SKOWHEGAN, Maine (AP) — Tasting maple syrup is a lot like tasting wine. Sniff the aroma, take a sip, hold it on your tongue and savor it for a moment to register the sensation.

At the International Maple Grading School, syrup is serious stuff. At a time when syrup production has boomed to its highest levels in decades, students here learn the sappy sweet nuances of the trade, such as how syrup flavour can be affected by soil type, weather, tree health, production practices and numerous other factors.

"If you think about all the variables that go into producing syrup, it's not surprising there are so many possibilities," Debra Hartford, who owns the Thurston and Peters Sugarhouse in Newfield, Maine, said after swirling and swallowing a sip of syrup to test her tasting abilities.

Let there be no mistake: The subject matter was sickly sweet at this unique school, a two-day session of instruction where syrup buyers, producers, inspectors and educators came together to learn the ins and outs of syrup regulations and the equipment that's used to measure a syrup's color, clarity and density.

They also learned about the complex regulations governing syrup, which vary among syrup-producing locales. For instance, different places have different names for the same syrup — the lightest grade is called "fancy" in Vermont, but is known as "light amber" in other states and "No. 1-AA" in Canada.

But the most important thing about syrup, as anyone will tell you, is flavour.

Late last week, students sampled more than three dozen syrups to differentiate the light varieties from the heavier styles and to identify the off-flavours of syrup that doesn't make the grade.

The syrup school, now in its sixth year, is sponsored by the Canada-based International Maple Syrup Institute and is held for two days at a different site each year. For the classes last week, about 20 participants gathered at the University of Maine Cooperative Extension offices in this central Maine town.

"When we first cooked up this idea, we thought it would have a one-year run," said Kathy Hopkins, a Cooperative Extension agent and the class instructor. "After the first year we had a waiting list. Now we're on the sixth year and we're booked four years out."

Maple syrup is big business.

Production in the US this year topped 2.3 million gallons, the highest total since 1944. Vermont is the number one state by far, followed by Maine and New York. Syrup is even bigger in Canada, which produces around 6 million gallons a year.

Between the US and Canada, maple syrup has grown to a $200 million industry.

The grading is important because it determines if the product can be sold for retail sales on store shelves, or for commercial use as an ingredient in products ranging from ice cream to bacon to beer. The better the syrup, the higher the price; in 2008, the US average price was $40.50 a gallon.

Lisa Mancuso, who's in charge of quality control at Bascom Maple Farm in Alstead, New Hampshire, said the grading system is important because customers have different needs.

For example, consumers generally like to douse their pancakes with a lighter syrup, which has a milder flavour. Food processors, meanwhile, may prefer darker and robust syrup.

"We have a lot of customers who buy in bulk from us for ingredient purposes — large bakeries, sausage companies — so they want a heavy maple flavour because they're blending it with other ingredients," Mancuso said.

But the product isn't always so sweet after sap is extracted from maple trees and boiled down into syrup every late winter and spring.

For their training, students took sips of syrup out of small paper cups that had hints of tree buds, chemicals, chlorine and other off-flavours. The taste can go bad from warm weather, how it is produced or even how sugarhouse operators clean their equipment, said Henry Marckres of the Vermont Agency of Agriculture, Food & Markets.

"I once tasted a barrel of syrup you could blow bubbles with," he said.