A timely failure to keep me humble. After a few successful runs which can be seen on Batch #37, Batch #39 fails on me. I guess I am a little bit harsh on myself because it was not a complete fail. I had mentioned previously that one important criteria for good natto was its silky long strings. I love to make a batch that has strong threads that hold the beans within the string even when lifted.
On Batch #39, the fermentation looked fine and I was looking forward to eating natto. One quick test is to take out a small portion of the freshly fermented natto and do the wet seasoning test. You can use shoyu or mentsuyu.
On a successful batch, the freshly made natto looks almost gummy and will stick to each other. When mentsuyu is added, it will still be gummy and sticky.
On the other hand, unsuccessful batches will almost have flaky natto growth with thin strings. When this is mixed with mentsuyu, the strings will have a wet look and will not have sticking power. You can see how the natto has not clumped together in the bowl after mixing.
I always thought this was due to a problem with the fermentation, but on this batch I used the same method as the successful batch #37. The only thing that I did different this time was to use a different starter. I had ran out of the old frozen natto so I used a new package. Unfortunately, I did not keep the outer label so I have no idea what the brand was. So assuming that I used a different brand of frozen natto starter, the outcome of the fermentation could be strongly linked to the strain of natto used for the fermentation. I am sure different brands use different strains of natto to establish their style.
I will start noting what brands I use as the starter to see if there are strains that are more stringy than others. I have heard that in Japan, there is a trend towards less stinky natto. There could be a correlation to natto that turns out less stringy when this less stinky natto is used.
This is a good remainder to me not to be discouraged by a bad batch. It is very possible that your natto did not turn stringy because of the starter used. If possible, try to use a different brand to see how the natto turns out.