Chinese Mustard Greens vs Mustard Greens: A Traditional Thai Fermentation Primer

What is the difference between Chinese mustard greens and other mustard greens in Thai fermented dishes?

Chinese mustard greens, known as gai choy (or phak kwang tung in Thai), are a specific variety of mustard green prized for their thick, crunchy stems and bold, peppery leaves. In Thai cooking, they are most famously used to make pickled mustard greens (phak kat dong), a tangy, salty ferment that adds sour punch to soups, stir-fries, and noodle dishes. Other mustard greens—such as the frilly mizuna, the broad-leafed tendergreen, or the smaller curly mustard—have thinner stems, softer leaves, and less bite, making them less ideal for the firm texture and sharp flavor needed in long ferments.

In traditional Thai fermentation, gai choy and similar large-stemmed varieties (like sawtooth mustard or large-petiole mustard) are preferred because their thick stalks stay crisp during brining, while their pungent leaves mellow into a deep mustardy tang. Other mustard greens, while delicious fresh or lightly pickled, tend to collapse into mush when fermented for weeks, and their milder flavor gets lost against garlic, rice bran, or chili.

Recognizing gai choy: leaf shape, stems, and visual character

Gai choy is easy to spot: it has broad, rumpled leaves with prominent white or pale-green veins, and thick, succulent stems that resemble celery or bok choy stalks but with a mustardy hairiness. The leaves can grow 30–60 cm long, forming a loose, upright head rather than a tight ball. The leaf shape is oblong with deeply toothed or frilled edges, and the surface is often bumpy or puckered. The color ranges from deep jade green to yellow-green, sometimes with a bluish cast.

By contrast, other mustard varieties have distinct silhouettes:

• Curly mustard (also called American mustard) has finely ruffled, bright green leaves and thin, flexible stems. It looks like a feathery lettuce but tastes hot.
• Mizuna (Japanese mustard) has narrow, deeply serrated leaves on very thin stalks; it is often sold as a salad green, with a mild, peppery flavor.
• Tendergreen (spinach mustard) has smooth, spoon-shaped leaves and slender stems; it is milder and more tender, similar to spinach.
• Red or purple mustard adds color but has stems too thin for pickling.

For Thai fermented pickles, look for mustard greens with thick, fleshy stems—at least 2–3 cm wide at the base—and leaves that hold their shape when squeezed.

How are Chinese mustard greens used in Thai fermentation?

Thai pickled mustard greens (phak kat dong) are made by a simple lactic acid fermentation. The process is similar to making sauerkraut or sour pickles, but Thai versions often include rice wash water or cooked rice to encourage a slightly sour, funky flavor.

Here is a basic traditional method:

1. Wash and wilt: Rinse whole gai choy plants thoroughly, then leave them in the sun for 2–4 hours (or overnight) until leaves are limp but not dry.
2. Salt and massage: In a large bowl, mix the wilted greens with coarse salt (about 2–3% of the weight of the greens). Rub the salt into the stems and leaves, squeezing gently to release some juices.
3. Pack into a jar: Press the salted greens tightly into a clean glass or ceramic jar, layering with optional aromatics like garlic, ginger, or bird's eye chili. Pour in any liquid released, plus enough cooled boiled water (or rice rinse water) to cover the greens completely.
4. Weigh down and ferment: Place a weight (a smaller jar filled with water, or a clean stone) on top to keep the greens submerged. Cover with a cloth and leave at room temperature (25–30°C is ideal) for 3–7 days, tasting from day 3.
5. Store: When the flavor is sour and mustardy to your liking, remove the weight, cap the jar loosely, and refrigerate. The pickles will keep for months and continue to sour slowly.

The result is a golden-green tangle of stems and leaves with a mouth-puckering sourness, salty depth, and a lingering mustard heat that softens into umami. The stems stay crunchy, while the leaves become silky and intense.

How do other mustard greens fare in pickling and fermentation?

Other mustard varieties can be fermented, but they require shorter brines and are eaten more like delicate kimchi than long-keeping pickles. Their thin stems soften quickly, and their flavor—though spicy when fresh—fades into a generic cabbage-like sourness. For example:

• Curly mustard makes a decent quick pickle (1–3 days) but loses its shape after a week.
• Mizuna is too thin for brining; better wilted or stir-fried.
• Tendergreen works in mixed vegetable ferments where crunch is less important.

If you cannot find gai choy, the closest substitutes for Thai fermentation are large-stemmed bok choy (especially Shanghai bok choy), napa cabbage (for a softer result), or collard greens (for sturdy leaves). However, these lack the distinct mustard sharpness, so add a teaspoon of ground mustard seeds or a spoonful of prepared mustard to the brine for a closer flavor.

Comparing Chinese mustard greens and other mustard varieties for Thai cooking

What do fermented gai choy and other pickled mustards look, smell, and taste like?

Fermented gai choy has a distinctive appearance: the leaves turn from bright green to a translucent olive or amber, and the thick stems become pale yellow with a glassy sheen. The brine is cloudy and slightly viscous. The aroma is sharply sour, earthy, and mustardy—like a cross between sauerkraut and horseradish, with fruity notes from natural fermentation. On the palate, the first hit is salty-sour, followed by a slow-building mustard heat that clears the sinuses but is not as fiery as fresh mustard. The texture is key: the stems should snap cleanly between the teeth, while the leaves chew like soft leather but with a juicy burst of brine.

Other pickled mustards (e.g., quick-pickled curly mustard) tend to be softer, less sour, and more uniformly cabbage-like. Their color stays greener, and their mustard flavor is milder, often masked by vinegar if made as refrigerator pickles. They lack the complex, funky depth that comes from longer fermentation with rice.

Simple tips for using pickled Chinese mustard greens in Thai dishes

Once you have a jar of phak kat dong, here are approachable ways to enjoy it:

• Rinse or not? If the pickle is very salty, rinse briefly before using; otherwise, squeeze out excess brine.
• Chop and add last: In stir-fries, add chopped pickled greens at the end to keep some crunch.
• Broth booster: Simmer a handful in chicken or pork broth for 10 minutes to create a sour, savory base (as in kaeng som or tom yum without lemongrass).
• Noodle garnish: Top khao soi or pad thai with a small mound for contrast against rich coconut or sweet tamarind.
• Rice side: Serve as a salty-sour condiment alongside grilled meats and jasmine rice, like a Thai version of pickled cucumbers.

If you don’t have gai choy pickles, chopped sour sauerkraut (rinsed) or Japanese takana pickles can stand in, though they lack the same mustard intensity. Add a pinch of dry mustard powder to approximate the heat.