by Tim Hall on Tue Dec 04, 2012 10:38 am
I have experience with this. Are you talking about using a working fridge so you can both heat and cool? Because this option isn't necessarily the cheapest.
If you just want an incubator (heating only), there are cheaper options than using a working fridge...so long as the ambient temperature in your space isn't over ~85F.
Also to culture koji (or any fungal culture, including tempeh and mushrooms) temperature is not the only factor that's important. You want high humidity, and a decent amount of fresh air exchange. But these can be accomplished cheaply and low-tech too. If I can get a better idea of how tightly you want to control the temp (heating only, or both heating and cooling) I can better explain your options.
My setup is designed for maximum versatility because I use it for koji and tempeh (usually only requires heating), brewing beer (requires cooling), growing mushrooms (requires both heating and cooling depending on species and ambient conditions), and theoretically I can use it as a cheese cave (requires cooling). BUT if koji is all you're interested in, you can probably get by with just heating, and you don't even need an operable fridge for this.