented in the order of glucose, maltose, sucrose, xylose, and fructose. After drying for 36 hrs, sucrose became the major sugar in the flesh of diied fruit, and maltose and xylose were not detectable. During di-ying process, the amount of glucose decreased throughout the dr5dng process, especially in the first 6 hrs. The amount of maltose also decreased rapidly and was not detectable after drying for 18 hrs. The amount of fructose increased and then decreased since drying for 18 hrs whereas the amount of sucrose had an opposite trend as that of fructose.

The change in free sugar content during drying process reveals that the decrease in glucose may have been caused by its isomerization to fructose as seen by the increase in fructose in the early stage of drying. The decrease in glucose might also be due to the Maillard reaction and/or carameUzation, which would result in the formation on new volatile compounds and imparted a brownish color to the flesh of Longan fruits.

360

Table 1. The free sugar content of Longan fruit dried at 70 °C. Content ( g / lOOg Longan fruit flesh, based on dry weight)

xylose fructose glucose maltose sucrose total sugar

0* 2.94 2.30 44.69 15.00 5.37 67.35 6 N.D.** 7.28 22.17 5.41 3.37 38.24

12 N.D. 18 N.D. 24 N.D. 30 N.D. 36 N.D.

* Drying time (h.

11.90 21.76 18.46 11.79 12.62

3urs). ** N.D. : Not Detectable.

15.84 12.54 9.77 13.75 11.79

4.09 N.D. N.D. N.D. N.D.

4.23 13.14 14.73 14.02 24.65

35.26 47.44 42.96 39.56 49.06

Table 2 shows the change in free amino acid content in Longan fruits during drying. It is observed that the predominant free amino acids in the flesh of raw fruits is glutamine and then follows by proUne, alanine, aspartic acid, tyrosine, serine, leucine, isoleucine, valine, and glycine. Some of these amino aicds, such as proline and leucine, significantly decreased in the amount after drying. The total amount of free amino acids decreased throughout the process. This result of decrease in the amount of free amino acids after drying is similar to that of free sugars. It suggests that the decrease in the amount of free amino acids might be due to Maillard reaction between the amino acids and reducing sugars. This might also explain the generation of the volatile compounds and the formation of brownish colors (see below).

3.3. Changes in volati le components of Longan fruits during drying process

The gas chromatogi-ams of the flavor isolates from the flesh of raw and 36-hr-dried Longan fruits are presented in Figure 6. It shows clearly visible differences between the gas chromatograms of the two fruit isolates. A total of 102 compounds (Table 3) were identified from the Longan fruit flesh isolates. Based on the functional groups, these flavor components are grouped into 9 classes: hydrocarbons, alcohols, acids, phenols, ketones, aldehydes, furans, esters, and miscellaneous compounds.

361

Table 2, The contents of free amino acids in the flesh of Longan fruits dried at 70 ^C.

Asp Ser Glu Pro Gly Ala Val ne

Leu Tyr Lys NH3 Total

Content ( 0*

10.06 5.06

31.25 25.91

1.42 20.14

1.54 1.79 2.39 8.42

N.D. 0.55

108.53

mg / lOOg Longan fruit flesh, based on dry weight) 6

8.74 2.57

18.97 20.73

1.16 12.85 0.67 1.79 1.71 9.42

N.D. 0.46

79.07

12 11.08 4.64

18.73 17.42

1.55 15.54

1.26 1.07 2.04 5.16 0.05 0.35

78.87

18 6.59 3.28

20.18 16.97 0.96

13.05 1.00 1.07 1.57 5.53

N.D. 0.51

70.71

24 4.19 1.87

11.56 11.15 0.54 8.69 0.54 0.64 0.86 3.16 0.05 0.18

43.42

30 3.38 1.16 7.79 7.09 0.38 4.90 0.36 0.64 0.64 2.31

N.D. 0.09

28.75

36 2.04 0.38 2.55 6.85

N.D.** 3.31 N.D. 0.47 0.32 2.74 0.16 0.16 18.98

* Drying time (hours). ** N.D. : Not Detectable.

CZ3

CD

o

<D

Q Q

CO

L-j

un 1

Lju-Jlj. JlLJu

CO

« '^ Raw

1 oo

ooco 1

uJUiJuuJiJLL

Ico 1 t—

UiX....j..-jL

^ ^

lLjLJLIL.>-^----L

c/a cj» ^ r— '«S«.-.CM "^ C 3 —<'«r

~ " " " D r i e d

1 , 1 jJJ 1. 1 o>

1

.UlllJj ..4,11 '^wiUL/^t- 1 1 50 100

Figure 6. The gas chromatograms of the flavor isolates from the flesh of raw and dried Longan fruits.

