Analysis of Rice Fermented Liquors

Analysis of Rice Fermented Liquors: Cheongju and Sake, The 1,000-Year Evolution of Fraternal Twins

The Wild Ecosystem of Nuruk vs. The Precision Control of Koji. The fermentation science of two liquors that started with the same rice but embarked on completely different microbiological journeys.

 

Korean Cheongju and Japanese Sake are fraternal twins sharing a common mother: "liquor brewed from rice." However, over a millennium, they have walked distinct evolutionary paths. One is the crystallization of naturalistic fermentation embracing the complexity of the wild, while the other is an engineering masterpiece pursuing purity through extreme control and refinement.

I cannot forget the shock of my first sip of Sake. From the colorless liquid in the clear glass rose aromas of apple and pear, and I was startled by the silk-like smoothness as it touched my lips. "This is made from rice?"

Yet, the Cheongju I drank at a Korean traditional liquor tasting a few days later was another world entirely. The nutty aroma of Nuruk, the heavy texture coating the tongue, and the long finish rising after swallowing—it was hard to believe such different liquors could come from the same rice.

That curiosity was resolved while reading microbiology papers. The difference lay in the 'Saccharifying Agent (Starter).' How one obtains the microorganisms that break down rice starch into sugar—this fundamental difference created the completely distinct characters of these two liquors.

Today, we will analyze Korean Cheongju (Yakju) and Japanese Sake through the lenses of microbiology, fermentation biochemistry, brewing engineering, and historical context. While there are many wine or whisky aficionados, few know about the liquor made from rice, our staple food. After reading this, every time you lift a glass, you will be reminded of the dozens of microbial species and hundreds of chemical reactions contained within it.

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Nuruk vs. Koji: The Fundamental Difference in Microbial Ecosystems

The core difference that determines the character of the liquor starts here. Rice is a lump of starch. Yeast cannot eat starch directly; it can only eat sugar. Therefore, someone must break down the starch into sugar. The entity playing this role is the mold that produces saccharifying enzymes.

However, the methods of obtaining this mold in Korea and Japan are completely different.

Nuruk: A Wild Orchestra Brewed by Nature

Korea's traditional Nuruk uses a 'Spontaneous Inoculation' method. Wheat is roughly ground, mixed with water to form a dough, shaped by stomping with feet (called Didim), and simply left in a warm place. Microorganisms floating in the air then naturally settle and colonize it.

🔬 Nuruk Manufacturing Process (Traditional Method)

• Raw Material: 90-100% Wheat or Wheat/Rice mix, coarsely ground to preserve bran and germ.
• Didim (Molding): Stomped into a round cake shape (Pressure 2-3 kg/cm²).
• Fermentation: Natural fermentation at 25-35°C for 7-14 days; airborne microbes settle naturally.
• Drying & Aging: Dried for 15-20 days, then aged for 1-6 months (some for years).

What is surprising is that an average of 50-100 species of microorganisms live in a single piece of Nuruk made this way. A study by Lee et al. published in Food Microbiology (2010) analyzed traditional Nuruk from 10 regions in Korea using 16S rRNA and ITS sequencing, and the results were fascinating.

Category	Major Genus	Detection Freq.	Role
Molds	Aspergillus	100%	Amylase production
	Rhizopus	87%	Amylase, Protease
	Mucor	73%	Lipase, Protease
	Absidia	45%	Auxiliary saccharification
Yeasts	Saccharomyces	92%	Alcohol fermentation
	Pichia	65%	Flavor generation
	Candida	58%	Auxiliary fermentation
Bacteria	Lactobacillus	88%	Lactic acid production
	Pediococcus	52%	Lactic acid production
	Bacillus	41%	Enzyme production

What does this mean? Nuruk is a single small ecosystem. It contains not only molds but also yeasts and bacteria. Each produces different enzymes and releases different metabolites. This is why liquor brewed with Nuruk has a complex and deep flavor.

Koji (麴): The Precision of a Controlled Single Strain

In contrast, Japanese Koji (Ip-guk) takes the opposite approach. It cultures a pure strain of a single mold species, Aspergillus oryzae. Only the spores of this mold are sprinkled onto steamed rice and grown in a cultivation room (Kojimuro) where temperature and humidity are strictly controlled.

🔬 Koji Manufacturing Process (Modern Method)

• Rice Prep: Uses Sakamai (sake rice), polished 60-70% (50% or less for premium), precisely washed, soaked for 12-18 hours.
• Steaming: Steamed with high-pressure steam for 60-90 mins (hard outside, soft inside).
• Inoculation: Spraying Aspergillus oryzae spores (300-500g per 1 ton of rice).
• Cultivation: Managed precisely within ±0.5°C for 45-50 hours with multiple mixings.

