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Volatile flavour compounds obtained from polished-graded flour and breads by a steam distillation method

J. H. Kim1, T. Maeda2 and N. Morita1

1Lab. of Food Chemistry, Graduate School of Life and Environmental Sciences, Osaka Prefecture University, 1-1, Gakuen-cho, Sakai, Osaka 599-8531, Japan.
2
Department of Life and Health Sciences, Hyogo University of Teacher Education, 942-1, Shimokume, Yashiro, Hyogo 673-1494, Japan.

Introduction

Polished-graded flours were obtained from the outer layer to the inner layer of whole-wheat grains using a gradually polishing method. Because the polishing procedure was conducted without debranning and removing germ and different from a conventional milling method, thus prepared flours have various benefits with high nutritional and functional values, as compared with commonly milled flours (CW). However, the presence of unfavorable bitter flavours derived from bran and germ in polished flours is one of some difficulties for its practical application to processed foods. To resolve the problem, it is necessary to identify flavour compounds of polished flours and breads. This study will describe volatile compounds and their identification from these samples by a steam distillation method.

Materials and methods

Wheat gain and flours

Polished flours were prepared from a hard-type wheat grain 1CW (No. 1 Canada Western Red Spring) using a rice polisher in the same polishing-grading method reported previously (Maeda et al, 2001). Eight fractions of polished fours were obtained from 1CW, however C-1 (100-90 %), C-5 (60-50 %) and C-8 (30-0 %) were used for the present experiments. Conventionally milled wheat flour prepared from 1CW (CW) was used as the control sample.

Bread baking

The breadmaking formula was 300 g of flour, 4.5 g of NaCl, 18 g sucrose, 6 g of dry yeast (J. T. Foods Co., Ltd., Sizuoka, Japan) and appropriate amount of water obtained from water absorption in farinograph mixing (AACC 54-21, 2000). The procedure and apparatus were the same as the international method (AACC 10-10B, 2000).

Isolation of volatile compounds by steam distillation

The volatile compounds contained in polished wheat flours and breads were extracted by the steam distillation method. Two hundred grams of flours or breads and 500 g of water were put into a 2-L round bottom flask, maintained in water bath at 30°C and the steam distillation was carried out for 3 hr. The volatile compounds were collected using the trap with dry ice, 86 % ethanol (-68°C) and liquid nitrogen (-196°C). Then, the trap was removed and collected materials were dissolved with 30-50 mL of freshly distilled diethyl ether for 2 hr. Thus obtained extract was dried over Na2SO4 and concentrated to ca. 100 µL using N2 gas flow, followed by storage at -20°C until the analysis.

Gas chromatograph-mass spectrometer (GC-MS) analysis

Isolated volatile compounds were analyzed using a Schimadzu 17A-GC gas chromatograph equipped with a QP-5000 mass spectrometer. A fused silica capillary column DB-1 (30 m × 0.25 mm i.d., film thickness 0.25 µm) was used under the splitless condition. Carrier gas (He) was flowed at a rate of 1.9 ml/min and temperatures of injector port and detector were 230°C. The column temperature was held at 30°C for 5min and programmed to 220°C at the rate of 3 oC/ min. The 10.0 µL of extract described above was injected into the GC-MS, and spectra were recorded in the electron-impact mode (MS/EI) at 70eV. Retention indices for all volatile compounds were determined according to the Kovats method using n-alkanes as standards (Kovats, 1965). To identify all compounds, the author applied the some methods that were matchings their fragmentation patterns to mass spectra obtained by NIST library, co-injection with an authentic specimen and published literatures. Authentic samples for the reference were obtained form commercial sources without purification.

Table 1. Volatile compounds identified in the polished-graded flours and breads.

Compounds

Flours

Breads

Peak area (%)

Peak area (%)

CW

C-1

C-5

C-8

CW

C-1

C-5

C-8

Alcohol

0.13

1.67

62.43

1.89

2.68

4.33

7.39

4.42

Aldehyde

54.60

26.04

9.32

65.22

14.52

50.04

48.79

9.10

Ketone

4.33

0.88

6.56

4.07

67.36

28.07

10.48

14.32

Acid

0.36

18.60

-

2.86

0.48

0.96

0.79

18.30

Ester

6.56

15.59

4.58

6.48

5.57

5.98

8.69

3.97

Others

32.77

11.81

9.67

9.59

5.51

1.66

13.04

21.44

Unknowns

1.26

25.41

7.44

9.89

3.88

8.96

10.82

28.45

CW, common hard-type wheat flour of 1CW; C-1, C-5, and C-8 were polished flours obtained from 100-90, 60-50 and 30-0 % layers of the 1CW whole grains, respectively.

