Lanna Fermented Soybean Disc (Tua Nao)

Ingredients

Equipment Needed

• •Large pot for boiling (at least 5-liter capacity)
• •Banana leaves for wrapping (the fermentation vessel and inoculant in one)
• •Kitchen twine or bamboo strips for tying parcels
• •Heavy granite mortar and pestle (krok hin) for pounding fermented beans
• •Bamboo drying tray or wire rack for sun-drying
• •Food thermometer (helpful but not essential)

Instructions

1. 1

   Soak the soybeans

   Put the dried soybeans in a large bowl and cover with water by at least three inches. They'll double in size overnight. Soak for a minimum of eight hours, up to twelve. When you split a bean with your fingernail, the interior should be uniformly pale and hydrated with no dry core. Drain and rinse. This step isn't optional and it isn't negotiable. Under-soaked beans won't soften properly in the pot, and under-softened beans won't ferment evenly.

   Use whole dried soybeans, not split. The skin helps hold the bean together during boiling and gives the bacteria more surface area to colonize during fermentation.
2. 2

   Boil until collapsing

   Transfer the soaked beans to a large pot. Cover with fresh water by two inches. Bring to a boil, then drop to a steady simmer. Cook for three to four hours. Yes, hours. You're not cooking beans for a salad. You're breaking down cell walls so that Bacillus subtilis can access the proteins inside. The beans are done when you can crush one between your thumb and finger with zero resistance. They should be mushy, almost falling apart, with skins splitting and separating. If there's any firmness at all, keep going. Drain completely and let them cool until they're warm but not hot, around 40-45°C. The bacteria need warmth to activate, but temperatures above 50°C will slow them down.

   A pressure cooker cuts boiling time to about 45 minutes at full pressure. Lanna grandmothers didn't have pressure cookers, but they'd have used them. The result is identical. The principle is soft beans, not suffering.
3. 3

   Wrap in banana leaves

   Pass each banana leaf over a gas flame or hot coil for a few seconds per side. The leaf should turn from matte to glossy and become pliable without tearing. This does two things: it sterilizes the surface of unwanted mold spores, and it makes the leaf flexible enough to wrap tightly. Pile the warm, drained beans onto the center of a banana leaf in a mound about two inches thick. Fold the leaf over to create a sealed parcel, tucking the edges underneath. Tie it shut with kitchen twine or bamboo strips. You want it snug but not airtight. The beans need some air exchange for the aerobic bacteria to work, but the enclosure traps heat and moisture. Make two or three parcels rather than one massive one. Even distribution means even fermentation.

   The banana leaf isn't just packaging. It's a fermentation vessel. Bacillus subtilis is naturally present on the leaf surface, so the wrapping material is also the inoculant. This is ancient biotechnology hiding in plain sight.
4. 4

   Ferment for two to three days

   Place the parcels in a warm spot. In Lanna villages, this means under the stilted house or near the kitchen hearth where ambient temperature sits around 30-40°C. At home, find the warmest corner of your kitchen, near the stove, on top of the refrigerator, or in an oven with just the light on. The target temperature is 30-40°C. Below 25°C and fermentation stalls. Above 45°C and you start killing the bacteria. After 24 hours, open one parcel and check. The beans should be starting to smell: sharp, pungent, like strong cheese or overripe fruit. By 48 hours, you'll see sticky, mucilaginous threads stretching between the beans when you pull them apart. That's the Bacillus subtilis producing polyglutamic acid. Those threads are proof that fermentation is working. At 72 hours, the smell will be intense, the threads thick, and the beans will have darkened to a deep brown. Taste a bean. It should be salty-savory with a deep, almost meaty umami. If it tastes only sour or bitter, the fermentation has gone wrong, likely contamination from other bacteria. Trust your nose and your tongue. Fermented: pungent but savory. Spoiled: acrid and sharp.

