Sweet potato fermentation food (sweet potato shochu)

Sweet potato fermentation food (sweet potato shochu)

CHAPTER 12 Sweet potato fermentation food (sweet potato shochu) Kazunori Takamine Division of Shochu Fermentation Technology, Education and Research ...

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Sweet potato fermentation food (sweet potato shochu) Kazunori Takamine Division of Shochu Fermentation Technology, Education and Research Center for Fermentation Studies, Faculty of Agriculture, Kagoshima University, Kagoshima, Japan

Introduction Shochu is a distilled alcoholic beverage that is produced in Japan, and sweet potato shochu, a shochu made from sweet potatoes, is a classic example of a fermented product that uses sweet potato as its base ingredient. Sweet potato shochu is made using a production process that is unique to Japan. Water is first added to koji (Aspergillus malt), which is produced by culturing koji fungi, at a water:koji ratio of 120:100 (v/w), and fermentation is allowed to proceed. After this, sweet potato is added at a ratio of 500:100 (v/w) with respect to the koji, and water is then added at a ratio of 280:100 (v/w) with respect to the koji, then further fermentation is performed. Distillation of this fermented liquid yields a sweet potato shochu with about 38% (v/v) ethanol. This shochu is diluted in water to give an alcohol content of 25% and then bottled (in Japan, one bottle of liquor is generally 900 mL or 1.8 L) and shipped. The overall manufacturing process can be broken down into a number of smaller processes as shown in Fig. 12.1: raw materials processing, a koji manufacturing process, a primary preparation process, a secondary preparation process, a distillation process, a purification process, and an aging process. Sweet potato shochu has an aroma that is derived from its raw materials, including the koji used for fermentation, yeast, and the distillation process itself. It is widely believed that drinking sweet potato shochu has health benefits. This chapter introduces the sweet potato shochu manufacturing methods, the factors affecting shochu aroma, and briefly discusses its potential health benefits.

Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00012-0

© 2019 Elsevier Inc. All rights reserved.



Sweet Potato

First stage moromi preparation process

Koji production process Traditional method

Raw material for koji (rice, barley, etc

Koji, water, yeast

automatic koji production

method Fermentation for 5 to 6 days

Raw material (sweet potato, brown sugar, rice, barley, etc)

Second stage moromi preparation process Raw material


First stage moromi

Fermentation for 8–15 days

Distillation process

Purification & aging process

Bottling process

Figure 12.1 Manufacturing process for shochu.

Raw material Raw sweet potato About 860,700 tons of sweet potato are harvested in Japan annually, with 322,800 tons (38%) being produced in Kagoshima prefecture, which is the primary production location for sweet potato shochu in Japan. Approximately 50% of sweet potatoes produced in Kagoshima are used in making shochu, 40% are used for starch production, and 10% are used for fresh consumption. In contrast, in other Japanese sweet potato production regions, such as Tochigi prefecture (172,000 tons) and Chiba prefecture (103,500 tons), 90% of sweet potatoes are harvested for fresh consumption (Crop yield of sweet potato, 2016). There are many types of sweet potatoes cultivated in Japan, including sweet potatoes used for fresh consumption, such as the “Beni-Satsuma,” which have reddish-purple skin and pale yellow flesh; sweet potatoes that contain anthocyanins, such as the “Aya-Murasaki,” which have purple flesh; and sweet potatoes that contain beta-carotene, such as the “Beni-Hayato,” which have yellow flesh. The sweet potato primarily used in the production of shochu is the “Kogane-Sengan,” which has been specifically bred to improve its starch production.

Sweet potato fermentation food (sweet potato shochu)


Cultivation period (days)

Sweet potatoes are highly nutritious with a well-balanced nutrient profile, making them a semicomplete nutritional food. The principal component in sweet potato is the storage carbohydrate starch, which is generally present at 25% 35%. Other sweet potato components include dietary fiber and minerals. Sweet potatoes also contain glucose, fructose, and sucrose at approximately 0.7%, 0.5%, and 3.0%, respectively. When sweet potatoes are steamed, a proportion of starch is converted into maltose as a result of beta-amylase activity inside. Because steamed sweet potatoes contain approximately 10% of maltose, they can also be considered to be a sugar source in addition to being a starch source. An interesting characteristic of sweet potatoes is that their water content does not change significantly after steaming. The following conditions must be met for sweet potatoes to be used in manufacturing sweet potato shochu: 1. They cannot be infected with black rot or soft rot. 2. They cannot be damaged by larvae of Scarabaeidae and other insects. 3. They must be as fresh as possible, since bruising progresses easily after harvesting. 4. They should have a high starch content. 5. They should have an appropriate size of 300 500 g. Fig. 12.2 shows the relationship between the cultivation period and size of sweet potato. In general, the percentage of medium-sized sweet potatoes (between 300 and 500 g in weight) and extra-large sized sweet potatoes (800 g or more) increases as the cultivation period increases.

120 XS S M L XL


180 0


40 60 Ratio (%)



Figure 12.2 Weight distributions used for the size categorization of sweet potatoes harvested 120, 150, and 180 days after planting. The sweet potatoes were divided into size categories of XS ( . 49 g), S (50 299 g), M (300 549 g), L (550 799 g), and XL ( . 799 g).


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Table 12.1 Starch value of sweet potatoes (%). Cultivation period

120 days 150 days 180 days

Sweet potato XS





25.7 28.3 28.1

27.9 30.9 27.2

28.4 27.2 27.5

24.6 27.5


, L and XL size sweet potatoes that harvested 120 days after being planted were absent in this study. XL size sweet potatoes that harvested 150 days after being planted could not be checked owing to sample shortage.

