Dưa muối – literally “salted vegetables” – is Vietnam’s everyday family of brine-fermented pickles, and the best known is dưa cải chua: mustard greens (cải bẹ, the broad-stemmed leaf mustard sold in Asian groceries as gai choy) wilted in the sun, packed in salted water and left until they turn golden-yellow, sour and crunchy. It’s served as a side with rich braises such as thịt kho trứng (pork and eggs braised in coconut water) and cá kho (caramelised fish), simmered into sour soups (canh dưa chua), and stir-fried with beef or pork. The same method gives dưa giá (bean sprouts) and dưa hành (shallots), and eggplant becomes cà muối.
What Makes It Sour
Dưa muối is a lactic ferment: no vinegar goes in, and the sourness comes from bacteria already on the greens. In 881 lactic acid bacteria isolated from Vietnamese fermented mustard, beet and eggplant, three species dominated – Lactobacillus fermentum (57%), L. pentosus (24%) and L. plantarum (17%) – with strains varying by vegetable and place of production (Nguyen et al., 2013). Fermented cabbage and bean sprouts bought in markets in Huế were dominated by Lactobacillus too, and the cabbage averaged pH 4.45 (Phan et al., 2017). That’s sour, but not by much: the line food-safety rules use to separate acid from low-acid foods is pH 4.6 (21 CFR 114), which is why the rule for dưa muối is to let it get properly sour before you eat it.
Why You Wait: Nitrite and “Dưa Khú”
Vietnamese cooks warn against eating dưa khú – greens that are still green and only half-sour – and the research on fermented greens points the same way. Leafy vegetables, mustard included, carry nitrate, and in the first days of fermentation bacteria turn some of it into nitrite; as the lactic acid bacteria take over, nitrite falls again. In a Chinese study, pickled mustard made with 6% salt held 258.8 mg/kg nitrite at day 8 and none at day 35; home-made leafy sauerkraut peaked at 258.7 mg/kg at day 6 and had none after 20 days, and washing removed about 91% of the nitrite from leafy pickles (Pu et al., 2001). A study of northeastern Chinese sauerkraut, which ferments in the cold, advised waiting four weeks (Zhou et al., 2018). The timing varies with vegetable, salt and temperature, so no study gives a safe day for a warm Vietnamese kitchen. The practical rules: wait until the greens are evenly yellow and clearly sour, don’t eat a half-done batch, and rinse before serving. Northeast China’s suan cai, soured napa cabbage, shows the same rise and fall of nitrite over its first weeks.
A Side Dish, Not a Staple
Salted, pickled vegetables are also part of the gastric cancer picture in East Asia. A meta-analysis of 60 observational studies found about 50% higher odds of gastric cancer among people who ate the most pickled vegetables, with stronger associations in Korea and China (Ren et al., 2012). Studies like these can’t prove cause, but they’re a good reason to enjoy dưa muối the traditional way – a few pieces beside rice and a braise – rather than by the bowl every day. Brine-fermented vegetables are also salty, so count them if you’re watching sodium.
Making It at Home
The recipe card weighs the salt: a 3% brine is 30 g of salt per litre of water. Spoon measures vary too much between salts to give a reliable percentage (see our fermented carrots for the same weighed-brine method). Households in Huế wilt the greens in the sun for three to four hours, submerge them in salt-and-sugar brine and ferment them for two or three days at 25–35°C (Phan et al., 2017); in a cooler kitchen it takes longer, so go by colour and taste, not the calendar. Wilting softens the greens so they pack tightly and stay under the brine. A little sugar is common in southern recipes and gives the bacteria a quick start. Keep everything under the brine: air is what grows mould, and a batch with fuzzy mould or a rotten smell goes in the bin (NCHFP). A white film is usually harmless kahm yeast; our troubleshooting guide shows how to tell them apart. Swiss chard ferments much the same way – see fermented Swiss chard.
Relatives
Fermented mustard greens are made all over Asia: Nepal’s sun-dried gundruk, Sichuan’s pao cai and the pressed mustard stem zha cai, and Korea’s kimchi, which shares the salting but adds a seasoning paste.