JP6911133B2 - A method for producing steamed soybeans for natto, a method for producing natto using steamed soybeans for natto obtained by the method, and natto. - Google Patents

Description

The present disclosure describes a method for producing steamed soybeans for natto (hereinafter, may be abbreviated as "steamed soybeans"), a method for producing natto using steamed soybeans for natto obtained by the method, and natto obtained thereby. Regarding.

Natto, which is fermented soybeans with Bacillus natto (a type of Bacillus subtilis), is an ancient Japanese fermented food that is rich in soybean protein and has high nutritional value. In addition to its nutritional value, natto is one of the daily foods of Japanese people because of its high palatability.
Furthermore, in recent years, it has been reported that natto has various health-promoting effects such as probiotic action, antibacterial action, and functional ingredients, and it is a food that is expected to increase in demand.

Natto has a unique flavor that is presented by complexly containing various aroma components and umami components produced by fermentation by natto bacteria, but users who are not accustomed to eating natto when the bitterness and harsh taste are too strong. Preference is reduced. In addition, it is preferable that the appearance of natto has a natural light brown color, and even if it is excessively colored due to heating or the like during production, consumer preference is reduced.

In the production of natto, a steaming step of softening soybeans by heating soybeans sufficiently immersed in water in a steaming facility is generally used before inoculation of natto bacteria. However, depending on the varieties of soybeans used as raw materials and storage conditions, soybeans are often overcolored in the process of softening to a hardness suitable for producing natto by steaming. Techniques for adjusting the color and hardness of soybeans are known.

For example, in Patent Document 1, by spraying a predetermined amount of hot water after steaming soybeans, the coloring component on the surface of soybeans adhered in the steaming step is removed without causing peeling or crushing of the soybeans, and it is appropriate. A method for producing steamed soybeans for natto having softness is disclosed.
Further, in Patent Document 2, by adding water after heating the soybeans and before the steaming step, the hardness of the steamed soybeans is high, and it is possible to reduce crushing and generation of broth when filling the container with the steamed soybeans. A method for producing steamed soybeans for natto is disclosed.

However, the production method of Patent Document 1 has a problem in the publication of Patent Document 2 that soybeans are steamed for a long time before water addition, so that the color of soybeans becomes dark and black, and soybeans cannot be sufficiently decolorized by water addition. Has been pointed out.
Further, even if the production method of Patent Document 2 is used, the coloring of soybean is not sufficiently suppressed, and further, the production methods of Patent Documents 1 and 2 all have the bitter taste and the harsh taste of natto produced using steamed soybean. The removal was not enough.

In Patent Document 3, the steaming step is divided into a first steaming step and a second steaming step, the steaming temperature of the first steaming step is higher than that of the second steaming step, and the watering step is the first steaming step. A method for producing steamed soybeans for natto is disclosed, in which natto with a plump texture can be obtained while ensuring the amount of water in the steamed soybeans by carrying out the process between the second steaming steps.

The claims of Patent Document 3 specify that the steaming temperature T1 of the first steaming step is 125 ° C. to 140 ° C. and the steaming temperature T2 of the second steaming step is 105 ° C. to 120 ° C.
Then, paragraph 0012 of the specification of Patent Document 3 states that the first and second steaming steps are carried out under pressurized conditions according to the temperature, respectively, and the pressure of the first steaming step is 230 to 230. It is described that the pressure in the second steaming step is 370 Pa and the pressure is 120 to 200 Pa.
Incidentally, in Example 1, "131.5 ° C. (pressure 285 Pa), 5 minutes first steaming step" and "115 ° C. (pressure 170 Pa), 45 minutes second steaming step" are described.
However, such temperature and pressure conditions could not be realized (implemented).
That is, the first steaming step was steamed at a pressure of 285 Pa for 5 minutes, but the temperature did not reach 131.5 ° C. On the contrary, the first steaming step was steamed at a temperature of 131.5 ° C. for 5 minutes, but the pressure did not reach 285 Pa. The same was true for the second steaming step.
Therefore, it was considered impossible to solve the problem by the method of Patent Document 3 (see Comparative Example 3 described later).

Japanese Unexamined Patent Publication No. 2004-24197 JP-A-2010-09032 Japanese Unexamined Patent Publication No. 2009-232753

An object of the present disclosure is to provide a method for producing steamed soybeans for natto, which not only has a bright color and an appropriate hardness when natto is produced, but also has less bitterness and harsh taste of natto. ..
Another object of the present disclosure is to provide a method for producing natto, which not only has a bright color and an appropriate hardness, but also can produce natto having less bitterness and harshness of natto.
Furthermore, an object of the present disclosure is to provide natto which is bright in color and has appropriate hardness, and also has less bitterness and harsh taste.

As a result of diligent studies to solve the above-mentioned conventional problems and examination of a feasible steaming method for soybeans used for producing natto, the present discloser surprisingly puts a watering process in between. When the steaming step is performed before and after that, the above-mentioned conventional problems are solved by performing the steaming step before water addition under specific conditions (mild) so as to be milder than the steaming step after water addition. We have found that this is possible, and based on this finding, we have completed this disclosure.

That is, the present disclosure relates to the following.
A method for producing steamed soybeans for natto.
(A) The first steaming process in which soybeans are steamed under pressure of 5 minutes or more, 15 minutes or less, 0.02 MPa or more and 0.18 MPa or less.
(B) The first watering step of adding water after the first steaming step,
(C) A second steaming step of pressurizing and steaming at a pressure of 10 minutes or more, 90 minutes or less, 0.06 MPa or more and 0.30 MPa or less after the first water addition step, and the first goal of the first steaming step. The present invention relates to a method for producing steamed soybeans for natto, wherein the pressure (P ₁ ) and the second target pressure (P ₂ ) in _{the second steaming step satisfy the relational expression of P 1} <P _2.

According to the present disclosure, by carrying out two steaming steps before and after the first watering step under specific conditions, the color is bright and moderately hard when natto is made as in the conventional steaming method with watering. It is possible to produce steamed soybeans for natto, which not only has hardness but also has less bitterness and harsh taste of natto.
Further, according to the present disclosure, there is provided a method for producing natto, which is capable of producing natto which is bright in color, has appropriate hardness, and has less bitterness and harsh taste.

