アメリカで納豆は簡単に作れます。
大豆はamazonで小粒の大豆を購入、最初の1回だけはASIANマーケットで納豆を購入します。
先ずは大豆を軽く洗います。（大豆は洗うと泡が出ます、でなくなるまで優しく浮いた豆などを取り除き洗います）
夜8時に大豆を水に12時間漬け込みます、朝8時からスロークッカーで12時間煮込みます。
指で摘んで押し潰れるくらい、柔らかくなった大豆を笊に取ります、その際、煮汁をスプーンに二 三杯取っておきます。
それから笊に取った大豆をトレイに載せます、その大豆にASIANマーケットで買った小箱の納豆を、半分練ってそれに先ほど取っておいた煮汁を加え、トレイの大豆に満遍なく混ぜてオーブンに入れます。
オーブンは照明だけにします、それでも温度は摂氏4５℃近くまで上がるので扉にタオルなどを挟んで摂氏40℃前後に調整して１２時間置いて完成です。
今回は大豆６００グラム作りましたが、量は結構いい加減です、トレイに２，３層と云いますが、山盛りになりました。
それでも大成功でした。過去に摂氏4３℃越えたときは余り上手く出来ませんでした、問題は温度だと思います、摂氏40℃前後を守るようにすれば誰がやっても成功すると思います。

Making natto in the U.S. is quite simple.
For the soybeans, I purchase small-grain varieties from Amazon; the only time I visit an Asian market is to buy the initial starter natto needed for the very first batch.
First, I give the soybeans a light rinse. (Washing soybeans tends to create foam; I continue rinsing gently—skimming off any floating beans or debris—until the foaming subsides.)
At 8:00 PM, I soak the soybeans in water for 12 hours; then, starting at 8:00 AM the following morning, I cook them in a slow cooker for another 12 hours.
Once the beans have become soft enough to be easily crushed between my fingers, I transfer them to a colander to drain. At this stage, I set aside two or three spoonfuls of the cooking liquid.
Next, I transfer the drained soybeans from the colander onto a tray. I take half of the small container of starter natto purchased from the Asian market, mash it up, and mix in the cooking liquid I had set aside earlier. I then thoroughly mix this mixture into the soybeans on the tray before placing the tray inside the oven.
I set the oven to use only the internal light (no heat). Even with just the light, the temperature can rise close to 45°C; therefore, I wedge a towel or similar item in the oven door to regulate the temperature to around 40°C. After leaving it like this for 12 hours, the natto is complete.
For this batch, I used 600 grams of soybeans. My measurements were actually quite approximate—although the general rule suggests keeping the beans to a depth of two or three layers on the tray, mine ended up piled high in a mound.
Nevertheless, it was a huge success! In the past, whenever the temperature exceeded 43°C, the results weren’t very good. I believe the key factor is the temperature; as long as you make sure to maintain it right around 40°C, I’m confident that anyone can successfully make natto.