Punch Up Your Meals With Bornean Fermented Mustard Greens

Kasam sabi—pleasantly tangy and salty fermented mustard greens—is commonly eaten as a side dish with rice or combined with meats on the hot and humid island of Borneo.

Overhead view of fermented Mustard Greens
Serious Eats / Amanda Suarez

On the hot and humid island of Borneo, lacto-fermentation is an advantageous food preservation technique found everywhere. While dehydration is certainly used to extend the life of some meats and fish, year-round humidity makes drying an inefficient method of preservation—which is why most meats are smoked, and many vegetables are fermented instead.

Lacto-fermentation's simplicity—along with the rare salt springs dotting the highlands and the robust salt trade between the coast and the islands—made it the most efficient method for preserving food before refrigeration. What started as a necessity soon overtook the local palate as people came to know and love the distinct taste and aroma of lacto-fermented food. Even with the availability of modern refrigeration today, lacto-fermented food remains an essential part of the Dayak (indigenous Bornean) diet.

Among the many lacto-ferments in Dayak cuisine, kasam sabi, or lacto-fermented mustard greens locally called ensabi, is one of the most popular. Kasam sabi’s signature sharp aroma comes from the vegetable itself. Ensabi, part of the Brassica juncea family of mustard greens, tastes slightly bitter with a mustardy tang. There is always an abundance of ensabi, thanks to it thriving in the hot and humid Bornean climate, but its delicate leaves and stem quickly go from bursting with water and crisp when plucked fresh to wilting after just a few hours. This makes it an ideal candidate for preservation. All you need to do is cast handfuls of seeds onto bare earth and in a few weeks, you’ll have bushels of the dark, leafy green to eat!

The Science of Lacto-Fermentation

Broadly speaking, lacto-fermentation happens when lactic acid bacteria consume carbohydrates and produce lactic acid. The bacteria is naturally present on the surface of vegetables and fruits, waiting for the right conditions to truly thrive. Those conditions include a high salinity and lack of oxygen, which is exactly what happens when you salt the vegetables: Salt draws moisture from the cell walls, submerging the vegetable in a brine of their own juices (if there aren't enough juices from the salted vegetables, you can make additional brine by dissolving salt in water and adding it to the mustard greens and their brine). This is the same process by which sauerkraut, kim chi, and yogurt are made, but in kasam sabi's case the result is a lacto-fermented mustard green that's pleasantly tangy and salty.

How to Make Kasam Sabi

Fermenting kasam sabi is quick and easy when your average daily temperature is 82ºF (28ºC), a warm temperature in which lactic acid bacteria kick into high gear. Though you’ll be able to ferment the vegetables at cooler temperatures like 70ºF (21ºC), the process will happen at a much slower pace.

The process is simple. After washing and drying the ensabi—which is kept whole, minus the roots—you mix the greens with 2% of its weight in salt. The salt not only draws out water in the mustard greens to form the brine in which the greens will ferment, but also minimizes risk of spoilage by preventing less salt-tolerant microorganisms that could cause spoilage or foodborne illness from becoming dominant.

Close up of mustard greens with salt
Serious Eats / Jordan Provost

There are several ways to make kasam sabi. Some recipes call for cooked white rice to be added to the mixture to provide additional sugars and nutrients for the lactic acid bacteria to metabolize, resulting in a more intense tanginess (if adding rice, you will need to increase the amount of salt in the recipe to maintain a 2% salinity for fermentation; the below recipe, which includes rice, has already factored this into the salt quantity, so you do not need to add extra salt).

If you’d like a more savory, less sour version, you can omit the rice, but having grown up with the rice version, I prefer its sharper salty-sour flavor. Another option is to sun-dry the ensabi, which allows the greens to wilt and results in kasam sabi with a drier, crisper texture (even when wilted, the greens still release enough liquid to form a brine).

Overhead view of released liquids
Serious Eats / Jordan Provost

Kasam sabi used to be fermented in ceramic jars, but most people today use airtight plastic or glass containers instead. Depending on the ambient temperature of your home, the kasam sabi’s fermentation is usually complete somewhere between four days (in hotter climates) and two weeks (in cooler ones). 

As for how to use it, kasam sabi is commonly eaten both as a side dish with rice during mealtimes or combined with meats (including smoked meats) and/or starches like rice or potatoes to make flavorful stews.

In a large bowl, wash the mustard leaves in several changes of cold water to remove any sand or grit. Using a salad spinner, spin dry.

Overhead view of mustards in a salad spinner
Serious Eats / Jordan Provost

In a medium bowl, massage the mustard leaves with salt, using your hands to lightly bruise the leaves without tearing them, until the leaves begin to turn limp and juices slowly build up, 3 to 5 minutes. Add the rice and mix well.

Four image collage of squeezing mustard greens with salt and adding ric
Serious Eats / Jordan Provost

In a clean, airtight fermentation vessel, pack in the mustard leaves and their brine while distributing the rice grains evenly throughout. Press down to submerge greens and rice with brine; if there is not enough natural brine to fully cover the solids, make additional brine by dissolving 1 teaspoon (3g) Diamond Crystal kosher salt (or 1/2 teaspoon table salt) in 1 cup (240ml) room temperature water, then pour this brine into the fermentation vessel until there is enough to fully submerge the solids. Press plastic wrap directly on the surface of the mustard greens and their brine to help keep them submerged, weighing it down if necessary, then close container.

Four image collage of adding mustard greens and rice to the jar along with liquid and covering with plastic wrap
Serious Eats / Jordan Provost

Allow mustard leaves to ferment at room temperature until the kasam sabi is pleasantly sour and salty, 4 days to 2 weeks (fermentation time can vary depending on the climate); make sure mustard greens remain submerged throughout the fermentation process as exposure to oxygen can lead to spoilage. Signs the ferment has gone wrong include an unpleasant, spoiled aroma, excessive visible mold (small amounts can be scraped off the surface), and any kind of slimy texture. Transfer to the refrigerator and consume within 2 to 3 months.

Side view of greens on table
Serious Eats / Jordan Provost

Special Equipment

Airtight food container for fermentation


In Borneo, the mustard leaves are a variety locally known as ensabi, which has a pungent aftertaste when consumed raw. It’s seldom found outside of Borneo, but you can substitute with any available mustard greens.

Kasam sabi can be used as a condiment to accompany stews or soups, or stir-fried with onions, garlic, and fresh red or green chiles to be eaten as a side with rice.

Make-Ahead and Storage

Once the kasam sabi has fermented, it can be kept in the refrigerator to slow its fermentation for up to 3 months (technically it can last even longer, but its flavor will begin to fade).