Open Access

Identification of the aroma compounds in Vitex doniana sweet: free and bound odorants

Chemistry Central Journal201711:19

https://doi.org/10.1186/s13065-017-0247-7

Received: 18 July 2016

Accepted: 14 February 2017

Published: 23 February 2017

Abstract

Background

Most often, the glycosidically-bound aroma compounds are released during industrial processing or pre-treatment of fruits. This usually introduces modification to the aroma notes of such fruits. Therefore, there is the need to understand the contribution of these bound aroma compounds to the overall aroma of a given fruit. In recent years research studies have reported on the free- and bound volatile compounds of several fruits. However, there is no report yet on Vitex doniana sweet.

Results

Results of gas chromatography–mass spectrometry (GC–MS) and gas chromatography–olfactometry (GC–O) of free and glycosidically-bound aroma-active compounds from Vitex doniana sweet revealed a total of 35 compounds in the free fraction, and 28 compounds were in the bound fraction respectively. Whilst the major group of compounds in the free fraction were terpenes, alcohols, and esters, the bound fraction consisted of ketones, alcohols, terpenes and norisoprenoids.

Conclusion

A comparative analysis of the aroma potencies of the free and bound volatile fractions revealed that; free fraction exhibited strong potency for the fruity and floral notes, and the bound fraction produced more of the flowery, caramel-like and cherry-like notes. In addition results of odour activity values showed that ethylbutanoate, β-damascenone, ethyl-2-methyl propionate, linalool, hexyl acetate and (Z)-rose oxide contributed highly to the sweet prune-like aroma of the fruit.

Keywords

Vitex doniana sweet Free and bound volatile compounds Odour activity values

Background

Vitex doniana sweet (Vds) is the edible fruit that belongs to the family Lamiaceae. There are about 250 species in this family [1]. V. doniana sweet is the most abundant and widespread of this genus in the Savannah regions. The fruit is commonly called ‘ucha koro’, ‘oori-nla’ and ‘mfudu’ or ‘mfulu’ in Swahili. V. doniana sweet is oblong, about 3 cm long. It is green when immature, and purplish-black on ripening with a starchy black pulp. Each fruit contains one hard conical seed which is about 1.5–2.0 cm long and 1–1.2 cm wide. The fruit which tastes like prunes is rich in nutrients including vitamins A (0.27 mg· 100−1g DB), B1 (18.33 mg· 100−1g DB), B2 (4.80 mg· 100−1g DB), B6 (20.45 mg· 100−1g DB) and C (35.58 mg· 100−1g DB) respectively [2]. The fruit which is consumed fresh can be made into jam and wine [3]. V. doniana sweet has a unique sweet prune-like aroma when ripened. Although, a number of sugars [4], amino acids and minerals [5] have been reported in Vds, however, there is no study yet on the components responsible for the unique sweet prune-like aroma of the Vds. Studies have shown that fruits’ aromatic components are either in the free form, or bound to sugar in the form of glycosides [68].

Most often, the glycosidically-bound aroma compounds are released during industrial processing or pre-treatment of fruits. This usually introduces modification to the aroma notes of such fruits [9]. Whilst several studies have reported on the free and glycosidically-bound volatiles in fruits such as strawberry [8], mango [10], raspberry [11], lychee [12], blackberry [6], acerola [7] and a host of other fruits, there has been no study on the volatile constituents of Vitex doniana sweet.

This study aimed at providing an insight into the free and glycosidically-bound aroma compounds of Vitex doniana sweet.

Results and discussion

The volatile fractions of both free and glycosidically bound V. doniana sweet, separated on two columns (DB-FFAP and SE-54) of different polarity are shown in Table 1 and Fig. 1. A total of 35 compounds were identified in the free fraction while only 28 compounds were detected in the bound fraction. In general, the aroma compounds identified in both fractions were made up of alcohols (7), aldehydes (2), acids (2), esters (11), terpenes (9), ketones (3), norisoprenoids (7), and a phenol. The most important ones in terms of concentration and the numbers identified in the free fraction were the terpenes (43%), alcohols (29%), and esters (25%). On the other hand, in the bound fraction, the ketones, were the most abundant (29%) followed by the alcohols (26%), terpenes (20%) and the norisoprenoids (13%).
Table 1

The concentration of volatile compounds (free and bound) identified in Vitex doniana sweet (µg kg−1 of pulp)