Larger sized sweet potatoes are, however, not as desirable in the manufacture of sweet potato shochu. They require significantly more time and effort in order to process them, since the steaming process takes longer time to heat the center of sweet potato and in addition the larger size means that the sweet potato needs to be physically cut into smaller sized pieces. Table 12.1 shows the influence of cultivation period and size of sweet potato on its starch value. The starch value of short-cultivated sweet potato (120 days) increases with its size, whereas that of long-cultivated sweet potato (150 or 180 days) is much lower with larger sweet potatoes (L or XL). Thus the relationships between the size and starch value of sweet potato are inconsistent. The crop yields are actually increased with longer cultivation, but the proportion of large sweet potato with lower starch value is increased. Therefore the moderate culture time for sweet potato production is approximately 150 days (Okutsu et al., 2016).

Raw material processing Like a lot of products sweet potatoes begin to deteriorate immediately after harvesting. They are initially susceptible to damage that can occur during transportation from the field to the factory as a result of physical interaction with the container and other sweet potatoes, as well as physical damage that can occur during unloading. Therefore the sweet potatoes must be processed as quickly as possible. After delivery, the sweet potatoes are thoroughly washed with water, and any defective portions, such as parts with black rot or soft rot, or with insect damage caused by Scarabaeidae, are removed. Since sweet potatoes are not uniformly shaped, raw material processing cannot be mechanized easily, so this relies on humans.

Sweet potato fermentation food (sweet potato shochu)


When the defective parts of sweet potatoes are removed, and also when the skin is peeled, the sweet potatoes will soon begin to dry and become bruised. As a result, they must be steamed as soon as possible. In general, a steaming duration of approximately 60 min is sufficient, assuming that appropriately sized sweet potatoes have been selected. After steaming, they are either air-cooled immediately and then prepared, or left to naturally cool overnight and then prepared the following morning. Preparation involves pulverizing them by machine to pieces with sizes of approximately 2 cm or less.

Koji production process Koji fungi Koji is the rice or barley that has been cultured with koji fungi, and it is broadly categorized into Chinese koji or Japanese koji. In the production of Chinese koji, barley, wheat, or peas are used as raw materials. The immersed raw materials are pulverized and solidified into either a brick or a ball shape. Fungi of the Rhizopus or Mucor genus, which are present in the raw materials or in the natural environment, are used in culture to produce koji which is referred to as Mochikoji, with the brick-shaped koji being referred to as Daqu and the ball-shaped koji being referred to as Xiaoqu. In contrast, Japanese koji uses steamed rice or barley cultured with koji fungi by inoculation, and it is referred to as “Bara-koji.” The color of a colony of Aspergillus is derived from the color of its conidia. When different species of Aspergillus genus are inoculated onto an agar medium there are numerous colors of the mature colony observed. Generally, black koji fungi (Aspergillus luchuensis) appears black-brown, yellow koji (Aspergillus oryzae) fungi appears green with some yellow coloration, and white koji fungi (A. luchuensis mut. kawachii) appears brown with some yellow coloration. However, it is important to be aware that even though a type of koji fungi may be referred to as yellow koji fungi, various different strains of yellow koji fungi exist, such as those used for sake, soy sauce, and miso, and the color of the conidia may be also different. For example, there is a white mutant strain of yellow koji fungi used for manufacturing miso, and it is even whiter in color than white koji fungi. Also black koji fungi is broadly classified into two different types: A. luchuensis var. awamori and A. luschuensis var. saitoi. In taxonomy, “var.” represents a variety. The former variety has a strong saccharifying power, and a low capacity of citric acid production, whereas the latter variant has


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a low saccharifying power, and a high citric acid production capacity. In the brewing of Awamori, two strains of koji fungi with different properties are generally mixed so that they are well-balanced, and often used in this combined state. Shochu koji is produced from a black koji fungi or white koji fungi. Yellow koji used for sake had been used for shochu making, but it was shifted to black koji in around 1919. Since black koji produces a considerable amount of citric acid compared to yellow koji, the pH of the moromi is decreased to 3 3.5. This pH decrement promotes the dominant growth of shochu yeast, which has acid tolerance without contamination. In 1945 White koji has started to be used for shochu making, because shochu made with white koji has a light and floral flavor compared to that with black koji. Meanwhile, black koji was revived for shochu making in the late 1980s.

Role of koji Koji has many roles, including the production of enzymes that degrade starch, protein, and other polymers in the raw material into small molecules, and the production of citric acid required for lowering the pH of the moromi (the liquid produced after adding water and yeast to the raw material and performing fermentation). In addition, the koji imparts an aroma to the shochu. Production of enzymes When koji fungi grows on rice or barley, it produces a wide variety of enzymes. The primary enzymes include amylolytic enzymes, proteolytic enzymes, and lipolytic enzymes. The pH of the moromi is low, usually between 3 and 3.5, so it is necessary for the enzymes produced by koji fungi to be acid-tolerant. The enzymes produced by the white and black koji used for shochu production have an optimal pH in the acidic range, and also have stable activities at low pH, providing them with excellent acid tolerance. In addition, the enzymatic activities of α-amylase in rice koji and barley koji are very small compared to those in yellow koji, the koji used for sake. The α-amylase activities in rice koji and barley koji are 1/8 and 1/14 of that in yellow koji, respectively. Despite this, the starch in the raw material dissolves and breaks down well in shochu moromi, so the enzymatic activity is sufficient for the production of shochu. The acidic proteases in rice koji and barley koji have very high activities compared to yellow koji being approximately 13-fold and 6-fold higher, respectively.