The Koji making process is truly engineering-based. The temperature is adjusted in 0.5°C increments by time, such as 30-32°C for 0-24 hours and 36-38°C for 24-36 hours. This is why brewmasters (Toji) in Japanese breweries stay up all night holding thermometers.

Genetics of Aspergillus oryzae: Thousands of Years of Artificial Selection

A study published in Nature (2005) decoded the entire genome of Aspergillus oryzae. The results were intriguing.

🧬 Aspergillus oryzae Genome Analysis (Machida et al., 2005)

• Genome Size: 37.2 Mb (Megabases)
• Number of Genes: Approx. 12,000
• Key Finding: Protease (protein-degrading enzyme) genes are partially down-regulated, resulting in 30-40% lower proteolytic activity compared to wild Aspergillus species.

This implies it is the result of thousands of years of artificial selection. Japanese brewers have discarded strains that break down too much protein (causing off-flavors) and selectively bred strains that cleanly convert only starch into sugar. This is the genetic basis of Sake's "clean taste."

Enzyme Activity Comparison: Differences by Numbers

Enzyme	Nuruk (U/g)	Koji (U/g)	Ratio
α-Amylase	1,200-1,800	2,500-3,500	Koji 2.0x
Glucoamylase	800-1,200	1,800-2,400	Koji 2.2x
Acid Protease	450-680	150-280	Nuruk 2.5x
Neutral Protease	280-420	80-150	Nuruk 3.3x

• U = Unit (Enzyme activity unit: Amount of enzyme converting 1 μmol of substrate per minute)

Key Differences Summary

• Nuruk:
  • High Protease Activity → Abundant amino acid production.
  • Result: Deep and rich taste, strong body, complex aromas (grain, earth, nuts). sometimes earthy notes (Nuruk-chi).
• Koji:
  • High Amylase Activity, Low Protease Activity.
  • Result: High sugar, low amino acids → Cleanliness. Fruity aromas (esters), light body.

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Polishing Ratio: The Difference in Philosophy of Cutting Rice

When talking about Sake, one cannot omit "Seimai-buai" (Rice Polishing Ratio). It is a figure indicating how much of the rice grain remains; 70% means 30% has been polished away.

At first, I didn't understand. "Why throw away 30% of expensive rice?" But I realized why after seeing a table analyzing the chemical composition of rice layers.

Part	Polishing Ratio	Starch (%)	Protein (%)	Fat (%)	Ash (%)
Brown Rice	100%	73.5	8.5	2.8	1.5
Outer Layer	90-100%	65.2	12.8	5.2	2.8
Middle	70-90%	76.8	7.2	1.8	0.9
Core (Shinpaku)	<70%	85.3	4.2	0.5	0.3

As shown, the outer layer of the rice grain is rich in protein and fat, while the center (Shinpaku) is pure starch. Protein breaks down into amino acids creating savory or off-flavors, and fat oxidizes to create stale odors. Therefore, to achieve a "clean taste," the outer layer must be shaved off.

The World of Extreme Polishing

Some top-tier Japanese Sakes pursue a polishing ratio of less than 1%. The famous Dassai Beyond has a polishing ratio of roughly 0.85% (marketed often around the concept of surpassing their 23%). This means discarding over 99% of the rice grain to use less than 1%.

⚠️ Extreme Polishing: Art or Waste?

• Result: 1 ton of rice → Only approx. 8.5kg used.
• Milling Time: 120-180 hours (5-7 days).
• Price: $500 - $1,000 per bottle.
• Controversy:
  • Pro: Extreme purity, artistic realm.
  • Con: Excessive resource waste, limited flavor improvement (many studies suggest differences are negligible below 50%).

Korean Cheongju's Baekse (百洗): The Aesthetics of Wholeness

In contrast, Korean traditional liquor did not obsess over polishing the rice. Instead, they used a method called "Baekse" (Hundred Washings), washing the rice 100 times (in reality, 20-30 times). With each wash, surface protein, fat, and dust are removed with the rinse water.

The advantage of this method is that rice loss is only 1-2%. In an era when rice was precious, throwing away 70% was unimaginable. Also, since some germ remains, Vitamin B complex is preserved.

However, there are downsides. It is labor-intensive, and protein and fat cannot be completely removed. This is why Korean traditional liquor has a heavier body than Sake and a strong nutty scent of Nuruk. Some call this "off-flavor," while others call it "deep flavor." It is a matter of perspective.

