Kokum is a relative of mangosteen that is used as a souring agent in some parts of India. The outer skin of the fruit is used in Konkani recipes as well as in Assamese cuisine. The skin is dried, which increases its tartness. Because of the relatively unknown nature of kokum, you may have trouble finding it outside of India; however, there are other traditional Indian souring agents that you can try. Consider some of the kokum substitutes below.
Your best bet: Tamarind
Tamarind is a legume from a tropical tree. Usually, tamarind is deseeded and dried before use. It can come in many forms including tamarind juice, paste, and powder. The powder form might be the best one for replacing kokum but the juice and paste are both effective stand-ins as well.
Like kokum, tamarind’s primary culinary application is for providing sourness to dishes. For Westerners, it may be most easily recognized as one of the main flavors in Worcestershire sauce.
Tamarind is most often used in dishes from the south of India. Tamarind is also used in the Caribbean and Mexico to make beverages, which is something else it has in common with kokum. Tamarind is sometimes called imli and gives found a pronounced sour flavor, though the tamarind from Thailand is considered relatively sweet when compared to Indian tamarind. Tamarind is used to flavor dishes like sambar and rasam.
A decent second choice: Mango powder
Mango powder is sometimes called amchur (or amchoor) and consists of raw green mangoes that have been dried and ground to a powder. The am in amchur means mango and the chur or choor means powder.
Mango powder provides the same strong sourness that you would get from kokum. Mango powder also has a mild sweetness that you might not get from kokum, but it is not enough to keep it from being an effective substitute. Traditional uses for mango powder include giving tartness to chutneys and in chaat masala.
In a pinch: Sour citrus juice
Lemons and limes are commonly used for adding acidity to Indian dishes. They have the benefit of being much easier to find in the west than Indian ingredients, though most of those listed here should be easy to find in most Indian grocery stores.
Unlike other souring agents, the juice from these fruits is not adding during the cooking process itself but at the end. Sour citrus juice is not usually cooked because heat can turn it bitter; you will probably want to reserve it for raw applications. In addition to their flavoring benefits, lemon and lime juice can both act as meat tenderizers.
Vinegar is a traditional Indian ingredient used to add sourness to some Indian dishes. It most likely came to India via the Portuguese, which is why it is most common in cuisine from the former Portuguese colony of Goa.
Tomato is a common ingredient in Indian food. It is used for its mild acidity as well as for its bright color.
Gamboge comes from Kerala and is also called fish tamarind. In some places, it is known as Malabar tamarind.
Anardana consists of sun-dried pomegranate seeds. The drying process turns pomegranate seeds from sweet to sour. The sour flavor is used widely in Punjabi dishes and dishes from Pakistan and Bangladesh. Along with its sourness, anardana has a sweet note that makes it similar to cranberries.