July 10th is 'Natto Day' | From a single soybean to the connection between fields and fermentation

In the morning, I open a pack of natto and stir it with chopsticks.

White threads stretch between the beans, and a unique aroma rises. Because it is such a familiar food, I don't usually think deeply about 'what natto is'.

However, if you trace that single bean back, it leads to soybeans growing in the field, the rains of the rainy season, the summer sun, the microorganisms living in the roots, and finally, fermentation by natto bacteria.

July 10th is 'Natto Day,' created from the wordplay of 'na' (7) and 'tou' (10).Generally, it is said that the anniversary, which began in the Kansai region in 1981, became a nationwide anniversary in 1992.

Moreover, around July 10th is also animportant seasonal turning point for the soybeansthat serve as the raw material.

In this article, we will organize the true nature of natto as a food, and trace its relationship to soybeans grown in home gardens and the climate of early July.

What exactly is natto?

What we usually eat is 'stringy natto' (itohiki natto).

Even though we just call it natto, there are actually several types.

The natto we see daily at the supermarket isofficially called 'stringy natto'. It is made by adding natto bacteria to steamed or boiled soybeans and fermenting them.

On the other hand, in Japan, there are also types of natto that do not pull threads, such as 'tera-natto' or 'hama-natto,' which are fermented using koji and salt. Even though they share the name natto, the microorganisms used, the production methods, and the flavors are quite different.

In this article, we will focus on the 'stringy natto' that we usually eat over rice.

Natto is a 'collaborative work of soybeans and natto bacteria'.

Natto bacteria are a type of bacteria called Bacillus subtilis. In the natural world, they exist in familiar places such as rice straw, soil, and the air.

In making natto, first, dried soybeans are soaked in water to let them absorb enough moisture. After that, the soybeans are steamed or boiled to soften them, and natto bacteria are added.

In factories, it is generally made in the following flow:

• Sort and wash the soybeans

• Soak in water to absorb moisture

• Steam or boil

• Add natto bacteria

• Pack into containers

• Manage temperature and humidity to ferment

• Cool and age at a low temperature

Natto bacteria are microorganisms that require oxygen. Therefore, the state in which the beans are moderately layered in the container and air can pass through is also related to fermentation.

The fermentation temperature is generally 38–42°C, and the time is about 16–24 hours. After that, it is cooled to 5°C or lower to stabilize the fermentation and allow it to age.

What are natto strings made of?

The center of the string is 'polyglutamic acid'

Those long, white strings that appear when you mix natto.

It is not because it is rotting and becoming sticky. It is a substance created by natto bacteria as they grow using the components of the soybeans.

The core of the stickiness is a substance called 'polyglutamic acid,' which is a long chain of glutamic acid. It also contains fructan, which is a chain of fructose.

Research has shown that polyglutamic acid is mainly responsible for the strong stickiness and stringiness characteristic of natto.

Even with the same natto, the strength of the strings and the stickiness differ slightly depending on the product because the bacterial strains used, the type of soybean, the moisture content, and the fermentation conditions vary.

There is a reason why there are so many small-bean natto products

In the natto section, you often see products labeled 'small bean' or 'extra small bean'.

In addition to being easy to eat with rice, small beans have a larger surface area than large beans for the same weight.Because the surface of the beans where the natto bacteria work is relatively larger, it is considered to be well-suited for natto processing. is said.

However, natto does not necessarily have to be made with small beans. There are also large-bean natto and black soybean natto, andthe texture and the way you perceive the sweetness of the beans change depending on the size of the beans..

Is natto a block of nutrition?

According to the Ministry of Education, Culture, Sports, Science and Technology's Food Composition Database, 100 grams of stringy natto contains the following nutritional components.

• Protein: 16.5 grams

• Dietary fiber: 9.5 grams

• Potassium: 660 milligrams

• Calcium: 90 milligrams

• Iron: 3.3 milligrams

• Vitamin K: 600 micrograms

• Vitamin B2: 0.56 milligrams

• Folic acid: 120 micrograms

Since most actual packs of natto are around 40 to 50 grams, the intake will be around half of this. Content and ingredients vary by product.

Natto is a food that makes it easy to consume plant-based protein and dietary fiber at the same time.