362

Table 3 Volatile compounds identified fi'om the flesh of Longan samples.

Peak No. Compounds

Hydrocarbon 1

11 13 15 18 19 39 40 52 53 69 83 84 92 93

Aldehyde 2 16 42 50 54 63

Alcohol 5 6 7 9 14 17 21 23 24 25 26 28 29 31 32 37 38

2-methyl-2-butene a-phellandrene a-terpinene P-phellandrene a-ocimene trans-P-ocimene p-mentha-l,5,8-triene (isomer I) cis-l,3-pentadiene trans-sabinene hydrate 2,4,6-trimethyl-1,3,5-trioxane 5-3-carene p-mentha-l,5,8-triene (isomer 2) l,2-dimethoxy-4-[2-propenyl]-benzene a-cedrene 1,2-dimethoxy-4-[ 1 -propenyl]-benzene

2-methyl propanal trans-2-hexenal furfural benzaldehyde 5-methyl furfural benzeneacetaldehyde

2-methyl-3-buten-2-ol isobutyl alcohol 2-pentanol 1-butanol isoamyl alcohol 3 -methyl-3 -buten-1 -ol 4-heptanol 4-methyl-1 -pentanol 2-heptanol 3 -methyl-2-buten-1 -ol 3 -methyl-1 -pentanol 1-hexanol 2-hexanol 3 -ethoxy-1 -propanol cis-3-hexen-l-ol l-octen-3-ol linalool oxide (isomer 1)

CAS No.

513-35-9 99-83-2 99-86-5

555-10-2 502-99-8

3779-61-1 21195-59-5

504-60-9 546-79-2 123-63-7

13466-78-9 21195-59-5

93-15-2 469-61-4 93-16-3

78-84-2 6728-26-3

98-01-1 100-52-7 620-02-0 122-78-1

115-18-4 78-83-1

6032-29-7 76-36-3 123-51-3 763-32-6 589-55-9 626-89-1 543-49-7 556-82-1 589-35-5 111-27-3 626-93-7 111-35-3 928-96-1

3391-86-4 5989-33-3

Formula

C5H10 C10H16 C10H16 C10H16 C10H16 C10H16 C10H14

C5H8 C10H18O C6H1203 C10H16 C10H14

C11H1402 C15H24

C11H1402

C4H80 C6H10O C5H402 C7H60

C6H602 C8H80

C5H10O C4H10O C5H120 C4H10O C5H10

C5H10O C7H160 C6H140 C7H160 C5H10O C6H140 C6H140 C6H140

C5H1202 C6H120 C8H160

C10H18O2

M.W.

70 136 136 136 136 136 134 68 154 132 136 134 178 204 178

72 98 96 106 110 120

86 74 88 74 70 86 116 102 116 86 102 102 102 104 100 128 170

363

(Table 3 continued)

Peak No.

41

43 51 56 58 60 61 61 64 66 71 72 76 78 79

Ketone 3 4 10 22 30 55 59 75 91 100

Ester 33 34 35 36 44 46 81 85 88 90 94 95

Compounds

4-methyl-l-[l-methyl ethyl], 3-cyclohexen-1-ol linalool oxide (isomer 2) 3,7-dimethyl-1,6-octadien-3 -ol 4-terpineol cis-p-2-menthen-1 -ol [5-methylcyclopent-1 -enyl] methanol p-mentha-trans-2,8-dien-1 -ol p-mentha-trans-2,8-dien-1 -ol 2-furan methanol cis-piperitol epoxylinalool d-nerolidol trans-geraniol benzenemethanol benzeneethanol

2-propanone 2,3-butanedione 2-heptanone 3 -hydroxy-2-butanone 4-hydroxy-4-methyl-2-pentanone E-6-methyl-3,5 -heptadien-2-one 5-methyl-3-hexen-2-one isopulegone pulegone 3,4-dihydro-8-hydroxy-3 -methyl-1H-benzopyran-1-one

methyl cis-2-butenoate methyl 2-hydroxy-3-methyl butyrate ethyl 3-hydroxy-3-methyl butyrate ethyl 2-hydroxy-3-methyl butyrate methyl 3-hydroxy butyrate methyl 2-hydroxy-3-methyl pentanoate methyl 2-hydroxy-5-methyl benzoate methyl 4-methyl benzoate methyl 2-methoxy benzoate ethyl 2-methoxy benzoate methyl hexadecanoate methyl 2-amino benzoate

CAS No.

