Learn the Aroma Components of Sake - Aged Aroma

Previously, high-end sake was mainly characterized by the high aroma and clear flavor produced by ginjo production, as we learned in the previous Ginjo Aroma article. In recent years, however, there has been a growing interest in aged sake, which has completely different characteristics.

In 2020, the Pasona Group launched the "Inishie no Bishu" brand of aged sake, and a group of sake brewers who envision the future of sake in the form of aged sake established the "Toki Sake Association" and began selling an aged sake set for 2.02 million yen.

In this second installment of the "Learning the Aroma of Sake" series, entitled Aged Aroma, I will explain from a scientific perspective the changes in aroma that accompany the aging process.

What is aging?

Definition of aging

Before we look at the science behind the aging aroma, we need to understand what exactly aging entails.

Generally, when foods ages there are two types of reactions that occur simultaneously:

1. Reactions that, over time, break down proteins through the action of enzymes in food.
2. Reactions in which substances in the food change over time without the action of microorganisms or enzymes.

When sake ages, only the second reaction occurs because sake undergoes pasteurization (hi-ire), stopping the enzymes from working. Nama (-zake), or unpasteurized sake, has not undergone this process, so active enzymes remain. In the case of nama, the enzymes break down the starch and sometimes make the sake sweeter.

In aging without enzymatic reactions, the main reaction that occurs is the Maillard reaction, in which sugars and amino acids react to produce a brown colored substance (melanoidin). Therefore, aged sake is often darker in color, or various substances combined with melanoidin may precipitate and precipitate as lees at the bottom.

In addition to these, a variety of other diverse and complex chemical reactions are involved in the aging process. There are many aspects of this process that are complex, and it is very difficult to control aging as there are many factors that impact the aging process, like the style of sake, storage temperature, and the length of time of aging. Among these, research is being conducted on the components that play a major role in maturation, and the mechanisms are gradually becoming clearer.

First, the following table summarizes roughly what aromas increase or decrease with aging.

This table shows the correlation between storage age and compositional change. The number of years of storage indicates the degree of change: ↑ for those that increase with age, and ↓ for those that decrease with age.

From here, I will introduce some of the chemical changes that occur with aging, including the Maillard reaction mentioned earlier.

Maillard reaction

Maillard reaction is a general term for reactions that occur when amino acids and sugar (which also requires water) are present at the same time, producing various aromatic components while synthesizing brown substances. Familiar examples of Maillard reactions include the phenomenon in which onions gradually turn brown when sauteed over time and the charring that occurs when meat is grilled (*1).

(*1) In addition to the Maillard reaction, phenomena such as carbonization are also involved in the charring that occurs in high-temperature cooking such as grilling meat.

In addition, a reaction that produces a substance called aldehyde as a side reaction of the Maillard reaction is called Strecker degradation.

These two types of reactions produce various aroma compounds in aging, as will be introduced later.

There are three factors that promote the Maillard reaction: the amount of amino acids and sugars, temperature, and time.

Regarding the amount of amino acids and sugars, for example, in sake such as junmai daiginjo, which is made from high-polished rice and contains relatively little amino acids, the color does not change much over a long period of time.

As for temperature, the higher the temperature, the faster the Maillard reaction proceeds. For example, when cooking meat, the Maillard reaction occurs quickly because of the high heat involved, and the meat gets charred. The higher the storage temperature of sake, the faster the Maillard reaction proceeds and the browner the color.

The last factor, time, is related to the other two factors (amount of amino acids and sugar, and temperature); the longer it takes, the more the Maillard reaction proceeds.

Ester bonding and decomposition

In simple terms, an ester bond is a combination of a type of acid and ethanol. The esters that increase during the aging of sake are organic acid esters. Organic acids refer to lactic acid, malic acid, succinic acid, and citric acid, which are the typical acidity components of sake, and these organic acids combine with ethanol over time.

It is also known that certain types of esters are hydrolyzed by aging and transformed into non-fragrance components. Isoamyl acetate and ethyl caproate, the two major components of ginjo aroma, are aromas that decompose and diminish over time.

Aged aroma components and their characteristics

This section will now focus on some of the components related to maturing aromas, including the characteristics of the aroma and how they are produced.

Sotron: aroma of caramel and dried fruits

Typical characteristic of maturing aroma. It is the main component of maturing aromas, which are produced to a level far exceeding the range value when maturing continues for a long period of time. It is said to be involved in the various aromas experienced as a result of ageing, such as caramel, dried fruit and honey-like aromas.

