The further south you go the brighter the colours, the broader the smiles, the more vibrant the flavours. Never have I found this to be more true than in India. I first visited Kerala - that lush coconut-fringed strip that runs down the southwest coast from Tellicherry to its capital, Thiruvananthapurum - six years ago, in an attempt to understand the intricacies of the spice trade. I have been back several times since, seduced by the Malabar coast's spice-laden tropical forests, its still, green backwaters, multicoloured festivals and bejewelled elephants and its life-affirming Ayervedic massages. Rarely has a place got so deeply under my skin.

Even now, I approach the dinner table there with caution. There are often no knives and forks. A banana leaf stands in for a plate. Scooping up the warm rice and hot pickle by hand feels natural enough till it gets to my mouth; then I am all fingers and thumbs, bits of curry-stained rice dropping into my lap, and still so obviously, embarrassingly, a tourist.

Benjy, a Cochin chef currently away from his hotel kitchen with a scalded foot, patiently shows me again how to eat my lunch. 'You smash your poppadom and crumble it over your rice, then, with your right hand, you mix a little from each of the tiny mounds of food laid out on the leaf.'

There is avival - a dice of vegetables bound with coconut, mustard seed and chilli, an earthy brown lentil dhal, a dry green bean curry called Thoren, a dab of hot pickle and a small pool of yoghurt and coriander chutney. Eaten individually they are too hot, too bland or too intense to cope with. Mashed together with the rice into a bite-sized ball, the searing heat of the pickle and the sour of the avival marry in a pleasing way, unthreatening enough for those who take authentic Indian cooking with trepidation.

We have driven for six hours to get to Benjy's cool, marble-floored villa in old Cochin. Kerala's silent green canals and mango plantations are forgotten in the terror of navigating southern India's nightmare roads. Knackered trucks with tired long-distance drivers, their cabs festooned with fairy lights, are piled high with fat sacks of rice, while thin, almost naked boys cling on for their life.

Here, and through much of India, you drive at full speed or not at all. There seems an unwritten rule that overtaking is only attempted on blind corners. I casually mention that I am agnostic and I regard this life as the only one, and not as a waiting room for the next. We pass burned-out trucks and traffic accidents that will probably haunt us for the rest of our days. Our ears sting from the constant horns. Tolerance is big here, no doubt due to centuries of contact with foreign spice dealers. Hindu, Christian and Muslim live side by side.

Now all is calm, save the rick-rick of some far-off, or not so far-off creature that I don't really want to know about. The ceiling fans are on, the droplets of condensation are running in rivulets down our lime sodas, the table is laid. As you might expect from a land that is predominantly paradise with occasional flashes of hell, the kitchen culture is built around the fruit of the coconut palm, seafood and spices and generous use of small, eye-wateringly hot chillies.

Kerala's cooking is fragrant with the flesh and juice of the coconuts that line the state's highways, side roads and beaches. Each time our truck had been caught in heavy traffic, young boys would run alongside offering green coconuts, shorn of their skin, a straw in the top to suck out the cool juice. The mild tartness of the thin liquid cut through the dust and diesel fumes. The grated flesh turns up in both curries and the local tooth-juddering sweetmeats in their tinsel bedecked boxes. Creamed coconut is a rich mixture of minced snow-white coconut flesh and water, and is used the way the French use cream. Like calamine lotion on burnt skin, it soothes the blistering heat of the local cooking.

Those who have only spent their days on the beach in Trivandrum - now tongue-twistingly re-christened Thiruvananthapurum - are probably unaware of the culinary treasures that are to be found further north. You cannot blame them. But a four of five hour journey by death-trap truck or over-crowded train will take you deep into Kerala's tropical forest. Here you will catch sight of few others. When you do, they will almost certainly be dangling from a tree, rope around their waist, harvesting some sort of spice or other. This is cardamom country, nutmeg too, where spices grow along the roadside, and where pepper vines wind their tendrils around banana palms and over camellia bushes. Though they appear untended and unclaimed, you should not pick them. You will be stealing someone's harvest. Someone's livelihood.

Cardamom scents everything from curries to steam room in India. It is my favourite spice. It is here, on the roadside near Cochin that I encounter the finest cardamom I have ever smelled. Plump, brilliant green, so fragrant as to be almost hallucinogenic. I am not sure whether to put it in my rice pudding or use it to clear my sinuses.

The epicentre of Keralas's spice trade is the dusty straggle of rickety warehouses and open-fronted shops known as Jewtown. At first, you cannot imagine this place has changed in hundreds of years, a film set where spices are set out in sacks on the floor for buyers to sniff and feel and where three-legged dogs chase rats along the open drains. The fragrances come at you in succession as you walk along. First the warm citrus of sun-dried ginger; then the earthy, almost medicinal hint of cinnamon and clove. You cannot fail to be taken by surprise that such a place still exists.