Compounds1

LR1

LR2

Free

Bound

Alcohols

 3-Methyl-but-3-en-1-ol

1209

720

1046 ± 33.0a

570 ± 23.6b

 2/3-Methyl-butanol

1213

738

153 ± 11.4a

102 ± 10.6b

 (Z)-3-Hexen-1-ol

1389

858

312 ± 17.2a

23 ± 2.0b

 Hexan-1-ol

1079

872

60 ± 3.5a

33 ± 1.5b

 2,6-Dimethylcyclohexanol

1112

979

tr

tr

 1-Octen-3-ol

1451

979

tr

tr

 2-Phenylethanol

1911

1117

2457 ± 151.0a

97 ± 5.9b

Aldehydes

 2-Phenylethanal

1037

tr

21 ± 2.1a

 Benzaldehyde

1524

1517

tr

35 ± 3.2a

Acids

 2-Ethyl hexanoic acid

1129

tr

Nd

 Acetic acid

1428

600

18 ± 2.7a

19 ± 0.8a

Esters

 Ethyl-2-methylpropionate

961

758

315 ± 26.0

Nd

 Methylbutanoate

981

723

205 ± 16.0a

tr

 Ethylbutanoate

1028

803

604 ± 112.0

Nd

 1-Pentyl acetate

1170

919

37 ± 4.3

Nd

 Methyl hexanoate

1000

433 ± 45.1

Nd

 Butyl butanoate

1218

995

65 ± 5.6

Nd

 2-Heptyl acetate

1259

1040

tr

tr

 Hexyl acetate

1270

1014

522 ± 101.6

Nd

 (Z)-3-Hexenyl acetate

1325

1007

125 ± 2.5a

tr

 Methyl octanoate

1137

475 ± 96.0a

35 ± 1.5b

 Ethyl cinnamate

2167

1469

715 ± 117.0

Nd

Terpenes

 Limonene

1185

1030

127 ± 9.3

Nd

 (E)-β-Ocimene

1250

1156

tr

Nd

 Borneol

1253

885

tr

tr

 (Z)-Rose oxide

1337

40 ± 5.0

Nd

 (E)-α-Bergamotene

1415

tr

Nd

 Linalool

1540

1103

5121 ± 107.0a

506 ± 19.4b

 α-Terpineol

1582

1195

216 ± 5.0a

57 ± 6.7b

 Geranial

1715

1277

114 ± 4.5

Nd

 Geraniol

1840

341 ± 13.4a

79 ± 8.6b

Ketones

 Acetophenone

1067

42 ± 6.0b

437 ± 15.6a

 4-Hydroxy-2,5-dimethyl-3(2H)-furanone

2038

1070

50 ± 2.6b

326 ± 15.0a

 ϒ-Jasmolactone

2176

Nd

186 ± 11.7

Phenol

 Guaiacol

1842

1089

Nd

231 ± 14.3

 Norisoprenoids

  Theaspirane isomer I

1280

Nd

tr

  Theaspirane isomer II

1308

Nd

tr

  β-Damascenone

1801

1389

tr

21 ± 1.7a

  4-Hydroxy-β-ionol

1601

Nd

162 ± 10

  β-Ionone

1933

1491

260 ± 12.0a

trb

  3-Oxo-α-ionol

1938

Nd

100 ± 12.5

  4-Oxo-β-ionol

1943

Nd

141 ± 7.9

 

Total

13,900 µg kg−1

3236 µg kg−1

Alcohols

29.1%

26.1%

Esters

25.2%

1.36%

Terpenes

43%

20.1%

Ketones

0.66%

29.3%

Nop.

1.91%

13.3%

Mean ± SD (n = 3) with different superscript along the same row are significantly different (P < 0.05)

LR1, DB-FFAP; LR2, SE-54; tr trace amount (<10 µg kg−1), Nd not detected, Nop norisoprenoids

LRI linear retention index on column 1, LR2 linear retention index on column 2

1Compounds were identified by comparing their retention indices on DB-FFAP and SE-54 columns, their mass spectra, and odour notes were compared with their respective reference odorants’ data