Therefore, according to the present disclosure, natto with less bitterness and harsh taste can be easily produced without changing any special production equipment. Here, "bitter taste" is a taste that makes the tongue feel unpleasant and refers to an unfavorable taste centered on burnt taste. In natto, a plurality of tastes such as soybean-derived polyphenols, melanoidins, and soybean peptides are used. It is believed that the substance produces a "bitter taste".
In addition, "egu taste" is a taste that makes the tongue feel rough, and refers to an unfavorable taste centered on astringent taste. It is thought that "egu taste" is produced.
According to the present disclosure, since natto with suppressed such "bitter taste" and "egg taste" is provided, consumers who are reluctant to eat natto feel resistance to eating natto. It will be reduced.
It is presumed that in the steamed soybeans for natto produced by the method of the present disclosure, the taste substances that cause "bitter taste" and "egg taste" are efficiently removed by the first watering step without using special equipment. It can be effectively used as a technique for producing higher quality natto.

The present disclosure will be described in detail below.
The first to fourth aspects of the present disclosure relate to a method for producing steamed soybeans for natto.
In the first to fourth aspects of the present disclosure, the methods for producing steamed soybeans for natto are basically common. Therefore, the common manufacturing method will be collectively described below.

In the first to fourth aspects of the present disclosure, the method for producing steamed soybeans for natto divides the steaming step into two steps, a first steaming step and a second steaming step, between the first steaming step and the second steaming step. It is characterized by the addition of water to the natto.
The method for producing steamed soybeans for natto will be illustrated below, but in the first to fourth aspects of the present disclosure, any production method usually used for the method for producing steamed soybeans for natto will be used. It can also be used by the method.

1. 1. Method for Producing Steamed Soybean for Natto In the method for producing steamed soybean for natto of the present disclosure, any raw material soybean can be used as long as it can be used for producing ordinary steamed soybean for natto.
As the raw material soybean, for example, whole soybean, half-split soybean, crushed soybean (raw material of ground natto), defatted soybean and the like can be used. Among these raw material soybeans, medium-grain and large-grain soybeans used in the production of high-quality threaded natto are particularly preferable.
These raw material soybeans can be used as they are, but it is common to use dried soybeans (dried products).

The raw material, dried whole soybeans, etc., is washed with water and then immersed in water according to a conventional method. For example, the raw soybean can be immersed in water at about 5 ° C. to 25 ° C. for about 5 to 24 hours. The soybeans obtained by this dipping step are subsequently subjected to a steaming step.

In the steaming step, a steaming facility generally used for producing steamed soybeans for natto may be used, but a steaming pot of a type capable of rotating in the vertical direction is preferable. As such a steaming pot, for example, there is one disclosed in Japanese Patent Application Laid-Open No. 11-225700.

In the production of steamed soybeans for natto of the present disclosure, the steaming step is divided into two steps before and after the first steaming step and the second steaming step, and a watering step (first watering) is performed between the first steaming step and the second steaming step. Process) is carried out.

Both the first steaming step and the second steaming step can be adopted by any general steaming method, and a typical steaming method is illustrated below.
First, pressurized steam is blown into the steaming pot containing soybeans, and the liquid discharge pipe provided at the bottom of the steaming pot is opened until the temperature inside the steaming pot reaches about 100 ° C. Discharge. When the temperature inside the steaming kettle reaches about 100 ° C, the valve of the liquid discharge pipe is closed and the predetermined pressure inside the kettle (hereinafter also referred to as "target pressure") and the predetermined temperature (hereinafter also referred to as "target temperature"). ) Is reached, and the pressurized steam is continuously blown.
After that, the steam is continuously blown while adjusting so that the above-mentioned predetermined pressure and the predetermined temperature are maintained, and this condition is maintained for a certain period of time (hereinafter, this holding time is also referred to as "steaming time").
Next, the blowing of the pressurized steam is stopped, the pressure and temperature in the steaming pot are lowered to about 100 ° C. at normal pressure, the liquid discharge pipe is opened, and the steaming process is completed.

It is essential that the first target pressure (P ₁ ) in the first steaming process and the second target pressure (P ₂ ) in _{the second steaming process satisfy the relational expression of P 1} <P ₂ , and the above relational expression is satisfied. In addition, the first target pressure P ₁ can be set to a desired pressure in the range of 0.02 MPa or more and 0.18 MPa or less, and the second target pressure P ₂ can be set to a desired pressure in the range of 0.06 MPa or more and 0.30 MPa or less.
That is, it is preferable that the second target pressure P ₂ is higher than the first target pressure P _1.

However, since the pressure is usually controlled by adjusting the amount of steam blown in the steaming step, a slight pressure fluctuation is unavoidable. Usually, especially for a few minutes immediately after the start of steaming, a little excess steam is blown into the steaming pot for the purpose of quickly shifting from the normal pressure state to the pressurized state at a predetermined pressure, so that the pressure is higher than the predetermined pressure. High pressure is inevitable.
In the present disclosure, for example, even if there is a slight pressure fluctuation (within ± 40%, preferably within ± 20%) and the pressure fluctuation is within a short time (for example, within 2 minutes, preferably within 1 minute). For example, it means that the pressure value was maintained at the above level.

In addition, the first target pressure P ₁ and the second target pressure P ₂ are second in the present disclosure so that the removal of unnecessary soybean-derived components can be promoted by adding water by applying an appropriate heat load before adding water. As shown, it is preferable to set so as to satisfy _{1.1 × P 1} ≦ P _2. From the viewpoint of optimizing the heat load before water addition, more preferably, _{the relational expression of 1.7 × P 1} ≤ P ₂ is satisfied, that is, the second target pressure P ₂ is 1.7 times or more the first target pressure P _1. It is more preferable to adjust so as to be.

Further, the first target pressure P ₁ is preferably set to a lower limit of 0.02 MPa or more in order to soften the soybeans soaked by applying an appropriate heat load before adding water. Further, in order to avoid excessive heating of the soybeans soaked before water addition, it is preferable to adjust the upper limit value to 0.18 MPa or less, preferably 0.14 MPa or less, more preferably 0.12 MPa or less.
On the other hand, since the second target pressure P ₂ sufficiently softens soybeans, the lower limit is preferably 0.06 MPa or more, preferably 0.09 MPa or more, and more preferably 0.12 MPa or more. Further, in order to maintain the texture of natto produced using steamed soybeans for natto, the second target pressure P ₂ can be set to an upper limit of 0.30 MPa or less, preferably 0.28 MPa or less, and more preferably 0.27 Mpa or less. preferable.

In the present disclosure, in order to avoid excessive heating of the soybeans soaked before water addition, the first target temperature (T ₁ ) of the first steaming step and the second target temperature (T ₂ ) of the second steaming step are referred to in the present disclosure. As shown in the third, it is preferable _{that T 1} <T _2. That is, it is preferable that the second target temperature T ₂ is higher than the first target temperature T _1.

