Research article
Received: 20 January 2016

Institute of Brewing & Distilling

Revised: 10 May 2016

Accepted: 15 May 2016

Published online in Wiley Online Library: 19 July 2016

(wileyonlinelibrary.com) DOI 10.1002/jib.334

Characteristic odour compounds in shochu
derived from rice koji
Yohei Shiraishi,1,2 Yumiko Yoshizaki,2* Toshifumi Ono,2 Hiroaki Yamato,2Kayu Okutsu,2 Hisanori Tamaki,2 Taiki Futagami,2 Sameshima Yoshihiro2
and Kazunori Takamine2
Shochu, a traditional Japanese distilled liquor, makes use of rice koji, which is koji mould grown on rice grain. Rice koji is an essential ingredient of Japanese liquors such as shochu, and it plays a role as a source of the enzyme to degrade starch. However,there has been no research on the effect of rice koji on the flavour of shochu. Therefore, in this study, the volatile compounds inshochu derived from rice koji were investigated. Two shochu samples were prepared to assess the contribution of rice koji to theflavour. One shochu sample was prepared from rice koji, yeast and water (rice koji-shochu). The other shochu sample was obtained
from steamed rice and various enzymes instead of rice koji (enzyme-shochu), along with yeast and water. The volatile compounds
were analysed by gas chromatography–mass spectrometry (GC-MS) and GC-MS/olfactometry with aroma extract dilution analysis. The results showed that enzyme-shochu had a higher flavour dilution value of dimethyl trisulphide and hexanal, whereas ricekoji-shochu had a higher flavour dilution value of some ester compounds that imparted aromatic odours such as fruity. Some
unknown peaks representing compounds that impart characteristic odours such as soda, potato, lavender, and tea-like were
specifically detected in rice koji-shochu. The concentrations and calculated odour active values of 14 compounds were measured.
These results showed that isovaleraldehyde, ethyl caprylate, ethyl caproate and ethyl 2-methylbutyrate played an important rolein imparting the specific odour of rice koji-shochu. Copyright © 2016 The Institute of Brewing & Distilling
Keywords: gas chromatography–mass spectrometry; gas chromatography–olfactometry; rice koji; shochu; volatile compounds

Introduction

J. Inst. Brew. 2016; 122: 381–387* Correspondence to: Yumiko Yoshizaki, Division of Shochu Fermentation
Technology, Education and Research Centre for Fermentation Studies, Facultyof Agriculture, Kagoshima University, 1-21-24 Korimoto, Kagoshima City
890-0065, Japan. E-mail: obana@ms.kagoshima-u.ac.jp
1

Bio’c, Co., Ltd, 111-1 Uchida, Muro-cho, Toyohashi City, Aichi, 441-8087, Japan

2

Division of Shochu Fermentation Technology, Education and ResearchCentre for Fermentation Studies, Faculty of Agriculture, Kagoshima University,
1-21-24 Korimoto, Kagoshima City 890-0065, Japan

Copyright © 2016 The Institute of Brewing & Distilling

381

Shochu, a traditional Japanese distilled liquor, is predominantly
produced in the southern part of Japan and is one of the mostimportant products that drive the economy in these prefectures.
The annual production volume of shochu in Japan from April
2013 to March 2014 was approximately 500 million litres, according to the public data of the National Tax Administration Agency,
Japan (1). The manufacture of Shochu has some differences fromthe manufacture of whisky, which is distilled liquor made fromgrain in Europe and America. There are two points that are themost important aspects of shochu manufacture. First, the shochu
is fermented using mould as the source of various enzymes.
Second, in shochu manufacture, the distillate of the shochu
moromi-mash is not separated and is thus collected together
throughout the distillation process.
Mould is previously cultured on steamed rice or barley grains sothat enzymes such as α-amylase, glucoamylase, protease andlipase required for the fermentation are produced (2–4). Rice grain
is mainly used as a culture for brewing. This type of solid culture isreferred to as rice koji. Rice koji plays a role similar to that of malt inwhisky production and is commonly used for making traditional
Japanese fermented foods such as shochu, soy sauce and sake (ricewine). Moulds are commonly used as sources of enzymes forfermentation in East Asian countries such as China and Korea;
however, in Japan, the scenario is different since Aspergillus sp. isused exclusively for the preparation of rice koji as the starter
mould. Specifically, yellow-koji mould, Aspergillus oryzae, has beenused for the production of various fermented foods, including
sake, soy sauce and miso. White-koji mould (A. luchuensis mut.

