Burong mustasa is a Filipino fermented pickle made from flat mustard greens, salt, and rice water. The entire process takes about a week, requires no special equipment, and produces a tangy, funky condiment that pairs perfectly with fried fish, tofu, and steamed rice. Here’s how to make it at home.
What You Need
The ingredient list is short. For a single jar, you’ll need about 8 ounces of fresh flat mustard greens, 2 tablespoons of coarse sea salt, and the starchy water left over from cooking rice. You’ll also need a clean glass jar with a tight-fitting lid, a mesh strainer or cheesecloth, and a sunny spot outdoors or near a window.
The rice water is the key ingredient most people overlook. It’s not just a liquid to submerge the greens. The starches dissolved in it serve as fuel for beneficial bacteria, giving them the carbon they need to multiply and drive fermentation. Without it, you’d get a basic salt brine pickle rather than a true ferment with that characteristic sour depth.
Preparing the Greens
Start by washing the mustard leaves thoroughly and patting them dry. Then lay them out in direct sunlight for about an hour, or until they’ve visibly wilted. This step removes excess moisture that could dilute your brine and slow fermentation. The leaves should look slightly limp and flexible, not crispy or dried out.
While the greens are wilting, cook your rice as you normally would. Before draining or once the rice is soft, reserve the cloudy cooking water and let it cool completely to room temperature. Hot liquid would kill the very bacteria you’re trying to cultivate.
Salting and Packing the Jar
Once the greens are wilted, combine them with the coarse sea salt in a large bowl. Gently mash the mixture with your hands or a wooden spoon. You’re not trying to pulverize the leaves, just bruise them enough to break down cell walls and release their natural juices. The salt draws out additional moisture through osmosis, creating an environment where beneficial bacteria thrive and harmful ones can’t compete.
Pack the salted greens tightly into your clean glass jar, pressing them down firmly to eliminate air pockets. Pour the cooled rice water over the greens until they’re fully submerged. Any leaves poking above the liquid line are exposed to oxygen and can develop mold. Place a mesh strainer or a small weight on top to keep everything below the surface, then seal the jar.
Fermentation Timeline
At tropical room temperatures (around 80 to 90°F), burong mustasa is typically ready in about seven days. If you live in a cooler climate, expect it to take a few days longer. The fermentation is driven primarily by Lactobacillus plantarum, a lactic acid bacterium that naturally lives on the surface of mustard greens and converts the sugars in rice water into lactic acid. This is the same species found in other traditional fermented vegetables worldwide, and research has shown it can adhere to the lining of the small intestine, functioning as a genuine probiotic.
During the first two to three days, you’ll notice small bubbles forming. This is normal and a good sign that fermentation is active. You can “burp” the jar once a day by briefly cracking the lid to release built-up carbon dioxide, then resealing it. By day four or five, the liquid will turn cloudy and the greens will shift from bright green to an olive or yellowish tone. Taste a small piece around day five. When the sourness reaches a level you enjoy, move the jar to the refrigerator to slow fermentation dramatically.
How to Tell If Something Goes Wrong
A thin, creamy white or beige film on the surface of your brine is almost certainly kahm yeast. It’s not dangerous, just unappealing. Kahm yeast stays at the top, looks smooth or slightly wavy, and never appears fuzzy. You can skim it off with a clean spoon and continue fermenting without worry. If it gets very thick, though, mold can grow on top of it, so skim early and often.
Actual mold looks distinctly different: fuzzy or hairy, and it comes in colors like green, black, blue, or sometimes white with a cottony texture. If you see this, discard the entire batch. Don’t try to scoop around it.
The acidity of properly fermented vegetables is your main safety net. The pH of a successful ferment drops below 4.6, a threshold that prevents the growth of Clostridium botulinum and other dangerous pathogens. Vegetables fermented in a simple salt and water brine (without added protein from dairy or soy) reliably reach this safe range. If your burong mustasa tastes distinctly sour, that’s the lactic acid doing its job.
Health Benefits Worth Knowing
Beyond the tangy flavor, burong mustasa carries real nutritional value. Mustard greens are rich in vitamins K and C, and the fermentation process makes certain nutrients more bioavailable while adding probiotic bacteria to the mix. Studies on lactic acid bacteria isolated from fermented mustard greens have identified strains of L. plantarum with measurable cholesterol-lowering properties in lab settings. Other species commonly found in fermented mustard greens, including Pediococcus acidilactici and Lactobacillus acidophilus, are well-established members of the probiotic family.
How to Serve Burong Mustasa
The simplest way to enjoy it is straight from the jar, chopped roughly and served alongside fried fish and steaming rice. The sourness and slight bitterness of the fermented greens cut through rich, oily dishes beautifully.
For a more composed dish, try ginisang burong mustasa at tokwa, a sautéed version with fried tofu that works as both a rice topping and a drinking snack. Fry cubed tofu in oil until golden and crispy on all sides, then set it aside. In the same pan, sauté sliced onion until translucent, add minced garlic, and cook until fragrant. Toss in chopped burong mustasa with a splash of its brine and stir just until warmed through. Add the fried tofu back in and mix everything together. The contrast between the crispy tofu and the funky, sour greens is exactly the kind of pairing that makes you reach for another cold beer.
You don’t even have to cook the greens if you prefer their raw fermented crunch. Just prepare the aromatics and tofu, turn off the heat, and fold the chopped burong mustasa in at the end. Both versions work, and it comes down to whether you want a softer, mellower flavor or a sharper, more acidic bite.