In the present disclosure, in order to prevent the surface of the soaked soybeans from being excessively colored due to an excessive heat load before water addition, the first steaming time (M ₁ ) of the first steaming step and the second steaming time (M 1) of the second steaming step ( For M ₂ ), _{it is preferable that M 1} <M _2, and more preferably 1.8 x M ₁ ≤ M ₂ .
That is, as shown in the fourth aspect of the present disclosure, it is preferable that the second steaming time M _{2 is longer than the first steaming time M 1} , and in particular, M ₂ is 1.8 times or more that of M _1. Is more preferable.
The first steaming time M ₁ and the second steaming time M ₂ can be any steaming time as long as they have the above relational expression. However, in order to avoid an excessive heat load before adding water, the first steaming time M _{1 can be} preferably 5 minutes or more and 15 minutes or less, and more preferably 5 minutes or more and 10 minutes or less. The second steaming time M ₂ is preferably 10 minutes or more and 90 minutes or less, more preferably 12 minutes or more and 40 minutes or less, and further preferably 12 minutes or more and 30 minutes in order to sufficiently soften the beans while suppressing the coloring of the surface of the soaked soybeans. It can be less than a minute.

Therefore, as a whole, it can be said that the first steaming step performed before watering is a milder condition than the second steaming step performed after watering.

Next, the first water addition step performed between the first steaming step and the second steaming step will be described.
The temperature of the water added in the first water addition step can be, for example, 10 ° C. or higher and 100 ° C. or lower. From the viewpoint of promoting the removal of unnecessary soybean-derived components, the temperature can be preferably 25 ° C. or higher and 95 ° C. or lower, and more preferably 50 ° C. or higher and 90 ° C. or lower.

The amount of water added at the time of watering can be, for example, 5% or more and 80% or less with respect to the weight of soybeans, but is preferably 15 from the viewpoint of promoting the removal of unnecessary soybean-derived components by watering. It can be% or more and 70% or less, more preferably 30% or more and 60% or less.

Any general method can be adopted as the method of watering, and either the method of watering directly in the steaming pot or the method of watering after transferring to another facility may be carried out. In order to efficiently permeate the steamed soybeans with water, a predetermined amount of water is added to the depressurized steaming pot through a pipe or the like, and then the water is added by rotating the steaming pot in the vertical direction. Is preferably evenly contacted with the steamed soybeans in the kettle.
The hydrated water is drained after the hydration and before the second steaming step. As a drainage method, for example, a method of draining water through a pipe at the bottom of the steaming pot can be exemplified.

After the first water addition step is performed as described above, the second steaming step is performed as described above.
That is, after draining water through the piping at the bottom of the steaming pot, pressurized steam is blown into the steaming pot again, and the liquid discharge pipe provided at the bottom of the steaming pot is opened until the temperature inside the steaming pot reaches about 100 ° C. Then, let the steamed soybean juice drain. When the temperature inside the steaming pot reaches about 100 ° C., close the valve of the liquid discharge pipe and continue blowing pressurized steam until the inside of the pot reaches the target pressure and the target temperature.
After that, while adjusting so that the above target pressure and target temperature are maintained, steam is continuously blown and steamed for a predetermined time.
Next, the blowing of the pressurized steam is stopped, the pressure and temperature in the steaming pot are lowered to about 100 ° C. at normal pressure, the liquid discharge pipe is opened, and the second steaming step is completed.

Next, a fifth aspect of the present disclosure will be described. A fifth aspect of the present disclosure is characterized in that water is added (second water addition step) after the second steaming step in producing steamed soybeans for natto.

The temperature of the water added in the second water addition step can be, for example, 10 ° C. or higher and 100 ° C. or lower. From the viewpoint of promoting the removal of unnecessary soybean-derived components, the temperature can be preferably 25 ° C. or higher and 95 ° C. or lower, and more preferably 50 ° C. or higher and 90 ° C. or lower.

Further, in the second water addition step, the amount of water added at the time of water addition is, for example, 5% or more and 80% or less, preferably 5% or more and 50% or less, and more preferably 5% or more and 30% or less with respect to the weight of the soaked soybeans. can do.
However, in general, when water is added using warm water having the same temperature, water is added only in the first watering step by dividing the watering into a first watering step and a second watering step. Even if the integrated value of the amount of water added is reduced, a sufficient effect can be obtained for improving the quality of steamed soybeans for natto.
Therefore, from the viewpoint of efficiently removing unnecessary soybean-derived components by watering, it is better to set the integrated value of the amount of water added in the first watering step and the second watering step to 10% or more and 65% or less. It is preferable, more preferably 15% or more and 60% or less, and particularly preferably 15% or more and 50% or less.

When producing steamed soybeans for natto on an industrial scale, large-scale equipment such as a boiler is required to heat the water to be added. The introduction cost of such equipment and the energy cost at the time of operation account for a large proportion of the manufacturing cost, and the present disclosure in which the water addition process is divided into two is industrially useful.

In the second watering step, any general method can be adopted as the watering method, and either the method of directly watering in the steaming pot or the method of watering after transferring to another facility is carried out. You may. In order to efficiently permeate the steamed soybeans with water, a predetermined amount of water is added to the depressurized steaming pot through a pipe or the like, and then the water is added by rotating the steaming pot in the vertical direction. Is preferably evenly contacted with the steamed soybeans in the kettle.
The hydrated water is drained after hydration and before the inoculation step described later. As a drainage method, for example, a method of draining water through a pipe at the bottom of the steaming pot can be exemplified.

The second watering step can be carried out at any time after the completion of the second steaming step and before the inoculation step described later, but since it is preferable to add water before the coloring of soybeans progresses by heating, the second steaming step is performed. It is preferable to add water within 1 hour after the completion of the step, further preferably within 30 minutes, and particularly preferably within 10 minutes.

Next, a sixth aspect of the present disclosure will be described.
A sixth aspect of the present disclosure relates to a method for producing natto, and in producing natto, steamed soybeans for natto produced by the production method according to any one of the first to fifth aspects of the present disclosure described above are used. It is characterized in that the soybean product temperature is maintained in the range of 37 ° C. or higher and 53 ° C. or lower for 10 hours or more and 24 hours or less in the natto fermentation process.

