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Why don't you just mix some natto that you previously bought with the pressure cooker steamed soy beans?
I did that in the past and it works.

Homemade natto (fermented soybeans)
Ingredients:

* two cups dried soybeans
* water
* one package commercial natto

Preparation time: 2-3 days
Directions

Note: Please read the entire recipe first and make a note of the ingredients and utensils that will be needed. The starter culture must be obtained from store-bought natto, unless you have other sources such as rice straw.

1. Soak two cups of dried soybeans overnight in ten cups of water.

2. Put the soybeans in a stainless steel basket (or colander) and cover it with a piece of cloth slightly larger than the basket.

3. Put the above in a pressure cooker with 3 cups water, and place it on the stove. Turn on the stove.

4. After the pressure cooker starts hissing, turn down the flame so that the hissing is at its minimum level.

5. Cook for 15 minutes (measured from the time the hissing has started).

6. DO NOT OPEN the pressure cooker yet. Allow the pressure cooker and its contents to cool down. If waiting is not your style, place the pressure cooker in the sink filled with cold water. (The soy beans are considered to have cooled down as long as its temperature is below 140 degrees F.)

7. Make sure that the kitchen counter and its surrounding area are absolutely clean. Sterilize a tablespoon with boiling water.

8. Wash your hands and arms - long-sleeved shirts not recommended.

9. Have a package of commercial natto ready.

10. Open the lid of the pressure cooker, peel back the cloth cover to one end of the basket, and using the tablespoon, quickly mix in about two spoonfuls of natto starter with the beans. Replace the cloth cover.

11. Close the pressure cooker lid with its air relief hole uncovered.

12. Place the cooker in a picnic ice-chest and place an electric heating pad over it. Replace the ice-chest cover.

The natto will be ready in between 24 and 48 hours, depending on the temperature of the heating pad. As an alternate heat source, a 7.5-watt lightbulb may be used. If you don't have a pressure cooker, a regular pot may be used. In this case, the cooking time will need to be increased to about two hours, and the amount of cooking water to about six cups.