Fig. 1

Characteristic gas chromatogram of solvent extracted sweet Vitex doniana

In the free fraction of the sweet black plum, the major aroma-active compounds (>300 µg kg−1) were linalool, 2-phenylethanol, 3-methyl-but-3-en-1-ol, ethyl cinnamate, ethylbutanoate, hexyl acetate, methyl octanoate, methyl hexanoate, ethyl-2-methylpropionate, geraniol, and (Z)-3-hexen-1-ol. These compounds accounted for 88.8% of the aroma in the free fraction. In addition, most of these compounds were previously reported in several fruits such as lychee, strawberry, cherry and oranges [8, 1214] either in the free or bound form. The identification of significant numbers of fatty acid esters such as methylbutanoate, ethylbutanoate and methyl hexanoate is an indication of the possible contribution of lipid metabolism in the biogenesis of Vds aroma. Volatile esters are produced by virtually all fruit species during ripening. Most volatile esters have flavour characteristics described as fruity [15]. Worthy of note was the high concentration of linalool (5121 µg kg−1) in the Vds. This floral-like terpene alcohol which is produced from isopentenyl pyrophosphate via the universal isoprenoid intermediate geranyl pyrophosphate, and membrane-bound enzymes such as linalool synthase [16] has been reported in lychee [17], Coastal Rican guava [18], mangaba fruit [19] and black velvet tamarind [20]. Another compound of interest is the honey-like 2-phenyl ethanol which produced a significant concentration in the free fraction. The odorant is an important flavour compound in the food and cosmetic industries.

The major volatile compounds in the bound fraction of the Vds were; 4-hydroxy-β-ionol, guaiacol, y-jasmolactone, 4-hydroxy-2,5-dimethyl-3(2H)-furanone, acetophenone, linalool and 3-methyl-but-3-en-1-ol (Table 1). In comparison to the free volatile compounds, which were mainly alcohols, esters and terpenes, the bound volatiles profiles included alcohols, ketones, and norisoprenoids. While most of the alcohols detected in the free fraction, were found in the bound form, there were fewer esters identified in the bound form. Only methyl octanoate was detected in both fractions. The reason for this observation is not farfetched because glycosidically bound volatiles are organic compounds in which the aglycone is volatile. This aglycone must be bounded to the sugar via ‘glycosidic bond’, for which these compounds have to have an –OH–, –SH, or –NH. Thus aldehydes, esters and terpenes are not able to form glycosidical bonds. Although, similar alcohol profiles were obtained from both free and bound fractions, the concentrations of the alcohols in the bound fraction were significantly (P < 0.05) lower to that of the free fraction. Of interest is the high abundance of 3-methyl-but-3-en-1-ol in both fractions. The presence of this compound in the bound form attested to the fact that it is an important intermediate in various biosynthetic pathways. In addition, significant numbers of odorous norisoprenoids were detected in the bound fraction. Among them were the floral 4-hydroxy-β-ionol, the spicy 3-oxo-α-ionol, 4-oxo-β-ionol and the flowery β-damascenone. Most of these compounds have been detected in several fruits such as grape [21], apple [22], raspberry [11] and passion fruit [23]. Also, identified in trace amounts (<10 µg kg−1) in the bound fraction were the two isomers (I & II) of theaspirane.

However, to gain an insight into the contribution of the aroma compounds to the aroma notes of the free and bound fractions, the 36 odorants detected through aroma extract dilution analysis (AEDA) as the key odorants were quantified. The flavour dilution (FD) factors obtained for the key odorants ranged from 2 to 512 (Table 2). Results revealed an array of aroma notes as shown in Table 2. The seventeen odorants with FD factors ≥16 were further investigated. The results of the quantitation showed that linalool was the predominant compound in both the free (5121 µg kg−1) and the bound (506 µg kg−1) fractions respectively (Table 3). This was followed by 2-phenyl ethanol (2457 µg kg−1) in the free fraction and acetophenone in the bound fraction. However, a comparative analysis of the aroma potencies revealed that the free volatile fraction of the Vds exhibited more potency for the ethyl-2-methylpropionate, β-damascenone and ethylbutanoate as exemplified by their high odour activity values (OAVs) (Table 3). On the other hand, the bound fraction recorded higher OAVs for β-damascenone and linalool respectively. Also, the OAVs indicated that hexyl acetate, ethyl-2-methylpropionate, ethylbutanoate, linalool, β-damacenone and (Z)-rose oxide contributed to the sweet prune-like aroma of the Vds. Interestingly, compounds with high concentration such as 2-phenyl ethanol (2457 µg kg−1), geraniol and methyl butanoate gave low OAVs. Therefore, their contribution to the aroma note of the Vds can be assumed to be low.
Table 2