2. Method for producing natto In the method for producing natto according to the sixth aspect of the present disclosure, natto bacteria are inoculated into steamed soybeans for natto produced by the method according to any one of the first to fifth aspects of the present disclosure. Then, it is fermented and aged to produce natto.

2-1. Inoculation Step of Bacillus natto First, a "inoculation step" of inoculating Bacillus natto is performed on steamed soybeans for natto produced by any of the first to fifth aspects of the present disclosure.
The state of Bacillus natto to be added is not particularly limited, but in order to prevent contamination by various germs, it is preferable to use a spore state that can be directly inoculated into high-temperature soybean immediately after steaming.

As the natto bacterium, any natto bacterium can be adopted. For example, general commercial bacteria Miyagino bacterium (trade name: Miyagino natto bacterium) (manufactured by Miyagino Seisakusho Co., Ltd.), Takahashi bacterium (trade name: natto element) (manufactured by Yuzo Takahashi Laboratory Co., Ltd.), Naruse bacterium (manufactured by Yuzo Takahashi Laboratory Co., Ltd.) Product name: Powdered natto bacteria) (manufactured by Naruse Fermentation Chemical Laboratory) and its mutants can be used.

In the inoculation of the above-mentioned natto bacteria into steamed soybeans, fermentation is carried out uniformly. Therefore, it is desirable to add the soybeans and the like so that the natto bacteria are uniform by inoculation or spraying, and then mix the soybeans. Preferably, it is preferable to prepare a spore suspension of the above-mentioned Bacillus natto and add it in a liquid state for use.

Here, as the spore suspension, a culture solution in which the above-mentioned Bacillus natto is cultured in a liquid medium having a component suitable for spore formation can be used.
The components of the liquid medium are particularly any liquid medium containing medium components such as a carbon source, a nitrogen source, and inorganic salts that enable spore formation and growth of Bacillus natto and are usually used for culturing Bacillus natto. It is not limited, and may be a synthetic medium or a natural medium.

The number of natto bacteria to be inoculated is not particularly limited depending on the bacterial concentration according to the conventional method, but is usually 10 ³ to 10 ⁶ per 1 g of steamed soybeans. The product temperature of soybeans at the time of inoculation is not particularly limited, but it is preferable to inoculate the soybeans at a high temperature of about 55 to 95 ° C. in order to prevent contamination by various germs at the time of inoculation.

The soybeans inoculated with the above natto bacteria are filled in individual containers for one to several servings (usually containers that can be filled with steamed soybeans of about 20 g to 100 g), and then fermentation is performed in the individual containers, which will be described later. Is preferable. In addition, as a traditional method, it is also possible to fill the boiled straw straw with it.
It is also possible to ferment in a container with a volume of several liters, but considering that when the volume value with respect to the surface area becomes large, it becomes difficult for the temperature change to be transmitted to the beans in the center, a large container is used. It is not preferable to use it.

Here, as the individual container, any container can be used as long as it can be filled with soybeans. Specifically, polystyrene-modified polyolefin resins commonly used in natto, polystyrene, high-impact polystyrene, polystyrene resins such as styrene-ethylene copolymers, polyethylene, polypropylene, ethylene-vinyl acetate copolymers, etc. A container formed of a foam sheet made of various synthetic resins such as a polyolefin resin and a polyester resin such as polystyrene terephthalate, and a cup-shaped paper container can be used.
Further, the shape of the container is preferably a shape that allows direct stirring (stirring) for eating using the container.
Further, after fermentation, those having a form that can be sealed with a lid or a sealing are preferable.

2-2. Natto Fermentation Step In the sixth aspect of the present disclosure, after performing the inoculation step of inoculating steamed soybeans with Bacillus natto, the natto fermentation step is carried out to ferment natto.
This natto fermentation step is usually carried out by putting the steamed soybeans into a fermentation room or the like (referring to a fermentation room, a constant temperature room, a constant temperature device, an incubator, etc.).

In the sixth aspect of the present disclosure, fermentation is carried out by maintaining the soybean product temperature in a specific temperature range for a predetermined time from the start of fermentation.
Specifically, in the fermentation of natto, the soybean product temperature is substantially kept in the temperature range of 37 ° C. or higher and 53 ° C. or lower, preferably in the temperature range of 40 ° C. or higher and 50 ° C. or lower, preferably 10 hours or more and 24 hours or less. Is performed by maintaining for 14 hours or more and 20 hours or less.
When the soybean product temperature is maintained in the above-mentioned specific temperature range and fermentation is carried out for the above-mentioned predetermined time, the inoculated Bacillus natto exhibits a fermentation characteristic in which the fermentation action is generally active. In general, the addition of natto-like flavor, the addition of stringiness, and the formation of bacterial membranes are promoted.

In the method for producing natto shown in the sixth aspect of the present disclosure, the room temperature of the fermentation chamber or the like is maintained in a temperature range of 30 ° C. or higher and 50 ° C. or lower, preferably 35 ° C. or higher and 50 ° C. or lower. , It becomes possible to maintain the product temperature of soybeans during fermentation in the above temperature range. This is because the product temperature of beans rises due to the heat of fermentation and is maintained in the above-mentioned natto fermentation temperature range.

In addition, "maintaining the product temperature of soybeans in a temperature range of 37 ° C or higher and 53 ° C or lower" does not mean that the temperature does not completely deviate from the temperature range. For example, if the temperature is within 2 ° C, preferably within 1 ° C) and for a short time (for example, within 10 minutes, preferably within 5 minutes), the product temperature may be out of the temperature range. , Means that the fermentation conditions are satisfied.
In the fermentation in the present disclosure, it is possible to carry out fermentation under conditions in which the temperature is raised and / or lowered in the middle as long as the temperature is included in the above temperature range.

In the present disclosure, as described above, it is essential to maintain the product temperature of soybeans inoculated with Bacillus natto in the range of 37 to 53 ° C. for a predetermined time. Normally, natto bacteria are inoculated on soybeans with a high temperature of about 55 to 95 ° C. Therefore, if the soybeans are moved to a fermentation room immediately after inoculation, the soybean product temperature exceeds 53 ° C for a long time. If the soybeans are left at room temperature and then moved to a fermentation room, etc., it is assumed that the soybean product temperature is less than 37 ° C. However, normally, the temperature control capacity of the fermentation chamber for inoculated soybeans filled in individual containers is sufficiently large, and the soybean product temperature should be adjusted to the range of 37 to 53 ° C within 30 minutes to 2 hours after moving to the fermentation chamber. Is possible.
Therefore, in the present disclosure, the time required to maintain the soybeans inoculated with Bacillus natto in the range of 37 to 53 ° C is the time required for the soybeans to reach the range of 37 to 53 ° C in the fermentation chamber. It can be set as appropriate after consideration.

