Indian curries change in flavour every few hundred kilometres, according to long-held wisdom. Furthermore, we recognise them by a minor variation in spicing and, more significantly, souring agents rather than by their "primary" ingredients.
When you consider complex cuisines like the several regional ones in India, where flavours are balanced in many different ways, doing this task becomes rather difficult. Unlike what many Westerners believe, a curry is not a universal cuisine. As many distinct curries, vegetable, meat, and seafood preparations, as well as lentil dishes made with various spice blends, may be found in country household cooking. The major component (jackfruit, shrimps, mutton, etc.) should not be used to define the preparation, even though some of them were defined and put into broad categories. Indian curries change in flavour every few hundred kilometres, according to long-held wisdom. Furthermore, we recognise them by a minor variation in spicing and, more significantly, souring agents rather than by their "primary" ingredients. Thus, grouping various Indian foods according to the souring chemicals they employ might be one method to organise them. Here we have different ingredients from the country that adds tartness to dishes.
Tomato: Of course, tomatoes are a mainstay of contemporary Indian cuisine. The traditional, nuanced gravies, where flavours were richer and more layered, are being replaced by the generic gravies they are added to. Souring agents in India have historically differed by location, depending on that region's geography and history. Yoghurt-based curries are more common in Delhi and Uttar Pradesh, it is undoubtedly due to the Mughal cuisine influence that permeated this region of the country. Similar to this, Goa's vinegar is a Portuguese import.
Kachampuli: However, the Kachampuli vinegar used in Coorg's pandi curry is a regional invention motivated by the desire to preserve fruit. Only available during the monsoon, the Kodampuli fruit is harvested in baskets and allowed to wilt into pulp and vinegar that can be used over the following months. This kachampuli vinegar is what gives the pork its distinctive flavour.
Kachri Powder: One of the most intriguing souring agents for cooking is this one. A sophisticated Delhi Kayasth cuisine like Badam Pasande uses dried and powdered kachri, a wild berry that grows in Rajasthan and a few locations in northern India. It functions as a souring agent and a meat tenderizer (yoghurt and it are both used in the marinade). In reality, the Kayasths' syncretic Ganga-Jamuni culinary tradition is evidenced by the use of the regional kachri in a dish that is obviously influenced by Mughal cooking. Even though the cuisine produced in Kayasth houses was complex, simple, everyday spices, tenderizers, aromatics, and souring agents were used. This distinguishes it from more "royal" Mughal cuisine, where meals were prepared.
Curd: The use of yoghurt as a souring agent is one of the characteristics of any meat dish with "courtly" Mughal/Nawabi/Nizami flavours. In Delhi, adding yoghurt and fried onions to the spices and nearly done meat in the last stages of cooking gave the curry body as well. On the other hand, the thin shorvas of traditional meat curries traditionally got their sourness from yoghurt rather than tomatoes. However, fish was never prepared in yoghurt. Perhaps as a result of the Ayurvedic foundations of Indian cuisine (fish and curd are a non-combination, according to Ayurveda).
Kokum: The mangosteen fruit's sun-dried outer fruit is a common souring agent in Maharashtra, Karnataka, and Assamese cuisine, highlighting the importance of geography and the availability of local ingredients in understanding pan-Indian cuisines.