562-74-3

5989-33-3 22564-99-4

562-74-3 35376-39-7 88125-84-2

~ -

98-00-0 16721-38-3 14049-11-7

142-50-7 106-24-1 100-51-6 60-12-8

67-64-1 431-03-8 110-43-0 513-86-0 123-42-2

16647-04-4 5166-53-0

29606-79-9 89-82-7

17397-85-2

4358-59-2 17417-00-4 18267-36-2 2441-06-7 1487-49-6

41654-19-7 22717-57-3

99-75-2 606-45-1 7335-26-4 112-39-0 134-20-3

Formula

C10H18O

C10H18O2 C10H18O C10H18O C10H18O C7H120

C10H16O C10H16O C5H602

C10H18O C10H18O2 C15H260 C10H18O

C7H80 C8H10O

C3H60 C4H602 C7H140 C4H802

C6H1202 C8H120 C7H120

C10H16O C10H16O

C10H10O3

C5H802 C6H1203 C7H1403 C7H1403 C5H10O3 C7H1403 C9H10O3 C9H10O2 C9H10O3

C10H12O3 C17H3402 C8H9N02

M.W.

154

170 154 154 154 112 152 152 98 154 170 222 154 108 122

58 86 114 88 116 124 112 152 152 178

100 132 146 146 118 146 166 150 166 180 270 151

364

(Table 3 (

Peak No.

48 67 73 74 96 101

Furan 20 45 47 49

62 68 70 97

Acid 65 87 98 99 102

Phenol 12 77 86 89

continued)

Compounds

ethyl 3-hydroxy butyrate methyl 3,7-dimethyl-2,6-octadienoate cis-3-hexenyl butyrate phenylethyl acetate ethyl hexadecanoate methyl 9,12-octadecadinoate

dihydro-2-methyl-3 [2H]-furanone 2,5-dihydrofuran 2-acetylfuran 3,6-dimethyl-2,3,3a,4,5,7a-hexahydrobenzofliran dihydro-2[3H]-furanone 5 -ethyldihy dro-2 [3 H] -fliranone 2-furancarboxylic acid 2,3-dihydrobenzofuran

isovaleric acid octanoic acid benzoic acid dodecanoic acid hexadecanoic acid

phenol 2-methoxyphenol 4-ethyl-2-methoxyphenol 4-methyl phenol

Miscellaneous 27 57 80 82

2,6-dimethylpyrazine 2-acetylpyridine benzenethiazole 2-acetylpyrrole

CAS No.

5405-41-4 2349-14-6 16491-36-4

103-45-7 628-97-7

2566-97-4

3188-00-9 1708-29-8 1192-62-7

70786-44-6

96-48-0 695-06-7 88-14-2

496-16-2

116-53-0 124-07-2 65-85-0 143-07-7 57-10-3

108-95-2 90-05-1

2785-89-9 106-44-5

108-50-9 1122-62-9 95-16-9

1072-83-9

Formula

C6H1203 C11H1802 C10H18O2 C10H12O2 C18H3602 C19H3402

C5H802 C4H60

C6H602 C10H16O

C4H602 C6H1202 C5H403 C8H80

C5H10O2 C8H1602 C7H602

C12H2402 C16H3202

C6H60 C7H802

C9H1202 C7H80

C6H8N2 C7H7NO C7H5NS C6H7NO

M.W.

132 182 170 164 284 294

100 70 110 152

86 114 112 120

102 144 122 200 256

94 124 152 108

108 121 135 109

365

The major volatile compounds found in the hydrocarbon grouping are the ocimenes. Ocimenes have been identified as the predominant volatile compounds in the raw fruit (Kuo et al., 1985). The major volatile compounds in the aldehyde grouping are furfural and 5-methyl furfural. These two compounds were beUeved to be generated fi'om sugars through carameUzation or thermal degradation. Most of the compounds in the alcohol grouping are short-chain alcohols. These alcohols, including isoamyl alcohol, linalool oxide, trans geraniol, and benzenemethanol, contribute to the flower smell and to the wine taste of the raw and dried fruits. Pulegone, the major volatile compound of the ketone grouping, was beheved to contribute a mint flavor. In the ester group, ethyl hexadecanoate is the representative compound; it contributes a mild, sweet taste. The compounds of the furan group, including 2-acetylfuran, 2-furancarboxyHc acid, and 2,5-dihydrofuran, were beUeved to be generated from the sugars through thermal degradation. In the miscellaneous group of compounds, 2-acetylpyrrol, represents a compound with baked or roasted flavor and was beheved to be generated fi'om Maillard reaction.