Here, examples of the room and apparatus for performing fermentation include a fermentation chamber, a constant temperature room, a constant temperature device, an incubator, and the like, which are referred to as "fermentation chambers and the like" as described above.
As a room or apparatus for performing such fermentation, when the aging step described later is also carried out with the same apparatus, one that can be adjusted in a temperature range of about 0 ° C. to 55 ° C. is used.

Specifically, for example, (i) means for controlling the temperature by using a fermentation chamber or the like having a temperature raising and cooling function can be adopted. When the means is adopted, the temperature condition can be adjusted accurately. In addition, when the fermentation temperature is changed significantly during fermentation, (ii) fermentation is first performed in a constant temperature zone set in the fermentation temperature zone, and after a predetermined time elapses, the constant temperature zone set in a different temperature zone, etc. By moving the beans together with the container, the temperature of the beans can be adjusted. When the temperature control means is adopted, by automating the movement of the container, waste of temperature control can be eliminated and mass production of natto can be efficiently performed.

2-3. Aging step After the fermentation step is completed, in order to suppress stringing deterioration due to secondary fermentation and quality deterioration such as ammonia production, it is usually 3 ° C or higher and lower than 10 ° C, preferably 3 ° C or higher and lower than 8 ° C, more preferably 3 The production of natto is completed by aging at a low temperature of ℃ or more and less than 6 ℃ for 6 hours to 3 days, preferably 8 hours to 2 days, more preferably about 24 hours.

2-4. Storage When storing natto after production, it is preferably 3 ° C or higher and lower than 10 ° C, more preferably, in order to suppress deterioration of the stringiness of natto due to secondary fermentation and quality deterioration such as precipitation of tyrosine. It is preferable to store in a refrigerated state so that the product temperature reaches 3 to 6 ° C.
As described above, natto can be produced according to the sixth aspect of the present disclosure.
The natto produced through the above steps is a natto having the properties shown in the seventh aspect of the present disclosure described below.

A seventh aspect of the present disclosure is the method for producing steamed soybeans for natto according to the first to fifth aspects of the present disclosure, wherein the surface color of the steamed soybeans for natto after production is the following relational expression 1 The present invention relates to a method for producing steamed soybeans for natto, which is characterized by satisfying the above conditions.

Relational expression 1
37 ≤ L ^* ≤ 53
8.0 ≤ a ^* ≤ 12.0
3.9 ≤ L ^* / a ^* ≤ 6.7
L ^* : L value (brightness) of steamed soybeans for natto
a ^* : A value (redness) of steamed soybeans for natto

In producing the steamed soybeans for natto according to the seventh aspect of the present disclosure, the color of the surface of the steamed soybeans for natto after production is adjusted so as to fall within the range of the above-mentioned relational expression 1. As a result, it is possible to produce highly palatable natto with a bright color and suppressed bitterness and harsh taste when natto is produced, without using special equipment.

In the method for producing steamed soybeans for natto shown in the seventh aspect of the present disclosure, it is said that the surface color of the steamed soybeans for natto after production suppresses bitterness and harshness while having appropriate brightness. From the viewpoint, it is preferable to satisfy the above-mentioned relational expression 1.

Further, in the method for producing steamed soybeans for natto shown in the seventh aspect of the present disclosure, the color of the surface of the steamed soybeans for natto after production satisfies the following relational expression 2 in order to improve the production efficiency. It is preferable, and it is more preferable that the relational expression 3 is satisfied.

Relational expression 2
37 ≤ L ^* ≤ 53
8.0 ≤ a ^* ≤ 12.0
4.3 ≤ L ^* / a ^* ≤ 6.0
L ^* : L value (brightness) of steamed soybeans for natto
a ^* : A value (redness) of steamed soybeans for natto

Relational expression 3
37 ≤ L ^* ≤ 53
8.0 ≤ a ^* ≤ 12.0
4.6 ≤ L ^* / a ^* ≤ 5.5
L ^* : L value (brightness) of steamed soybeans for natto
a ^* : A value (redness) of steamed soybeans for natto

When adjusting the surface color of steamed soybeans for natto, use a general colorimeter (for example, SD-3000 manufactured by Nippon Denshoku Industries Co., Ltd.) in the L ^* a ^* b ^* colorimeter measurement mode. The measurement can be performed and the obtained L ^* (L value) and a ^* (a value) can be used as indicators for adjustment. The color can be measured with either natto in the middle of the steaming process or steamed soybeans for natto cooled after the steaming process, but after the steaming process is completed, it reflects the quality of the steamed soybeans for natto during the production of natto. It is preferable to evaluate with steamed soybeans for natto.
In the case of industrial production of steamed soybeans for natto, a pretest is performed on a small scale to ensure that the steamed soybeans for natto after the steaming process are included in the above relational expression. It is more efficient to carry out production.

Next, the eighth aspect of the present disclosure relates to natto produced using steamed soybeans for natto whose surface color of the steamed soybeans for natto after production satisfies the above relational expression 1.
That is, the natto according to the eighth aspect of the present disclosure is a natto produced using steamed soybeans for natto whose surface color of the steamed soybeans for natto after production satisfies the above relational expression 1, and has specific characteristics. It is a natto that is unique in that it uses steamed soybeans for natto.
In consideration of production efficiency, it is more preferable that the surface color of the steamed soybeans for natto after production is natto produced using steamed soybeans for natto satisfying the above relational expression 2, and further the relational expression 3 is applied. It is more preferable to satisfy.
According to the eighth aspect of the present disclosure, the surface color of the steamed soybeans for natto after production is produced using steamed soybeans for natto satisfying the above relational expression 1, so that no special equipment is used. When natto is produced, the color is bright, and natto with high palatability with suppressed bitterness and harsh taste is provided.

Further, a ninth aspect of the present disclosure relates to a method for suppressing the bitterness of natto, which comprises using steamed soybeans for natto whose surface color of the steamed soybeans for natto after production satisfies the above relational expression 1.
In consideration of production efficiency, it is more preferable to use steamed soybeans for natto whose surface color of the steamed soybeans for natto after production satisfies the above relational expression 2, and it is more preferable to satisfy the relational expression 3.
According to the ninth aspect of the present disclosure, the surface color of the steamed soybean for natto after production is produced by using the steamed soybean for natto satisfying the above relational expression 1 without using special equipment. The color of natto is bright, and bitterness and harshness can be suppressed.