5. 5

   Pound into flat discs

   Unwrap the fermented beans. They'll be sticky, dark, and intensely aromatic. If you want to add a small amount of salt for preservation, mix it in now. Using a mortar and pestle or your hands (oiled lightly to prevent sticking), pound or mash the beans into a rough paste. You're not making it smooth. You want a thick, chunky mass with some texture remaining. Take a golf-ball-sized portion and press it flat between your palms or onto a banana leaf into a disc about 8-10 centimeters across and half a centimeter thick. The thinner the disc, the faster and more evenly it dries. Repeat until all the paste is shaped.

   Ajarn always said that preservation is just fermentation's next chapter. Forming the discs thin and uniform is the same principle as slicing meat thin for drying: surface area is everything. More surface means faster moisture loss, which means the fermentation stops cleanly and the product is shelf-stable.
6. 6

   Sun-dry the discs

   Lay the discs on a bamboo tray, a wire rack, or clean banana leaves set in direct sunlight. You need strong sun and good airflow. In Northern Thailand, this takes one to two full days of sun exposure, flipping the discs once halfway through. The discs are done when they're hard, dry, and snap cleanly when you bend them. They should feel like a firm cracker, not a flexible wafer. If they're still bendy, they need more time. If your climate is humid or overcast, use a food dehydrator at 50-55°C for 8-12 hours, or an oven at its lowest setting with the door cracked open. The goal is dehydration, not cooking. You're removing moisture to halt bacterial activity and prevent mold during storage.

   In rainy season, Lanna cooks sometimes roast the discs over low charcoal instead of sun-drying. The light smokiness this adds is considered a feature, not a compromise. Smoke has antimicrobial properties that aid preservation.

Chef Tips

• •Tua nao is to Lanna cooking what kapi is to Central Thai cooking and pla ra is to Isan: the umami backbone of an entire regional cuisine. Same structural role, different protein source. The principle is that every Thai regional tradition has a fermented product providing salinity and savory depth. The system adapts to geography. Mountains don't have oceans. So the North fermented beans.
• •The stringy, mucilaginous threads you see during fermentation are polyglutamic acid produced by Bacillus subtilis. This is the same organism and the same biochemistry behind Japanese natto. The flavor profiles are cousins: deep umami, slightly funky, intensely savory. If you've had natto and liked it, tua nao will feel familiar. If you haven't, prepare yourself. The smell is strong. The flavor is stronger. And once it's in a nam prik or a gaeng khae, you'll understand why the Lanna people built their kitchen around it.
• •Dried tua nao discs store for months in a cool, dry place. Wrap them in paper or cloth, not plastic. They need to breathe. Before using, toast a disc over low flame or dry-roast it in a pan for a minute per side. This wakes up the aroma and makes it easier to crumble into dishes. You can also rehydrate a disc in warm water for ten minutes if you want to pound it into a paste for nam prik tua nao.
• •Safety note: fermentation relies on the right bacteria dominating. If at any point during the 48-72 hour window the beans develop pink, green, or black mold, or smell like ammonia or rotten eggs rather than pungent cheese, discard everything. Good tua nao fermentation smells strong but savory. Bad fermentation smells chemical and acrid. Your nose is your best safety tool. If it makes you recoil, it's gone wrong.

Advance Preparation

• •Soybeans must be soaked overnight (8-12 hours) before boiling. Start the soak the night before you plan to begin.
• •The full process from soaking to finished dried discs takes 4-5 days. Plan accordingly: Day 1 soak and boil, Days 2-4 ferment, Days 4-5 dry. This is not a weekend project you start on Saturday afternoon.
• •Dried tua nao discs keep for 3-6 months stored in a cool, dry place wrapped in paper or cloth. In Lanna villages, they're hung in the kitchen rafters where smoke from the cooking fire provides additional preservation. At home, a paper bag in a cupboard works fine.

Frequently Asked Questions

Nutrition Information

1 serving (about 25g)