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Multiple Parallel Fermentation: A Miracle in One Tank

Beer or whisky utilizes 'Sequential Fermentation.' First, malt breaks down starch into sugar, and then yeast is added to ferment it into alcohol. The two processes happen sequentially.

Wine is simpler. Grapes already have sugar, so you just add yeast. This is 'Single Fermentation.'

However, rice liquor is different. Saccharification (starch to sugar) and Alcohol Fermentation (sugar to alcohol by yeast) happen simultaneously in one tank. This is called 'Multiple Parallel Fermentation.'

1. Stage 1: Starch Hydrolysis (Amylase)
   • $(C_6H_{10}O_5)_n + nH_2O \rightarrow nC_6H_{12}O_6$
2. Stage 2: Alcohol Fermentation (Yeast)
   • $C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2 + Heat$

Overall Reaction:

• $(C_6H_{10}O_5)_n + nH_2O \rightarrow 2nC_2H_5OH + 2nCO_2 + Heat$

This is important because it allows for a constant sugar concentration. If the speed at which mold creates sugar and the speed at which yeast consumes sugar are balanced, the yeast can always work in an optimal environment. This is why Sake can produce alcohol levels of 18-20%, much higher than wine (12-14%) or beer (4-6%), without distillation.

Sake's 3-Step Mash (Sandan Shikomi): Wisdom of Gradual Yeast Increase

One of the key techniques in Sake brewing is the '3-Step Mash.' Ingredients are not added all at once but divided over 3 days.

Stage	Japanese	Time	Steamed Rice (kg)	Koji (kg)	Water (L)
Hatsu-zoe	初添え	Day 0	60	30	70
(Rest)	踊り (Odori)	Day 1	-	-	-
Naka-zoe	仲添え	Day 2	120	60	100
Tome-zoe	留添え	Day 3	180	90	130

Why do this? If ingredients are added all at once, the yeast density is drastically diluted, creating an opportunity for spoilage bacteria to breed. However, by adding ingredients in stages, the yeast can maintain a sufficient population and dominance. Truly clever.

Ginjo-zukuri (吟醸造り): The Science of Low-Temperature Long Fermentation

Premium Sake (Ginjo, Daiginjo) is fermented slowly for 30-40 days at extremely low temperatures of 5-10°C. Why?

📊 Low-Temp Fermentation & Ester Production (Yoshizawa, 1999)

Comparing fermentation of same ingredients/yeast at 15°C vs. 7°C:

Compound	15°C (mg/L)	7°C (mg/L)	Aroma Characteristics
Isoamyl acetate	1.8	4.2	Banana, Pear
Ethyl caproate	0.3	1.8	Apple, Anise
Isoamyl alcohol	180	220	Grain, Solvent

Conclusion: At low temperatures, yeast gets stressed and over-expresses ester synthesis enzymes (Alcohol acetyltransferase, ATF). This is a defense mechanism to increase cell membrane fluidity for survival, but the byproduct is the creation of flamboyant fruity aromas.

In other words, Sake's luxurious apple and pear scents are the 'products of stress' created by yeast shivering in the cold. Biologically, it's quite ironic.

Korean Cheongju: Diverse Brewing Methods

Korean liquor is named and characterized by the number of brewing steps.

Category	Steps	Fermentation	Alcohol %	Characteristics
Danyangju	1 time	5-7 days	8-12%	High sweetness, strong acidity, quick consumption
Iyangju	2 times	10-15 days	13-16%	Most common, balanced taste
Samyangju	3 times	15-21 days	16-18%	High alcohol, smooth, deep aroma
Sayangju	4 times	21-28 days	17-19%	Extremely smooth, rare

Iyangju (Two-step fermentation) is the most common, where a starter mash (Mitsul) is made first, and when yeast propagates sufficiently, the main mash (Deotsul) is added. The principle is similar to Sake's 3-step mash, but the number of times and timing differ.

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The Scar of 1916: What the Liquor Tax Law Left Behind

This part is history, not science, but it must be addressed because it is the decisive event that shaped today's Korean traditional liquor.

📜 History of Korean Traditional Liquor: Prosperity and Disconnection

• Joseon Dynasty (1392-1910): Unique home-brewing (Gayangju) traditions in every noble household. Hundreds of recipes recorded in Eumsik Dimibang (1670), Gyuhap Chongseo (1809).
• 1909 - Liquor Tax Law: Japanese colonial government introduces tax law, switching to a permit system.
• 1916 - Liquor Tax Decree: Home brewing completely banned. Destruction of Gayangju culture. Only licensed breweries could produce liquor.
• 1934 - Nuruk Control: Japan enforces the use of Ip-guk (Japanese style Koji). Manufacture of traditional Nuruk banned.
• 1965 - Grain Management Act: Rice liquor production banned due to rice shortage. Only wheat Makgeolli and diluted Soju allowed.
• 1995 - Rice Liquor Allowed Again: Rice liquor production re-legalized after 79 years with the abolition of the Grain Management Act.