The present disclosure is as described above.
In the examples of the present disclosure described later, amino acids and their measuring methods and various saccharides and their measuring methods are as follows.

2-5. Amino Acid Measurement Method In this disclosure, "total amount of amino acids" means the total value of the amino acids on the right (in parentheses) contained in the sample (aspartic acid, threonine, serine, glutamic acid, proline, glycine, alanine, valine, Cistine, methionine, isoleucine, leucine, tyrosine, phenylalanine, histidine, lysine, tryptophan, arginine).
Further, in the present disclosure, the "basic amino acid amount" means the total value of lysine, histidine, and arginine among the above amino acids.

The method for measuring amino acids is not particularly limited, but HPLC analysis by the post-column derivative method using ninhydrin can be adopted. As a measuring device, a high-speed amino acid analyzer L-8900 (Hitachi High-Tech Science Corporation) can be exemplified. The method of sample pretreatment is not particularly limited, but when steamed soybeans or natto are used as samples, 50% ethanol is added to the crushed product crushed in a dairy pot or homogenizer, ultrasonic treatment is performed, and the liquid phase is performed. An example is a method in which an extract obtained by extracting a water-soluble component is filtered, further diluted with a lithium citrate buffer solution, and used for measurement.

2-6. Methods for Measuring Various Sugars In this disclosure, "total sugar amount" means the total value of the sugars on the right (in parentheses) contained in the sample (glycerin, fructose, glucose, sucrose, maltose, raffinose, stachyose).

The method for measuring saccharides is not particularly limited, and for example, HPLC analysis using a column for polymer-based hydrophilic interaction chromatography and a differential refractometer can be adopted. Examples of columns that can be used include HILICpak VG-50 4E (Showa Denko KK). The method of sample pretreatment is not particularly limited, but when steamed soybeans or natto are used as samples, 50% ethanol is added to the crushed product crushed in a dairy pot or homogenizer, and sonication is performed to perform the liquid phase. An example is a method in which an extract obtained by extracting a water-soluble component is filtered, further diluted with water, and used for measurement.

Hereinafter, the present disclosure will be described with reference to examples, but the scope of the present disclosure is not limited thereto.

I. Examples 1 to 3 and Comparative Examples 1 to 4
Steamed soybeans and natto are produced under the various production conditions shown in Table 1, and the obtained steamed soybeans and natto are measured for amino acids and various sugars, and the surface color, hardness, quality, etc. are evaluated. (Examples 1 to 3 and Comparative Examples 1 to 4). These conditions and results are shown in Table 1, Table 2, and Table 3.
The preparation of steamed soybeans, the preparation of natto, the analysis of amino acids, the analysis of various sugars, the measurement of hardness, the measurement of surface color, and the quality evaluation were carried out as follows.

I-1. Preparation of Steamed Soybeans The steamed soybeans of Examples 1 to 3 and Comparative Examples 1 to 4 were prepared by the following methods.
First, the raw material, dried whole soybeans, was lightly washed with water and immersed in water at room temperature (about 25 ° C.) for a whole day and night to allow the soybeans to absorb water sufficiently, and then drained with a gold colander.
Subsequently, in Examples 1 to 3 and Comparative Examples 1, 3 and 4, the soybeans were subjected to the first steaming step. Specifically, put the soaked soybeans in a metal container, put them in a steaming pot (Harada Sangyo's test steaming pot), and set the pressure (P ₁ ) and steaming time (M ₁ ) shown in Table 1. It was served for steaming.
On the other hand, in Comparative Example 2, the first steaming step was not carried out, and only heating was performed until the temperature inside the pot reached 90 ° C.
_{Under the above conditions, the steaming temperature (T 1} ) in the first steaming steps of Examples 1 to 3 and Comparative Examples 1, 3 and 4 was as shown in Table 1.
Regarding Comparative Example 3 which was carried out assuming Patent Document 3, as described above, the steaming temperature and the steaming pressure could not be adjusted within a predetermined range, and it was impossible to carry out the subsequent steps. Not implemented.

Subsequently, the soybeans after the completion of the first steaming step were subjected to the first watering step except for Comparative Example 3. Specifically, after the completion of the first steaming, depressurization is performed, the lid of the steaming pot is opened, and warm water at about 75 ° C. is added to the ratio shown in "Amount of water added"(%; mass ratio of steamed soybeans) in Table 1. Water was poured into the container containing the soaked soybeans. Approximately 1 minute after water injection, warm water hydrated with a metal colander was drained to complete the first hydration step.

The soybeans after the completion of the first watering step were further subjected to the second steaming step. Specifically, after closing the lid of the kettle tightly, set the pressure (P ₂ ), steaming time (M ₂ ), and steaming temperature (T ₂ ) shown in Table 1 to be used for the second steaming. This completes the steaming process.
_{Under the above conditions, (T 2} ) of Examples 1 to 3 and Comparative Examples 1, 2 and 4 are as shown in Table 1.
The quality of steamed soybeans was evaluated by using a sample refrigerated at 4 ° C. for 3 days immediately after production by the above method.

II. Example 4
The steamed soybean of Example 4 was prepared by the following method.
First, the raw material, dried whole soybeans, was lightly washed with water and immersed in water at room temperature (about 25 ° C.) for a whole day and night to allow the soybeans to absorb water sufficiently, and then drained with a gold colander.
Subsequently, under the conditions shown in Table 4, after being subjected to the first steaming step, the first watering step, and the second steaming step, water was further added immediately after the completion of the second steaming step (second watering step).
The watering and steaming methods and the quality evaluation of the steamed soybeans were carried out in the same manner as in Examples 1 to 3 and Comparative Examples 1, 2, and 4.

II-1. Natto fermentation The steamed soybeans in a high temperature state immediately after steaming in the above procedure, and the steamed soybeans drained immediately after the second water addition step in Example 4, were fermented with natto according to the following procedure.
First, a Bacillus natto starter diluted with water so as to contain about 5,000 Bacillus natto per gram of steamed soybean was added and lightly homogenized. Bacillus subtilis No. 7 strain (NITE BP-01805) was used as the natto bacterium, and the spore suspension itself obtained by culturing the Bacillus subtilis No. 7 strain in a liquid medium was used as the natto bacterium starter.
After that, 45 g of soybeans were placed in a polystyrene natto container, covered, and fermented in a natto fermentation room (test fermentation room manufactured by Harada Corporation). Fermentation of each Example and Comparative Example was carried out under the conditions described in the "room temperature" and "fermentation time" columns of Table 1 or Table 4.
After the fermentation process was completed, the natto was aged by cooling in a refrigerator at 4 ° C or 10 ° C.
For quality evaluation of natto, a sample stored at 4 ° C for 3 days assuming aging at the time of normal shipment, a sample stored at 4 ° C for 7 days assuming storage equivalent to the expiration date, and then further stored at 10 ° C for 13 days. Each sample stored for one day was evaluated.

