The Science of Hiire

In sake, “hiire” refers to heat pasteurization.

This technique was actually practiced in Japan more than 300 years before Pasteur discovered low-temperature pasteurization. Why is sake heated, and what happens if it isn’t? Let’s explore the science of hiire.

What Is Hiire?

Basic Definition

Hiire is the process of heating sake to approximately 60-65°C.

Purpose

• Enzyme deactivation (stopping their activity)
• Killing microorganisms
• Stabilizing flavor and aroma

Freshly pressed sake still contains living enzymes and microorganisms. If left untreated, the sake quality will change. Hiire prevents this.

Timing of Hiire

Typically, sake undergoes hiire twice.

First Time: Before Storage Sake is heated before being transferred to storage tanks. This prevents quality changes during storage.

Second Time: Before Shipping Sake is heated just before bottling. This stabilizes quality during distribution.

Types Based on Hiire

The name of the sake changes depending on the number of hiire treatments and the method used.

Type	Number of Hiire	Characteristics
Regular sake	2 times	Stable quality
Nama-sake	0 times	Fresh, requires refrigeration
Nama-chozo-shu	1 time (before shipping only)	Slightly nama-like flavor
Nama-zume-shu	1 time (before storage only)	Such as hiyaoroshi

Scientific Mechanisms of Hiire

Enzyme Deactivation

The most important purpose of hiire is to deactivate enzymes.

Amylase (Saccharification Enzyme) An enzyme that breaks down starch into sugar. Without hiire, remaining starch continues to convert to sugar, increasing sweetness.

Protease (Protein-Degrading Enzyme) An enzyme that breaks down proteins into amino acids. If it continues to work, amino acids increase excessively, causing off-flavors.

Deactivation Temperature These enzymes stop working when heated at 60-65°C for several minutes, as their protein structure denatures.

Sterilization of Microorganisms

Microorganisms remaining in the sake are killed.

Hi-ochi Bacteria Lactic acid bacteria specific to sake, and the most problematic microorganism. The main target of hiire.

Yeast Even yeast that has finished fermentation may become active again under the right conditions.

Sterilization Temperature and Time Heating at 60-65°C for several minutes kills most microorganisms. However, some heat-resistant bacteria may survive.

Protein Denaturation

Heating denatures proteins in the sake.

Clarification Effect Denatured proteins coagulate and settle more easily. This has a clarifying effect on the sake.

Impact on Flavor Protein denaturation also affects the sake’s flavor profile. Some flavors are created by the heating process itself.

Methods of Hiire

Bin-kan (Bottle Pasteurization)

A method where bottled sake is heated in a hot water bath.

Advantages

• Minimizes oxidation
• Easy quality control
• Uniform heating of each bottle

Disadvantages

• Time-consuming and labor-intensive
• Not suitable for mass production

Often used for high-quality sake.

Plate Heat Exchanger

A method where sake flows through thin layers while being heated.

Advantages

• Quick heating
• Suitable for mass production
• Efficient

Disadvantages

• Requires capital investment
• Possible uneven heating

Widely used in large and medium-sized breweries.

Pasteurizer

A method where bottled sake is heated by shower-like warm water.

Advantages

• Continuous processing possible
• Suitable for mass production

Disadvantages

• High equipment costs
• Possible uneven heating

Mainly used by major manufacturers.

Temperature and Time for Hiire

Standard Conditions

Typical hiire conditions are:

Temperature: 60-65°C Time: Several minutes to about 30 minutes

The balance of temperature and time is important. Both high-temperature short-time and low-temperature long-time can be effective, but they affect sake quality differently.

Differences by Temperature

Low Temperature (Around 60°C)

• Takes longer to deactivate enzymes
• Less impact on aroma
• Suitable for delicate sake

High Temperature (65°C and Above)

• Effects achieved in shorter time
• More likely to develop heated odors
• Can cause hineka (aged off-flavor)

Importance of Rapid Cooling

After hiire, cooling should be done as quickly as possible.

Reasons

• Minimize heating time
• Prevent development of heated odors
• Suppress oxidation

The presence or absence of rapid cooling equipment significantly affects sake quality.

What Happens Without Hiire?

Characteristics of Nama-sake

Sake without hiire is called “nama-sake.”

Advantages

• Fresh and refreshing aroma
• Lively taste
• Flavors from enzymes and yeast

Disadvantages

• Quality changes easily
• Requires refrigeration (5°C or below)
• Short shelf life

Quality Changes in Nama-sake

If nama-sake is left at room temperature, the following changes occur.

