Does Natto Go Bad

In this brief article, we will be discussing the question, “does natto go bad?” and discuss how to know if natto has gone bad, the shelf life of natto and the benefits and risks of eating natto.  

Does natto go bad?

Yes, natto can go bad. During storage, the action of the fermentation bacteria Bacillus subtilis leads to the breakdown of proteins and generation of amino acids and ammonia, which may lead to bitterness and off-odors. 

In addition, changes in the texture, such as hardness, and color, due to Maillard reactions, turning the natto darker, lead to the loss of quality of natto (4).  

However, spoilage caused by pathogenic bacteria is rare, since natto and other fermented soybean foods contain substances produced during the fermentation process which are able to inhibit the development of many microorganisms, including Salmonella, Listeria, Bacillus cereus and Escherichia coli (6).

How to know if natto has gone bad?

To know if natto has gone bad, you should be able to identify the possible signs of spoilage in natto. Natto is produced by the fermentation of soybeans and contains amino acids which are generated through the breakdown of soybean proteins.

As a result, natto has a sticky texture and characteristic odor and flavor of these amino acids, such as umami and musty. However, When natto goes bad, it will be characterized by an ammonia-like odor and loss of stickiness, as well as loss of hardness (1,3,4). 

What is the shelf life of Natto?

The shelf life of natto is 10 days at 5°C (40°F) and 5 days at 10°C (50°F). It is not recommended to store natto above 10°C (4). 

Refrigeration can reduce the rate in which reactions occur, which cause the loss of the quality of natto. At low temperature, the action of the proteases produced by the fermentation bacteria Bacillus subtilis is slowed down. 

The loss of hardness is also reduced when temperature is kept at refrigerated temperature.

Can you eat natto after the expiration date?

Yes, you can eat natto after the expiration date. However, due to the critical changes during the prolonged shelf life, natto can become untasty after its expiration date (4).

As mentioned earlier in this article, soybean fermented foods have antimicrobial activity. In fact, they are often used as preservation aids in foods to prevent the spoilage through pathogens, and therefore these fermented soybean products, including natto, present a reduced risk of foodborne illness if consumed after the expiration date (6). 

Can natto be frozen?

Yes, it is possible to freeze natto. Freezing food is one of the most effective and used practices to improve the shelf life of foods (7), as it can significantly reduce the chemical reactions and halt the microbial growth.

A study showed that the vitamin  K2 content in natto was generally 2–4 times higher in frozen products than the content in refrigerated products (5).

What are the benefits and risks of eating natto?

The benefits of eating natto are related to the several compounds with improved biological and positive effects on health, such as soybean isoflavones, vitamin K, bioactive peptides, dietary fibers and many others (2).

Eating natto daily (in recommended amounts) can (2):

  • strengthen the immune system
  • toughen bones
  • provide protection against heart disease 
  • help the body digest foods more easily
  • reduce blood pressure
  • dissolve blood clot
  • prevent against cancer
  • reduce the risk of Alzheimer’s disease

The risks of eating natto are related to  possible allergic effects. Although the soybean allergens are significantly reduced during the fermentation process and production of natto, studies report cases of severe allergic reactions after the ingestion of natto (8).


In this brief article, we answered the question, “does natto go bad?” and discussed how to know if natto is bad, the shelf life of natto and the benefits and risks of eating natto.   


  1. Weng, Tien Man, and Ming Tsao Chen. Changes of Protein in Natto (a fermented soybean food) Affected by Fermenting Time. Food sci technol res, 2010, 16, 537-542.
  2. Liu, Yanchun, et al. Analysis of Main Components and Prospects of Natto. Adv Enzyme Res, 2021, 9, 1-9.
  3. Hosoi, Tomohiro, and Kan Kiuchi. Natto–a food made by fermenting cooked soybeans with Bacillus subtilis (natto). Handbook of fermented functional foods, 2003, 227-245.
  4. Kubo, Yuji, Tomotsugu Noguchi, and Keitarou Kimura. Storage temperature and quality changes of natto. Food Sci Technol Res, 2021, 27, 497-504.  
  5. Jeong, IS., Gu, SY., Park, KH. et al. A simultaneous determination and monitoring of vitamin K1 (phylloquinone) and vitamin K2 (menaquinone) in vegetable drinks and natto sold on the Korean market. Food Measure, 2022, 16, 248–257. 
  6. Sopheap, Ek, et al. Application of antimicrobial Bacillus subtilis strain as a starter culture to improve qualities and safety of fermented soybean (SIENG) produced in Cambodia. J Food Eng, 2019, 9, 333-353.
  7. Evans, Judith Anne, ed. Frozen food science and technology. Blackwell, 2008.
  8. Inomata, Naoko, et al. Late-onset anaphylaxis after ingestion of Bacillus Subtilis-fermented soybeans (Natto): clinical review of 7 patients. Allergol int, 2007, 56, 257-261.

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