⚠️ The Missing Link: 1916-1995 (79 Years)

What was lost:

• Gayangju Culture: Extinction of unique recipes passed down in families.
• Nuruk Diversity: Disconnection of regional and familial Nuruk manufacturing skills.
• Rice Varieties: Disappearance of native rice varieties suitable for brewing.
• Vessels: Decline of traditional vessel technologies like Onggi and jars.

Even more ironic is that under current Korean law, "Cheongju (淸酒)" refers to liquor made following the Japanese method (using Koji/Ip-guk). Clear liquor made using traditional Nuruk is classified as "Yakju (藥酒)." The legacy of colonial rule still remains in the legal system.

However, there is hope. Since the 2010s, young brewers have been restoring ancient texts, reviving traditional Nuruk, and attempting creative innovations. Excellent traditional liquors are emerging from breweries like Baesangmyun Brewery, Kooksoondang, and Boksoondoga.

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Flavor Chemistry: The Brilliance of Esters vs. The Depth of Amino Acids

When you drink Sake, fruity aromas rise flamboyantly; when you drink Cheongju, the nuttiness of grain and savory umami are felt. Where does this difference come from? It is because the composition of flavor components is completely different.

Sake's Aroma Profile: A Feast of Esters

The flamboyant aroma of Sake comes mainly from Ester compounds. In particular, Ginjo Sake features a unique fruity scent called 'Ginjo-ka (吟醸香)', which is a combination of Isoamyl acetate and Ethyl caproate.

Compound	Conc. (mg/L)	Threshold (μg/L)	Aroma Characteristics
Isoamyl acetate	1.5-4.5	30	Banana, Pear
Ethyl caproate	0.5-2.0	1.8	Apple, Anise
Phenylethyl acetate	0.1-0.5	250	Rose, Honey
Phenylethyl alcohol	20-80	10,000	Rose

Especially, Ethyl caproate has an extremely low threshold (the minimum concentration detectable by humans) of 1.8 μg/L, so even a tiny amount produces an intense apple scent.

Cheongju's Flavor Profile: Complex Harmony

Cheongju is characterized by a complex harmony of amino acids, organic acids, and higher alcohols rather than esters.

Korean Cheongju/Yakju

• Organic Acids: Lactic acid, acetic acid, citric acid (0.3-0.8%) → Acidity, freshness.
• Amino Acids: Glutamic acid, alanine (100-400 mg/100mL) → Umami, body.
• Higher Alcohols: Isoamyl alcohol (150-300 mg/L) → Grain aroma.
• Esters: Low (20-60 mg/L) → Weak fruit aroma.

Japanese Sake

• Esters: High (50-150 mg/L) → Flamboyant fruit aroma.
• Amino Acids: Low (80-200 mg/100mL) → Light body.
• Organic Acids: Low (Trace lactic acid) → Cleanliness.
• Aroma Direction: Apple, pear, melon, banana.

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Practical Guide: How to Drink?

Theory is enough. Now, let's look at how to enjoy the liquor in practice.

Storage Method

📦 Optimal Storage Conditions

• Sake:
  • Temp: 5-10°C (Refrigerated), Namazake (Unpasteurized) at 0-5°C.
  • Light: Block completely (Sensitive to UV, prevents yellowing).
  • After Opening: Consume within 1-2 weeks (Ginjo), 1 month (Junmai).
• Cheongju/Yakju:
  • Temp: 10-15°C (Refrigeration recommended).
  • Traditional liquors often contain live cultures, so refrigeration is essential.
  • After Opening: Consume within 2-3 weeks.

Serving Temperature

Sake becomes a completely different drink depending on the temperature. In Japan, there are even names for each temperature range.

Temp	Japanese	Suitable Sake	Characteristics
5°C	Yukibie (Snow cold)	Daiginjo, Namazake	Aroma preservation, maximizes crispness
10°C	Hanabie (Flower cold)	Ginjo, Raw Sake	Distinct fruit aroma, refreshing
15°C	Suzubie (Cool breeze)	Junmai, Honjozo	Balanced taste
40°C	Hitohadakan (Skin warm)	Junmai, Kimoto	Rich aroma, full body

Temp Selection Guide

• Want to enjoy flamboyant aroma: 5-10°C (Cold)
• Want to enjoy rich taste: 30-40°C (Warm)
• Want balance: 15-20°C (Room temp)
• Korean traditional liquor is usually best drunk at room temperature (15-20°C) or slightly chilled (12-15°C). This reveals the complex flavors of Nuruk well.