kawachii) and black-koji mould (A. luchuensis) have been used onlyin the manufacture of shochu. The shochu manufacturing process
is briefly described below. Rice koji prepared using koji mould
starter is mixed with water and yeast for the preparation of the firstmoromi-mash. The starch value in rice koji is almost the same asthat in rice grain, even though the former is mould-grown (5). Thus,
rice koji can also be used as an ingredient for fermentation; starchsaccharification and alcohol fermentation proceed simultaneously
in the moromi-mash. After 5 days, water and steamed sweet potato, barley, rice or buckwheat (known as the main material) areadded to the first moromi-mash (second moromi-mash). Fermentation of the second moromi-mash is continued for an additional 10
days. Alternatively, if the main material is not added and thefermentation of the first moromi-mash is continued, it becomesthe mash of a kind of shochu called awamori. The fermented mash
is finally distilled using a pot still. Therefore, the use of rice koji for
brewing is one of the main reasons the characteristics of shochu
are distinct from those of other distilled liquors available aroundthe world.

Y. Shiraishi et al.

Institute of Brewing & Distilling

Shochu is characterized by the flavour associated with the ingredients. The odour compounds derived from the main material
in the second moromi-mash have been identified and quantified
in previous research (6,7). To date, rice koji and koji mould haveattracted attention because of the mass production and secretion of various enzymes outside of fungus bodies. Therefore,much research has been devoted to understanding the molecular biological mechanism of the secretion of enzymes and theapplication to the mass production of beneficial enzymes(8–10). The role of rice koji is recognized to be supplementary
to that of the enzymes. On the other hand, the effect of rice koji
on the flavour of Japanese liquor has not been researched indetail. Rice koji manufacture requires ~45 h after inoculating
the koji mould on the steamed rice grain. During this period,
the odour becomes different from that of steamed rice. Finally,rice koji assumes a distinctive flavour of chestnuts, mushrooms
and Indian ink. The quality of rice koji depends on the growthof the koji mould – in other words, the enzyme activity – and thisdirectly affects the fermentation. The flavours play a role indetermining the quality of the rice koji. Therefore, research was
carried out to identify the key volatile compounds in rice koji,
and isobutyraldehyde, isovaleraldehyde, 1-octen-3-ol andphenylacetaldehyde were identified as the main contributors to
the typical odours of rice koji (11–13). The term ‘koji’ was usedin the sensory evaluation of sake by the National Research Institute of Brewing, Japan in 2006 (14,15). It is strongly suggested
that rice koji has a marked effect on the flavour of Japaneseliquors such as sake and shochu.
To study the effect of rice koji on the flavour of shochu, two typesof shochu were prepared; one was made exclusively from rice kojiwith yeast and water (rice koji-shochu), whereas the other was
made from steamed rice and some enzymes instead of rice koji
(enzyme-shochu). The odours and volatile compounds in the
shochu were analysed by gas chromatography–mass spectrometry(GC-MS) and GC-MS/olfactometry with aroma extract dilution
analysis (AEDA) and sensory evaluation.

increased beyond 38°C. Then, the rice koji was stored at
until further use.

