The Science of Sake Aroma The Science of Sake Aroma

The Science of Sake Aroma

In the worlds of sake and wine alike, descriptions like “luscious fruit” and “warm spices” adorn labels and crop up in discussion describing the “nose” of particular drinks. But what exactly imbues a beverage with these unmistakable aromas without such fruits or spices even being present? It’s time for a chemistry lesson as we take a deep dive into the topic of sake aroma!


Fruity, nutty, floral, caramel, earthy — these are all terms used to describe aromas in sake. They also showcase the breadth and variety of sake characteristics. Chemically speaking, though, all sake share about a half dozen basic aromatic components. And the differences in each label’s aromatic characteristics are merely due to the ratios of each component in a given brew. To better understand how this works, and discuss the importance of aroma in sake, we spoke with Shunsuke Miyao, Executive Research & Development Officer at KIKUSUI Brewery in Shibata, Niigata.

a conference table of sake aroma reagents

Standing at the head of a conference table with his set of sake aroma reagents before him, Miyao explains that the sense of taste is rather universal — biting into a lemon is recognized as sour whether you are from Nairobi or Stockholm, and a spoonful of honey is perceived as sweet whether you’re seven or seventy years old. The sense of smell, however, is largely a product of experience, Miyao says. The types of foods we are familiar with, among other olfactory associations we form throughout our lives, determine how we judge and react to different smells – and thus, how we perceive a certain food or drink. In other words, perception of aroma is a complex and nuanced process that’s informed in part by cultural influences and past experiences.

Yeast and Aroma

Yeast is the strongest factor that determines a sake’s aromatic profile. Historically, breweries used wild yeast strains that grew organically in their facility to brew sake, which meant that the brewing process had minimal human intervention but was also a largely unpredictable and haphazard undertaking. The advent of microbiology in the early twentieth century, though, led to standardized sake brewing yeasts. The Brewing Society of Japan, at the time, succeeded in isolating and cultivating a strain that was specifically suitable for sake brewing, after collecting samples from award-winning sake and isolating their yeast cultures.

A number of yeast strains were added to the list in the subsequent decades, where some were lab replications of wild yeast from certain breweries like Miyasaka Jozo or Aramasa Shuzo, while others were even created by individual prefectures as a “key ingredient” to give their regional labels a signature character. For example, Aramasa, where Kyokai Kobo 6 (Yeast #6 from the Brewing Society of Japan) originated, still proudly features the number six on their labels while prefectural research institutes have developed their own proprietary mutations like the light and elegant HD-1, representative of sake from Shizuoka.

This commercialization of different yeast strains enabled brewers to safely experiment with yeast types that were unknown or unavailable to them previously, and eventually gave birth to the idea of “designing” a sake’s flavor. Now brewers can shop for a type of yeast that yields the aromatic profile they’re looking for, sometimes even blending more than one strain to achieve a desired result.

The big ginjo trend of the past decade or two owes a lot to these Kyokai Kobo yeast strains, which have enabled breweries hoping to make a name for themselves in annual competitions to select the yeast types that bring out the best aromatic qualities in their offerings, resulting in an increased presence of ginjo-style products in the market, bringing about today’s ginjo boom. The insatiable demand for ever-fruitier sake has even led to the invention of a number of modern yeast strains, some of which are created from unconventional sources like flowers as showcased in the Amabuki label series.

Label of Aramasa, using Kyokai Kobo 6, and Amabuki label series

With the prevalence of extremely aromatic sake in the market today, it’s no surprise that fruity sake is sometimes thought to have been directly infused with fruits and herbs — think orange peels in Belgian beer or juniper berries in gin. Another reason behind this perception may be that, as sake picks up traction in overseas markets, there’s been a marked emergence of sake-based mixed drinks around the world, further adding to consumers’ confusion. In any case, it’s worthwhile to clear the air on sake aroma once and for all. Sake, by definition, can’t have any additional ingredients to enhance aroma and flavor. It’s all the work of yeast!

The Makeup of “Ginjo-ka”

Anyone who has tried a fragrant ginjo will know what ginjoka smells like. It’s that signature clean and fruity aroma, often likened to ripe tropical fruits like bananas, melons, or pineapples. Less is known about the two main chemical substances that make up what we sense as ginjo-ka. The amount and balance in which these two substances exist in a sake determines whether it smells more like a tropical fruit salad or a delicate bouquet of white flowers.