Sotrone is produced by the Maillard reaction pathway and increases with aging time. It is also formed when alpha-ketobutyric acid, a breakdown product of amino acids, combines with acetaldehyde, which is formed during fermentation.

Incidentally, the Maillard reaction and caramelisation are considered separate reactions.

Isovaleraldehyde: burnt smell

Strecker degradation produces a variety of aldehydes during maturation. This section introduces isovaleraldehyde, an aldehyde that is deeply involved in maturation.

Isobarrel aldehyde is a component produced by the Strecker degradation of leucine, a type of amino acid. While it can be perceived as a nutty or burnt aroma, it is also professionally described as a sweltering, sweet-sour, burnt odour, known as mure-ka. It is also the aroma produced when nama is left at room temperature and is the main component responsible for the nama-hineka (off-flavor specific to nama). It is important to note that this aroma can significantly upset the balance of the fruity aroma.

［Isobarrelaldehyde formation process]
Isovaleraldehyde ⇄ Isoamyl alcohol

It is a precursor of isoamyl alcohol, which appeared in the ginjo aroma article, and is more likely to appear in stored nama zake, because if the nama is kept at room temperature, an enzyme promotes a reaction that changes isoamyl alcohol to isovaleraldehyde. When stored at low temperatures, such as -5°C, this reaction is suppressed because the enzymes are not active (refrigerated storage is especially important for nama).

In rare cases, the aroma of some nama is not destroyed by ageing at room temperature, but it is likely that many such sake are made intentionally with a low level of isoamyl alcohol.

Pyrazines: savoury aroma like hojicha tea

Pyrazines are the main components of the aromatic flavour of hojicha tea and are also responsible for the aromatic flavour in aged sake. Pyrazines are formed by the reaction of the products of Strecker degradation.

Diethyl succinate: honey-like aroma

Among the organic acid esters produced by ester bonding, diethyl succinate is a characteristic one.

This substance is succinic acid bound to two ethanols , which is considered to be the main component of the honey-like aroma sometimes detected in aged sake.

Furfural: hard to detect on its own

Furfural is a substance with a caramel or burnt aroma. Like sotrone, it is produced in the Maillard reaction.

As seen above, it is a component that increases significantly with aging, but none of the sake samples used in the literature referred to here exceeded the threshold value, so it is assumed that it is difficult to detect the aroma on its own. However, recent studies on beer, for example, have pointed out the possibility that components below the threshold level may have a synergistic effect on the aroma, and furfural, which increases significantly with age, may also influence the formation of the aroma of matured sake.

*Reference: Toru Kishimoto, Beer Aroma: Reconstruction of Beer Aroma by 76 Components to Reveal Its Structure, JSBBA, Chemistry and Biology 56-10, 2018

Polysulphide: pickle-like aroma

Polysulphide is a substance with multiple sulfur bonds (two DMDS, three DMTS) and is known as pickle odor in the quality assessment terminology of the NRIB. Polysulphide has a very low range value and a relatively more perceptible odor than other substances, so it is also a substance thought to play a major role in the hineka odor, as we will see later in more detail.

Polysulphide is thought to be formed by polymerisation (*2) of its precursor (*3) with oxidation during storage.

(*2) Research has shown that the precursor of polysulphide (DMTS-P1) is produced when yeast metabolises methionine, a type of amino acid.

(*3) Polymerisation: a reaction in which two or more molecules of a compound combine to form a new compound of several times the molecular weight.

Vanillin: vanilla aroma produced by a change from smoky aroma

It is known that 4VG (4-vinylguaiacol), known as the off-flavor of smokiness in sake, changes to vanillin, the main component of the vanilla aroma, upon aging. This reaction is not often seen in the aging of sake, but in awamori, this aroma is the characteristic aroma of aged style.

Since 4VG is considered an off-flavor and avoided in the sake making process, there are not many sake in which the presence of 4VG can be detected, but if you detect it, it may be interesting to try aging the sake a little.

What is the difference between aged aroma and hineka?

Definition of aged aroma and hineka

In sake, there is a term related to aged aroma called “hineka" (hine means old, and ka means aroma). There are various interpretations of the difference between these, and for example, as with the difference between "fermentation" and "putrefaction,” the former may be considered a favorable change over time for people, while the latter may be considered something that is not so favorable.

