Fermented Sweet Rice (Khao Mak)

Ingredients

Equipment Needed

• •Thai bamboo steamer basket (huad) or regular steamer with cheesecloth
• •Clean glass jar or food-safe plastic container with tight-fitting lid
• •Granite mortar and pestle (krok hin) for crushing luk paeng
• •Large clean tray for cooling rice
• •Banana leaves for wrapping (optional, traditional)

Instructions

1. 1

   Soak the glutinous rice

   Rinse the glutinous rice (khao niew) three to four times until the water runs mostly clear. Cover with room temperature water and soak overnight, at least six hours. The grains should be swollen and opaque white, easy to crush between your fingers. This hydration step is critical. Undersoaked rice steams unevenly, and uneven moisture means uneven fermentation. The mold needs consistent access to starch throughout every grain.

   Use Thai glutinous rice, not Japanese short-grain or regular jasmine. Glutinous rice is almost entirely amylopectin starch, which is what the Amylomyces mold in the luk paeng is designed to break down. Regular rice has too much amylose and won't ferment the same way.
2. 2

   Steam the rice

   Drain the soaked rice thoroughly. Line a Thai bamboo steamer basket (huad) or regular steamer with cheesecloth. Spread the rice in an even layer, no thicker than two inches. Steam over rapidly boiling water for 20 to 25 minutes. Halfway through, flip the rice or sprinkle a few tablespoons of water over the top and redistribute to ensure even cooking. The rice is done when it's translucent, tender, and sticky but not mushy. Each grain should hold its shape when pressed.

3. 3

   Cool completely

   Spread the steamed rice on a large clean tray or baking sheet. Let it cool completely to room temperature. This is non-negotiable. If you add the luk paeng to warm rice, you kill the yeast. Saccharomyces dies above 40°C. The mold is more heat-tolerant but still works best at body temperature or below. Spread it thin so it cools faster. Use a clean fan if you want. Just don't touch the rice with dirty hands. From this point forward, everything that contacts the rice must be scrupulously clean. Fermentation amplifies whatever is present: the right organisms make khao mak, the wrong ones make garbage.

   Contamination is the only thing that can ruin this dish. Wash your hands. Clean your utensils. Boil your water and let it cool. The luk paeng organisms are robust, but they need a head start over competing bacteria. Cleanliness gives them that advantage.
4. 4

   Crush the luk paeng

   Place the luk paeng balls in a mortar and pound them to a fine powder. Krok ก่อน. The finer the powder, the more evenly the organisms distribute across the rice. Lumps mean pockets of intense fermentation next to pockets of nothing. You want consistency. The powder should be pale, dry, and have a slightly yeasty, herbal smell. That smell is millions of dormant organisms waiting to wake up.

5. 5

   Inoculate the rice

   Sprinkle about two-thirds of the crushed luk paeng powder evenly over the cooled rice. Sprinkle the cool boiled water over the rice, just enough to lightly moisten the surface. Using clean hands, gently toss and fold the rice to distribute the powder throughout. Don't mash. Don't compress. The grains should remain separate and coated. Every grain needs contact with the starter culture. Think of it as seeding a field. Even coverage is everything.

6. 6

   Pack and ferment

   Transfer the inoculated rice into a clean glass jar or plastic container. Press it gently into an even layer, then use your finger or a chopstick to poke a hole in the center all the way to the bottom. This hole serves two purposes: it lets you monitor liquid accumulation (the sweet liquid that collects is called nam khao mak, and it's a sign fermentation is working), and it provides a slight oxygen channel for the initial mold growth phase. Sprinkle the remaining luk paeng powder on top. Cover tightly with a lid or cling wrap. Place in a warm spot, 28 to 32°C is ideal. In Thailand, that's any corner of the kitchen. In cooler climates, near the oven, on top of the fridge, or in a turned-off oven with the light on.