II-2. Pretreatment of samples for analysis of amino acids and analysis of various sugars The analysis samples of amino acids and various sugars for steamed soybeans are pretreated by the following methods, and then the analysis of amino acids and various sugars are performed. Used for analysis.
First, as a sample, steamed soybeans were crushed with a homogenizer, sufficiently homogenized, and then 10 g of the crushed sample was weighed with precision and transferred to a 100 ml volumetric flask. The reason why the collected amount is measured with precision is to accurately calculate the dilution rate of the sample. After putting the sample, 50% ethanol was added to the volumetric flask and filled up to 100 ml. The volumetric flask containing the sample and 50% ethanol was sonicated as it was with a sonicator for 30 minutes, and the water-soluble components in the sample were extracted into the liquid phase. The sample after sonication was filtered through filter paper (No. 2) to remove solids, and then subjected to amino acid analysis and various sugar composition analysis, respectively.

II-3. Analysis of amino acids In the analysis of amino acids, the sample subjected to the above pretreatment was diluted with lithium citrate buffer (diluted for steamed soybeans by 2), and then the diluted solution filtered with a 0.45 μm filter was used for measurement. did. The measurement was carried out by HPLC analysis by the post-column derivative method using ninhydrin, and a high-performance amino acid analyzer JLC-500 / V (JEOL Ltd.) was used as the measuring instrument.
The standard product of amino acids for measurement is the amino acid mixed standard solution AN-II for automatic amino acid analysis.
Type (Wako Pure Chemical Industries), Amino Acid Mixing Standard Solution for Automatic Amino Acid Analysis Type B (Wako Pure Chemical Industries), L-Aspartic Acid Standard Solution for Automatic Amino Acid Analysis (Wako Pure Chemical Industries), L-Glutamine (Wako Pure Chemical Industries), L- It was prepared by diluting tryptophan (Wako Pure Chemical Industries, Ltd.) with a lithium citrate solution (pH 2.2) to prepare a mixed standard solution.
The measured values obtained were converted according to the dilution rate of the sample to calculate the measured values. Total amino acids were calculated by adding up the measured values of amino acids on the right (in parentheses) (aspartic acid, threonine, serine, glutamic acid, proline, glycine, alanine, valine, cystine, methionine, isoleucine, leucine, tyrosine, phenylalanine. , Histidine, lysine, tryptophan, arginine). For basic amino acids, the total value of the amino acids shown on the right (in parentheses) was calculated (lysine, histidine, arginine).

II-4. Analysis of various saccharides For the analysis of various saccharides, the filtrate obtained by filtering the pretreated sample with a 0.45 μm filter was subjected to HPLC measurement. The HPLC system used was Prominence (Shimadzu Corporation).
The measurement column was HILICpak VG-50 4E HPLC, 80% acetonitrile was used as the mobile phase, and detection was performed with a differential refractometer (Shodex RI-201H).
Standard products include glycerin (Kanto Chemical), fructose (Sigma-Aldrich), glucose (Sigma-Aldrich), sucrose (Sigma-Aldrich), maltose (Wako Pure Chemical Industries), raffinose (Wako Pure Chemical Industries), and stachyose (MA Biomedicals). ) Was mixed and a mixed standard solution was used.
The measured values obtained were converted according to the dilution rate of the sample to calculate the measured values. The total amount of sugar was calculated by adding up the measured values of sugar on the right (in parentheses) (glycerin, fructose, glucose, sucrose, maltose, raffinose, stachyose).

II-5. Measurement of hardness The hardness of steamed soybeans and natto was measured by the following method using a digital force gauge (Nippon Densan Symposium FGP-5).
First, install the digital force gauge on the upper part of the measuring table, and with one grain of steamed soybean or natto as a sample installed in the Kubomi of the measuring part of the measuring table, make sure that the force gauge plunger pierces the center of the sample. The position was adjusted. Then, 14 grains of steamed soybeans or natto were measured per sample, and the average value of 10 points excluding the maximum value of 2 points and the minimum value of 2 points was used as the measured value.

II-6. Measurement of the surface color of steamed soybean surface and natto surface The color of the surface of steamed soybean or natto was measured using a colorimeter (SD-3000 manufactured by Nippon Denshoku Kogyo Co., Ltd.). Specifically, after installing steamed soybeans or natto in the measuring section of the colorimeter, measurement is performed in ^{the L *} a ^* b ^{* colorimeter measurement mode.} The obtained L ^* (L value) is an index of brightness, a ^* (a value) is an index of redness (red-green direction), and b ^* (b value) is an index of yellowness (yellow-blue direction). ^{C *} (C value) calculated by the formula [C ^* = ((a ^* ) ² + (b ^* ) ² ) ^1/2 ] was used as an index of saturation.

II-7. Quality evaluation The quality evaluation of natto obtained in the examples and comparative examples was carried out by a total of eight people who had experience in natto-related work and were fully familiar with the quality of steamed soybeans and natto, based on the following scale. Then, the arithmetic average value of the evaluation result was used as the score.

The stringing of natto was evaluated in the following five stages according to the degree of difficulty of dropping the natto grains from the chopsticks when the natto grains were lifted after stirring the natto with chopsticks 30 times / 10 seconds. ..

5: Very strong (when a lump of mixed natto grains is lifted with chopsticks, it is held in the air and does not fall for more than 5 seconds)
4: Strong (when a lump of mixed natto grains is lifted with chopsticks, it is held in the air and takes 3 seconds to fall)
3: Normal (When a lump of mixed natto grains is lifted with chopsticks, it cannot be held in the air and the natto grains fall off)
2: Weak (pulls the thread but does not form a lump of natto grains)
1: Very weak (does not pull thread)

The bitterness of natto was evaluated on the following five levels.

5: Good with no bitterness, 4: Good with little bitterness, 3: Normal 2: Slightly bitter and unpleasant, 1: Strong bitterness and unpleasant

The natto taste was evaluated on the following five levels.

5: Good with no harsh taste 4: Good with little harsh taste 3: Normal 2: Slightly harsh taste and unpleasant, 1: Strong and unpleasant harsh taste

The hardness (texture) of natto was evaluated in the following five stages.