Sweet potato fermentation food (sweet potato shochu)


Shochu koji has an important characteristic, a raw starch degrading enzyme, that yellow koji does not have it (Iwano et al., 1986). Amylolytic enzymes Amylolytic enzymes are critical for producing shochu using raw materials in which starch is the main ingredient, because glucose is produced by the hydrolysis of starch in the raw material and then metabolized by yeast to produce alcohol. Amylolytic enzymes produced by koji fungi that are important in degrading starch include α-amylase, glucoamylase, and α-glucosidase. α-Amylase Starch has a structure whereby glucose molecules are linked together with α-1,4 bonds. α-Amylase is an endoenzyme that can randomly hydrolyze these bonds. As it can rapidly reduce the viscosity of starch solutions, α-amylase is also referred to as a liquefying enzyme. Glucoamylase Glucoamylase is an exoenzyme that hydrolyzes the α-1,4 bonds of the starch backbone into glucose units starting from the nonreducing end, and because it produces glucose, it is also referred to as a saccharifying enzyme. This enzyme can also hydrolyze α-1,6 bonds, which are associated with the branches present in starch. Proteolytic enzymes Proteolytic enzymes hydrolyze protein and peptides to produce amino acids. The resulting amino acids are further metabolized by yeast to produce aromatic components such as higher alcohols, as well as their esters and aldehydes. In addition, some of the amino acids are involved in forming aromatic compounds, such as aldehydes, by undergoing nonenzymatic thermal reactions (e.g., the Maillard reaction, Strecker degradation, etc.) in the acidic moromi environment during distillation. Acidic protease Acidic protease is an endoenzyme that can randomly hydrolyze peptide bonds in proteins and peptides. The proteases present in white koji, black koji, and sake yellow koji are called acidic protease, because their optimal pH is 3.0. This enzyme is principally responsible for producing the peptides and amino acids present in the moromi. Acidic carboxypeptidase Acidic carboxypeptidase is an exoenzyme that hydrolyzes carboxy-terminal peptide bonds in proteins and peptides into amino acids. The optimal pH of the carboxypeptidases in white koji, black koji, or sake yellow koji are approximately 3.0 3.5, so for this reason


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these enzymes are referred to as acidic carboxypeptidases. This enzyme is also important in producing the peptides and amino acids present in the moromi. Lipolytic enzymes Lipolytic enzymes produce fatty acids by hydrolyzing the ester bonds present in lipids. Among the fatty acids that are produced, palmitic acid, stearic acid, oleic acid, linoleic acid, and their ethyl ester compounds contribute to the physical taste of shochu, giving it a roundness and a mellow sharp flavor. These fatty acids are the principal lipid components in shochu. β-Glucosidase β-Glucosidase is an enzyme that hydrolyzes linked structures containing β-1,4 bonds, such as cellulose. The monoterpene alcohol that gives sweet potato shochu its characteristic aroma is present in sweet potato as a monoterpene glycoside linked to glucose through a β-1,4 bond. During fermentation, β-glucosidase acts on monoterpene glycoside, and produces monoterpene alcohols such as geraniol, which provide the characteristic aroma of sweet potato shochu. For further details, please refer to the section titled “Shochu aroma.” Production of citric acid The citric acid produced by koji fungi is responsible for decreasing the pH of the moromi to between 3 and 3.5, and in doing so, prevents the moromi from rotting. Citric acid is a nonvolatile organic acid that can be separated during the distillation step, so that it does not become an ingredient in shochu. The quantity of citric acid contained in the koji requires a koji acidity of 5 7. Koji acidity is defined as the quantity of 0.1 N NaOH required to neutralize 10 mL of filtrate after 100 mL of distilled water has been added to 20 g of koji which has been allowed to leach over a 3-h period at room temperature with periodic stirring. Citric acid is produced from the metabolism of glucose, since glucose is required to produce alcohol. The higher the amount of citric acid produced, the lower the yield of shochu. The citric acid contained in 100 kg of koji is calculated to be 320 g (1.67 mol) per koji acidity level.

Sweet potato fermentation food (sweet potato shochu)


Flavor contributions Koji fungi significantly affects the flavor of shochu. For example, sweet potato shochu manufactured using yellow koji has the splendid characteristic aroma of yellow koji. Shochu manufactured using white koji and black koji shares a fruity aroma and a sweet burnt aroma, whereas shochu produced with white koji is light and elegant, and shochu produced with black koji is rich and mellow.

Koji production Koji is produced either by a traditional method that uses a koji board or by an automatic koji production method that uses mechanized equipment. The conventional methods include the board koji method, the box koji method, and the floor koji method. The automatic koji production methods include a ventilated koji production device that uses a rotating drum and a triangular shelf, a fully-automated koji production device that uses a rotating drum and a disk-type automatic koji production device. All of these devices require approximately 43 h to produce koji. The raw materials, rice and barley are first steamed, and the starter koji is then used to inoculate them at 38°C. Approximately after 27 h, the temperature is reduced to approximately 35°C and maintained at that level for the remainder of the process. This reduction in temperature and its maintenance enables the koji fungi to produce citric acid. Starter koji is generally rice koji, in which koji fungi has been cultured on rice and the growth of spores is allowed to occur. The starter koji is used for inoculating spores into the raw material being used for koji production. There are two types of starter koji, namely, granular starter koji and powder starter koji (Fig. 12.3). The amount of starter koji dispersed during inoculation is 1:1000 with respect to the weight of the koji raw material in the case of granular starter koji, and 1:500 with respect to the weight of the koji raw material in the case of powder starter koji. The raw materials for the production of koji are generally rice and barley. This is because a water content of 40% or less is desirable as the koji fungi culture environment. However, the water content of sweet potatoes is very high, typically from 60% to 70%, which makes culturing koji fungi difficult. Also the specific surface area of sweet potato is very small compared with rice and other cereals, which decreases the quantity of enzymes and citric acid produced. These problems were resolved by developing modern technologies in which sweet potatoes are cut into dice shapes