80°C

Moromi-mash of rice koji-shochu. Approximately 1.6 kg of the
prepared rice koji (corresponding to 1.3 kg of polished rice) was
added to 2.4 L of water and 40 mL of yeast seed medium andfermented for 11 days at 30°C.
Moromi-mash of enzyme-shochu. Approximately 1.8 kg ofsteamed rice (corresponding to 1.3 kg of polished rice) was added
to 2.2 L of 80 mM citrate buffer (pH 4.0). Glucoamylase, α-amylase
and protease corresponding to the enzyme activity in rice koji
were added to the moromi-mash. Yeast seed medium (40 mL)
was added to the moromi-mash and fermented for 12 days at
30°C.
Monitoring of the alcohol fermentation. The alcohol fermentation was monitored by measuring the amount of CO2 gas
generated. The initial total weight of the containers wasmeasured after preparing the moromi-mash. The total weightof the moromi-mash container was measured each day. The
difference between the initial weight and the weight after
incubation (the decrease in weight) was used to determine theamount of CO2 gas generation. The integration curve for weight
reduction was plotted on a graph.
Distillation of moromi-mash. Shochu was made according to
the method used in our previous studies (7). Shochu was obtained from single-batch distillation in a glass distillation apparatus (a glass pot still coupled to a glass column). Approximately 1 kgof moromi-mash was distilled using the steam generated fromwater in a round-bottomed flask heated by a mantle heater. The
distillate was then water-cooled. The end point of distillation was
the point at which the alcohol content in the bundled distillate
reached ~38%. The distillate was filtered and diluted to 25%
alcohol. The shochu samples were stored at room temperature
prior to analysis.
Large volume static headspace samplingMaterials and methods
Chemicals and strain
White-koji mould starter (A. luchuensis mut. kawachii) and polishedrice were purchased from a local company and market in Japan,
respectively. The yeast strain Kagoshima-5 (16) was supplied by
the Kagoshima Prefectural Brewing Association (Kagoshima,
Japan). Dextrozyme-glucoamylase ( glucoamylase), Sumizyme
Shochu (α-amylase) and Orientase 20A (protease) were suppliedby Nippon Denpun Kogyo Corp. (Kagoshima, Japan), Shin Nihon
Chemical Co. Ltd (Aichi, Japan) and HBI Enzymes Inc. (Hyogo,
Japan), respectively.

382

Rice koji preparation. Rice koji was prepared according to themethod adopted in one of our previous studies (11). Polished rice
was washed and soaked in water for 1 h. Then, the water was
drained off, and the rice was steamed for 1 h and subsequently
cooled to ~43°C. White-koji mould starter was inoculated onto
the steamed rice, mixed well, and incubated for 45 h at 35°C anda relative humidity of 90%. During the preparation of rice koji,
the temperature of the inoculated rice was measured using athermometer at a central point of the rice mass. The inoculated
rice was repeatedly stirred and cooled when the temperature

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Headspace volatile components were collected in a large volume
static headspace (LVSH) system (Entech 7100A series; Entech Instruments Inc., Simi Valley, CA, USA). The shochu samples (10 mL)
were transferred to a 200 mL sample bottle. For quantitative determination, 1 mL of 1-pentanol (10 mg/L) was added to the samples
as an internal standard. After incubation at 30°C, 100 mL ofheadspace gas was vacuum-extracted from the sample bottle.
The volatile compounds were adsorbed onto two tandemly
arranged commercial traps (Entech Instruments Inc.), which were
packed with different stationary phases; trap 1 was packed witha glass beads/Tenax mixture resin and trap 2 was packed with
Tenax resin. The volatile compounds were desorbed bythermodesorption using an Entech 7100A preconcentrator andapplied to the GC-MS system.
GC-MS/O. Quantification of the volatile compounds was performed using an Agilent 6890 series gas chromatograph (Agilent
Technologies Inc., Palo Alto, CA, USA) equipped with an Agilent
5975B mass-selective detector and a sniffing port (Gerstel,
Mülheim an der Ruhr, Germany). One half of the column flow
was directed to the MS system, while the other half was directed
to a heated sniffing port. The GC-MS system was equipped witha DB-WAX column (60 m × 0.25 mm i.d., 0.25 μm film thickness;
Agilent Technologies Inc.). The GC operation conditions were an

