From Fish to Pla Ra and Pla Som: Understanding Thai Fermented Fish Dishes

Pla som is a Thai fermented fish dish made by packing fresh fish with salt, cooked rice, and garlic, then letting it ferment at room temperature for 2–5 days until it turns pleasantly sour. It is one of the most accessible introductions to Thai fermented fish because it is quick to make, mildly tangy, and often eaten raw or lightly cooked. This article focuses on how pla som is prepared and how it compares to the stronger, longer-fermented pla ra, while explaining the ingredients and tools that create these distinctive flavors.

What is Pla Som? A Mildly Sour Fermented Fish

Pla som (ปลาส้ม) literally means "sour fish" in Thai. It is a short fermentation, lasting only a few days, that produces a tender, slightly sour fish with a clean, lactic tang. The texture remains firm but softens compared to fresh fish, and the aroma is pleasantly yeasty and garlicky, not overwhelming. Pla som is often made from freshwater fish like snakehead fish, tilapia, or small carp, but any firm white fish works. The key is using very fresh fish, because the fermentation is brief and does not mask off flavors.

Unlike pla ra, which ferments for months and develops a strong, pungent, almost cheesy aroma, pla som is mild and approachable. It is commonly eaten as a side dish with sticky rice, or fried briefly until golden. In northeastern Thailand (Isan), pla som is a beloved regional staple, sometimes wrapped in banana leaves and grilled.

How Pla Som is Made: The Basic Fermentation Process

Pla som relies on lactic acid fermentation, similar to sauerkraut or yogurt. The fish is mixed with salt, cooked rice, and garlic. The salt draws moisture from the fish, creating a brine, while the cooked rice provides sugars that feed naturally occurring lactic acid bacteria. These bacteria produce lactic acid, which lowers the pH and preserves the fish, giving it a sour taste. The garlic adds flavor and has mild antimicrobial properties.

Here is a typical sequence for making pla som at home:

1. Prepare the fish: Clean and fillet firm white fish, removing bones and scaling. Cut into bite-sized pieces, about 1–2 inches wide.
2. Mix the dry ingredients: For every 1 kg (2.2 lbs) of fish, combine about 100 g (3.5 oz) of cooked jasmine rice (cooled), 50 g (1.8 oz) of minced garlic, and 25 g (0.9 oz) of salt. Adjust salt to taste; it should be noticeable but not harsh.
3. Coat the fish: Toss the fish pieces with the rice mixture until evenly coated. The salt will start to draw out liquid.
4. Pack tightly: Press the mixture into a clean glass or ceramic jar, removing air pockets. The fish should be submerged in its own juices. If needed, add a little cooled boiled water to cover.
5. Ferment at room temperature: Cover the jar with a lid or cloth. Keep at around 25–30°C (77–86°F) for 2–5 days. Check daily: small bubbles may appear, and the aroma should become pleasantly sour, not rotten.
6. Store: Once the desired sourness is reached, refrigerate to slow fermentation. Pla som keeps for about 1–2 weeks in the fridge.

A common mistake is using too little salt or too much rice, which can lead to spoilage. If the fish smells putrid or develops a slimy texture, discard it.

Key Ingredients: Fish, Rice, Salt, and Garlic

Each ingredient in pla som plays a specific role:

• Fish: The main protein. Firm, white-fleshed fish like tilapia, snakehead, or sea bass holds its shape during fermentation. Freshness is crucial; if the fish smells fishy before fermenting, it will only get worse.
• Rice: Usually cooked jasmine or sticky rice. It provides starch, which breaks down into sugars that feed the lactic acid bacteria. The rice also adds a subtle sweetness and helps create a slightly sticky coating.
• Salt: It inhibits harmful bacteria, draws moisture from the fish to create a brine, and seasons the final product. Use non-iodized salt (sea salt or kosher salt) because iodine can interfere with fermentation.
• Garlic: Adds a pungent, aromatic flavor that complements the sourness. It also contributes antimicrobial compounds that help keep the fermentation safe.