Sweet Potato

Figure 12.3 Two types of starter koji.

with each side being 5 mm in length, which are then either immediately dried, or steamed and then dried. These dried, dice-shaped sweet potatoes are then allowed to absorb water until the water content is approximately 40%. After this, they are steamed and the koji is produced. The sweet potato shochu manufactured using this koji has a sweeter sweet potato aroma, and a more refreshing taste than that made with rice koji.

Preparation process The shochu preparation method is a unique method, consisting of a primary preparation process and a secondary preparation process. First, the primary preparation process is performed using koji, water, and yeast, allowing the fermentation to proceed for 5 days to obtain a primary moromi. Then steamed/pulverized sweet potato and water are added to this primary moromi, and a second preparation process is performed.

Yeast From one molecule of glucose, yeast produces two molecules of ethanol and two molecules of carbon dioxide. This phenomenon is referred to as ethanol fermentation, and it is carried out by yeast under anaerobic conditions. It has also been discovered that various aromatic compounds are produced as a result of yeast metabolism. For further details, please refer to the section, “Shochu aroma.”

Sweet potato fermentation food (sweet potato shochu)


The characteristics of shochu yeast (Saccharomyces cerevisiae) are as follows: 1. It can grow even at high temperatures, and has a high fermentation capacity. 2. It can grow even at a low moromi pH (3 3.5). 3. It has a capacity for growth/fermentation even in the presence of ethanol (3% 5%). 4. It can survive even in high ethanol concentrations (18% 20%). 5. It contributes to the desirable flavors in the shochu. 6. It can bring out the properties of the base material in each shochu. Several types of yeast are used for shochu making, such as Miyazaki yeast, Kumamoto yeast, Awamori No. 1, Shochu yeast Kyokai No.2 and No.3, and Kagoshima yeast (Ko). In addition, Kagoshima No. 2 (K2) was selected from Ko in the 1970s, and Kagoshima No. 4 (C4) and No. 5 (H5) were isolated from shochu mash in 1995. These yeast strains are also used for shochu making. Sweet potato shochu, which uses C4, produces a high quantity of higher alcohols and their esters, and is prized as being “splendid” and having a “soft flavor.” The use of H5 gives an approximately 3% improvement in alcohol yield compared to the use of K2 (Takamine et al., 1994).

Primary preparation process In the primary preparation process, water is added to the koji base material at a ratio of 120:100 (water:koji). Cultured yeast solution (200 mL) per 100 kg of koji is then added to this mixture. Following this the moromi is fermented by controlling the temperature to prevent it from exceeding 32°C. The pH of the primary moromi is reduced to pH 3 3.5 by the citric acid produced by the koji, which then inhibits the proliferation of bacteria. Meanwhile, the shochu yeast with superior acid resistance grow preferentially, and the moromi is fermented safely without the risk of bacterial contamination. The primary moromi fermented over a period of approximately 5 days is then used in the secondary preparation process. Yeast concentration The yeast concentration immediately after preparation is approximately 1 to 2 3 105 cells/mL, however after 2 days of preparation, the yeast proliferates to a concentration of 2 to 4 3 108 cells/mL. The level of viable yeast is the highest from 3 to 4 days, but it is desirable to use the moromi for the secondary preparation process after 5 days have passed. One reason


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for this is that, immediately after initiating the second preparation process, any young yeast present that have high fermentation capacity initiate a burst of fermentation, which can cause a spike in the temperature of the secondary moromi that then makes it impossible to suppress the temperature of the moromi. This leads to the death of the yeast, which can lead to problems with fermentation. Acidity The acidity is approximately the koji acidity value multiplied by four, and the standard range is 20 27. Therefore if the moromi acidity falls below 5, acid supplementation is required. The maximum value occurs 2 days after preparation, and it falls by about one to two levels after 5 days. Alcohol concentration The alcohol concentration rises to 6% 8% after 2 days of preparation, and is 15% 17% by 5 days. Excess alcohol production inhibits yeast proliferation, leading to death and affecting the fermentation capacity of the secondary moromi. Therefore strategic measures are required to suppress alcohol production, such as adjusting the temperature of the moromi to be between 20°C and 25°C, beginning 4 days after preparation. Temperature The standard preparation temperature is 20°C 25°C, which is adjusted according to the time it takes to reach ambient temperature, or for the moromi to begin fermenting, as well as other factors. When the ambient temperature is expected to be getting cool, measures such as wrapping insulating material around the tank are usually undertaken. The maximum temperature of the moromi after 2 days of preparation is controlled to be between 28°C and 32°C. The temperature of the moromi immediately before the secondary preparation step is controlled from 20°C to 25°C.

