This is Kudam Puli belonging to the Mangosteen family. It is known as Garcinia Cambogia or Gambooge.
It is often mistaken for kokum. Kokum (Garcinia Indica) also belongs to the same family but it is dark red to purple in colour. Kokum is used mostly in Maharashtrian and Konkan cooking while Gambooge is used in Kerala cooking.This variety is green which turns yellow when it ripens. This is a very important ingredient in Ayurvedic medicines for weight loss. This also aids in digestion.
I can vouch for its weight loss properties. I used to cook fish curry with this daily and used to roll the gamboge inside my chapati and have it daily. Yes, it did help me reduce my weight.
During summer, hundreds of fruits are produced from a single tree. This is plucked when ripe and halved. The outer cover of the fruit is sundried for a day or two.
These are then smoked till dark. See the picture below.
Once it is smoked to perfection, it is rubbed with oil and then stored away for later use. It stays fresh for a very very long time.
It is used as a sour spice, as a substitute for tamarind for making fish curry and sometimes while making bitter gourd and colocasia too.
The picture below is the dried rind. This has to be washed properly before using it for cooking. Thats because it is sundried and smoked- ofcourse, possibilities of dust and even stones in between the rinds are
there. So again, wash properly!!!
The washed gamboge is then soaked in warm water for a few minutes and then added to the curry along with the water.
The picture below is travancore style fish curry made with Kudam Puli or Gambooge (see the black rind in the curry?)