Copyright © 2016 The Institute of Brewing & Distilling

J. Inst. Brew. 2016; 122: 381–387

Odour compounds in shochu from rice kojiInstitute of Brewing & Distilling

injector temperature of 220°C, transfer line temperature of 250°C,
quadrupole ion trap temperature of 150°C and ion source
temperature of 250°C. Analyses were carried out using helium as
the carrier gas at a flow rate of 1.0 mL/min with the following
temperature programme: 40°C for 5 min then increased at 3°
C/min to 240°C. The injector and detector were held at 250°C
and 300°C, respectively. All mass spectra were acquired in the electron impact mode. Quantitative determinations were obtainedusing 1-pentanol (m/z 55). The volatile compound content wascalculated from the GC-peak areas relative to the GC-peak areaof the internal standard. The panellists recorded the retention timeby using an olfactory detector port recorder software (Gerstel) andas per the description of the aroma compounds. Each sample wassniffed four times. When a volatile compound was detected atleast three times, this analyte was determined to be a declaredaroma compound.
Qualitative and quantitative analysis
For compound identification, the mass spectra and retention times
were compared with those of authentic standards or the retention
index in the database of the Aromaoffice software (Nishikawa
Keisoku Co. Ltd, Tokyo, Japan). Retention indices were calculatedby analysing a series of n-alkanes (C7–C33; Shimadzu GLC Ltd,Tokyo, Japan). All standards were prepared and diluted with a
25% ethanol solution to obtain the standard solutions. The peakareas were used for quantification. The calibration curves for individual compounds were constructed by plotting the area ratio of
the target compounds to the internal standard. The odour active
value (OAV) for each volatile compound was calculated from theformula below:
OAV = concentration in shochu/odour threshold value (17,18)

Figure 1. Time course of the alcohol fermentation. The alcohol fermentation wasmonitored by observing the amount of CO2 gas generated. The total weight of themoromi-mash container was measured each day. The difference between the initial
weight and the weight after incubation (the decrease in weight) was used to determine the amount of CO2 gas generation. The integration curve for weight reduction
was plotted on a graph. The squares and triangles represent the moromi-mash of ricekoji- and enzyme-shochu, respectively.

AEDA. The flavour dilution (FD) values of the odour-active
compounds were determined by AEDA (19). Each extract wasserially diluted 3-, 9-, 27-, 91- and 273-fold using 25% ethanol as
the diluent. Each sample was sniffed four times. When a volatilecompound was detected three or more times, the odorant was
defined as having been detected. FD is defined as the ratio of
the concentration of a compound in the initial extract to that in
the most diluted extract in which the odour was detected by
GC-MS/O.
Sensory evaluation. Rice koji-shochu and enzyme-shochu were
evaluated in terms of odour and taste. Sensory profile analysis was
conducted by nine assessors (six females and three males) from
Kagoshima University. Most of the assessors were previously
trained using sensory evaluation techniques. The intensity of eachcharacteristic was evaluated on a scale of 0–5 (0 = not detected,
1 = very weak intensity, 2 = weak intensity, 3 = moderate intensity,
4 = strong intensity, 5 = very strong intensity). The results of thesensory profile analysis were averaged for each odour and plottedin a spider web diagram.

Results and discussion
Preparation of rice koji-shochu and enzyme-shochu

J. Inst. Brew. 2016; 122: 381–387

Figure 2. Spider plot for sensory evaluation of (A) odours and (B) tastes of rice kojishochu and enzyme-shochu. Rice koji-shochu and enzyme-shochu are indicated by
the solid line and dotted line, respectively. Asterisks indicate the significant differences
between rice koji-shochu and enzyme-shochu as calculated by the t-test (p < 0.05).