Secondary preparation process The secondary preparation process is the operation of adding the base ingredient, sweet potato, and water to the primary moromi. The primary moromi used in the secondary preparation process has an acidity of approximately 25, a pH of approximately 3 3.2, an alcohol content of approximately 16%, a sugar concentration of 6% 10%, and a yeast concentration of 2 to 4 3 108 cells/mL. The largest factor in safely fermenting the primary moromi without bacterial contamination is keeping the pH of the

Sweet potato fermentation food (sweet potato shochu)


moromi low enough to suppress bacterial proliferation while still being able to preferentially grow only the shochu yeast that have superior acid tolerance. Sweet potato that has been steamed, cooled, and pulverized, and water are then added to the koji raw material with stirring. The sweet potato is added at a ratio of 500 100, and the water is added at a ratio of 280 100, each with respect to the koji raw material. Stirring, either mechanical stirring or air stirring, is then performed to prevent clumping of the pulverized sweet potato. The inside of any clump does not come in contact with the primary moromi, which contains alcohol and citric acid, so it is easy for bacteria, especially lactic acid bacteria, to proliferate there. Therefore attention must be paid to ensure that clumping does not occur. A pulverizer is used to pulverize the sweet potato to a particle size of 2 cm or less. Sweet potato with a high starch value, such as “KoganeSengan,” has a low water content, so it can be pulverized easily. However, sweet potatoes that are used for raw food have high water content and therefore are sticky, so particular attention must be paid in order to prevent clumping when pulverizing. The β-amylase activity in the sweet potatoes causes approximately one-third of the starch to convert to maltose when steamed, so as a result it becomes sweet. When water and steamed sweet potato containing approximately 10% maltose are added to the primary moromi, fermentation is rapidly promoted immediately after preparation. As sweet potato shochu has a high viscosity, the carbon dioxide gas generated during fermentation builds up in the secondary moromi, causing it to swell, and when the inside of the moromi becomes saturated with carbon dioxide, the gas violently erupts, causing natural agitation to occur. Generally, the initial preparation temperature is approximately 25°C, so after 2 3 days of preparation, it rises to 32°C. The rate at which yeast die increases at 35°C, causing not only poor fermentation, but also the production of acetic acid, which can decrease the quality of the shochu. The moromi alcohol concentration typically reaches 14% 15%.

Distillation process Atmospheric distillation Methods of heating the moromi include direct heating, direct steaming, indirect steaming, and a combination of direct and indirect steaming. In addition to ethanol and water, shochu contains trace components, such as higher alcohols, fatty acid esters, organic acids, and minerals, and despite


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the fact that their total content is only approximately 0.2%, they are very important in the flavor of shochu. In fact, the important differences in flavor between sweet potato shochu base materials and sweet potato shochu from each manufacturer are derived from these trace components, which affect the base material properties and the shochu quality. In addition to originating from the base materials, or being produced during the fermentation process, these components are also produced by thermal reactions that occur during the distillation process, meaning that the distillation operations can also affect the quality of the shochu. With respect to the distillation time, it takes about 30 min from steam being blown onto the moromi to the shochu beginning to distill (begins dripping), and the quantity of steam is adjusted so that the distillation will finish approximately 180 min after it begins. The end point of the distillation is generally at the time that the alcohol content of the distillate reaches 8% 10%. If it is set lower, then it is easier for the “final distillate aroma,” the characteristic scent of the distillate near the end of distillation, to be expressed. However, if there is only a small amount of distillate near the end of the distillation, the flavor tends to be light.

Vacuum distillation In vacuum distillation, the moromi is distilled at a reduced pressure of approximately 100 Torr at 40°C 50°C, so it is difficult for thermal reactions to occur. As a result, a light and soft shochu is produced, which is completely different from the shochu obtained with atmospheric distillation. The shochu quality is therefore intimately related to the moromi temperature, so each company must set the distillation conditions required for the shochu quality of their own shochu brand. The moromi is not heated by steam blowing, but rather by indirect heating using a coiled tube-type or a jacket-type pot still. During distillation, the quantity of the moromi will gradually decrease. If the liquid surface of the moromi is lower than the height of the steam-heated surface, the moromi will burn, so the amount of moromi added to the pot still must be carefully adjusted.

Purification process The properties of shochu obtained by atmospheric distillation and vacuum distillation vary greatly, so the purification methods are also different. In the case of shochu obtained by atmospheric distillation, the unique flavor of the base ingredient is strong, the scent of strong-smelling gases, such as

Sweet potato fermentation food (sweet potato shochu)


aldehydes or sulfur compounds, is present, and lipid components, comprising primarily higher fatty acids and their ethyl esters, make the shochu cloudy and they float to the surface. In contrast, shochu obtained by vacuum distillation has hardly any strong-smelling gas odors, and few lipid components. The strong-smelling gas odors and lipid components, if present, are primarily removed during the purification process.

Gas components The gas produced consists of a mixture of aldehydes and sulfur compounds. Typically, shochu distilled under atmospheric pressure contains a significant amount of gas, while shochu distilled under a vacuum contains relatively little gas. The alcohol steam that is produced during distillation is cooled and becomes the shochu. At this time, the alcohol steam is cooled gradually, and it is necessary to adjust the cooling water so that the temperature of the shochu that comes out of the condenser is approximately 30°C. By slowly cooling, it is possible to minimize the amounts of strong-smelling gas components that dissolve in the shochu. If it is cooled too rapidly, it is easy for strong-smelling gas components, such as aldehydes, to dissolve in the shochu because they have a low boiling point, and therefore it takes time to remove the strong-smelling gas components contained in the shochu. The dissolved strong-smelling gas components will naturally evaporate during storage, but several methods can be used to force gas removal including by causing agitation by transferring shochu between tanks, stirring with a paddle, or using an air pump or circulation pump. Regardless, the strong-smelling gas will eventually be removed and the product quality stabilizes in approximately 2 4 months. However, larger tanks may require more time to become stabilized. In the case of sweet potato shochu, these gas components still remain after about 1 month of short-term storage following distillation, but this product may be shipped and sold as “new sake,” which has a rich sweet potato aroma.

