Food and Chemical Toxicology 48 (2010) 2005–2010
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Food and Chemical Toxicology
journal homepage: www.elsevier.com/locate/foodchemtox
Determination of non-volatile and volatile organic acids in Korean traditional
fermented soybean paste (Doenjang)
Shruti Shukla a,1, Tae Bong Choi b,1, Hae-Kyong Park a, Myunghee Kim a,*, In Koo Lee b, Jong-Kyu Kim a
a
b
Department of Food Science and Technology, Yeungnam University, 214-1 Dae-dong, Gyeongsan-si, Gyeongsangbuk-do 712-749, Republic of Korea
Department of Agricultural Chemistry, Kyungpook National University, 1370 Sankyuk-dong, Buk-gu, Daegu-si 702-701, Republic of Korea
a r t i c l e
i n f o
Article history:
Received 25 February 2010
Accepted 22 April 2010
Keywords:
GC/HPLC determination
Organic acids
Soybean fermented paste
Doenjang
a b s t r a c t
Organic acids are formed in food as a result of metabolism of large molecular mass compounds. These
organic acids play an important role in the taste and aroma of fermented food products. Doenjang is a
traditional Korean fermented soybean paste product that provides a major source of protein. The quantitative data for volatile and non-volatile organic acid contents of 18 samples of Doenjang were determined by comparing the abundances of each peak by gas (GC) and high performance liquid
chromatography (HPLC). The mean values of volatile organic acids (acetic acid, butyric acid, propionic
acid and 3-methyl butanoic acid), determined in 18 Doenjang samples, were found to be 91.73, 29.54,
70.07 and 19.80 mg%, respectively, whereas the mean values of non-volatile organic acids, such as oxalic
acid, citric acid, lactic acid and succinic acid, were noted to be 14.69, 5.56, 9.95 and 0.21 mg%, respectively. Malonic and glutaric acids were absent in all the tested samples of Doenjang. The findings of this
study suggest that determination of organic acid contents by GC and HPLC can be considered as an affective approach to evaluate the quality characteristics of fermented food products.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
Organic acids occur in fermented products as a result of hydrolysis, biochemical metabolism and microbial activity. Quantitative
determination of organic acids is of great importance in fermented
foods due to technical, nutritional, sensorial and microbial reasons.
The volatile and non-volatile organic acid compounds in various
fermented soybean products, such as Japanese miso and natto, Chinese sufu and Thai thua nao, have been studied extensively (Chung
et al., 2005; Leejeerajumnean et al., 2001). These organic acids,
mainly acetic acid, lactic acid and less often propionic acid and butyric acid, may originate from raw materials or be generated by fermentation during processing and storage. Studies have shown that
the most frequently reported compounds in fermented food products include esters (ethyl 2-methyl butyrate, ethyl hexanoate),
acids (acetic acid, 2/3-methyl butanoic acid), pyrazines and phenolic compounds. However, the volatile component profiles of fermented food products vary with the microflora involved, as well
as by the processing conditions such as fermentation, drying, brining or ageing (Leejeerajumnean et al., 2001).
Some studies have examined the volatile organic acid contents
in Korean fermented soybean pastes using different types of micro* Corresponding author. Tel.: +82 53 810 2958; fax: +82 53 810 4662.
E-mail address: [email protected] (M. Kim).
1
First and second authors equally contributed to this work.
0278-6915/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.fct.2010.04.034
bial strains (Seo et al., 1996), as well as different extraction procedures (Park et al., 2003) and ageing and mixing methods (Choi
et al., 1997). A commonly used method for the determination of
acid content is alkaline titration using an appropriate visual indicator. However, the titration methods are generally not selective nor
sufficiently sensitive or precise to detect a small acid content
(Casella and Gatta, 2002). To avoid these problems, gas chromatography (GC) and high performance liquid chromatography (HPLC)
methods have gained importance in organic acid analysis. Because
of the speed, selectivity, sensitivity, reliability and simple sample
preparation methods involved in GC and HPLC, these are the most
widely used techniques (AOAC, 1975; Blanco Gomis and Mangas
Alonso, 1996). HPLC has been used with refractive index, UV detection and pulsed electrochemical detection as one of the most
important modern analytical techniques for the determination of
volatile organic acids in fermented food products (Mendlicott
and Thompson, 1985).
