The notion of adding an acidic tang to recipes has been popular for thousands of years, and arguably the most used acidic spice of all is tamarind.
The tamarind tree (Tamarindus indica) is large and spreading, growing up to 20 metres in height. These trees provide a wonderful canopy of shade, punctuated by small clusters of red-striped, yellow flowers when in bloom.
My most vivid recollection of the majestic tamarind tree was when Liz and I led a spice tour to India in 1991. After a long, hot and dusty drive, we had a picnic lunch outside Hyderabad on an enormous handmade rug spread out underneath the cooling shade of a tamarind tree.
There is a belief that tamarind trees emit harmful, acrid vapours, making it unsafe to sleep under them and that plants will not grow there due to the acidity exhaled from the tree overnight. This may be why there is usually little vegetation around their bases, creating an ideal picnic spot in the middle of a hot Indian March day.
After flowering, tamarind pods form as 10 centimetre long fruits that are knobbly and light-brown. These contain an acidic pale brown pulp that surrounds about 10 shiny, smooth, dark brown, angular seeds. These bulbous, knuckle-like pods have a brittle shell, which when broken away reveals a pale-tan, sticky mass with longitudinal strings and fibrous veins attached. Upon coming into contact with the air, the pulp begins to oxidise and turns dark brown and almost black. The aroma of the oxidized pulp is vaguely fruity and sharp while the flavour is intensely acidic, tingling, refreshing and reminiscent of dried stone fruit.
Tamarind is very sharp-tasting because of its high tartaric acid content, and is one of the most popular souring agents used in dishes in the majority of tropical countries. Although it may often seem very moist, tamarind should never go mouldy as its high level of acidity acts as a preservative.
Recipes will generally call for a quantity of tamarind water, typically two tablespoons to a cup, to be added during cooking. To make tamarind water from the hard tamarind block, break off a walnut-sized piece (a 2 cm diameter ball) and put it into half a cup of hot water. Stir it around with a spoon and leave for about 15 minutes. Strain the liquid off, squeezing the remaining pulp as dry as possible before discarding it. Tamarind water can be made in large batches and frozen into ice cubes to drop into cooking whenever the fruity tang of tamarind is required.
Tamarind concentrate from India is a thick, black, molasses-type liquid, and is made by boiling down an extract of oxidized tamarind paste that has been strained to remove the seeds and fibre. This is a somewhat more convenient way to add tamarind to cooking.
There is also a light brown tamarind paste available in many Asian grocery stores. However, this paste is not suitable for Indian recipes, as it is made by adding salt and food acid (to assist preservation) and prevent the fresh pulp from oxidizing.
Some South Indian recipes may call for Fish Tamarind, which is not tamarind at all. Rather, it is the outer skin of a plum-like fruit called kokam (Garcinia indica Choisy). Kokam may be used instead of tamarind, but the flavour is less acidic and slightly fruitier.
For more detailed information about tamarind and kokam, including recipes, refer to The Spice & Herb Bible 3rd Edition by Ian Hemphill with recipes by Kate Hemphill.