Some recipes add a little sugar or cooked sticky rice only, but the core is always fish, rice, salt, and garlic.

Pla Som vs. Pla Ra: Two Ends of the Fermentation Spectrum

While pla som is a short, mild fermentation, pla ra (ปลาร้า) is a long, intense one. Pla ra is made by fermenting fish, often with roasted rice bran or rice, and a higher salt concentration, for several months to over a year. The result is a dark, liquidy paste with a strong, funky, almost ammonia-like aroma. Pla ra is used as a cooking ingredient—a little goes a long way in dishes like som tam (green papaya salad) or nam phrik (chili dips)—whereas pla som is often eaten as a dish on its own.

The table below summarizes the differences:

Understanding this spectrum helps you appreciate how fermentation time and salt concentration transform the same raw material into very different products.

Kitchen Tools for Making Pla Som

You don't need special equipment, but a few basics help:

• Glass or ceramic jar: Non-reactive and easy to clean. A wide-mouth jar makes packing and removing fish easier. Avoid metal containers, as salt and acids can react with them.
• Weights or a fermentation lid: Keeping the fish submerged under the brine prevents mold. You can use a small plate or a zip-top bag filled with water as a weight.
• Clean hands or gloves: Mixing by hand ensures even coating of the rice mixture. Use food-safe gloves if you prefer, but clean hands are traditional.
• Knife and cutting board: For cleaning and cutting the fish. A sharp knife makes removing bones easier.

No special fermentation crock is needed; a simple mason jar works well for small batches.

How to Eat Pla Som

Pla som is versatile. Here are common ways to enjoy it:

• Raw as a side dish: Sliced pla som is often served with fresh herbs (mint, cilantro), sliced shallots, ginger, and Thai chilies. Wrap a piece in a lettuce leaf with some herbs and sticky rice for a burst of sour, salty, and spicy flavors.
• Fried pla som: Coat pieces in a little rice flour and shallow-fry until golden and crispy. The sourness mellows and the outside becomes crunchy. Serve with a squeeze of lime and a dipping sauce of fish sauce, chilies, and garlic.
• In salads: Shredded pla som can be tossed with green mango, shallots, and a lime-fish sauce dressing, similar to a Thai sour fish salad.
• With sticky rice: The classic Isan pairing: a ball of sticky rice dipped into the juices of pla som.

If you are new to fermented fish, start with a small piece to get accustomed to the sourness. The flavor is bright and tangy, not fishy in the way you might expect.

Substitutes and Adjustments

If you cannot find traditional freshwater fish, any firm white fish works. Avoid oily fish like mackerel, as their fat can go rancid during fermentation. If you are concerned about eating raw fermented fish, you can cook pla som thoroughly: simmer in coconut milk or fry until the internal temperature reaches 63°C (145°F) for safety.

For the rice, jasmine rice is commonly used, but sticky rice gives a slightly sweeter result. In a pinch, cooked white rice from any long-grain variety works, but the texture may differ. Glutinous rice is not essential.

Garlic can be adjusted to taste; some recipes use a lot, while others use just a hint. Do not omit it entirely, because it contributes to the overall flavor and safety.

Safety and Troubleshooting

Because pla som involves raw fish and a short fermentation, proper hygiene is essential. Always use clean utensils and containers. If you see mold on the surface, discard the batch—mold can produce harmful toxins. A white film on the brine is often kahm yeast, which is harmless but can affect flavor; skim it off. The fish should smell sour and garlicky, never rotten. When in doubt, throw it out.

If the fish is too salty, rinse it briefly before eating. If it is not sour enough, let it ferment another day. Temperature affects speed: warmer fermentation is faster, but temperatures above 35°C (95°F) may encourage spoilage.

Making pla som at home is a rewarding way to explore Thai fermented foods. It requires only a few ingredients, a little patience, and a sense of culinary adventure. The result is a vibrant, tangy dish that showcases the transformative power of fermentation.