Table 4 shows the temporal change in the content of volatile components in Longan fruit during drying process. The alcohol grouping is the major class of volatile compounds in raw fruit flesh, with a value of 32,177.5 ppb. The other grouping, including esters, hydrocarbons, and ketones, have the values of 31,177.7, 30,261.1, and 26,842.4 ppb, respectively. After 36-hr drying, the major group of the volatile compounds of the dried fruit flesh shifted from the alcohols to the esters group, with a 36 hour value of 54,210.4 ppb. The other groups, including hydrocarbons, alcohols, andfurans, had the values of 28,588.6, 13,885.4, and 13,838.3 ppb, respectively after 36 hours of drjang.

Table 4. The contents of volatile compounds, classified by the functional groups, in the flesh of Longan fruit dried at 70 °C.

Compound Type Phenol Furan Acid Aldehyde Ketone Alcohol Hydrocarbon Ester Miscellaneous

Total

0*

956.9 1,512.5 3,938.8 1,870.8

26,842.4 32,177.5 30,261.1 31,177.7

1,943.9

130,681.6

Concentration (ppb, based 6

551.6 393.2

2,287.3 312.7

1,848.7 14,439.8 18,005.9 33,263.8

861.5

71,964.5

12

440.7 2,468.4

990.0 950.0 420.0

9,200.2 14,080.2 21,403.3

281.5

50,234.3

18

901.9 959.8

3,409.5 1,893.8 3,157.0 6,303.4

10,589.5 24,143.8

625.9

51,984.5

on dry weight) 24

399.2 1,718.7

996.6 1,749.6 1,154.1 3,216.1

45,805.0 17,969.6

416.5

73,425.5

30

629.7 4,320.1 1,559.8 2,780.1 1,785.8 6,001.4

45,288.0 44,363.5

248.6

106,977.0

36

961.7 13,838.3

1,173.6 3,438.5 4,741.9

13,885.4 28,588.6 54,210.4

374.6

121,212.8

Drying time (hours)

366

The data in Table 4 show that during drying process, the total amount of the volatile compounds decreased sharply in the first 6-hr drying. This reduction in the total amount might be caused by the evaporation of the volatile compounds in the raw fruits. During the 6-18 hour drying period, the total amount of the volatile compounds varied a small range, this may be due to the balance between the evaporation and the generation of volatile compounds. In later stages of the drying period (24-36 hrs), the total amount of the volatile compounds increased to a value of 121,212.8 ppb. This might be a result of the prosperous generation of many volatile compounds in the groups, including phenols, furans, aldehydes, ketones, alcohols, and esters.

3.4. Comparison of the Contents of the Volati le Compounds in the Flesh of Raw and Dried Longan Fruits

Figure 7 depicts the volatile compound content placed in functional groups in the flesh of raw and 36-hr-diied fruits. It is found that the furans grouping increased to a great extent through drying. The esters grouping had the same trend as the furans grouping. Inversely, the concentration of ketones and alcohols groupings decreased after di*ying process. The volatile compounds of other groupings didn't changed significantly in the quantity after di*ying process. The total amount of the volatile compounds in the 36-hr-dried fruit flesh was smaller than that of the raw fruit flesh.

1.4e+5

^ 1.2e+5

t, l.Oe+5 •o G

° 8.0e+4 -\ o. a. ^ 6.0e+4 e "c O 4.0e+4

2.0e+4

O.Oe+0

Raw 36 hrs-Dried

T — ^ n- -T 1

Figure 7. Contents of volatile compounds in the flesh of raw and 36 hrs-dried Longan fruits

367

By comparing the changes of the free sugars, free amino acids and volatile compounds of Longan fruits before and after drying process, it was found that the difference between the fresh and dried Longan fruits in flavor was due to the higher contents in furans and esters in dried Longan fruits. The volatile compounds of furans and esters groupings were most likely generated from the sugars through many reactions, especially Maillard reaction and thermal degradation and complexation.

4. ACKNOWLEDGEMENTS

This work was supported by a grant (NSC85-2321-B-212-002) from the National Science Council, Executive Yuan, Repubhc of China.

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