   The center hole is traditional and functional. After 24 hours you should see clear, sweet liquid pooling in it. That's glucose dissolved in water, produced by the mold's amylase enzymes breaking down starch. If you see liquid, the fermentation is alive. If the hole is dry after 36 hours, something went wrong: dead starter, rice too hot when inoculated, or contamination.
7. 7

   Monitor the fermentation

   Don't open the container for the first 24 hours. The organisms need an undisturbed environment to colonize the rice. After 24 hours, check without touching: you should see a slight white fuzz on the surface (that's the mold, Amylomyces and Rhizopus, doing their work) and smell a sweet, slightly fruity aroma. Clear liquid should be visible in the center hole. After 36 to 48 hours, taste a grain. It should be noticeably sweet, soft, and have a gentle alcoholic tingle. At 48 to 72 hours, the sweetness peaks and the alcohol becomes more pronounced. This is your window.

8. 8

   Stop and store

   When the khao mak reaches your preferred balance of sweetness and booziness, move it to the refrigerator. Cold temperatures slow the organisms dramatically, effectively pausing the fermentation. Left at room temperature, the yeast keeps converting sugar to alcohol and eventually to vinegar via acetobacter bacteria. You end up with sour rice wine, not dessert. Refrigerated khao mak keeps for up to a week, though it's best in the first three to four days. Serve at room temperature or slightly chilled. In banana leaf packets is traditional. In a bowl with crushed ice is the modern street stall method.

Chef Tips

• •Luk paeng (ลูกแป้ง) is the soul of this dish. It's a dried ball of mixed cultures: Amylomyces rouxii (the mold that produces amylase to break starch into sugar), Rhizopus oryzae (another starch-digesting mold), and Saccharomyces cerevisiae (the yeast that converts sugar into alcohol). You can find luk paeng at any Thai or Chinese grocery, usually sold in small bags near the rice section. They look like dusty white ping-pong balls. Different brands produce different flavor profiles, some sweeter, some boozier. Experiment.
• •Temperature controls the speed and character of fermentation. At 28 to 32°C, the mold and yeast work at a balanced pace, producing sweetness first and alcohol second. Above 35°C, bacteria can outcompete the desired organisms and produce off flavors, sourness, or sliminess. Below 25°C, fermentation crawls and may stall entirely. Thai room temperature is perfect for khao mak. If you live somewhere cooler, find a consistently warm spot.
• •The sweet liquid that collects (nam khao mak, น้ำข้าวหมาก) is a drink in its own right. Vendors sell it cold, slightly fizzy from residual carbon dioxide, and sweet with a light alcoholic kick. Don't throw it away. It's the liquid proof of fermentation at work. Some people drink it straight. Some mix it over crushed ice. Both are correct.
• •If you see pink, black, or green mold, or if the rice smells sour, vinegary, or rotten at any point, discard everything and start over. White fuzz is good: that's your Rhizopus and Amylomyces. Anything else means contamination won. Cleanliness at every step is the only prevention.
• •Wrapping individual portions in banana leaves is the traditional Central Thai presentation. The banana leaf imparts a faint green, grassy fragrance and keeps each serving self-contained. Street vendors fold them into neat rectangular packets sealed with toothpicks. Eating khao mak from a banana leaf is part of the experience. The leaf isn't just packaging. It's part of the flavor.

Advance Preparation

• •The glutinous rice must be soaked overnight (at least 6 hours) before steaming. Plan accordingly.
• •Fermentation takes 48 to 72 hours at Thai room temperature (28 to 32°C). In cooler environments, allow up to 4 days. This is not a same-day dish. Start it three days before you want to eat it.
• •Once fermented to your preferred sweetness, refrigerate immediately. Khao mak keeps for up to a week in the fridge but is best within 3 to 4 days. The yeast continues working slowly even in the cold, so it will gradually become more alcoholic and less sweet over time.

Frequently Asked Questions

Nutrition Information

1 serving (about 190g)