Lipid components Lipid components serve to smooth out the physical taste of shochu, giving it a roundness and a tempering of sharp flavors. However, to some extent, the lipid components must also be removed, owing primarily to two negative effects. The first negative effect is that they precipitate during storage and form white, thread-like aggregates that float around in the shochu. The white, floating material is a precipitate/aggregate formed when lipid


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components in the shochu bind to the mineral (metal) components, such as calcium, magnesium, copper, and iron, present in the water used to dilute the raw alcohol to 25% (v/v) ethanol. The second negative effect is that the lipid components produce an oily odor when they are oxidized. Therefore successful retention of the ideal amount of lipids in the shochu reflects the fine balancing act between their positive attributes (i.e., roundness of flavor and a tempering of sharp flavors) and their negative attributes (i.e., precipitation and oxidation). Filtration is used to appropriately eliminate the oily component ethyl linoleate, in particular. Ethyl linoleate, itself, possesses only a weak oily smell, so it is not directly responsible of the oily odor. However, upon decomposition ethyl linoleate breakdown products, namely alzeic acid semialdehyde monoethyl ester, n-hexanal, 2,4-nonadienal, and pimelic acid semialdehyde ethyl ester, are produced, which are responsible for the oily smell (Nishiya and Sugama, 1978). There are two principal methods used for removing oily components. The first is the skimming method. Oily components have a low specific gravity and therefore float to the surface of the tank following cooling during storage. Utilizing this property, the oily components on the surface of the shochu, can be skimmed away using filter paper or a flannel cloth. Although it is difficult to skim off transparent oily components, they can also be removed by causing them to adhere to the food wrap used for food storage. The second method is the cold filtration method. The temperature of the shochu affects the solubility of ethyl linoleate, the main precursor substances responsible for the oily aroma in shochu. The lower the temperature, the lower the solubility of shochu. After using a cooling apparatus to decrease the temperature of the shochu, the ethyl linoleate can be easily removed by filtration using filter paper.

Shochu aroma In addition to ethanol and water, shochu contains trace components, such as higher alcohols, fatty acid esters, organic acids, and minerals, but their total content is low, being approximately 0.2% 0.5%. However, these trace components play important roles in shochu. The differences in the flavor for each base material (such as sweet potato shochu or brown sugar shochu) from each manufacturer are due to these trace components. The oily components temper the sharp taste of shochu and give it a

Sweet potato fermentation food (sweet potato shochu)


rounded flavor, so they are an essential ingredient of shochu that contribute to its physical taste. Higher alcohols include isoamyl alcohol, activated amyl alcohol, isobutyl alcohol, normalpropyl alcohol, β-phenethyl alcohol, etc. These components primarily exhibit an alcohol-like aroma, whereas β-phenethyl alcohol has a rose-like aroma. The presence of higher alcohols is intimately related to amino acid metabolism in yeast, and they are produced from intermediates in the pathways by which yeast conduct the biosynthesis or degradation (Ehrlich pathway) of amino acids. Specifically, isoamyl alcohol is produced from leucine, activated amyl alcohol is produced from isoleucine, isobutyl alcohol is produced from valine, n-propyl alcohol is produced from threonine, and β-phenylalanine is produced from phenylalanine. Whether or not alcohols are generated by any of these biosynthetic pathways depends on the amino acid content of the moromi. When the amino acid content in the moromi is low, amino acids must by biosynthesized using the nitrogen sources taken up by the yeast, and higher alcohols are produced as by-products (amino acid biosynthetic pathway). However, when the amino acid content in the moromi is high, the yeast acquires the nitrogen components from the amino acids that are taken up, converts the remaining keto acids to higher alcohols containing one less carbon atom, and releases them outside the fungal body (Ehrlich pathway). Isoamyl acetate, which has an apple-like aroma, and β-phenyl acetate, which has a rose-like aroma, are also produced by yeast, and the amount produced varies depending on the type of yeast. As a result, shochu with various alcohol qualities can be produced by yeast. The characteristic aroma of sweet potato shochu is reported to be caused by the presence of monoterpene alcohols, such as linalool, α-terpineol, citronellol, geraniol, and isoeugenol, as well as rose oxide and β-damascenone (Kamiwatari et al., 2006; Kuriyama et al., 2005; Ota, 1991; Takamine et al., 2011). In comparison, these components are barely present in rice shochu and barley shochu. Monoterpene alcohols exist as monoterpene glycosides in sweet potatoes, and they are hydrolyzed and liberated by β-glucosidase derived from koji during fermentation. In addition, some of the geraniol and nerol are converted to citronellol by yeast during fermentation and to linalool and α-terpineol by acid and heat during distillation. β-Damascenone is an important characteristic aromatic component that contributes to the sweet scent of sweet potato shochu. Both monoterpene alcohols and β-damascenone are said to have a soothing effect, and these sweet potato shochu-specific components are believed


Sweet Potato

% of each total content

70 60 50 40 30 20 10 0 Nerol









Figure 12.4 Content distribution of monoterpene alcohol in various parts of sweet potato.