Copyright © 2016 The Institute of Brewing & Distilling

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383

The white-koji mould was selected to prepare rice koji since thismould is popular for shochu manufacture. Although most typesof shochu are produced by adding a steamed main material suchas sweet potato, barley or rice, the shochu was prepared without
addition of the main material. This manufacturing method was

Y. Shiraishi et al.

Institute of Brewing & Distilling

Table 1. Odour intensity and description of odour active compounds in rice koji-shochu or enzyme-shochu
Compounds

Alcohols
Isoamyl alcohol
Ester compounds
Ethyl acetate
Ethyl propionate
Ethyl isobutyrate
Isobutyl acetate
Ethyl butyrate
Ethyl 2-methyl butyrate
Ethyl isovalarate
Isoamyl acetate
Ethyl caproate
Ethyl caprylate
Ethyl nonanoate
Ethyl caprate
Diethyl succinate
Ethyl laurate
Aldehydes
Acetaldehyde
AcetalIsovaleraldehyde
HexanalSulphuric compounds
Dimethyl trisuphideUnknown compounds
Unknown 1
Unknown 2
Unknown 3
Unknown 4
Unknown 5
Unknown 6
Unknown 7
Unknown 8
Unknown 9
Unknown 10
Unknown 11

RI*

RIref¶

Identification†

Odour description

FD

Reference

Rice koji-madeEnzyme-made

1217

1224

MS, RI

Characteristic odour

27

81

(20)

858
939
949
1009
1030
1047
1062
1114
1235
1435
1537
1635
1658
1872

892
999
1007
1034
1055
1069
1083
1137
1251
1441
1541
1643
1655
1847

MS, RIMS, RIMS, RIMS, RIMS, RIMS, RIMS, RIMS, RIMS, RIMS, RIMS, RIMS, RIMS, RIMS, RI

Sweet
FruityFruityRaisin, pear
FruityStrawberry, pineapple
Apple
Banana, pear
Sweet
Fruity, soapy
CerealFruityBaked apple, roasted
Soapy

3
9
81
3
27
81
81
3
27
1
3
1
9
1

3
3
243
9
27
81
81
1
27
ND
1
ND
9
ND

(20)
(20)
(20)
(20)
(20)
(20)
(20)
(20)
(20)
(21)
(21)
(21)
(22)
(21)

766
865
890
1072

713
889
915
1094

MS, RIMS, RIMS, RIMS, RI

Fruity, acrid
Sweet,vinyl
Acrid
Green, grass

1
27
3
3

1
243
1
27

(23)
(23)
(22)
(21)

1374

1385

MS, RI

Pickles

3

3

(24)

1024
1116
1126
1173
1285
1301
1320
1464
1559
1660
1825

-

-

81
1
3
243
3
9
ND
9
1
1
1

ND
3
ND
243
ND
27
1
ND
ND
1
ND

FruityFruityVinyl
Potato, roasted
Soda
Perilla
Sweet
Fried potato
LavenderFloralTea-like* RI, retention index calculated on a DB-WAX capillary column with a series of n-alkanes (C7–C33).
¶
RIRef, retention index from each reference (21–27). ND, not detected.
†
Identification proposal is indicated by the following: MS, identification by comparing the mass spectrum with Nist05a spectral
database; RI, identification by retention indexes with literature data.

384

the same as that for awamori. It was thought that this would makethe identification of key volatile compounds in shochu from ricekoji clearer, as it would not contain the volatile compounds fromthe main materials. Furthermore, another shochu was preparedfrom steamed rice and enzymes instead of rice koji as a control.
Since white-koji mould produces and secretes a large amount of
citric acid, the pH of the mash becomes low during fermentationof the shochu moromi-mash. Thus, a citrate buffer (pH 4.0) was also
used to adjust the concentration of the citric acid and the pH in accordance with that of rice koji. This shochu was expected to containvolatile compounds from rice and the metabolites of yeast. It was
confirmed that the moromi-mash made from steamed rice and enzymes was nearly fermented with the desired outcome, although

