Disclaimer: I am not a microbiologist or food safety expert. All of these procedures are based on research I have done myself and experiments that have worked successfully for me. You can attempt a procedure like this at your own risk. That being said, the reason I personally feel comfortable fermenting meats in this way is that spoilage bacteria is predominantly on the surface of meat. So when treated correctly I am confident that Koji, which is a beneficial fungus, can essentially completely take over the microbial landscape in the timescale of these projects. However, it would be beneficial to do your own research on the topic of more conventional charcuterie/meat fermentation as well as dry aging at home.
Video Tutorial. For Koji fermented meats I put things into two broad categories, Koji aging and Koji charcuterie. Koji Aging is when whole pieces of fresh meat are cultured with Koji for a relatively short period of time (on the scale of days) before being cooked and eaten. In the context of beef this is meant to somewhat replicate dry aging, although it can be done to essentially any type of meat. Koji Charcuterie is when a standard cured meat procedure has a Koji fermentation step added. Many types of charcuterie already involve fermentation, which generally involves purposefully adding a bacterial or fungal culture to the meat (salami as an example). For this type of charcuterie I would say the Koji would be replacing the standard cultures that are traditionally used. In the case of cured meat that isn’t intentionally fermented (although many forms of cured meat rely on the growth of certain beneficial molds spontaneously) a Koji fermentation Step would be added after curing but before aging.
Find a source of Koji. This can be as simple as buying spores online, which you can use directly on meat. However, my preferred method is to grow the spores on some sort of substrate, like rice or grains for at least 24 hours. And then use those Koji substances to inoculate your meat. If you’ve never done that before, I also have a written tutorial for that procedure.
Prepare your meat.
In the case of Koji charcuterie this means following a standard recipe, including using the appropriate amount of normal and curing salts, up until the point where other cultures are introduced and/or the meat is aged.
For Koji aging you want to take your meat and still lightly cure it in a mixture of equal parts salt and sugar by weight. You want enough mixture to completely bury your meat, and this is done based on time. The goal is not to thoroughly or precisely season the meat, but simply expose the surface tissue to some intense salt and sugar. This will temporarily kill or inhibit other microbes, giving Koji time to take over. The amount of time needed is going to depend on the size and surface area you are working with, but in general I would say between 10 to 20 minutes is a good range. After the curing time is done you want to thoroughly rinse off any excess salt and sugar, but don’t worry about drying it off.
Coat your meat in rice flour. No matter what style of Koji meat you are making you want to thoroughly coat the pieces in rice flour, and it can be helpful if your product is still slightly damp from any type of curing procedure. In a pinch you can also use regular wheat flour, but rice flour is my preference. I prefer the way it acts as a coating when cooking the final product.
Inoculate your meat. As I mentioned earlier in this guide, my preference is using Koji fermented rice or grains as the source for inoculation. In doing this, simply spread your Koji fermented substance on top and around your pieces of meat that have been covered in rice flour. The easiest way to do this would probably be a gloved hand. If using spores directly, you want to dilute them in about a cup of rice flour and simply apply a second layer of this flour/spore mixture.
Incubate your meat. Of course my preference is an immersion circulator in a large hotel pan or basin. Your water temperature should be set between 30-33 Celsius and all of your meat should be placed in inserts, pots, or trays that fit comfortably inside the water or just above it (without letting water directly in of course). Each tray or container should be individually wrapped in foil to keep in warmth and then the entire sous vide setup should also be wrapped in foil. It’s also important that you do not crowd your pieces of meat, if possible don’t have them overlap at all.
Incubate for a total of 48-72 hours. In general I find it only takes 24 hours for some noticeable fungal growth and the generally after 48 hours things are thoroughly fuzzy and ready to stop incubating. It’s a good habit to check on them once or twice a day during the incubation period. You can also add some extra rice flour to spots if some falls off.
Congratulations! Your Koji fermented meat is now done.
For Koji aging I would let it sit in the fridge on a tray with plenty of air circulation for at least a day. It will also continue to very slowly age in the fridge like this for several days. When cooking meat fermented with Koji, it’s important to note that it’s going to be significantly more tender so you may need to reduce cooking times, especially for things like tough cut. Remember that your product has already been lightly cured, so it probably doesn’t need much seasoning. And be aware that Koji fermented meat develops a very dark crust from searing much more quickly than normal meat. Flavor outcomes are going to depend on a wide number of variables but you will frequently get notes of other Koji products like miso and soy sauce, but there’s plenty of room for more complexity and experimentation.
For Koji charcuterie it’s now time to age your product. Ideal conditions will depend on what you are making, obviously consult whichever recipe you are using for ideal temperature and humidity for aging. However, I will say anecdotally I have had success aging Koji based charcuterie in nonideal conditions (a cool part of my basement but no environmental control). If the goal of your charcuterie is a certain level of moisture loss I would check it fairly frequently. Koji tends to accelerate the aging process, sometimes quite significantly. Flavor wise, again there’s going to be a lot of variables. In my limited experience I would say the Koji adds distinct flavors when the charcuterie is on the smaller side, such as pork belly. Whereas with larger products like prosciutto, the impact is mainly in aging acceleration and not as much adding a distinct flavor.