We will look at the natto fermentation from the perspective of our main character, Bacillus subtilis natto. It is a bacteria commonly found in nature and for this reason is also referred to as hay bacillus or grass bacillus. The relation to hay goes back to 1086 AD in Japan when soybeans spontaneously fermented when wrapped in hay (Natto, wikipedia). Due to this spontaneous nature, I am sure that the origin of natto fermetation goes back even further.
Natto is unique in its fermentation as it does not produce acids when fermenting with lactobacillus ie. yogurt and pickles. It also does not produce alcohol as is the case with yeast fermentation. Natto produces stringyness, which comes from long-chain polysaccharides. The stringiness is very unique to fermented products and is what makes natto so cool!
The essential conditions to produce this wonderful stringiness are as follows,
-Properly Soaked and Cooked Beans
Soybeans have a really tough skin and take a long time to cook. The natto-kin (natto bacteria) has to penetrate to skin in order to get to the nutrients inside. I do not know if the natto-kin actually grow into the soybean or if they absorb the nutrients and grow on the surface, but the result is the same. The nutrients have to be available for the natto-kin and in order to do this, the soybeans have to be soaked long enough for full hydration. I soak the beans for 10-12 hours in the summer and 20-24 hours in the winter time. A good way to tell is to split the soybean in half to see if the two halves touch in the middle. If there is a gap, they need to be soaked longer for full hydration.
Boiling the soybeans is usually the first method people try for its ease. The disadvantage is that the flavors leach out of the bean and the amount of time it takes, 4-6 hours. The next solution is to steam the beans over boiling water. This will make tastier natto, but it also takes hours to cook. Then there is boiling the soybeans under pressure. This will take 40 minutes, but again one runs into the problem of leaching flavors out of the bean. The best method and the only method I use now is to steam to soybeans under pressure. This solves both problems of time and flavor. Steaming under pressure will retain the flavors and take 40 minutes. From the natto-kin perspective, this is important because more flavor equals more food for them and softness of the beans means that nutrients will be easier to access from within the bean.
-Optimum Temperature
Natto-kin is interesting that although it is tolerant to extreme conditions, it grows slowly and is vulnerable to other bacteria in the initial stages of growth. I have not experienced this personally, but there are anecdotes of contamination in the fermentation. To favor ideal growth conditions, natto should be fermented at a higher temperature when compared to a lactic fermentation. I usually shoot for 95F-108F (34C-42C). It is important to keep this temperature the initial 6 hours of fermentation to establish a strong natto-kin population. After 12 hours, the natto-kin start to generate heat and so one can be more relaxed about maintaining temperature.
-Moisture and Air
This relationship was really difficult to maintain as there is an inverse relationship. Many natto makers stress the importance of having good ventilation so the natto-kin can breath. This makes absolute sense, as B. subtilis is classified as an obligate aerobe, meaning it needs air to grow. Unfortunately, moisture is impossible to maintain if you keep the fermentation container open. The surface of the natto would become dehydrated and brown and the growth of the natto-kin would decrease. I took a hint from packaged natto which is packaged in a styrofoam container. On top of that, the natto is covered directly with a plastic sheet with small holes. This is how I came up with the double cling-wrap method. I put a cling-wrap with small holes, poked with a tooth pick, directly over the soybeans and another with holes on the baking sheet. Having a cling-wrap directly over the soybeans really helps to prevent drying of the surface. The air gap formed between the sheets also helps slow down evaporation.
-Time
The intial 6 hours as mentioned above are very crucial to establish a healthy population of natto-kin. Not much is visible, but the natto-kin is busy getting settled in, absorbing nutrients and later starting to multiply. Then in the 6-12 hours of fermentation, the bacteria go through a population explosion and you can start smelling the aromas coming from the fermentation. After 12 hours of so, the natto start to generate heat from metabolism and will continue fermenting until they start to slow down due to overcrowding and depletion of nutrients at around 18-20 hours. From experience, I have found that under ideal conditions, the fermentation will be done in 18 hours.
Not mentioned here is about the starter used and this deserves a post of its own.