to play an important role in the “relief” that is felt with an evening drink of sweet potato shochu. In particular, drinking diluted shochu with hot water allows the aroma to stand out more easily, and is believed to bring out the sweetness of the shochu and its soothing effects. The monoterpene alcohol that gives sweet potato shochu its characteristic aroma is derived from sweet potatoes. However, these are not free molecules since they all exist as glycosides. Fig. 12.4 shows the distribution of monoterpene glycosides in “Kogane-Sengan,” a sweet potato variety used as the base ingredient for sweet potato shochu. In this analysis, the “Kogane-Sengan” sweet potato was divided into five distinct parts as follows. The upper and lower 10% of the length were cut off and defined as the upper and lower part, respectively. The remaining parts were subsequently divided into three parts: central part (defined within cambium layer), cambium part, and skin part (outside of cambium layer). The monoterpene glycoside fraction was extracted from each of these sweet potato parts. The parent monoterpene alcohols were then liberated by the actions of β-glucosidase and β-primeverosidase on these monoterpene glycoside fractions. The liberated monoterpene alcohols were then measured using GC-MS, and the values were used as the basis for calculating the monoterpene glycoside distribution shown in Fig. 12.4. Nerol was found to be present at 8.4% at the top part, and was found at the highest percentage, 38.6%, in the central part. Geraniol, linalool, and α-terpineol were found at the highest concentration in the skin, being 37.4%, 65.9%,

Sweet potato fermentation food (sweet potato shochu)


and 60.3%, respectively. Linalool was not detected in the central part, but α-terpineol was found, although at a low percentage of 1.7% (Takamine et al., 2012).

Sweet potato varieties and shochu quality Joy white The first sweet potato variety bred as the base ingredient for shochu was the “Joy White.” When sweet potato shochu was produced using this variety, the monoterpene alcohol content was high, and a characteristic fruity and strong citrus-like shochu quality was provided. Linalool in shochu with sweet potato variety “Joy White,” in particular, was present at an approximately fivefold higher concentration compared with shochu made with the sweet potato variety “Kogane-Sengan,” which is generally used in sweet potato shochu manufacturing. This led to the discovery that monoterpene alcohols are important compounds that affect the quality of sweet potato shochu.

Colored sweet potatoes Some sweet potato shochus use sweet potato varieties that have bright orange or purple flesh. Sweet potato shochu manufactured with orangecolored sweet potatoes are prized for having a “heated carrot aroma” or “a steamed squash scent” and β-ionone compounds, which have the aroma of violets, and have been specifically detected in this sweet potato shochu. In addition, sweet potato shochu manufactured with purple sweet potatoes is said to have a “yogurt aroma” or a “red wine aroma,” and diacetyl compounds have been found to contribute to these aromas (Kamiwatari et al., 2006).

Health properties of shochu J-curve effect of alcohol It is said that if alcohol is drunk well it is the best of all medicines. Moderate alcohol consumption has mental and physical effects, such as increasing appetite, promoting sleep, and eliminating stress, and it is socially useful for easing human interactions, maintaining cultural practices, and social customs. Those who have been drinking small quantities of alcohol for many years are said to have a lower rate of death from heart disease, cancer, and other diseases as compared with people who do not drink any alcohol, or


Sweet Potato


Relative risk

1.6 1.4 1.2 1.0 Male



0.6 0


10–19 20–29 30–39 40–49 50–59


Drinking volume (g/d) Figure 12.5 J-curve effect of alcohol.

who drink large quantities of alcohol. Relative risk of all-cause mortality in male drinkers compared with abstainers fell to 0.84 of alcohol at 10 19 g of alcohol per day, in female drinkers the lowest relative risk of 0.88 was at 0 9 g of alcohol per day (Fig. 12.5) (Holman et al., 1996). This is known as the “J-curve effect of alcohol.” This theory was first proposed in 1981 by Dr. Marmot in England in an epidemiological study.

Thrombolytic effects of shochu An alcohol consumption study was conducted, over 15 years, in healthy subjects using shochu, sake, wine, beer, and whiskey, each containing 60 mL of ethanol, and a very large amount of hematological data was collected (Sumi et al., 1988; Sumi, 2001). After measuring the fibrinolysin activity in the blood 1 h after drinking alcohol, the consumption of alcoholic beverages was found to increase this activity, with shochu increasing it the most. It was notable that shochu did not suppress the formation of blood clots, but only degraded blood clots that had already been formed. In other words, the consumption of shochu does not affect “hemostasis,” which is a process essential for life.

Blood glucose lowering effect of shochu In several epidemiological studies a moderate quantity of alcohol was effective in preventing diabetes mellitus (Mackenzie et al., 2006). As to the relationship between alcohol and blood glucose, it has been reported

Sweet potato fermentation food (sweet potato shochu)


that drinking large amounts of alcohol, under fasting conditions, causes serious hypoglycemia, and that alcohol consumption inhibits gluconeogenesis in the liver and suppresses the secretion of insulin (Steiner et al., 2015). However, these studies only examined the effect of alcohol alone, they did not examine the effect of alcohol on blood sugar elevation following the consumption of food. As a result, Kido et al. (2014) researched how blood glucose values and insulin secretion changes when alcohol is consumed with a meal. In addition, they showed how those effects differed according to alcohol type. In this study, there were five healthy subjects (three men and two women) who were asked to drink beer, sake, shochu, or water with a meal, and blood glucose values and blood insulin concentration were measured immediately before eating and at 1, 2, and 12 h after eating. Since alcohol concentration varies for each beverage, the quantity of each drink was adjusted so that each volunteer consumed 40 g of alcohol. The results showed that blood glucose values and insulin concentrations differed depending on the type of alcohol (Fig. 12.6). The alcohol associated with the highest blood glucose quantity was beer. In contrast, the shochu or sake drinking groups resulted in a lower blood glucose level than in those groups drinking water and beer. In addition, shochu resulted in lower insulin levels compared with the other groups. One conceivable reason for this is that the carbohydrate content differs depending on the type of alcohol. The carbohydrate content of shochu is 0 g; whereas in beer it is 31 g, and in sake it is 13.3 g. These carbohydrate values are lower than that of carbohydrates derived from food (approximately 100 g), but carbohydrates in alcoholic beverages have been shown to potentially affect postprandial blood glucose levels. Another interesting result in this study was the fact that when shochu was consumed, postprandial blood glucose levels and blood insulin concentration were lower than they were after the consumption of water, which similarly contains no carbohydrates. There are two mechanisms that could be responsible for this. The first is the possibility that components in shochu increase the effects of insulin (i.e., they may act as insulin sensitizers to improve glucose tolerance), thereby blunting the increases in blood glucose. The other possibility is that the components in shochu might inhibit the absorption of carbohydrates. Indeed, alcohol has been reported to inhibit the motility of the gastrointestinal tract, which would inhibit the absorption of carbohydrates. However, in this study, the same quantity of alcohol was consumed, so it cannot be concluded that alcohol alone in shochu