Key odorants (free and bound) detected in Vitex doniana sweet

No

Compound

Odour impression

DB-FFAP

FD

1

Ethyl-2-methylpropionatea

Fruity

961

32

2

Methylbutanoatea

Fruity

981

128

3

Ethylbutanoatea

Banana-like

1028

16

4

2-Phenylethanalb

Honey-like

1037

4

5

Acetophenonea

Cherry-like

1067

512

6

Hexan-1-ola

Green, blooming

1079

2

7

2,6-Dimethylcyclohexanolc

112

Nd

8

2-Ethyl hexanoic acida

1129

Nd

9

1-Pentyl acetatea

Herbal-like

1170

2

10

Limonenea

Orange-like

1185

16

11

3-Methylbut-3-en-1-ola

Slightly apple-like

1209

8

12

2/3-Methylbutanola

Solvent

1213

4

13

Butyl butanoatea

Fruity, pineapple

1218

32

14

(E)-β-Ocimeneb

Flowery, blooming

1250

64

15

Borneolb

Camphor-like

1253

2

16

2-Heptyl acetatea

Woody, rum-like

1259

2

17

Hexyl acetatea

Fruity

1270

16

18

(Z)-3-Hexenyl acetatea

Fresh, pear-like

1337

8

19

(Z)-Rose oxidea

Rose-like

1337

16

20

(Z)-3-Hexen-1-ola

Green

1389

8

21

(E)-α-Bergamoteneb

floral

1415

8

22

Acetic acida

Sweaty

1428

4

23

1-Octen-3-ola

Mushroom-like

1451

2

24

Benzaldehydea

Almond-like

1521

16

25

Linaloola

Flowery

1540

16

26

α-Terpineola

Floral

1582

8

27

4-Hydroxy-β-ionola

Floral

1601

16

28

Geraniala

Rose-like

1715

8

29

β-Damascenonea

Flowery

1801

16

30

Geraniola

Rose-like

1840

16

31

Guaiacola

Smoky

1842

4

32

2-Phenylethanola

Honey-like

1911

16

33

β-Iononea

Floral, violet-like

1933

4

34

3-Oxo-α-ionolc

Spicy

1938

2

35

4-Hydroxy-2,5-dimethyl-3(2H)-furanonea

Caramel-like

2038

16

36

Ethyl cinnamatea

Flowery, sweet

2167

32

Nd not determined, FD flavour dilution

aGC retention and MS data in agreement with that of the reference odorants

bGC retention and MS data in agreement with spectra found in the library

cTentatively identified by MS matching with library spectra

Table 3

A comparative analysis of the aroma potency of compounds with flavour dilution (FD) values ≥16 in Vitex doniana sweet

No

Compounds

Conc.(µg kg−1fresh fruit) of fractions

Threshold (µg kg−1 of H2O) [ref.]

OAVs

Free

Bound

Free

Bound

1

Ethyl-2-methylpropionate

315

Nd

0.1 [4]

3150

Nd

2

Methylbutanoate

205

<10

28 [4]

7

<1

3

Ethylbutanoate

604

Nd

5 x 10−2 [4]

120,800

Nd

4

Acetophenone

42

437

65 [5]

<1

7

5

Limonene

127

Nd

210 [1]

<1

Nd

6

Butylbutanoate

65

Nd

100 [2]

<1

Nd

7

(E)-β-Ocimene

<10

Nd

Nd

Nd

8

Hexyl acetate

522

Nd

2 [4]

261

Nd

9

(Z)-Rose oxide

40

Nd

0.5 [1]

80

Nd

10

Benzaldehyde

<10

35

350 [5]

<1

<1

11

Linalool

5121

506

15 [3]

341

34

12

4-Hydroxy-β-ionol

Nd

162

Nd

Nd

13

Geraniol

79

341

40 [4]

2

9

14

β-Damascenone

<10

26

2 x 10−3 [4]

5000

10,500

15

2-Phenylethanol

2457

97

1000 [4]

3

<1

16

4-Hydroxy-2,5-dimethyl-3(2H)-furanone

50

326

40 [4]

1

8

17

Ethyl cinnamate

715

Nd

Nd

Nd

Nd not detected, OAVs odour activity values

[1] Maarse [29], [2] Takeoka et al. [30], [3] Lasekan & Ng [20], [4] Rychlik et al. [31], [5] Buttery et al. [32]