Traditional Korean soybean paste, Doenjang is primarily made
with meju, which typically uses natural microflora and soybeans
as the basic ingredient (Park et al., 2003). Due to the unequal fermentation progression that occurs with natural microflora, the traditional method of making Korean soybean paste of Doenjang has
been adapted for mass production. However, there is increasing
consumer interest for traditionally made soybean pastes that possess significant health effects as well as complex aroma characteristics (Shin et al., 2001).
2006
S. Shukla et al. / Food and Chemical Toxicology 48 (2010) 2005–2010
To the best of our knowledge, only fewer reports are available
on quantitative determination of organic acid contents in Doenjang
samples. Therefore, the objective of the present study was to determine the contents of the non-volatile and volatile organic acids in
various traditional fermented Korean soybean paste samples of
Doenjang by HPLC and GC techniques.
2. Materials and methods
the solution of 0.1% H3PO4/dH2O at 1 ml/min flow rate. The peak heights of standard
solutions were identified to determine the organic acid concentrations in all Doenjang samples.
2.6.2. GC equipment and conditions
The GC analysis conditions for the determination of volatile organic acids were
as follows: instrument: Hewlett–Packard gas chromatograph model 5890 II with
flame ionization detector, column: 10% PEG6000 in glass column (1.2 mm 1.5 m),
oven temperature: 170 °C, injector temperature: 200 °C, detector temperature:
200 °C and carrier gas: He (0.9 ml/min).
2.1. Samples
A total of 18 traditional Korean fermented soybean paste samples of Doenjang
were analyzed. All samples were collected from different area of Daegu-si and
Gyeongsangbuk-do districts, Korea. Fermentation period of all Doenjang samples
was near about one year. Collected samples were stored at 4 °C until analysis.
2.2. Standards and reagents
All chemicals and solvents used in this study were of analytical and chromatographic grade. Non-volatile organic acids (oxalic acid, malonic acid and glutaric
acid) and volatile organic acids (propionic acid, butyric acid and 3-methyl butanoic
acid) were purchased from Sigma–Aldrich (St. Louis, MO, USA). Other standards
such as citric and succinic acids were purchased from Duksan and Daejung Chemicals (Korea), respectively, while lactic and acetic acids were purchased from Oriental Chemicals (Korea).
2.3. Preparation of standard solution
Standard solution of all mixed non-volatile organic acids (oxalic acid, citric acid,
lactic acid, succinic acid, malonic acid and glutaric acid) was prepared at 0.5% concentration in 100 ml distilled water, whereas standard mix solution of volatile organic acids (acetic acid, propionic acid and butyric acid) was prepared at 0.1%
concentration in 100 ml distilled water. 3-Methyl butanoic acid is not soluble in
distilled water, therefore, it was dissolved separately in ethanol at the same concentration. All solutions were stored at 4 °C until the use.
2.4. Measurement of non-volatile organic acids
One hundred grams of each soybean paste sample of Doenjang was reflux extracted with 150 ml of 86.5% ethanol for 1 h at 65 °C, and filtrated with Whatman
filter paper No. 2. The residue was again extracted twice with equal volume of
65% ethanol for 1 h. The filtrate was centrifuged at 8000 rpm for 20 min at 4 °C.
The upper solution having ethanol was vacuum-evaporated. Three hundred sixty
ml of methanol (90%) was added (three times) to desalt the samples followed by filtration. The filtrate was vacuum-evaporated followed by dissolving it in 100 ml distilled water. Extracted components were purified by ion-exchange column
chromatography. Amberlite IR-120 and amberlite IRA-400 were used as cation
and anion exchangers for chromatographic separation, respectively. Twenty milliliters extracted and desalted samples were eluted with distilled water through cation
and anion exchange columns and free sugar components were collected. The
remaining sample components, absorbed on amberlite IR-400 were eluted with
150 ml of eluting solvent, 2 N (NH4)2CO3 at slow flow rate. Eluent was recovered,
vacuum-evaporated and dried under P2O5 desiccator for overnight. Non-volatile organic acids were dissolved in 15–20 ml of diethyl ether and then methylated
according to the adopted method of Schlenk and Gellerman (1960). Finally, 10 ll
sample was injected into HPLC and chromatographs were analyzed.