Several Days to One Week

• Aroma begins to change
• Sweetness increases (enzyme activity)
• May become cloudy

Several Weeks to One Month

• Acidity increases
• Color deepens
• Hineka (aged off-flavor) develops

After More Time

• Deteriorates to undrinkable state
• Off-odors develop
• May experience hi-ochi

What Is Hi-ochi?

“Hi-ochi” refers to sake becoming cloudy and sour due to hi-ochi bacteria.

Symptoms

• Sake becomes cloudy white
• Sour smell develops
• Taste deteriorates

Cause Hi-ochi bacteria (lactic acid bacteria of the Lactobacillus genus) proliferate. They have high alcohol tolerance and can grow even in sake.

Hiire is the most important process to prevent hi-ochi.

Relationship Between Hiire and Flavor

Changes from Hiire

Hiire changes the sake’s flavor as follows.

Aroma

• Fresh character unique to nama-sake decreases
• Aged notes from heating are added
• Overall becomes more mellow

Taste

• Becomes smoother with rough edges removed
• Acidity softens
• Flavor becomes more unified

Color

• May become slightly deeper in color
• Clarity increases (protein precipitation)

Comparing Nama-sake and Hiire Sake

Even with the same sake, the impression differs depending on whether hiire was performed.

Aspect	Nama-sake	Hiire Sake
Aroma	Fresh, vibrant	Calm, aged notes
Taste	Lively, effervescent	Mellow, smooth
Storage	Requires refrigeration, short term	Room temperature OK, long-term storage possible
Drinking Time	Immediately	Can be aged

Neither is necessarily better; each has its own appeal.

History of Hiire

Origins in Japan

Hiire was already being practiced during the Muromachi period (around the 15th century).

The “Goshu no Nikki” (1489) contains descriptions of heating sake to stabilize quality. It was empirically known that heating would make sake last longer.

300 Years Before Pasteur

Louis Pasteur of France discovered pasteurization in 1866.

Japan’s hiire had been practiced about 300 years earlier. This was the wisdom of ancestors who knew its effectiveness through experience, even without understanding the scientific principles.

Modern Hiire

Today, hiire is performed based on scientific understanding.

Temperature and time management, the importance of rapid cooling, and choosing between bin-kan and heat exchangers—more precise control has become possible.

Trends in Hiire

Low-Temperature Hiire

In recent years, more breweries are performing hiire at lower temperatures.

Purpose

• Preserve aroma
• Retain freshness
• Maintain flavors close to nama-sake

Hiire at around 55°C over a longer time. This achieves both enzyme deactivation and flavor preservation.

Rediscovery of Bin-kan

Although labor-intensive, bin-kan is being reconsidered for its quality advantages.

Especially for high-quality sake, more breweries are adopting bin-kan. Some brands market “bin-kan hiire” as a selling point.

Improved Distribution of Nama-sake

With the development of cold chain logistics, nama-sake can now be delivered reliably.

The spread of refrigerated delivery and improved cold storage at sake retailers—the environment for enjoying nama-sake has improved.

Relationship to Home Storage

Storing Hiire Sake

Sake that has undergone hiire is relatively easy to store.

Unopened

• About 1 year in a cool, dark place
• Even longer in the refrigerator
• Avoid direct sunlight and high temperatures

After Opening

• Store in the refrigerator
• Consume within 2 weeks to 1 month
• Oxidation progresses when exposed to air

Storing Nama-sake

Nama-sake requires more delicate management than hiire sake.

Unopened

• Must be stored in the refrigerator
• Within 1-3 months of purchase
• Avoid temperature fluctuations

After Opening

• Store in the refrigerator
• Consume as soon as possible
• Ideally within 1 week

Summary

Key points about the science of hiire:

Purpose of Hiire

• Enzyme deactivation
• Sterilization of microorganisms
• Stabilization of flavor and aroma

Hiire Conditions

• Temperature: 60-65°C
• Time: Several minutes to about 30 minutes
• Rapid cooling is important

Without Hiire

• Quality changes easily
• Risk of hi-ochi
• Requires refrigeration, short term only

Hiire is Japanese wisdom that has continued for over 300 years. This scientifically sound technique supports the quality of sake.

---

For more about nama-sake, see What Is Nama-sake.

For detailed sake storage methods, see Storage Methods.