Before millions of Americans can present their loved ones with a bouquet of Valentine's Day roses, most of the flowers are flown from Colombia and Ecuador to Miami, many in the bellies of passenger planes. There, cargo handlers and customs agents — call them Cupid's helpers — ensure that the deep red petals stay perfect until they reach their final destination.

In the weeks leading up to Valentine's Day, about 738 million flowers — 85 percent of imported flowers — come through the Miami International Airport. Los Angeles is a distant second, with 44 million. The roses, carnations, hydrangeas, sunflowers and other varieties are rushed by forklift from planes to chilled warehouses and then onto refrigerated trucks or other planes and eventually delivered to florists, gas stations and grocery stores across the country.

"We always joke that a passenger gets themselves to the next flight while a bit of cargo does not," says Jim Butler, president of cargo operations at American Airlines.

Cargo is a small, but increasingly important part of U.S. airlines' business. New jets are built with more freight space and the airlines are adding new non-stop international routes popular with shippers.

Most airline passengers focus on what's visible to them, like the amount of legroom and the space in the overhead bins. Few think about what's beneath the cabin floor.

There's fresh Alaskan salmon, this season's latest luxury clothing from Milan and plenty of Peruvian asparagus heading to London. Then there are the more unusual items like human corneas, the occasional live cheetah or lion and large shipments of gold and diamonds.

And there are the flowers.

Valentine's Day is a big day for flowers, topped only by Mother's Day, and cargo teams work extra hours ahead of both to ensure on-time deliveries.

"There's a spark in the air while loading these," says Andy Kirschner, director of cargo sales for Delta Air Lines. "You know this is going to loved ones."

Worldwide, airlines and air shippers carried about 52 million tons of freight representing $6 trillion worth of goods last year, according to the International Air Transport Association, the airlines' trade group. That was up 1.4 percent from the prior year. The amount of air cargo is expected to climb 17 percent in the next five years.

Shipping by air costs about 10 times more than by sea, says David G. Ross, a transportation analyst at Stifel. So, plane rides are reserved for trendy high-end fashion items, the hottest electronics or perishable foods and flowers.

"If it's the new product on the block and everybody wants it, then you can ship it by air," Ross says.

Most non-perishables, such as T-shirts, jeans and even mass-produced flat-screen TVs, travel by ship.

"If you have a low price point on it, you don't have room for expensive transportation," says Ross.

That's been the philosophy of many corporations coming out of the recession — and has made for rough going for the air cargo business. Low interest rates have also factored into companies choosing to take a few extra weeks to ship products to the marketplace by sea.

As a result, air cargo rates have been depressed. Air shippers worldwide took in $59 billion in revenue last year, down 12 percent from two years ago.

For the biggest U.S. airlines — American Airlines, Delta and United Airlines — cargo accounted for just 2.3 percent of their overall revenue last year, down from 2.5 percent in 2012 and 2.8 percent in 2011. United's cargo revenue fell 13.4 percent last year, while Delta's fell 5.4 percent. American's remained virtually flat, thanks in part to its dominance on South American routes. It's the largest carrier in Miami.

The airlines don't break out cargo costs but the side business is said to be profitable. They already have the jets and are paying the pilots, and they fill planes with enough passengers to cover their expenses. Plus, there's plenty of space next to the passenger luggage in a wide-body jet like the Boeing 777.

"It's incremental revenue. You're already paying for the airplane to go," says Brandon Fried, executive director of the Airforwarders Association, the trade group for shippers. Plus, "freight doesn't complain like passengers do at times."

Delta considered replacing the 777s it uses between Los Angles and Sydney with 747s, which seat 107 extra passengers. But that would have reduced the capacity for the strawberries, asparagus, green onions, lettuce and other perishable items it ships from California to Australia.

The cargo business isn't just about the space in a plane's belly. There needs to also be precision handling on the ground, especially with a product that can spoil.

With flowers, as soon as they're cut a clock starts ticking. And nobody wants to give wilted roses on Valentine's Day.

Heat is the enemy. When a plane touches down in Miami, the flowers are rushed to a nearby warehouse where a parade of forklifts carry them into giant coolers — really rooms — set at 35 degrees. Every time the giant cooler doors open up, fog rolls out as the frigid air hits the Florida humidity.

Inside, big vacuums suck the hot air out of flower boxes and bring in the surrounding cold air. In one hour, the core temperature of flowers, vegetables or other perishables drops 46 degrees.

"It's like it cryogenically extends the life," says Nathaniel R. Miller, a supervisor with Perishable Handling Specialists, which operates American's Miami coolers.

Before the flowers can be sent to stores across the country, U.S. Customs and Border Protection must sign off. Agents check tax documentation, ensure that drugs aren't being smuggled and inspect petals and stems for pests like moths, leaf-miner flies and spider mites, which can ruin crops in American fields.

The bugs — some as small as a period — can't be detected by X-ray machines. So a team of agents travels from warehouse to warehouse, looking at a sample of flowers. Bouquets are turned upside down, hit on the side.

Thump, thump, thump.

Dirt, leaves and other debris fall onto tables covered in white paper. Magnifying loops are used to inspect the specks. Any bugs discovered are dropped into test tubes and sent off to a lab.

The job has hazards: roses come with plenty of thorns and some officers wear masks to protect against the pollen. Their uniforms include hats and gloves.

"It's like working in a meat locker," says Michael DiBlasi, a Customs and Border Protection agriculture specialist. "We love our job. You have to, to work in a cooler."

Based on reporting by the Associated Press.