2.5. Measurement of volatile organic acids
Extraction of fermented soybean paste samples of Doenjang and GC determination of volatile organic acids were carried out according to the modified procedure
developed by Park et al. (2002). For the extraction of volatile organic acids, 10 g of
each sample was manually homogenized with 20 ml of triple distilled pure water
and then filtered with Whatman paper No. 2, followed by centrifugation at
10,000g for 30 min. Subsequently, 0.37 ll of 2% H2SO4 was added to 0.7 ml of the
filtrate, to make the final concentration 0.1% of H2SO4 and filtered through
0.45 lm pore-size membrane syringe filter (Pall Co., Acrodisc Syringe Filters,
25 mm, 0.2 lm, Woongki Science Co., Korea). Finally, 2 ll solution of each sample
and standard were injected into GC separately.
3. Results and discussion
3.1. Determination of non-volatile organic acids
The HPLC technique was used to determine non-volatile organic
acid contents in soybean fermented samples of Doenjang. Standards solutions of individual non-volatile and volatile organic acids
were chromatographed separately and mixed to determine the
retention times and the response of each organic acid (Table 1).
The individual non-volatile organic acids were qualitatively determined by comparison with standard chromatograms and by doping the samples with standard reference acids (Fig. 1). Table 2
shows the composition of non-volatile organic acids in each type
of Doenjang sample. A total of six non-volatile organic acids,
including oxalic acid, citric acid, lactic acid, succinic acid, malonic
acid and glutaric acid, were analyzed. The mean values of non-volatile organic acids, such as oxalic acid, citric acid, lactic acid and
succinic acid, determined in 18 Doenjang samples were found to
be 14.69, 5.56, 9.95 and 0.21 mg%, respectively (Table 2). None of
the samples contained malonic and glutaric acids. Choi and Bajpai
(2010) compared the organic acid contents in non-germinated
meju (MNG), germinated meju under light (MGL) and dark
(MGD) conditions and observed that total of six organic acids,
including tartaric acid, malic acid, lactic acid, acetic acid, citric acid
and succinic acid, were detected. The compositions of total organic
acids in MNG, MGD and MGL were ranged 942.2 ± 70.0,
1075.6 ± 79.5 and 1019.7 ± 82.1 mg%, respectively (Choi and Bajpai, 2010). A number of factors, such as fermentation environment,
types of microorganisms and the state of raw materials can affect
the ratio of the contents of organic acids (Choi et al., 2007). As
shown in Fig. 1, the major non-volatile organic acid in 18 Doenjang
samples was found to be lactic acid. However, oxalic, citric and
succinic acids were present in a very small amount. To produce lactic acid through the biological route, there are various kinds of lactic acid producing microorganisms. Lactic acid bacteria convert
glucose into lactic acid through the homo-fermentative pathway
(Hofvendahl and Hagerda, 2000). However, there have been no reports published on homo-fermentative lactic acid bacteria isolated
from fermented soybean paste (Doenjang), for the production of a
high concentration of lactic acid. The major microorganism involved in this process is Bacillus subtilis which produces lactate
Table 1
Retention times of non-volatile and volatile organic acids separated by HPLC and GC
analysis.
Organic acids
Non-volatile
2.6. Chromatographic conditions
2.6.1. HPLC equipment and conditions
Quantitative analysis of non-volatile organic acids was carried out through high
performance liquid chromatography, using Waters HPLC unit coupled with refractive index detector. Separation of non-volatile organic acids was achieved using
an RSpak KC-811 column (8.0 300 mm) at 40 °C. The mobile phase consisted of
Volatile
Retention time (min)
Oxalic acid
Citric acid
Malonic acid
Succinic acid
Lactic acid
Glutaric acid
Acetic acid
Propionic acid
Butyric acid
3-Methyl butanoic acid
5.7
6.8
7.8
9.0
9.5
10.3
3.8
5.1
7.0
7.9
S. Shukla et al. / Food and Chemical Toxicology 48 (2010) 2005–2010
2007
Fig. 1. HPLC chromatograms relative to: (A) Standard solution of six non-volatile organic acids; (B) Doenjang sample; (C) Doenjang sample (1: oxalic acid; 2: citric acid; 3:
malonic acid; 4: succinic acid; 5: lactic acid; 6: glutaric acid).
through various metabolic pathways. It has been reported that
average values of total non-volatile organic acid contents for fermented soybean paste, prepared with B. subtilis alone, improved
meju and traditional meju were found to be 6.1%, 1.48% and
2.01%, respectively (Kim, 1998, 2000). In addition to this, previously we reported the contents of non-volatile organic acids in
commercial soybean paste. It was found that contents of non-volatile organic acids such as lactic acid, oxalic acid, malonic acid, succinic acid, glutaric acid and citric acid, determined in commercial
soybean paste were found to be 0.6, 0.8, 0.9, 0.8, 23.4 and
54.5 mg%, respectively (Park et al., 2002). However, these findings
were not in agreement with the results obtained in the present
study. This might be attributed to the sample variations in fermen-
tation period, conditions, different microbial strains and storing
temperatures.