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it was inferior to the moromi-mash made from rice koji (Fig. 1).
These shochu samples were referred to as rice koji-shochu andenzyme-shochu, respectively, in this study.
Flavour characterization of rice koji-shochu and enzymeshochu
Rice koji-shochu and enzyme-shochu were compared by sensory
evaluation. The assessors selected a perceived term for eachshochu from the 21 odour and seven taste terms that were selected from the flavour wheel of sake (14,15). The odour terms
were fruity, alcohol, woody, green, spicy, floral, cereal, rice bran,rice koji, steamed rice, sweet, caramel, roasted, sulphuric,

Copyright © 2016 The Institute of Brewing & Distilling

J. Inst. Brew. 2016; 122: 381–387

Odour compounds in shochu from rice kojiInstitute of Brewing & Distillingmushroom, rubber, mouldy, earthy, soapy, oily and sour, while thetaste terms were sour, sweet, salty, bitter, astringent, sharp andheavy. Sulphuric and rubber were not selected by any of the assessors. Many assessors selected seven odour and five taste terms.
Furthermore, many assessors pointed out the unlisted odour of‘pickle-like’ for the enzyme-shochu. Therefore, eight odour terms
were compared by adding ‘pickle-like’ and five taste terms for eachshochu (Fig. 2). Rice koji-shochu had strong sweet, caramel androasted odours, and strong heavy tastes. Significant differenceswere found in sweet, caramel, and roasted odours between ricekoji-shochu and enzyme-shochu. On the other hand, theenzyme-shochu had a strong woody and pickle-like odour.
Although rice koji-shochu prepared in this study was not identified
as ‘rice koji’ by most of the assessors, three out of the nine assessors identified it as having a ‘rice koji’ odour. It is considered thatthese differences were derived from the odour of the rice koji
and the effects of the small amount of enzymes in the rice koji.
GC-MS/O and AEDA
To identify the key volatile compounds in rice koji-shochu,
GC-MS/O and AEDA analyses were performed. Rice koji-shochu
and enzyme-shochu exhibited 31 and 22 odour peaks, respectively(Table 1) (20–24). Eleven odour peaks could not be identified bymass spectrometry. These peaks were referred to as unknowns
1–11. Isoamyl alcohol, ethyl isobutyrate, isobutyl acetate, acetal,
hexanal, unknown 2, unknown 6 and unknown 7 were detected
at higher dilution in enzyme-shochu than in rice koji-shochu. In
contrast, ethyl propionate, isoamyl acetate and isovaleraldehyde
were detected at higher dilution in rice koji-shochu than in
enzyme-shochu. Ethyl caprylate, ethyl caproate, ethyl laurate
and unknowns 1, 3, 5, 8, 9 and 11 were detected in rice kojishochu only. Among them, unknowns 5, 8, 9 and 11 that impart
characteristic odours such as soda, potato, lavender and tea-likewere specifically detected in rice koji-shochu. These unknown
odour peaks do not have higher FD values but may contributethe characteristic odours. It has been reported previously thatshochu prepared using a saccharification enzyme as an alternative to rice koji has a dry taste (25,26). Although that shochu
was prepared by adding sweet potato as the main material, it
is consistent with our results. Isovaleraldehyde has been previously identified and regarded as a characteristic compound inrice koji (11). Therefore, it was confirmed that isovaleraldehyde
also affects the flavour of shochu.
Quantification and OAVTo obtain insight into the contribution of each volatile compound to the aroma, a total of 14 aroma-active compounds were
quantified. Isovaleraldehyde, ethyl 2-methylbutyrate, ethyl
caproate (C6), ethyl caprylate (C8) and ethyl laurate (C12) weredetected at higher concentrations in rice koji-shochu than in
enzyme-shochu (Table 2). These results showed that ethyl estersof medium-chain fatty acids (C6–C12) had a higher content in
the rice koji-shochu. The lipase activity of rice koji has been
detected in the extract of intact rice koji (27). In that study, it
was considered that the lipase in rice koji contributed to the
production of ethyl esters of long-chain fatty acids. It was revealed that the enzyme from rice koji may affect the productionof ethyl esters of medium-chain fatty acids directly or indirectly.
Therefore, most fatty acid-ethyl ester compounds imparted astrong sweet odour to rice koji-shochu. The concentrations of
isoamyl alcohol and isoamyl acetate were higher in the
enzyme-shochu than in the rice koji-shochu. It is considered thatisoamyl acetate facilitates the production of high concentrationsof isoamyl alcohol (28). All compounds, except dimethyl
trisulphide, had an OAV >1. Isovaleraldehyde and ethyl caprylate
showed the highest OAVs and concentrations in both shochus. It
is suggested that these compounds contribute to the flavour ofshochu made from rice koji. This result also supports the assertion that isovaleraldehyde is one of the key flavour compounds
in shochu made from rice koji. Isovaleraldehyde has also been