Much of the spice is taken by road, piled into sacks and shipped to Jewtown to the pepper market. A little, mostly cinnamon and nutmeg travels by boat, winding its way through the miles of backwaters. Some is traded locally in the markets.

It is something of a cliché to say that to know about a country's food you should start in its street market, but it makes sense. Yes, we found the shallow wicker baskets of green peppercorns and tiny red onions, minuscule limes and tiny oval mangoes that let you know instantly you are in India, but there was more to it than that. Kerala's markets are greener and more lush than further north, in fact, with their pineapples, coconuts, bananas and starfruit they look more Thai than Indian.

Curry leaves, coriander, coconut, tamarind and limes. These are the tart, cooling flavours you expect further east, yet a Keralan fish stew may be scented with them all. The fish in the market is good enough, the usual Indian blue-grey pomfret, giant eel and the area's famous prawns - after all we are not far from the sea - but we can do better. Down by the sea, past the fishing boats at Cochin, you can also buy it from the boats down by the harbour in Cochin.

My hosts for dinner, Sanjay and his wife Ella are embarrassed that I want to eat at the open-air restaurant next to the fish stall. They consider eating outside in this way uncultured, and not for the likes of them. But I insist. (I later learn they were 'gutted' to be seen eating here and much prefer the snacks and pastries at the coffee shop of the local five star hotel.) Still, we choose our fish from the boxes on the dock. There must be three or four different sizes and prices of prawns. There are a couple of majestic crayfish almost as long as my forearm and, of course, the ubiquitous pomfret. There are crabs too, more spiky and longer in the leg than the fat, rounded snappers we know. I settled on a chilli-hot fish soup and a plate of grilled prawns with chilli and lime. Despite their initial reluctance to sample the local food we all ended up laughing about it.

This not the India I know, it is a more colourful, vibrant India, where the food is almost Thai in its freshness and heat. And, I remain convinced that in Kerala the smiles are bigger too.

Nigel Slater's Kerala fish stew with lime and curry leaves

Serves 4

750g mixed fish, such as haddock, mullet
a little turmeric
juice of a lime
3 tbsps of coconut, vegetable or groundnut oil
an onion, peeled and finely chopped
4 large, or 6 small green chillies, seeded and finely chopped
4 cloves of garlic, peeled and crushed
a piece of ginger as big as your thumb, peeled and finely grated
6 green cardamoms
20 fresh curry leaves
400ml tin coconut milk
rice to serve

Rinse the fish, taking care to remove any loose scales or bones. Pat it dry with kitchen paper, put it in a shallow dish and dust with a couple of pinches of turmeric and the same of salt. Squeeze over the lime juice and set aside for half an hour or so.

Heat the oil in a large, shallow pan, adding the onion as it warms. Cook over a low to moderate heat until the onion is soft, stirring from time to time, then add the chillies, garlic and ginger, continuing till all has softened.

Break open the cardamom pods, crush the seeds slightly, and add them to the onion mixture with the curry leaves. Stir in the coconut milk and an equal amount of water. Simmer for 8-10 minutes, making sure it does not boil. (It will curdle if it does.)

Cut the fish into large, meaty chunks then slide them into the sauce. Let it cook gently, barely bubbling, until tender and easily parted from its skin or bone. This will take about 4 or 5 minutes depending on the thickness and variety of your fish. Taste for seasoning, adding more lime juice or salt as you wish.

Nigel Slater's grilled prawns

Benjy's recipe for spiced prawns changes according to his whim, and the flavour does change from day to day, but this is the recipe, almost as he gave it to me. Serves 4 as a starter

For the masala:

2 medium sized, fairly hot red chillies
2 cloves garlic, peeled
a thumb-sized lump of fresh ginger, peeled
6 black peppercorns, coarsely ground
2 tsps cumin seed
half tsp ground turmeric
1 tsp coriander seeds, crushed
50g creamed coconut
water or natural yoghurt to mix
500g large, juicy prawns (shelled)
to serve - a little chopped coriander leaves and some wedges of lime

Slice the chillies in half, scraping out the seeds if you want a milder spice mixture. Put the chillies into a blender or food processor with the garlic, the ginger thinly sliced, the peppercorns, cumin seed, turmeric, coriander seed and creamed coconut. Blitz to a thick mush, adding a little water or yoghurt until you have a loose paste. Scrape into a medium sized bowl.

Rinse and dry the prawns then toss them in the masala paste and set aside for an hour or so. During this time the prawns will take up some of the flavours from the masala. Get an overhead grill hot. Thread the prawns, three or four at a time, onto skewers, a bit messy this, but it makes the prawns much easier to turn. Cook them under the grill for a couple of minutes or so each side, until they are sizzling and fully opaque. A little browning here and there will be a good thing.

Serve them, with a little chopped coriander leaves and lime juice.