Shio koji, or 塩麹 (salted koji) is a going through a major boom in Japan right now. What is it? It is a mix of salt and koji mold. Wiki says, "Koji (Aspergillus oryzae) is a mold used in Chinese and Japanese cuisine to ferment soybeans. It is also used in the making of alcoholic beverages such as sake, and shochu."
Basically koji is used to make miso and sake, to get its distinctive flavors. Shio koji has been used widely in rural areas in Japan for a long time, but recently it has become a national sensation. Why is it so popular? Because it's an ingredient with super powers -- it tenderizes meat and adds sweet and salty flavors. You can only understand it if you use it.
It's yet not commercially available in the States (correct me if I'm wrong), but it's super easy to make, so I made it the other day.
All you need is koji and sea salt. You can get koji at Japanese grocery stores near the area where they sell miso. Sometimes you can find one in the rice aisle as well.
I bought this organic expensive koji brown rice, but if you can a find cheaper one, that's totally fine.
Mix koji with salt. The ratio is about 30-40% salt to the koji rice.
Mix them together well, and pour in water. That's basically it.
How much water? Make sure all the koji is submerged in the water.
Then let it sit in room temperature for about 10 days. For the first few days, you will want to add more water so that all the koji are submerged in the water.
Even if rice isn't covered in water after a couple of days, it's ok. You also want to mix this daily. In the beginning, it's just salty water, but after about 10 days, it smells sweet -- somewhat similar to miso or sake (or some people say banana). You can also easily break the rice grains by pressing them with your fingers.
This was day two:
Once you make shio koji, keep it in the fridge. It should last for about half a year, so you can make it in bulk.
This was about 10 days later. Smells very sweet, but tasted very mild -- not overpowering in saltiness at all.
What do you use it for? Anything. Think of this as a salt substitute, but better. Marinate chicken (even breast meat) with about 2 tablespoons of it for an hour. It never makes dry chicken. Marinate fish with it and it will add a very nice sweet/salty flavor. Marinate tofu (you want to press the tofu to get rid of as much water as possible) and wrap it with the koji for a week, it changes the texture and flavor of tofu completely. It makes something similar to cream cheese, but without any fat in it!!
I made koji tofu, and it really changed the texture.
The options are limitless. Shio koji produces a truly a magical flavor. It's apparently reduces salt intake as well since the salt is diluted with water and koji rice. This is definitely the easiest recipe, so if you can find koji rice near you, you should definitely try it.
Having said that, I made a second batch using koji from Cold Mountain (a brand name). Even after two weeks, it wasn't easily breakable with fingers (although itsmelled and tasted good). I heard it was because koji mold wasn't properly applied onto the rice. Something to think about.