Until about twenty years ago, a chemical compound called isoamyl acetate was most prevalent in ginjo and daiginjo sake. This substance has a sweet nose of bananas and a sharp structure of honeydew melons. Sake high in isoamyl acetate generally has a fruity nose with a clean palate. With time, however, the trend has shifted towards a preference for a more pronounced, fruitier nose produced by a substance called ethyl caproate. This compound, made up of caproic acid and ethyl alcohol, has a more extravagant aromatic quality, bringing with it notes of ripe red apples, pineapples, and pears, and generally has a more robust, sweeter mouthfeel.

While the lush fragrance may be ginjo’s biggest strength, it could also be its biggest weakness; Both of these substances dissipate very easily, meaning it’s prone to fading. You might suggest ramping up the ginjo-ka so there’s plenty to remain over time, but it’s not that simple. Ethyl caproate is a double-edged sword, because with increased fruity notes comes an inevitable oily and damp smell, like that of goat’s fur or chevre cheese. It’s even in the name, “capra,” meaning “goat” in Latin. When you’ve got to take the good with the bad, or the fruity with the goat-y, just how much of the bad can you have in pursuit of the good? That delicate balancing act is left to the skilled direction of the toji, or brewmaster, who is tasked with designing a sake’s flavor profile by manipulating myriad variables.

It’s interesting to note that, according to Miyao, overseas markets tend to prefer a bolder ethyl caproate profile, and seem to be less put off by its musky aroma in larger concentrations — another example of how our aromatic senses are influenced by our cultural environment. Some breweries eager to market to overseas consumers have since adjusted their export products by deliberately increasing ethyl caproate and making the sake more fruit-forward for their international customers.

Back on the topic of dissipation, with sake that have a high level of ethyl caproate, dissipation of the fruity aromas can be particularly detrimental. Once ethyl alcohol, the more volatile component, evaporates, sake is left with the foul-smelling caproic acid, completely tainting the drink’s overall characteristics. As sake enthusiasts, not knowing this can have grave consequences; it means to never let a vacuum wine preserver near a bottle of sake. This is due to how these two drinks deteriorate. While deterioration for wine happens mostly through oxidation, sake, especially ginjo or daiginjo sake, deteriorate through dissipation of aroma. So by sucking out the air inside of the bottle, all of the delicate aromatic qualities are being extracted out of the sake, leaving it stale at best, and smelling like a barnyard at worst.

This has happened in the bar scene in Japan where a system known as “bottle keep” – where patrons purchase a bottle of liquor or sake, and keep the remainder at the bar for a next visit – is common. Miyao cautions gravely that this has led to customer complaints on some occasions, where a customer returns to the bar only to find his expensive bottle of daiginjo gone to waste. The rule of thumb in enjoying aromatic sake is to drink it quickly; sake continues to change in its qualities and lose some of its aromatic luster as soon as it’s been bottled. So it’s best to enjoy aromatic sake while it’s fresh rather than to hold on to it for multiple seasons. If keeping an opened bottle for another sitting, try minimizing contact with air by transferring the remainder to a smaller bottle and keeping it well refrigerated. In any case, finish the sake in a week to two, at most, if you want to enjoy it as intended.

Aromas in Nama Sake

As in the case with ethyl caproate, there’s a list of other aromatic substances in sake that require a delicate balance. One such example is the compound that exists in nama-zake, fresh sake that haven’t been pasteurized. While nama-zake has been acquiring new sake fans overseas with its vibrant and zippy flavor profile, the general palate in Japan calls for a more subtle expression. The three substances below create a unique “nama-esque” aroma that, in some cases, give the sake its very own identity, but can also overpower the sake if not kept in check.

The first of these is ethyl acetate. This compound, as the name suggests, gives off the unmistakable smell of nail polish remover. This aroma can bring out pineapple notes when present in small amounts, but anything more is regarded as a bad thing in sake tasting.