However, eating only specific foods will not make you healthy. It is natural to think of natto as part of a meal combined with staple foods, vegetables, meat, fish, and so on.

The surprisingly close relationship between edamame, soybeans, and natto

Edamame and soybeans are actually the same plant

Edamame is a familiar sight in home gardens.

In fact, edamame and soybeans are the same thing as plants .

Edamame is harvested while the pods are still green and the beans are soft. Soybeans are what you get when you let them grow in the field without harvesting, allowing the beans to fully mature and dry out .

In other words, in a home garden, the path diverges depending on the harvest time.

• Harvest while green and soft
  → Eat as edamame

• Wait until the leaves turn yellow and the pods and beans mature
  → Harvest as dried soybeans

• Ferment those dried soybeans
  → Become natto

When looking at edamame and thinking, 'If I wait a little longer, it will become a soybean,' it feels a bit mysterious.

However, there are now many varieties specifically bred for their flavor as edamame, so there can be differences in characteristics between varieties harvested young and those specifically for edamame.

How to grow 'beans for natto' in a home garden

First, choose a variety that suits your region

Soybeans vary significantly in terms of suitable cultivation regions and sowing times depending on the variety.

Small or extra-small grain varieties are often used for natto, but it doesn't mean 'any small grain will grow anywhere.' For example, 'Suzuotome' is a small-grain variety for natto bred specifically for cultivation in warmer regions like Kyushu.

In a home garden, rather than deciding based solely on the variety name, it is important to check the following using seed packets or local cultivation information:

• Can it be grown in your region?

• When is the optimal sowing time?

• Will it get cold before it matures?

• Is it an edamame variety or a dry soybean variety?

It is important to check these.

If making natto, go past the 'edamame harvest period'

When harvesting soybeans for natto, you do not harvest them while the pods are still green like edamame.

You continue to grow them, waiting until the beans are fully mature, hard, and dry.

In a home garden, this 'waiting' process is surprisingly difficult. When you see edamame with plump pods, you want to eat them, but if you want to harvest soybeans for natto or boiled beans, you must leave them until they are fully ripe.

You can also harvest some as edamame and grow the rest into soybeans. However, the more young pods you harvest, the lower the yield of fully mature soybeans will be.

Three points to be especially careful about in a home garden

1. Excessive moisture immediately after sowing

Soybeans absorb a lot of water when they germinate.

However, if you sow seeds in soil that is too saturated during the rainy season, the beans may absorb water too rapidly and become damaged, or the lack of oxygen in the soil may lead to uneven germination.

Since the soybean sowing season falls between late June and late July, which often overlaps with the rainy season, the importance of drainage measures is emphasized. If excessive moisture persists after growth, it can weaken not only the roots but also the function of the root nodules, potentially leading to yellowing or wilting of the leaves.

In a home garden, keep the following points in mind.

• Avoid areas where water tends to pool

• Make the ridges slightly higher

• Avoid sowing seeds right before heavy rain

• Do not add extra water when the soil is excessively damp

You cannot simply categorize soybeans as liking either water or dryness. They need water during germination, but they also need oxygen in the soil.

2. Bird damage after germination

Soybeans can be targeted by birds during the period from sowing until the cotyledons open.

This is because the beans themselves are large and remain conspicuous even just after sprouting. In home gardens, protecting them with non-woven fabric or bird netting immediately after germination helps stabilize growth.

However, once the leaves begin to spread, check to ensure the netting is not hindering the growth of the plants.

3. Dryness after flowering

While excessive moisture is a problem immediately after germination, dryness becomes the issue once the flowers start to bloom and the pods begin to swell.

Soybeans and edamame require a lot of water from just before flowering until the pods develop. If severe dryness continues during this period, flowers and young pods are more likely to drop, which can lead to poor pod filling.

The key is not to water every day just by looking at the surface of the soil, but to check the moisture level of the soil a few centimeters below.

If watering is necessary, do it in the morning before the temperature rises fully, and apply it slowly to the base of the plants.

Around July 10th, what stage are the soybeans in the field at?

There is no uniform answer nationwide

'What growth stage are soybeans at on July 10th?'