Results and discussion

Various flours

A total of thirty-one compounds including 5 alcohols, 8 aldehydes, 2 ketones, 5 acids, 4 esters and 7 others were identified in all flour samples. The composition of volatile compounds was very different between the CW and polished flours as shown in Table 1. Aldehydes and alcohols were the most abundant compounds detected from CW and C-8, and from C-5, respectively. C-1 comprised aldehydes, acids, esters and others. The 1,6-heptadien-4-ol, 3-hydroxy-2-butanone, diallyl carbonate, γ-nonalactone, 1,1-diethoxy-ethane, 3-methylindole and α-ionone were found in all flour samples. Major compounds identified in CW were (E)-2-heptenal, 3-hydroxy-2-butanone, diallyl carbonate, γ-nonalactone, 1,1-diethoxyethane, 1,3-dimethyl-naphthalene and α-ionone. The C-8 of the innermost fraction included (E)-2-heptenal, 3-hydroxy-2-butanone, diallyl carbonate and α-ionone. The compounds identified in C-5 were primarily alcohols and aldehydes: the most abundant alcohols were hexanol, 3-methyl-1-pentatnol, 1,6-heptadien-4-ol and benzyl alcohol were less abundant, and the abundant aldehydes were (E,E)-2,4-hexadienal, 4-methyl-2-phenylhex-2-enal and dodecanal.

Figure 1. Alcohols and aldehydes found in the common wheat flour and polished flours.

The (E)-2-heptenal was found in CW, C-1 and C-8 with a high proportion, but not found in C-5. Major components identified in C-1 included (E)-2-heptenal, decanoic acid and diallyl carbonate. Results of peak area for alcohols, aldehydes, ketones, acids, esters and others in flour samples are shown in Figures 1-3. Alcohols and aldehydes were found in larger amounts in polished flours than in CW (Figure 1). Especially, C-5 contained larger amount of alcohol including hexanol, 3-methyl-1-pentanol and benzyl alcohol. Identified aldehydes in the flour samples were the most predominant in C-1. However, (E)-2-heptenal was definitely the highest in C-8, and dodecanal was higher in C-5 than in C-1 and C-8. 3-Hydroxy-2-butanone that was identified from all flour samples was also higher in polished flours than in CW (Figure 2). 2-Undecanone and acids were not found in C-5. Decanoic acid was definitely the highest in C-1, and pentanoic and undecanoic acid were the highest in C-8. Figure 3 shows esters and others found in various flour samples, and C-1 had the highest levels of linalyl acetate and diallyl carbonate. From these results, differences in volatile compounds between the common flour and polished flours were clear, and polished wheat flours greatly enhanced the levels of volatile compounds, as compared with common wheat flour.

Bread made from various flours

A total of forty-three compounds including 9 alcohols, 11 aldehydes, 6 ketones, 4 acids, 4 esters and 9 others were identified from all flour samples. The bread samples contained more additional compounds that might be formed by fermentation or baking process under the heating condition, as compared with the flour samples. The composition of identified volatile compounds in bread samples is summarized in Table 1. Proportion of alcohols in the bread samples was quite higher than those of flour samples, except for C-5. The identified compounds in the bread samples were primarily aldehydes and ketones. Their proportion in CW, C-1 and C-5 was more 70 %, whereas C-8 contained about 23 % of carbonyl compounds.

Figure 2. Ketones and acids found in the common wheat flour and polished wheat flours.

Figure 3. Esters and others found in the common wheat flour and polished wheat flours.

Major compounds in the flour samples (i.e. (E)-2-heptenal, 3-hydroxy-2- butanone, 1,6-heptadien-4-ol, diallyl carbonate) were also found in all bread samples with a high level.

Furthermore, compounds identified as bread flavour (i.e. 1-octanol, 1-nonanol, benzaldehyde, (E)-2-nonenal, heptanal, decanal, phenylacetaldehyde, dodecanal, 3-methyl-2- butenoic acid, nonanoic acid, decanoic acid, ethyl acetate, γ-nonalactone) in the present study coincided with those reported by Grosch and Schieberle (1997) and Martínez-Anaya (1996). However, some compounds reported as fresh bread-like aromas, such as 2-phenylethanol, 3-methylbutanal, (E,E)-2,4-decadienal, 2-acetyl-1-pyrroline and 6-acetyltetrahydropyridine (Grosch and Schieberle, 1997) were not found in the present analysis. The reasons are considered as follows: 1) volatile compounds with low molecular weight would be disappeared in the extraction and concentration processes; 2) extraction method used in the present study was relatively less sensible to that used in previous study.

Acknowledgements

The authors wish to thank the Miyake Flour Milling Co., Ltd. (Osaka, Japan) for supplying wheat grains; J. T. Foods Co., Ltd. (Shizuoka, Japan) for providing yeast and Itomen Co., Ltd. (Hyogo, Japan) for preparing polished flours.

References

American Association of Cereal Chemists (2000). Method 10-10B; 54-21.

Grosch, W., and Schieberle, P. (1997). Cereal Chemistry 74: 91-97.

Kovats E. sz. (1965). Adv.Chromatogr. 1: 229-247.

Maeda, T., Maeda, N., and Morita, N. (2001). Journal Applied Glycoscience 48: 27-36.

Martínez-Anaya, M. A. (1996). Journal Agricultural Food Chemistry 44: 2469-2480.

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