Updated: Aug 2, 2021
You often find the mold growing on the surface of the miso, which is not pleasant looking for sure. No need to either freak out or dump whole the miso as long as you know how to handle it correctly.
Why mold growing?
There are tons of microbes floating around in the air as well as things that surround us. (Even the average human has over 100 trillion microbes in and on their body) When those microbes found a cozy place to settle on, they start inhabiting what is called mold.
Most molds need oxygen and moisture to live, which means different molds can be grown based on the environment. That's why a majority of food products including miso packed and sealed to prevent air contact, added preservatives and went through a pasteurizing process.
Good mold & Bad mold
The most common mold you would see from miso is a white one, which is not mold but a type of yeast called 'film yeast'. If you see film yeast forming, gently skim them off as you would with the jam because it is not a serious deal.
* Film yeast is also commonly seen in other fermented foods such as soy sauce, wine.
Film yeast on soybean miso
However, if the color of the mold is rather colorful(pink, green), scrap it not just the surface where you can see but at least 1 inch (2.5 cm) around and below the mold. Make sure to keep an eye on them.
How to store
Hand-made miso is more likely to experience mold forming in a high chance, so make sure to scatter salt around the surface and cover with cling film before applying weight on the top.
Regarding miso products from a supermarket, make sure the miso container is properly sealed (if the package cannot be sealed, move to the one that can) and store them in a fridge.