Acetaldehyde, sometimes fingered as the cause of hangovers, is an inevitable by-product of alcoholic fermentation, and adds a unique aroma to sake. It has a notable green and woody smell, but the aroma can otherwise be hard to pin down. Some sense fresh cut grass, Granny Smith apples, wood, or even medicine boxes. Its presence has to therefore be extremely well calculated and managed; No one wants to be reminded of being at a dentist’s office as they sip their nama-zake. Unlike other aromas covered later, whose presence can be judged favorably depending on the drink, acetaldehyde, at least in large quantities, is considered an off flavor across all beverages including beer and wine.

In contrast to the universally unpleasant medicinal nose of acetaldehyde, another compound found in nama is likely to evoke a ubiquitously happy sentiment. The compound, called isovaleraldehyde, boasts an aroma that’s a dead ringer for brownies. in fact it smells precisely like the slightly burnt edges of a fresh tray. It’s a smell that takes many of us straight back to our childhoods. As sweet as the smell is, however, an excessive amount of isovaleraldehyde is considered undesirable in nama-zake, often labeled as “nama hine ka” or “mure ka” (faulty nama smell, or musty smell). Unsurprisingly, this aroma, in excess, can easily overpower a sake’s overall profile.

According to Miyao, KIKUSUI developed a technology to completely rid the sake of isovaleraldehyde several years ago. Considering it’s regarded as an unwanted smell, this technology should, at least in theory, improve the quality of KIKUSUI sake. Confident in the discovery, KIKUSUI used the technique on their Funaguchi sake, a household brand name for their nama sake series that comes in a signature 200 ml can. Sampled by a group of loyal fans, the feedback was a surprise to KIKUSUI engineers: Many of the samplers preferred the original Funaguchi flavor, stating that the new and supposedly improved version without isovaleraldehyde lacked character, and was therefore no longer recognizable as Funaguchi. Moral of the story: these aromatic substances are inherently neutral in most cases – but it’s up to the brewer to carefully balance all of these elements to create a pleasant flavor that’s unique and memorable.

kikusui funaguchi ichibanshibori

Undesirable Aromas in Sake

While all sake, regardless of grade or ingredients, have these substances to varying degrees, there are certain smells only caused by production defects or poor handling. One example is diacetyl; this chemical compound has a distinct cheesy smell — think mozzarella or cottage cheese, or the whey that collects atop yogurt — and is a telltale sign of a production failure that’s prematurely killed off the yeast. This can happen in particular to ginjo grade moromi, where the rice used is highly polished, leaving few nutrients for the yeast to feed on. The low temperature required to brew a delicate ginjo inhibits yeast’s growth and further threatens its survival, too. Drawing this fine line is all up to the brewers’ skill and experience.

Although the cheesy nose is considered a fault in sake, it’s not necessarily a sign of contamination. In fact, diacetyl is what gives butter its “buttery” characteristic, and is therefore artificially added to products like margarine. And when it comes to big-boned Californian Chardonnay, some brewers tweak the production process to deliberately promote the production of diacetyl. Here, too, is a case of cultural differences: Japanese consumers, on the whole, can’t stand this telltale aroma, while western consumers seem to have a higher tolerance – and may even enjoy it.

As much as we’d love to believe otherwise, sake isn’t immune to contamination. A chemical compound called 2-Methoxy-4-vinylphenol, more commonly referred to as 4VG (4-Vinylguaiacol) in sake-speak, can sometimes be produced during fermentation, and it’s an indication of foreign microorganisms present in the moromi. This substance has a clove-like smell, and can be easily detected in the finished sake if present in large enough quantities.

The science behind the emergence of 4VG had long been a mystery as all of the sake yeast strains cultivated and distributed by the Brewing Society of Japan have a genetic mutation that, in theory, disables them from producing the chemical compound – a result of decades of research working to perfect the best types of yeasts for brewing sake.

It was several years ago that the Niigata Prefectural Institute of Brewing illuminated bacteria and foreign yeast contamination during the process of making koji as the culprit in the presence of unpleasant 4VG in modern sake. This makes sense, of course: koji-making requires an environment that bacteria love. A specially designed, tiny wooden room where brewers sprinkle koji mold onto steamed rice during the brewing process is kept warm and humid, creating a virtual sauna. While this type of environment is essential in facilitating even and thorough koji mold growth, this part of the process has traditionally been conducted by bare-handed brewery workers – presenting a contamination risk, to say the least.

