The delicious flavour of Thai food has its basis in a combination of four tastes _ sour, salty, sweet, and spicy _ which are balanced differently depending on the dish. In some foods sweetness dominates _the thick, sweet, coconut cream-based curries made with pork, chicken, or beef called phanaeng, are a good example, as are the Chinese aromatic stewed pork belly dish called palo moo sam chan and the sugary shredded pork known as moo wan. Others put the stress on chilli heat. Pad cha pla krabane (a spicy stir-fry of stingray, herbs, and potent seasonings including chilli), pla duc pad phet (catfish stir-fried with chilli and other seasonings), or the fiery southern curry called kaeng tai pla.

PUCKER UP: ‘Taling pling’ is a super sour fruit that you won’t find in the local market.
Salty dishes include tom khem recipes in which fish like mackerel or carp are cooked in a salty broth. For sourness, there are the soup-like kaeng som and its southern Thai version known as kaeng lueang, kaeng khua-type curries like kaeng phet pet yang, the pineapple and horseshoe crab egg curry called kaeng khua sapparot kap khai maeng da, and kaeng moo sam chan kap pak boong (a curry made from pork belly meat and the shoots of a morning glory-like vine). The sour-hot salads called yam in Thai include all four tastes, with the sourness kept from becoming too prominent.
Among these four basic tastes, saltiness usually comes from nam pla or plain salt and sweetness from palm or cane sugar. There are not many other sources. But the ways to make food spicy or sour are many. Heat can come from different kinds of chillies, including the fiery prik kee nu, the less potent prik chee fa, or dried chillies that have been ground or pounded, as well as from other spicy ingredients.
But there are even more ways to give a dish a sour bite, because the sourness comes from vegetables or fruits whose flavour and acidity differ. There are many of these, including pineapple, lime, kaffir lime, the shiny green fruit called madan in Thai, sour tamarind, starfruit, certain unripe oranges, the tiny sour tomatoes called makheua prio, the ultra-sour cucumber-shaped fruit called taling pling, rosella fruit and leaves, and the southern citrus called som khaek.

Limes are at the top of the list because in addition to their intense sourness there is the added attraction of the fragrance their peels release when they are squeezed. However, limes are not appropriate for use in every dish that requires sourness. They don't work, for example, in kaeng som, which should be made with sour tamarind. The sour-hot-salty sauce eaten with fried fish, too, should be seasoned only with tamarind. For khanom jeen nam prik and kaeng massaman, the sourness should come from kaffir lime juice, and the crisp-fried noodle dish called mee krawp is flavoured with the skin of the rough-peeled citrus called som sa.
Lime is used to make nam prik dishes sour, but in some of them the chief sour ingredient is some other fruit with the lime used only as an auxiliary seasoning. Nam prik makham sot, for example, is made with tamarind fruit, and nam prik madan gets its sourness primarily from the madan fruit. The sour-hot soups called tom yam are generally made with lime juice, but a regional variant native to Samut Songkhram made with mackerel uses madan instead and is called pla thu tom madan. The banana flower salad called yam hua plee gets its sourness from tamarind.
Thais vary from place to place and season to season in their choice of sour fruits to season food. When pad thai is made in some rural areas seasonal fruits are used in place of lime to give it the proper sourness. Slivered unripe mango or star fruit might be scattered over it to give it a sour bite that is less aggressive than when lime is squeezed over the noodles.
Taling pling has been used for a very long time in Thai cooking, but now few people are familiar with it. It is rarely sold in markets, but farmers know it and use it in the kitchen when it is appropriate. The tree bears fruit during the hot season, when limes are in short supply.
When the pork belly curry called kaeng khua moo sam chan is made in rural kitchens they add a combination of sour fruits _ the fuzzy ma-ueak (an aubergine relative), makhuea prio and taling pling. Since the delectable dish is rarely prepared this way in the capital, the people who get to enjoy it are farmers, not Bangkok foodies.
But it isn't only country people who use taling pling for cooking. A dish made in the royal palace called paeng sip sai pla nueng (a steamed fish recipe) is eaten together with taling pling fruits. They add a sour accent and mask the smell of the fish. Chopped taling pling is also welcome in pad thai and some nam prik dishes.
The fibrous, sour green fruit called makawk in Thai has thick, crunchy pulp that can be chopped and made into a sour and spicy salad with crispy smoked sheatfish. The dish has a sour taste very different from that of lime and tamarind.
The same is true of mangoes of the type called mamuang kaew in Thai. There is enough pulp on them when they are still sour and unripe to chop up and eat with catfish that has been teased up with a fork and deep fried to crispness. It is also good sprinkled over a fried pla samlee, a salt-water fish with plenty of firm white meat.
Nothing else is as suitable as this raw mango for giving the dish a mouthwatering sour tang.
The curry called kaeng khua made with horseshoe crab eggs or dried mussels has to be seasoned with pineapple, as does the sao nam sauce eaten together with fermented rice noodles to make khanom jeen sao nam. It harmonises well with the seasoning sauce of chopped ginger and chillies in nam pla eaten with the dish.
So when it comes to the sour segment of the spectrum of flavours that give such endless variety of Thai food, there is much more to it than the vinegar and lemon that some other cuisines rely on to enliven recipes.
A good cook knows how to use taling pling, madan, sour tamarind or mango, and other fruits that combine sourness with unique tastes and fragrances to give each dish its special character.