Plaa-som (pla som)

What plaa-som is

Plaa-som is a Thai fermented fish with a sour, salty taste and a firm texture. Freshwater fish are salted, mixed with cooked rice and garlic and left in a jar until they turn sour; the finished fish is greyish-white. It is made as a home industry in Thailand's Northeast, and it is a different product from the longer-fermented, more heavily salted fish sauces and pastes of mainland Southeast Asia.

Names and forms

The Thai name is ปลาส้ม. The Japanese agricultural research centre JIRCAS records the local names plaa-som, plaa-khao-suk and plaa-prieo for the fish form, and koong-som and koong-dorng for a shrimp form. The shrimp form is pink where the fish form is greyish-white. The two share a process family but remain different products, and these records do not show that every form shares one preparation or culture.

JIRCAS lists the fish normally used as Probarbus jullieni, Puntius gonionotus, Pangasius hypophthalmus, Labiobarbus leptocheilus and Cirrhinus microlepsis, and the shrimp Macrobrachium lanchesteri.

The method as recorded

The JIRCAS method has two stages. The fish is cleaned and mixed with salt at 8 parts fish to 1 part salt by weight, then left overnight. Next morning cooked rice and minced garlic are added in a 20:4:1 weight ratio, the mixture is packed into a jar and kept at ambient temperature. Fermentation takes 5 to 7 days and the entry gives a storage life of about 3 weeks. The entry prints the ratio without saying which number belongs to which ingredient, and it gives no absolute quantities or temperatures, so the recipe above does not assign them.

A study of the Thai-Yor community of Tha Khon Yang village in Maha Sarakham describes the same family of ingredients in local terms: small freshwater fish mixed with steamed glutinous rice, garlic and salt. The community keeps its recipe ratio secret. A local news report describes a pla-som training project in Nakhon Phanom. These are specific community records, not a national standard or a claim about origin.

What happens in the jar

Plaa-som is a lactic-acid fermentation. The salt draws liquid from the fish and holds back many spoilage organisms, and the cooked rice adds starch and sugars for the bacteria to use. Lactic-acid bacteria then acidify the mixture, which is what makes the fish sour. A study of four batches made with different salt concentrations reported different acidification and microbial patterns at different salt levels, so the amount of salt shapes how quickly and how far the souring goes.

The microflora is not a single organism. JIRCAS lists many bacteria and yeasts reported from plaa-som, among them Lactobacillus plantarum, Pediococcus pentosaceus, Weissella cibaria and Zygosaccharomyces rouxii, and a separate study describes Kazachstania yeast in sampled products from northeastern Thailand. These observations concern the batches and samples studied. They do not make a species name a starter recommendation or create universal fermentation limits.

Starter culture trials

Traditional plaa-som is made without an added starter, and several laboratory studies have tested whether one helps. A 2010 study used a mixed culture of Lactobacillus plantarum IFRPD P15 and Lactobacillus reuteri IFRPD P17, both isolated from natural plaa-som fermentation. The culture was air-dried with sterilised rice grains as filler and stayed stored at 4 degrees C for more than 20 weeks. In the trial P15 was the main acid producer and P17 suppressed pathogens such as E. coli within 24 hours; a single-strain starter did not fully suppress pathogens. The authors suggest the mixed starter may shorten fermentation.

Other work selected lactic-acid bacteria from products made by producers in Phayao, and a 2023 paper tested an encapsulated bacterial powder in another batch. These are distinct experimental systems; none establishes a single culture for all plaa-som.

Eating plaa-som

Plaa-som is usually cooked before it is eaten. JIRCAS records it fried, topped with chopped shallot and bird's eye chilli, and eaten with rice.

What remains open

The sources reviewed do not establish a complete, independently reviewed household method, absolute quantities, a temperature range, seasonal patterns or many community-specific variations. The studies of starters and microflora describe the batches they analysed. Further work needs full experimental methods, Thai-language primary sources and qualified seafood-process review. Smell, taste or sourness alone do not establish that a fermented fish is safe to eat.