There is no single answer to this question that applies nationwide.
This is because soybean growth varies significantly based on the following conditions.

• Cultivation region

• Variety

• Date of sowing

• Whether sown in spring or after the wheat harvest

• Temperature

• Daylight hours

• Soil moisture

In a typical cultivation calendar for temperate regions, June to July is the period for germination and seedling establishment, July to August is the flowering period, August to September is the pod formation period, September to October is the seed enlargement period, and October to November is the maturation period.

Therefore, around July 10th, broadly speaking, the following types of plants can be found in the fields.

• Plants sown from May to early June
  → Stems and leaves have grown large, and early varieties are nearing flowering

• Plants sown in late June
  → Early growth stage with true leaves unfolding

• Regions where sowing occurs in July, such as after the wheat harvest
  → From just after sowing to the germination stage

• Edamame sown early in spring
  → Pods are swelling and nearing harvest time

In other words, while July 10th is a season when 'soybeans are in the fields' in many regions, there are fields with still-small seedlings, as well as fields where flowers are beginning to appear.

Soybeans bloom by sensing the 'length of the day'

Soybeans are generally 'short-day plants' whose flower bud formation progresses as days get shorter. However, the strength of this reaction varies depending on the variety.

Varieties intended for northern Japan have a nature that makes them prone to flowering even during relatively long days. On the other hand, many varieties suitable for southern regions have a strong reaction to shortening days.

July 10th is just after the summer solstice.

In Tokyo on July 10, 2026, sunrise is at 4:33 AM and sunset is at 6:59 PM, making the day about 14 hours and 26 minutes long. Although the day is still very long, it has been gradually shortening since the summer solstice.

However, this does not mean that 'flowers bloom all at once just because it is July 10th'.

Soybeans are influenced not only by day length but also by temperature, variety, and sowing time. It is best to think of early July as the season when the internal clock for flowering is gradually advancing.

July 10, 2026, is the boundary between the rainy season and midsummer

Part of western Japan has ended the rainy season, while eastern Japan is still in the middle of it

As of the Japan Meteorological Agency's announcement on July 8, 2026, northern Kyushu, Chugoku, and Kinki are all considered to have ended the rainy season 'around July 8th'.

On the other hand, as of the same date, the end of the rainy season had not been announced for Shikoku, Tokai, Kanto-Koshin, Hokuriku, or Tohoku. Note that the start and end of the rainy season are not phenomena that switch suddenly in a single day, and the Japan Meteorological Agency explains that they include a transition period of approximately five days before and after.

In other words, July 10, 2026, is a time with significant regional differences where:

• intense sunlight immediately after the end of the rainy season in parts of western Japan

• muggy heat at the end of the rainy season in Tokai and Kanto

• the influence of the rainy season front and humid air in northern Japan

all exist simultaneously.

For soybeans, it is a 'difficult time for water management'

Managing soybeans during this period is difficult because problems arise whether there is too much rain or if it suddenly clears up and becomes dry.

Heavy rain at the end of the rainy season causes problems such as:

• seeds being damaged

• uneven germination

• roots becoming oxygen-deprived

• reduced function of root nodules

• increased susceptibility to disease

.

On the other hand, if strong sunlight and high temperatures continue immediately after the rainy season ends, the topsoil will dry out rapidly. For plants that have entered the flowering stage, dryness may cause flowers or young pods to drop.

In the one-month forecast released by the Japan Meteorological Agency on July 9, 2026, it was indicated that temperatures in northern, eastern, and western Japan are likely to be significantly higher than average during the first half of the period from July 11 to August 10.

In a home garden, you should not decide on watering based solely on the calendar, but rather judge it by combining:

• rainfall

• soil moisture levels

• plant growth stage

• next day's temperature

• sunlight and wind

as factors.

Home Garden Checklist for around July 10th

If you have just sown seeds

• Is water pooling around the ridges?

• Is the soil becoming waterlogged?

• Are the newly sprouted shoots being targeted by birds?

• Has the soil hardened due to heavy rain?

Before and after heavy rain, prioritize checking drainage over additional watering.

If true leaves are increasing

• Are the bases of the plants becoming prone to falling over?

