The recipe for the fish head curry sold by Muthu's Curry is known only to two people - the sons of its founder, Mr Ayyakkannu S.
It is so well guarded that brothers Kasivishvanaath A. and Srinivasan A. mix the spice blend for the dish on their own in a room in their central kitchen in Tuas.
They do so about thrice a week.
The mix includes cumin, chilli powder and coriander, Mr Kasivishvanaath's wife, Mrs Veshali Visvanaath, 40, the chain's marketing director, told The Straits Times. The chain's main outlet is in Little India, and it has branches in Dempsey Hill and Suntec City.
The story of Muthu's Curry is captured in the upcoming book Delicious Heirlooms, which documents Singapore's food heritage and oldest family-run restaurants.
Mr Ayyakkannu, the founder, thought of starting a business as he wanted to do more than wash cars and cook staff meals with his father at a provision shop.
In 1969, he rented a stall in a coffee shop in Klang Lane, in Little India, to sell fish head curry, to which he had given a South Indian twist typical of his home region in Chettinad, Tamil Nadu. Apart from the secret spice blend, the dish includes fenugreek seeds, curry leaves, tamarind juice and coconut milk.
The book tells of how the business got its name - the stall had no name until a customer complimented Mr Ayyakkannu's version of fish head curry as "muttha erukku", which means having the richness of the pearl in Tamil.
The name Muthu stuck.
Mrs Veshali said her late father-in-law was a hardworking man, and added that the family is "incredibly proud to continue his legacy".