3.2. Determination of volatile organic acids
The mean values of volatile organic acids such as acetic acid, butyric acid, propionic acid and 3-methyl butanoic acid, determined
in 18 Doenjang samples, were found to be 91.73, 29.54, 70.07
and 19.80 mg%, respectively (Table 3). The average value of total
contents of volatile organic acids in 18 Doenjang samples was
found to be 211.14 mg%. Fig. 2 showed the comparison of standard
chromatogram of a mixture of the volatile organic acids as well as
volatile organic acids present in 18 Doenjang samples. The average
2008
S. Shukla et al. / Food and Chemical Toxicology 48 (2010) 2005–2010
Table 2
Analysis of non-volatile organic acid contents in traditional Korean fermented
soybean paste samples of Doenjang (mg/100 g).
Sample no.
OXA
CIT
LAC
SUC
MAL
GLU
Total
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Mean
ND
27.53
1.84
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
14.69
ND
5.56
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
5.56
ND
7.11
13.02
51.73
6.95
ND
7.69
9.48
5.40
ND
4.87
6.42
0.86
8.72
11.85
7.55
6.11
1.53
9.95
ND
ND
0.25
ND
ND
ND
ND
ND
ND
ND
ND
ND
0.17
ND
ND
ND
ND
ND
0.21
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
40.20
15.11
51.73
6.95
0.00
7.69
9.48
5.40
0.00
4.87
6.42
1.03
8.72
11.85
7.55
6.11
1.53
10.86
OXA: oxalic acid; CIT: citric acid; LAC: lactic acid; SUC: succinic acid; MAL: malonic
acid; GLU: glutaric acid.
ND: not detected.
Table 3
Analysis of volatile organic acid contents in traditional Korean fermented soybean
paste samples of Doenjang (mg/100 g).
Sample no.
ACE
BUT
PRO
3-MET
Total
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Mean
63.84
151.84
62.05
79.19
136.96
50.22
42.05
84.78
81.38
63.82
63.29
31.74
178.28
105.04
117.65
77.14
139.10
122.76
91.73
68.94
42.46
52.00
1.06
15.42
0.00
0.00
0.00
0.00
23.81
0.00
0.00
126.36
53.09
39.00
22.87
48.48
38.22
29.54
122.74
100.63
34.15
47.69
37.21
32.34
16.49
59.26
33.50
30.99
30.69
40.49
218.20
109.09
91.16
75.84
124.19
56.65
70.07
8.94
8.60
6.69
3.82
26.29
7.32
59.04
16.70
43.04
36.55
29.28
14.44
14.31
14.58
16.13
16.62
15.89
18.08
19.80
264.45
303.52
154.90
131.75
215.89
89.88
117.58
160.74
157.92
155.17
123.26
86.67
537.16
281.80
263.94
192.47
327.65
235.71
211.14
ACE: acetic acid; BUT: butyric acid; PRO: propionic acid; 3-MET: 3-methyl butanoic
acid.
ND: not detected.
value of propionic acid was higher than butyric acid and 3-methyl
butanoic acid (Table 3). Previously, propionic acid has been found
to detect in Korean fermented and non-fermented marketed food
products (Lee et al., 2010). As reported, among 11 kinds of Korean
marketed food samples including fermented soybean paste, the
detection rate of propionic acid in salted and fermented fish sauce
and Chunggukjang (product of soybean with shorter fermentation
period) was higher than that of others (Lee et al., 2010). Montano
et al. (2003) reported that propionic acid was found in Spanishstyle fermented green olives by the growth of Propioni bacterium
species. Accordingly, organic acids including propionic acid in fermented products of Deonjang varied by a pre-dominant microorganism. Lee et al. (2002) reported that organic acids including
propionic acid were found in Cnidium officinale, a Korean medicinal
plant, attributing to typical flavor. These results indicated that
naturally occurring propionic acid might be produced by not only
fermentation but also flavor components. However, various microorganisms can grow during fermentation, and are attributed to
specific metabolism of microbial enzyme and organic acids that
can also change the pH of the samples (Kim, 1996).