OAVs, calculated by dividing concentration with threshold value in water

Sensory evaluation of both bound and free odorants of V. doniana sweet revealed distinct aroma characteristics. For instance, while the free fraction was characterised by the flowery and fruity notes, the bound fraction exhibited cherry-like, flowery, and caramel notes (Fig. 2). However to determine which compounds are responsible for the perceived aroma notes, a more detailed analysis on aroma models and omission test will be required.
Fig. 2

Comparative aroma profiles of bound and free compounds in Vitex doniana sweet

Conclusion

The study has revealed for the first time the aroma profiles of the free and glycosidically bound fractions of V. doniana sweet. In the free fraction, the predominant compounds were the terpenes, alcohols and esters. The glycosidically bound fraction was composed of ketones, alcohols, terpenes and norisoprenoids. Results of the OAVs revealed that while the free volatile fraction of the V. doniana sweet exhibited strong potency for the fruity and floral notes; the bound volatile fraction produced more of flowery, caramel and cherry-like notes. In addition, results have shown that ethylbutanoate, β-damascenone, ethyl-2-methyl propionate, linalool, hexyl acetate and (Z)-rose oxide contributed highly to the sweet prune-like aroma of V. doniana sweet.

Materials and methods

Fruit material

Freshly harvested ripe Vitex doniana sweet (purple–black in colour) (Fig. 3) (300 fruits) grown in Owo, southwest Nigeria, were purchased from a local producer and stored (20 °C, 85% RH). The fruits were 2.8–3.2 cm in length, 1.2–1.4 cm in width and contained one hard conical seed each which is about 1.5–2.0 cm long and 1.0–1.2 cm wide. Quartering method [24] was used to select fruits for aroma analysis. At harvest, fruit had 10.5o brix and a titratable acidity of 0.86% malic acid equivalent.
Fig. 3

Ripened Vitex doniana sweet

Reagents and standards

Ethanol, methanol and dichloromethane were purchased from Merck (Darmstadt, Germany), while sodium dihydrogen phosphate-1-hydrate,l- (+) -ascorbic acid, and citric acid were obtained from Panreac (Barcelona, Spain). Sodium fluoride and ethyl acetate were purchased from Fluka (Buchs, Switzerland). Almond β-glucosidase was obtained from Sigma Chemical (St. Louis, MO). Amberlite XAD-2 resins were purchased from Sigma-Aldrich (Poole, Dorset, UK) and pure water was from a Milli-Q purification system (Millipore, Bedford, MA, USA). An alkane solution (C8–C24; 20 mgL−1 dichloromethane) was used to calculate the linear retention index (LRI) for each analyte. Other reagents were of analytical grade.

The following reference chemicals: Acetic acid, methyl butanoate, ethyl-2-methyl propionate, ethyl butanoate, 2-ethylhexanoic acid, 3-methylbutanol, (Z)-3-hexen-1-ol, hexanol, octen-3-ol, benzaldehyde, 3-methyl-but-3-en-1-ol, 2-phenylethanol, 1-pentyl acetate, limonene, 3-methylbut-3-en-1ol, acetophenone, butylbutanoate, (E)-β-ocimene, 2-heptyl acetate, hexyl acetate, (Z)-3-hexenyl acetate, (Z)-rose oxide, (Z)-3-hexenol, (E)-α-bergamotene, 1-octen-3-ol, linalool, α-terpineol, 4-hydroxy-β-ionol, geranial, geraniol, guaiacol, β-damascenone, β-ionone, 4-hydroxy-2,5-dimethyl-3(2H)-furanone, ethylcinnamate were from Sigma-Aldrich (St. Louis, MO). Stock standard solutions of 103 or 104 µg mL−1 of each compound was prepared as described earlier [25].

Fractionation of free aroma compounds of sweet black plum

Fruit pulp (500 g) was blended with 700 mL of distilled water. After 30 s, the mixture was centrifuged at 3000×g and 4 °C for 15 min. The supernatant was filtered through a bed of Celite. The clear Vds juice (300 mL) was applied onto an Amberlite XAD-2 adsorbent in a (30 × 2 cm) glass column. The column was washed with 250 mL of deionised water and 200 mL of n-pentane/diethyl ether mixture (1/1 v/v). The eluted extract was dried over anhydrous sodium sulphate and concentrated to 1 mL [26]. The concentrated extract (i.e. free fraction of the sweet black plum) was used for the GC–MS and GC–O analyses. The experiment was carried out in triplicate.

