kashk
A Persian dairy product made from soured, reduced yogurt or whey, sold as a thick liquid in jars or as dried pieces. Strongly sour, salty and savoury, like an aged cheese.
Kashk is made from yogurt or the whey left over from churning butter, which is reduced, drained and often dried into balls or chunks. It is sold either as a creamy, thick paste in jars or in dried form, which has to be soaked and worked smooth before use. It tastes distinctly sour, salty and deeply savoury, with a slightly fermented note reminiscent of aged cheese. For that reason it is used sparingly as a seasoning and sauce, for instance over aubergine dishes and soups, rather than eaten like yogurt. It is available in Persian and Middle Eastern grocery shops, occasionally in well-stocked Turkish or Arab supermarkets, and by mail order.
Substitute:
There is no true equivalent. Strained Greek yogurt or sour cream mixed with a little salt, lemon juice and finely grated feta or Parmesan roughly matches the sourness and saltiness, but not the fermented depth and savoury bite of kashk, so the dish will taste milder and fresher.