With this discovery, KIKUSUI Brewery, along with other breweries in the prefecture, began enforcing the use of surgical gloves during koji-making. But this solution comes with a caveat; koji-making is a delicate process that relies heavily on the brewers’ sensory assessment of the rice’s condition, such as its temperature and level of humidity, the wafting aroma, and the quickly-changing texture of the rice. Asking brewers to wear gloves will effectively block these tactile senses that they depend on in brewing. Needless to say, many breweries that tried to introduce a similar measure were met with passionate resistance from the resident brewers.

The resistance is understandable, as koji-making is arguably the most critical process in sake brewing. The koji lays the foundation for the subsequent fermentation, and sets the direction of the overall result of the batch. It’s so important that many breweries have a designated team of koji-making specialists who are entrusted with this time and energy-consuming process.

With the modernization of the brewing process happening across the industry, though, more breweries are moving away from the stubborn-to-a-fault artsmanship of the past to joining the likes of KIKUSUI in becoming science-driven and hygienically controlled production facilities.

the 4VG is regarded as a sign of koji contamination in sake

While the 4VG is regarded as a sign of koji contamination in sake, the substance itself isn’t necessarily an evil; As was the case with diacetyl and wine, 4VG is a legitimate by-product of a beer yeast used to brew Weizenbier, giving it its signature clove aroma. It appears organically in highly aged Awamori (a Japanese liquor), and thus is regarded here as a favorable trait for the spirit.

Last, and the least palatable, is the mold odor caused by using chloride on wood. This odor is so powerful that it will taint all the sake – even the stuff that’s already been bottled – in its proximity. Unlike the two previous substances that are rather subtle in most cases and may require a trained sampler to accurately detect in sake, this mold odor doesn’t take an expert to spot immediately. This type of contamination is detrimental to breweries because the chemical compound that causes this odor, 2,4,6-Trichloroanisole (TCA), is extremely difficult to eradicate – usually requiring the brewer to discard all contaminated wooden equipment and, in a few unfortunate cases, to rebuild their wooden koji room from scratch.

Although the majority of TCA contamination happens during production, anyone who works with sake can potentially introduce the contaminant. Miyao references a real-life example where an overseas distributor found some mold on the wooden pallet upon receiving their sake and, out of solely good intentions, decided to scrub it with bleach, supposedly spoiling an entire lot of perfectly good sake.

Putting Knowledge to Use

So, does all this sake science have a practical use for sake drinkers? For starters, as we learned earlier, the general rule is to consume aromatic sake as quickly as possible, and, if possible, transfer it into a small, narrow container when keeping some for another day. Lucky for us, many breweries are now offering a range of small, “single-serving” products in addition the conventional 720 or 1800 ml. Funaguchi, for one, is known in many international markets for its “bullet sized” cans, while other brands, like Ozeki, offer their “OneCup,” sized at 180 ml.

When you can’t finish in one sitting, though, sake should generally be stored in a refrigerator. In fact, temperature has a great influence on the speed at which sake (or any drink, really) matures. Miyao emphasizes that all sake is at its best condition at the time of bottling. That’s not to discourage experimenting with aging sake at home – a practice that’s steadily gaining popularity in recent years – but the resulting sake will be a completely different creature, unrecognizable from where it started. In other words, sake at the time of bottling is the true embodiment of what the brewer intended.

KIKUSUI, for one, store their sake at zero degrees celsius (32F) at their Setsugoro-gura  facility while their sake go through the necessary maturing process. The low temperature can help hinder unwanted aging while allowing the sake to become better blended and more rounded with time. That being said, sake is a living thing and it will inevitably continue to develop, however slowly. As a general rule of thumb, Miyao recommends enjoying sake within one year of bottling, regardless of its storage condition, to ensure enjoyment.

Storage of sake

Lastly, a set of sake aroma reagents are available through The Brewing Society of Japan for purchase for anyone serious enough to channel their inner chemistry geek. Scientific analysis of aromas in sake is a discipline that requires theoretical understanding as well as sensory training, and is highly recommended for anyone seeking to level up on their sake journey.