Image caption Madam Chew has sold food from her stall since the 1980s

The mouth-watering food served up by South East Asia's street food vendors is a vital part of region's character and tourist appeal. So what happens if you make them pack up and move indoors?

In the tiny, two-metre square kitchen where Melvin works alongside his mother, there's barely space for both to stand between the boiling vats of braising gravy, the buckets of offal, bags of rice and a row of suspended duck carcasses.

Melvin grew up watching his parents run the stall in Singapore, instilling in him a passion for hawker food.

"I love this job. I love being a hawker," he says, before admitting it's a "very tough" way of earning a living.

Image caption Melvin joined his mother running the business after his father's death

They don't take short cuts, so work starts early, shortly after 7am, collecting fresh ingredients, rolling up the rattling metal shutters, then laboriously washing out tubes of pig intestine and fatty strips of pork skin in plenty of cold water. Eggs must be parboiled, vegetables and tofu chopped, the meat cooked and portioned, sauces and garnishes prepared.

But somehow by lunchtime their stall is ready to churn out anything up to 200 portions a day of kway chap and braised duck, a dish originating from Guangdong in southern China, and rich in the flavours of orange peel, star anise, chilli and cinnamon.

In most Asian cities food hawkers line the streets, with pungent cooking aromas enticing passers-by. But Melvin's customers tuck into their meals under cover, on the second floor of a purpose-built shopping complex. They eat at formica-topped tables and cooking smells are sucked away by a humming extractor fan.

"Street food is no longer street-side food here in Singapore. It's a style of food that came from the street," says KF Seetoh, a culinary consultant and unofficial spokesperson for the city state's food hawkers.

Singapore is a city built on migration. Waves of Indians, Chinese, Indonesians and others have flooded into the region's most prosperous city, joining the native Malays. But from the start, migrant workers craved familiar dishes from home.

"In the 1950s there were 22,000 itinerant vendors on the streets," says Mr Seetoh. "Some people said get rid of them."

Instead, he says, the authorities made "a very practical decision" - to bring them indoors.

Image caption KF Seetoh is concerned about the future of Singapore's hawkers

It is an approach that has served Singapore well. The country is renowned for its rich variety of affordable food. Two of the city's hawkers have been awarded coveted Michelin stars.

Several have created lucrative enterprises, a few have even expanded abroad. Singapore's government has applied for hawker culture to be recognised by Unesco for "intangible cultural heritage" status, alongside the likes of Belgium's beers and Turkish coffee houses.

That success, Mr Seetoh argues, is largely due to Singapore's decision to sanitise and regulate the sector.

"They said if you are going to eat it, it better be safe, because we don't know where these folks on the street got their supplies from. Did they wash their hands? Are they hygienic?"

From the 1960s onwards, vendors were installed in purpose-built hawker centres across the rapidly developing city state, provided with running water, electricity, grey water drainage and extraction hoods.

Along with the new facilities came regulations. Don't chop and prepare food on porous wood - surfaces must be stainless steel. Keep cooked and uncooked meat separate, and stored at precise temperatures. Wear gloves. Check the source of your ingredients.

Each stall is given a hygiene rating.

"It is what we expect from this uber-efficient government," Mr Seetoh says. "We call that peace of mind. Who doesn't want that?

Image caption Singapore's purpose-built hawker centres provide stall-holders with facilities like ventilation

Image caption ...while cleaners wipe tables and mop floors

The answer is plenty of others in the region. Malaysians scoffed when Singapore applied for Unesco heritage recognition, arguing the city's food had lost its authenticity since coming off the streets.

Bangkok is now attempting to follow in Singapore's footsteps by bringing its own street food vendors into purpose-built centres in a bid to clear space on the pavements. However, the authorities are facing widespread opposition from those who say the character and appeal of Thailand's capital will be lost if its food hawkers are brought indoors.

Image caption One Singapore hawker's stall that received a Michelin star has now become a chain of restaurants

Jorge Carillo, an anthropologist who studies South East Asia's street food sector, says moves like this are being encouraged by a preference amongst a new generation of customers in countries like Vietnam for higher hygiene standards and air-conditioned shopping centres. On top of that, costs are rising, while customers expect prices for street food to remain low.

But, above all, what is undermining the sector's prospects is the day-to-day reality of the job, he says.

"I have this issue with some people, because they keep pushing to keep street food because it gives the city character," says Mr Carillo. "The reality is that selling street food is very hard work."

"What is changing is job opportunities are coming up, and people simply are stopping to do this very hard work" he says. One woman hawker in Bangkok he interviewed starts shopping for ingredients at 3am, goes to her stall at 7am and then sells food for eight to 10 hours.

"If she gets another opportunity to do another job of course she's going to stop," he says.

The same is true in Singapore, as the first generation of hawkers, now in their 60s and 70s retires. It is proving difficult to persuade millennials, many of whom have degrees, and aspire to be entrepreneurs or work in Singapore's air-conditioned high-rises, to take up the baton. It's not what their parents want for them either.

Melvin is an exception. "Initially my mother was very against her children being hawkers because of the long hours," he says. "She wanted me to work in an office." When his father passed away Melvin couldn't bear to think of the family business closing but they now take home only around 6,000 Singapore dollars (US$4,200; £3,500) a month between them for a six-day working week.

"There are so many challenges, and these are very stacked up against the hawkers," says KF Seetoh. He adds that he's been "shouting and crying about the impending demise of this food culture" for some time - all because of this generational threat that no-one seems to know how to tackle.

Image caption Michelle Yee and her husband Alan aren't typical of a generation that would mostly prefer an office job

There are bright spots of hope though - newcomers such as Michelle Yee Yuan and her husband Alan who gave up office jobs to open a stall not far from Melvin's. They serve a Korean-style ham cha, a mix of vegetables, peanuts, and rice combined with a bitter herb soup.

The ingredients are designed to appeal to health-conscious younger consumers, and they work hard to push their presence on social media, fighting for every customer.

"To get a new person to try our food is one of the challenges," says Michelle. She says it has been hard, working 12-14 hours a day sometimes, at the beginning she was so exhausted she fell ill a lot. She and Alan are taking home about half of their previous joint income.

Everything, says Mr Seetoh, is stacked up against new entrants like Michelle and Alan. And yet Michelle says she loves the job, the environment and working alongside her husband all day.

"And I am working for my own creation," she says.

You can listen to the accompanying radio programme on this topic from BBC World Service's The Food Chain.