Bound aroma compounds of the V. doniana sweet

After the free fraction was obtained from the Amberlite XAD-2 glass column, the glycosidic extract adsorbed on the column was collected by washing it with 250 mL of methanol. The obtained extract was dried over anhydrous sodium sulphate and similarly concentrated as the free fraction. The concentrated bound fraction was re-dissolved in 100 mL of phosphate-citrate buffer (0.2 M, pH 5.0) and washed (2×) with 45 mL of n-pentane/diethyl ether (1/1, v/v) to remove any free fraction. One mililiter of an almond β-glucosidase solution (5 unit mg−1 solid, concentration of 1 unit mL−1 buffer) was added to the glycosidic extract and incubated overnight at 37 °C [27]. The liberated aglycones were extracted with 30 mL of n-pentane/diethyl ether (1/1, v/v) (2×). The combined extracts were dried over anhydrous sodium sulphate, filtered and concentrated as described earlier [26]. The concentrated extract was used for the GC–MS analysis and the experiment was carried out in triplicate.

GC–MS and GC–FID analyses

A Shimadzu (Kyoto, Japan) QP-5050A GC–MS equipped with a GC-17 A Ver.3, a flame ionization detector (FID) and fitted differently with columns DB-FFAP and SE-54 (each, 30 m × 0.32 mm i.d., film thickness 0.25 µm; Scientific Instrument Services, Inc., Ringoes, NJ) was employed. The gas chromatographic and mass spectrometric conditions were the same as described previously by Lasekan & Ng, [20]. The HP Chemstation Software was employed for the data acquisition and mass spectra were identified using the NIST/NB575K database.

Gas chromatography–olfactometry

A Trace Ultra 1300 gas chromatograph (Thermo Scientific, Waltham, MA, USA) fitted with a DB-FFAP column (30 m × 0.32 mm i.d., film thickness, 0.25 µm, Scientific Instrument Services, Inc., Ringoes, NJ) and an ODP 3 olfactory Detector Port (Gerstel, Mulheim, Germany), with additional supply of humidified purge air, was operated as earlier reported by Lasekan et al. [25]. The split ratio between the sniffing port and the FID detector was 1:1. Two replicate samples were sniffed by three trained panellists who presented normalised responses, reproducibility and agreement with one another. The GC–O analysis was divided into three parts of 20 min and each panellist participated in the sniffing. An aroma note is valid only when the three panellists were able to detect the odour note.

Identification and quantification

The linear retention indices were calculated according to Kovats method using a mixture of normal paraffin C6–C28 as external references. The identification of volatiles was carried out by comparing their retention indices, mass spectra data and odour notes with those of the reference odorants, literature data or with the data bank (NIST/NB575K). Quantitative data were obtained by relating the peak area of each odorant to that of the corresponding external standard and were expressed as µg kg−1.

Aroma extracts dilution analysis (AEDA)

The extracts of the free and bound fractions were diluted step wise twofold with dichloromethane by volume to obtain dilutions of 1:2, 1:4, 1:8, and 1:16 and so on. Each obtained dilution was injected into the GC–O. The highest dilution in which an aroma compound was observed is referred to as the FD factor of that compound [28].

Aroma profile determination

Fresh Vds (40 g) were placed inside glass containers (7 cm × 3.5 cm) and were orthonasally analysed as described earlier [20]. Reference odorants used were: Acetophenone (cherry-like), linalool (Flowery), (Z)-rose oxide (rose-like), 4-hydroxy-2,5-dimethyl-3(2H)-furanone (caramel-like) and hexyl acetate (fruity). Panellists rated the intensities of each descriptor on an unstructured scale from 0 to 10, where 0 = not detectable, 5 = weak, and 10 = strong. Final results were presented in a web plot.

Statistical analysis

Statistical analyses were carried out with SPSS version 16.0 Windows (SPSS Inc., Chicago, IL). Significance of differences between means was tested by one-way analysis of variance (ANOVA). Results were expressed as mean ± SD (standard deviation) of triplicate analyses.

Declarations

Acknowledgements

The author is grateful for the extensive financial support of the Fundamental Research Scheme (No. 5524558) at the University Putra Malaysia.

Competing interests

The author declares that he has no competing interests.

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Authors’ Affiliations

(1)
Department of Food Technology, University Putra Malaysia

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