• Are they being deprived of light and wind by weeds?

• Are the leaves suddenly turning yellow?

• Is the soil moist not just on the surface, but also inside?

Perform light hilling as needed, but avoid tilling muddy soil immediately after rain, as it will cause the soil to become compacted.

When flowers begin to appear

• Observe the condition of the leaves and flowers in the morning

• Check if the soil has been completely dry for a long period

• Make watering a morning routine

• Avoid strenuous work during the high temperatures of the day

The fact that 'flowers have bloomed' is a signal that the plant is entering the stage of forming pods. From this period on, the need to be careful about water shortages increases.

When edamame pods are swelling

If you harvest them now, they are edamame.

If you leave them as they are and wait until the leaves turn yellow and the beans become hard, they will become dried soybeans.

In a home garden, it is easier to manage if you divide the purpose for each plant.

• This plant is for edamame

• This plant is for soybeans

• This one is for observation next year

If you mark them, you can prevent a situation where family members mistakenly harvest everything as edamame.

What kind of season is summer for natto as a food product?

We have looked at soybeans in the field so far, but for finished natto in the refrigerator, the story is a bit different.

Natto bacteria are active at around 40°C. However, commercially available natto is a food product that has been cooled after fermentation to stabilize its quality at low temperatures is.

Therefore, the heat of midsummer is not an 'environment that will make it ferment even more deliciously.' If left in a high-temperature place for a long time, the activity of the natto bacteria will become active again,

• The aroma becomes too strong

• An ammonia smell develops

• The beans become too soft

• The container expands

• Quality declines

These changes are likely to occur. It is important that natto is distributed and stored under refrigeration after production. When shopping in the summer, try not to leave natto in the car for a long time after purchasing it, and put it in the refrigerator as soon as you get home.

While the high temperatures of July help soybeans in the field grow, for finished natto, they are a factor that causes fermentation to progress too far. Even for the same natto, the meaning of temperature changes between the 'stage in the field' and the 'stage as a food product'.

Summary | Natto is a food that connects fields, microorganisms, and the seasons

Natto is not simply soybeans that have been softened and seasoned.

In the field, seeds absorb water, sprout, and spread their leaves. Root nodules form on the roots, supporting the growth of the soybean. As the days gradually shorten after the summer solstice, the plants bloom, form pods, and mature their beans according to the characteristics of each variety.

Those fully ripened soybeans are then steamed and handed over to the natto bacteria.

Natto bacteria multiply while utilizing the components of the soybeans, creating the aroma, flavor, and characteristic threads. Finally, by cooling it to stabilize the fermentation, it becomes the natto we eat every day.

July 10th, 'Natto Day,' is an anniversary born from a pun.

However, when viewed from the perspective of the seasons, early July is also the time when the raw soybeans are stretching their stems and leaves while receiving the rain of the rainy season, eventually heading toward flowering.

The single bean inside a pack of natto did not just suddenly appear there.

The rain of the rainy season, the summer sun, the air in the soil, the judgment of the person managing the field, and the work of invisible microorganisms. All of these are connected to form a single white thread.

Today, when you mix your natto, please take a moment to think about the summer soybean field that lies beyond those threads.

Main References

• Japan Natto Cooperative Society Federation, 'Natto Day' and 'How Natto is Made'

• Ministry of Education, Culture, Sports, Science and Technology, 'Standard Tables of Food Composition in Japan - Stringy Natto'

• Ministry of Agriculture, Forestry and Fisheries: 'Differences between edamame and soybeans', 'Natto and Tera-natto'

• NARO / Chugoku-Shikoku Regional Agricultural Administration Office: 'Cultivation techniques for high-yield soybeans in warm regions'

• Chiba Prefecture: 'Countermeasures against moisture damage in soybeans', 'Countermeasures against high temperatures and dryness for edamame'

• Japan Meteorological Agency: 'Start and end of the rainy season in 2026', 'One-month forecast'

• National Astronomical Observatory of Japan: 'Sunrise and sunset in Tokyo'

#NattoDay #Natto #Soybeans #Edamame #HomeGardening #FermentedFoods #SummerHomeGardening #VegetableGrowing #WeatherAndAgriculture #July10