5: Soft and not chewy (6 out of 10 beans are crushed when crushing beans one by one inside the tongue and mouth) 4: No chewy (however, it is not easily crushed inside the tongue and mouth) ) 3: Slightly chewy, 2: Feeling chewy, 1: Feeling strongly chewy

The color of natto was evaluated in the following five stages.

5: Very bright and moderate redness 4: Slightly bright and moderate redness 3: Normal 2: Slightly dark and slightly excessive redness, 1: Dark and excessive redness

The scent of natto was evaluated in the following three stages.

3: No offensive odor such as ammonia, 2: Slight offensive odor such as ammonia, 1: Offensive odor such as ammonia is felt

II-8. Evaluation Results of Steamed Soybeans for Natto Table 1 shows the evaluation results of the steamed soybeans for natto produced in Examples 1 to 3 and Comparative Examples 1, 2 and 4. Evaluation was carried out on steamed soybeans stored at 4 ° C. for 3 days after production.
The steamed soybeans for natto produced as Examples 1 to 3 have a large L value and a small a value as compared with the steamed soybeans for natto produced as Comparative Examples 1, 2 and 4, and therefore have L ^* / a ^* . The value was large. The cause of this is unknown, but it is presumed that the Maillard reaction caused by heating progresses more slowly in the examples because the total amount of amino acids and sugars is smaller and the amount of sucrose is particularly small.
The C value of the steamed soybean was almost the same in all the measured samples, and the hardness was almost the same except for Comparative Example 4 in which the water addition step was not performed.

II-9. Evaluation Results of Natto The evaluation results of the natto produced in Examples 1 to 3 and Comparative Examples 1, 2 and 4 are as shown in Tables 2 and 3.
For quality evaluation of natto, a sample stored at 4 ° C for 3 days (shown in Table 2) assuming aging at the time of normal shipment and another 10 after storage at 4 ° C for 7 days assuming storage equivalent to the expiration date. Evaluation was performed on two points of the sample (listed in Table 3) stored at ° C for 13 days.
As a result, in the sample stored at 4 ° C. for 3 days, the natto produced as Examples 1 to 3 had a larger L value and a smaller a value than the natto produced as Comparative Examples 1, 2 and 4. The value of L ^* / a ^{* was large.}
Further, in the quality evaluation of natto, the natto produced as Examples 1 to 3 were clearly superior in the evaluation of bitterness, harsh taste, and color as compared with Comparative Examples 1, 2, and 4.
On the other hand, the quality evaluation of stringing and fragrance was almost the same in Examples 1 to 3 and Comparative Examples 1, 2 and 4. The quality evaluation of hardness was almost the same except for Comparative Example 4 in which the water addition step was not performed.
Next, the sample stored at 4 ° C for 7 days and then stored at 10 ° C for 13 days had a smaller L value and a larger a value as a whole than the sample stored at 4 ° C for 3 days ^. The value of / a ^* became smaller, and the evaluation of bitterness, harshness, and color decreased in the quality evaluation.
However, in Examples 1 to 3, all the scores were 3 points or more, and it was confirmed that they had sufficient palatability even when stored until the expiration date.
As described above, with respect to Comparative Example 3 which was carried out assuming Patent Document 3, as described above, the steaming temperature and the steaming pressure could not be adjusted within a predetermined range, and thus it was impossible to carry out. , In Tables 2 and 3, it is described as "measurement not performed because it cannot be performed".

The evaluation results of Example 4 are as shown in Tables 4 to 6. Regarding steamed soybeans for natto, it seemed that there were more amino acids than in Examples 1 to 3, but the L value, a value, and L ^* / a ^* values were the same, and the color was bright and good natto. Steamed soybeans for use were obtained.
In the quality evaluation of natto, the sensory evaluation values were clearly superior to those of Comparative Examples 1, 2 and 4 in the same manner as in Examples 1 to 3, and 7 at 4 ° C. assuming storage equivalent to the expiration date. After storing for 1 day, even if it was stored at 10 ° C for 13 days, all the scores were 3 points or more.

Cross-reference of related applications

This application claims priority based on Japanese Patent Application No. 2017-179726 filed with the Japan Patent Office on September 20, 2017, the entire disclosure of which is incorporated herein by reference in its entirety.

Claims (7)

A method for producing steamed soybeans for natto.
(A) The first steaming process in which soybeans are steamed under pressure of 5 minutes or more, 15 minutes or less, 0.02 MPa or more and 0.18 MPa or less.
(B) The first watering step of adding water after the first steaming step,
(C) A second steaming step of pressurizing and steaming at a pressure of 10 minutes or more, 90 minutes or less, 0.06 MPa or more and 0.30 MPa or less after the first water addition step, and the first goal of the first steaming step. A method for producing steamed soybeans for natto, in which the pressure (P ₁ ) and the second target pressure (P ₂ ) in _{the second steaming step satisfy the relational expression of P 1} <P _2. The method for producing steamed soybeans for natto according to claim 1, wherein the first target pressure (P ₁ ) and the second target pressure (P ₂ ) satisfy the relational expression of 1.1 × P ₁ ≤ P _2. The natto according to claim 1 or 2, wherein the first target temperature (T ₁ ) of the first steaming step and the second target temperature (T ₂ ) of _{the second steaming step satisfy the relational expression of T 1} <T _2. How to make steamed soybeans. The first steaming time (M ₁ ) of the first steaming step and the second steaming time (M ₂ ) of _{the second steaming step satisfy the relational expression of M 1} <M ₂ , according to any one of claims 1 to 3. The method for producing steamed soybeans for natto described. The method for producing steamed soybeans for natto according to any one of claims 1 to 4, further comprising a second watering step of adding water after the second steaming step. Using steamed soybeans for natto produced by the production method according to any one of claims 1 to 5, in the natto fermentation step, the soybean product temperature is substantially set to a temperature range of 37 ° C. or higher and 53 ° C. or lower for 10 hours. A method of manufacturing natto that is maintained within 24 hours. The method for producing steamed soybeans for natto according to any one of claims 1 to 5, wherein the surface color of the steamed soybeans for natto after production satisfies the following relational expression 1.
(Relational formula 1)
37 ≤ L * ≤ 53
8.0 ≤ a * ≤ 12.0
3.9 ≤ L * / a * ≤ 6.7
L *: L value (brightness) of steamed soybeans for natto
a *: a value (redness) of steamed soybeans for natto