On the other hand, a study and paper by the NRIB defines it as follows:
Aged - one in which the flavor or aroma has changed due to the intended long-term storage
Hine - A (unintentional) change that has occurred during the process of distribution or storage

*Reference: Atsuko Isotani, The Aromatic Components Involved in the Aged Sake and Their Formation Mechanisms (1) - Aroma of Aged Sake and Old Aroma, Journal of the Brewing Society of Japan 104-11, 2009

However, there are various other interpretations, such as the components related to aging include hine or not only the aroma but also the taste is affected, which may lead one to wonder what exactly is the difference between aged aroma and hineka. The following is a scientific explanation of the difference between them.

Scientific difference between aged aroma and hineka

The graph below shows the results obtained from a study on the difference between hineka and aged aroma. First, an appraiser from the National Tax Agency, a professional in sake quality evaluation, determines whether or not a sake has hineka, and analyzes the aroma components of each sake (20 sake types that have a lot of hineka and 20 sake types that do not). 15 types of sake that have been aged for more than 3 years (intended long-aged sake) are also analyzed for their components.

The horizontal axis of the graph shows the overall amount of ripening aroma components. The vertical axis shows the balance between polysulfides and sotrone. (The graph is plotted in the + direction (up) when polysulfides are high, and in the - direction (down) when carbonyl compounds such as sotolone and diethyl succinate are high.)

From this graph, it can be seen that the sake with many indications of hineka (indicated by red circles) tends to have less overall aged aroma components, and polysulfides are relatively stronger than other aromas. Sake with no hineka (indicated by green triangles) has less overall aged aroma components and less polysulfide, indicating that polysulfide plays a major role in hineka.

In the intended long-aged sake (indicated by the dark blue diamond-shaped mark), we can see that the overall aged aroma component is increasing in proportion to the aging period. On the other hand, the balance between polysulfides and sotolones varies. Extreme amounts of polysulfides can be perceived as hineka, but depending on the balance, they can also emphasize the characteristic aroma of sotolon and other aromas. The Maillard reaction that produces sotolon takes a long time, so even a type of sake that has hineka may be balanced after a long period of aging.

In addition to the above points, nama is particularly susceptible to alteration, and even a slight rise in temperature can affect the balance of aroma and flavor. In particular, isovaleraldehyde, which we saw earlier in 2-2., is considered the main component of nama-hineka and is strongly related to aged hineka.

Summary

In this article, I have introduced some of the chemical reactions that make up aged aroma. However, one study found that the typical aroma components of aged aroma did not become like that of aged sake when added to unaged sake.

In addition, (I did not write about it in depth this time because I focused on the relationship between aging and aroma,) besides the change in aroma, the color of the liquid turns brown due to the formation of a brown pigment called melanoidin. Various reactions occur, including a decrease in sharply perceived acidity such as citric acid and malic acid, and a decrease in sweetness due to the decomposition of glucose.

One of the most interesting aspects of aged sake is the complex chemical reactions that occur during aging, resulting in an unpredictable complex flavor. Take the time to savor the complex flavors that have changed over the years and can only be tasted at that moment.

References

• Atsuko Isoya, 清酒の熟成に関与する香気成分およびその生成機構について（1）―清酒の古酒の香りと老香 [The Aromatic Components Involved in the Aged Sake and Their Formation Mechanisms (1) - Aroma of Aged Sake and Old Aroma], Journal of the Brewing Society of Japan 104-11, 2009
• Hiroshi Iwata, 清酒の熟成について [On the Aging of Sake], Report on the 38th Lecture Meeting of the National Research Institute of Brewing, 2002
• 清酒のにおいとその由来について [The Smell of Sake and Its Origin], National Research Institute of Brewing Official Website
• Atsuko Isoya, 清酒の「老香」とその前駆物質 [Hineka and its Precursors in Sake], Tips for BFD Vol. 28, Kita Sangyo
• Toru Kishimoto, ビールの香り：その‘構造’を解き明かす76成分によるビール香気の再構築 [Beer aroma: Reconstruction of Beer Aroma by 76 Components to Reveal Its Structure], JSBBA, Chemistry and Biology 56-10, 2018
• Atsuko Isoya, 清酒の品質と香気成分 [Sake Quality and Aroma], Gas Chromatography Research Group Special Lecture Vol. 334, 2014
• Atsuko Isoya, 清酒の熟成に関する香気成分 [Aroma Components Related to Sake Aging], Journal of The Society for Biotechnology, 89-12, 2016