When the rainbow-coloured iridescent bubbles start floating on the rasam, you know it is ready to be removed from the stove. It's then time for the all-familiar collective aroma of ghee, coriander and curry leaves to tantalise your taste buds.

Rasam is unique to south Indian cuisine. But, according to food connoisseur Revathi Shanmugam, there is a distinct difference in flavour when it is prepared in eiyachombu (pronounced ee-ya-chom-bu), a traditional utensil made of tin. “When cooked in the vessel, the rasam acquires a unique smell and flavour,” she says.

S. Narayanan, who runs his grandfather's business of selling tin utensils in Triplicane, rues that the lack of knowledge about the useful properties of the metal and has led to the decline in its use. “Cans that are used to store fruits and vegetables are actually coated with tin. Even brass vessels are coated with tin to prevent food poisoning. But unfortunately people have mistaken tin for lead,” says Mr. Narayanan. “I am 72 and am talking to you now. Every day, rasam at my home is cooked in eiyachombu and I am still healthy. It is not at all true that cooking in tin utensils can cause food poisoning.”

The tin is mostly imported from Malaysia and sold through a government quota system. The selling price of the metal is Rs.1,800-Rs.2,000 a kg.

“It requires care while using the vessel, that's why people don't want to use it,” says R. Venugopal, proprietor of a small utensil shop on South Usman Road that stocks just a dozen vessels in a crevice on the compound wall of the Shiva Vishnu temple.

The metal's melting point is around 200 degree Celsius. So, if you place the empty vessel on the stove, you might end up with a blob of silvery-white metal in a matter of minutes.

Even though the utensil is gradually vanishing from kitchens in the city, the taste of rasam cooked in eiyachombu lingers in many mouths. So much so that at wedding feasts, chefs often seek to remind guests of the traditional flavour of rasam by investing in the tin utensil: no, not to cook the rasam in it but to drop the utensil into the boiling pot of rasam so that the metal lends its flavour to the dish.

“That way, the rasam tastes as if it has been prepared in an eiyachombu,” says Arusuvai N. Kumar, a caterer.