Table 2. Odour intensity and description of odour active compounds in rice koji-shochu or enzyme-shochu
Compounds

Isovaleraldehyde
Ethyl caprylate
AcetalEthyl caproate
Ethyl 2-methyl butyrate
Ethyl isobutyrate
Ethyl acetate
HexanalIsoamyl alcohol
Isoamyl acetate
Ethyl butyrate
Ethyl isovalarate
Dimethyl trisulfide
Ethyl laurate

OAV¶

Concentration
Rice koji-shochu

Enzyme-shochu

Rice koji-shochu

Enzyme-shochu

230
350
2,600
90
16
110
39,00021
139,000
130
70
7
0.13
110

80
230
3,800
40
6
90
30,00025
278,000
310
50
9
0.16
80

1150
175
52.0
18
16
7.3
5.2
4.7
4.6
4.3
3.5
2.3
0.7
-

400
115
76.0
8
6
6.0
4.0
5.6
9.3
10.3
2.5
3.0
0.8
-

Threshold*(μg/L)

Reference

0.2
2
50
5
1
15
7,500
5
30,000
30
20
3
0.20
-

(17)
(18)
(18)
(18)
(18)
(18)
(18)
(18)
(18)
(18)
(18)
(18)
(18)
-

J. Inst. Brew. 2016; 122: 381–387

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wileyonlinelibrary.com/journal/jib385* Odour threshold were taken from the literature (17,18). (17) Odour thresholds were determined in water. (18) Odour thresholds were
determined in 10% (w/w) ethanol/water solution.
¶
OAV were calculated by dividing the concentration by the odour thresholds.

Y. Shiraishi et al.

Institute of Brewing & Distilling

detected in other liquors such as sherry wine, pear brandy andbourbon whisky (29–31). It was previously revealed thatisovaleraldehyde is formed during fermentation as an intermediate of isoamyl alcohol production by yeast (32,33), and formed
during distillation through the Maillard reaction (34,35). In addition, in Japanese sake, it has been reported that isovaleraldehyde
is produced from isoamyl alcohol by the alcohol oxidase of kojimould during storage and commercial distribution at room
temperature and that the active enzyme in sake is inactive
during the heating process (36). With respect to the fact thatshochu is a distilled liquor, the increment of isovaleraldehyde
during storage in shochu can be eliminated. Therefore,
isovaleraldehyde in enzyme-shochu might be mostly producedby yeast metabolites and distillation. The Maillard reaction during
distillation is also a process that can produce isovaleraldehyde.
However, when one considers that distillation is a common
process between rice koji-shochu and enzyme-shochu, it issuggested that the higher concentration of isovaleraldehyde inrice koji-shochu is derived from rice koji itself and produced
during fermentation by enzymes from rice koji. The results of thisstudy suggest that the isovaleraldehyde content in rice kojiaffects the concentration in shochu and the shochu flavour. In
the future, it will be necessary to identify the formation mechanisms of isovaleraldehyde during the making of rice koji andthe effect of isovaleraldehyde on the flavour of shochu.
Acknowledgements
This work was carried out with the support of the ‘Cooperative
Research Program for Agriculture Science and Technology
Development’ (project no. PJ008600), Rural Development
Administration, Republic of Korea and JSPS KAKENHI grant
number 24700804.

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