Doenjang is traditionally prepared by soaking, steaming and
fermenting soybeans in a humid closed space maintained at
30 °C for specific periods. During fermentation, proteins and carbohydrates are degraded by B. subtilis, which is the primary bacterium involved in Doenjang fermentation, producing sugars, small
peptides and amino acids that contribute to the flavor and taste
of the samples. In addition, sugars and amino acids are further
metabolized during fermentation, forming pyruvic acid as a key
intermediate in several organic acid-forming pathways (Skeie
and Ardo, 2000). Organic acids are then produced by the Embden
Meyerhof Parnas pathway and hexose monophosphate pathway.
Pyruvic acid is a critical intermediate, leading to numerous volatile
organic acids, thereby assuming that among the volatile organic
acids found in Doenjang samples, acetic acid, propionic and butanoic acid can be formed from pyruvic acid via five or six successive
reactions during the fermentation of propanoic and butanoic acids.
Moreover, branched-chained organic acids, such as 2-methyl propanoic acid and 3-methyl butanoic acid, can be formed mainly from
branched-chained amino acids, including valine, leucine and isoleucine. In particular, 2-methyl propanoic acid was produced from
valine (Beck, 2005), whereas 3-methyl butanoic acid was produced
by leucine catabolism via transamination, followed by oxidation
steps (Czerny and Schieberle, 2005).
Choi and Ji (1989) investigated the changes of flavor in Cheonggukjang during fermentation, and found that two volatile organic
acids, 2-methyl propanoic acid and pentanoic acid, were important
for flavor changes. On the other hand, volatile organic acids, such
as acetic acid, propionic acid, butanoic acid and 3-methyl butanoic
acid, were subsequently identified by Choi et al. (1998), in which
butanoic and 3-methyl butanoic acids were found to affect the
quality of Cheonggukjang. These findings suggest that volatile organic acids, such as acetic acid, propionic acid, butyric acid and
3-methyl butanoic acid present in Doenjang, can make an important contribution to the flavor characteristics. In particular, butanoic and 3-methyl butanoic acids have characteristic odors, being
rancid, sweaty and cheese-like. Therefore, it is worth quantifying
the contents of volatile organic acids that strongly affect the quality of Doenjang.
In case of butyric acid synthesis, some lactate bacterium especially Lactobacillus plantarum converts lipids into butyric acid
through the activity of intracellular enzymes (Azarnia et al.,
2006) and, then this again forms 3-methyl butanoic acid. In our
tested samples, the average contents of butyric acid (29.54 mg%)
and 3-methyl butanoic acid (19.80 mg%) were lesser than acetic
acid (91.73 mg%) and propionic acid (70.07 mg%), and these results
are in strong agreement with previous findings of Choi et al.
(1998). Also it has been reported that the contents of acetic acid,
butanoic acid and 3-methyl butanoic acid were enhanced during
fermentation of Korean soybean meju as compared to propionic
acid (Choi et al., 1998). These discrepancies with the results of this
study may be due to different cultivating conditions and different
microbial strains used as starter for fermentation process.
4. Conclusion
Based on the aforementioned results, it can be concluded that
both non-volatile and volatile organic acids can have potential
influence on the quality characteristics of food products during
fermentation process. Thus, these findings reinforce the suggestions that evaluation of organic acid contents in fermented food
S. Shukla et al. / Food and Chemical Toxicology 48 (2010) 2005–2010
2009
Fig. 2. GC chromatograms relative to: (A) Standard solution of three organic acids; (B) Doenjang sample; (C) Doenjang sample (1: acetic acid; 2: propionic acid; 3: butyric
acid).
products can be included as an effective addition to food industry
to determine the quality and flavor of Doenjang samples with useful information related to food quality. However, further research
including a correlation between the formation of organic acids
and microbial species in Korean fermented soybean paste is
needed.
Acknowledgements
This study was supported by Technology Development Program
for Agriculture and Forestry, Ministry for Food, Agriculture, Forestry and Fisheries, Republic of Korea, in 2009.
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Conflict of Interest
The authors declare that there are no conflicts of interest.
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