Pairing: Harmony with Food

Just as wine has pairings, rice liquor has pairings.

🍱 Sake Pairing

• Daiginjo/Ginjo: Sashimi, oysters, scallops, cheese, light salads.
• Junmai: Grilled fish, Yakitori, miso dishes, mushroom dishes.
• Kimoto/Yamahai: Steak, grilled eel, aged cheese, rich broth ramen.

🍲 Cheongju/Yakju Pairing

• Clean Style: Jeon (pancakes), Namul (seasoned greens), Sashimi, Tofu dishes.
• Rich Style: Bulgogi, Galbijjim (braised ribs), Kimchi-jjigae, Makchang/Gopchang (tripe).

General Principle:

• Spicy food → Liquor with acidity.
• Greasy food → High alcohol liquor.
• Bland food → Light liquor.

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Conclusion: Coexistence of Diversity and Precision

Let's go back to the first question. Between Korean Cheongju and Japanese Sake, which is better?

The answer is "Both are excellent, but their directions are different."

🍶 If Sake is the 'Aesthetics of Subtraction', Korean Traditional Cheongju is the 'Aesthetics of Inclusion'.

Characteristics of Sake:

• Koji (Aspergillus oryzae single strain)
• High Rice Polishing (50% or less)
• Low-Temp Long Fermentation (5-10°C, 30-40 days)
• Ester-focused flamboyant aroma
• Clean and delicate taste
• Goal: Purity

Characteristics of Korean Traditional Cheongju:

• Nuruk (Complex ecosystem of 50-100 microbial species)
• Baekse (Washing) or minimal polishing
• Room Temp Fermentation (18-25°C, 10-20 days)
• Complex flavor focused on amino acids & organic acids
• Deep and rich taste
• Goal: Diversity

There is no scientific superiority. There is only a difference in philosophy and environment in handling microorganisms.

Sake aims for a pure and clear taste by polishing rice, using a single strain, and fermenting at low temperatures to remove off-flavors to the extreme. This aligns with Japan's Zen philosophy or food culture like Sushi.

On the other hand, Korean Cheongju uses whole grains and embraces wild microorganisms to aim for a colorful and deep taste. Bitterness, sweetness, acidity, and umami blend together to harmonize perfectly with strong-flavored Korean food.

Speaking from personal experience, I was first captivated by the flamboyant aroma of Sake. But as I learned more about Korean traditional liquor, I marveled at the depth of its complex flavors. Now, I love both. When it's hot, I drink cold Ginjo Sake; on winter nights, I drink slightly warmed Junmai or Korean Yakju.

Now, every time you lift a glass, think about it. Inside that transparent liquid lies the 12,000 genes of Aspergillus oryzae, or a complex ecosystem of 50 microbial species; the precision temperature control of 45 hours, or the courage to embrace nature's randomness.

And above all, the journey of Korean traditional liquor, which is resurrecting right now, overcoming the 79-year vacuum severed by the 1916 Liquor Tax Decree.

Liquor is not just an alcoholic beverage. It is a history of fermentation created together by microbes and humans over thousands of years, and an artifact containing the aesthetics and philosophy of each culture.

Cheers!

Key References:

• Machida, M., et al. (2005). "Genome sequencing and analysis of Aspergillus oryzae." Nature, 438(7071), 1157-1161.
• Lee, J.W., et al. (2010). "Microbial succession and metabolite changes during fermentation of Korean traditional wheat-based Nuruk." Food Microbiology, 27(4), 505-513.
• Yoshizawa, K. (1999). "Sake: Production and properties of major Japanese alcoholic beverage." Food Reviews International, 15(1), 83-107.
• Park, H., et al. (2015). "Traditional knowledge and uses of Korean traditional rice wine, Yakju." Journal of Ethnic Foods, 2(1), 10-17.
• National Tax Agency Japan. (2018). "Sake Brewing Guideline and Quality Standards." Tokyo: Government Publication.
• McGee, Harold. (2004). On Food and Cooking: The Science and Lore of the Kitchen. New York: Scribner.
• Korea Food Research Institute. (2020). "Research on Modernization of Traditional Liquor Manufacturing Technology." Korea: MAFRA.
• Kim, H.R., et al. (2016). "Characterization of Aspergillus species from traditional Korean Nuruk." Journal of Microbiology and Biotechnology, 26(10), 1767-1776.
• Sato, M., et al. (2008). "Effect of temperature on the formation of flavor components in Japanese sake." Journal of Bioscience and Bioengineering, 105(2), 166-171.