Blood alcohol (mg/dL)


Sweet Potato

(B) 1











*** *** ***


0.4 0.2




Time (h)





** ### 0




Time (h) Water Beer

100 Insulin (μU/mL)



0 (C)




Water Beer Shochu Sake

** Blood glucose (mg/dL)





60 40 20 0


1 2 Time (h)


Figure 12.6 Blood glucose and insulin levels after drinking four types of beverages in healthy subjects. Blood alcohol levels (A), blood glucose levels (B), blood insulin levels (C) after drinking four types of beverages in healthy subjects. Data are expressed as mean standard error (n 6 5). Two-way repeated measures analysis of variance and post hoc analysis using least significant differences were used to compare clinical data between the beverages. P , .05, P , .01, P , .001, compared with water; #P , .05, ##P , .01, ###P , .001, compared with beer.

suppressed gastrointestinal motility. Other components besides alcohol might be involved. In the future, we will anticipate discovering the identity of the components that contribute to this effect and the mechanisms underlying their effect.

References Crop yield of sweet potato, 2016. ,http://www.maff.go.jp/j/tokei/sokuhou/sakumotu/ sakkyou_kome/kansyo/h28/index.html. (accessed 30.01.18). Holman, C.D., English, D.R., Mine, E., Winter, M.G., 1996. Meta-analysis of alcohol and all-cause mortality: a validation of NHMRC recommendations. Med. J. Aust. 164, 141 145.

Sweet potato fermentation food (sweet potato shochu)


Iwano, K., Mikami, S., Fukuda, K., Shiinoki, S., Shimada, T., Obata, T., et al., 1986. Distribution of enzyme activities of shochu koji. J. Inst. Brewing 81, 495 498. Kamiwatari, T., Setoguchi, S., Kanda, J., Setoguchi, T., Ogata, S., 2006. Effects of a sweetpotato cultivar on the quality of Imo-shochu with references to the characteristic flavor. J. Brewing Soc. Jpn 101, 437 445. Kido, M., Asakawa, A., Koyama, K.K., Takaoka, T., Tajima, A., Takaoka, S., et al., 2014. Acute effect of traditional Japanese alcohol beverages on blood glucose and polysomnography levels in healthy subjects. Peer J. 1853. Available from: https://doi.org/ 10.7717/peerj. Kuriyama, K., Nagatomo, M., Yamanaka, H., Yoshihama, Y., Watanabe, Y., 2005. Relationships between panel preference and volatile compounds in various types of shochu. J. Brewing Soc. Jpn 100, 817 823. Mackenzie, T., Brooks, B., O’connor, G., 2006. Beverage intake, diabetes, and glucose control of adults in America. Ann. Epidemiol. 16, 688 691. Nishiya, N., Sugama, S., 1978. Oil flavor developed during aging process of honkaku shochu, the traditional distilled liquor. J. Soc. Brewing Jpn 73, 844 849. Okutsu, K., Yoshizaki, Y., Kojima, M., Yoshitake, K., Tamaki, T., Takamine, K., 2016. Effects of the cultivation period of sweet potato on the sensory quality of imo-shochu, a Japanese traditional spirit. J. Brewing Soc. Jpn. 122, 168 174. Ota, T., 1991. Characteristic flavor of kansho-shochu (Sweet potato shochu). J. Brewing Soc. Jpn. 86, 250 254. Steiner, J.L., Crowell, K.T., Lang, C.H., 2015. Impact of alcohol on glycemic control and insulin action. Biomolecules 5, 2223 2246. Sumi, H., 2001. Physiological functions of traditional shochu and awamori. J. Brewing Soc. Jpn. 96, 513 519. Sumi, H., Hamada, H., Tsushima, H., Mihara, H., 1988. Urokinase-like plasminogen activator increase plasma after alcohol drinking. Alcohol Alcoholism 23, 33 43. Takamine, K., Setoguchi, S., Kamesawa, H., Hamasaki, Y., 1994. Study on screening of shochu yeast. Annu. Rep. Kagoshima Inst. Ind. Technol. Cent. 8, 1 6. Takamine, K., Yoshizaki, Y., Shimada, S., Takaya, S., Tamaki, H., Ito, K., et al., 2011. Estimation of the mechanism for cis and trans rose oxides formation in sweet potato shochu. J. Brewing Soc. Jpn. 106, 50 57. Takamine, K., Yoshizaki, Y., Yamamoto, Y., Yoshitake, K., Hashimoto, F., Tamaki, H., et al., 2012. Distribution of monoterpene glycosides in sweet potato. J. Brewing Soc. Jpn. 107, 782 787.