When Matt Nadeau from Rock Art Brewery is brewing up a seasonal beer there are a few things on his mind: What's fresh now, his beer base, other add-ins -- and a lot of math.
"You need to see what that flavor is going to mix in, but a lot of it comes down to a lot of math formulas," he says.
Anything very outside the box gets tested in house and may not make it to the bottle.
"We've got a pine beer we just brewed that we're going to be trying that in house and we'll see how that comes out," he says.
And brewers also pay attention to the temperatures because they say what people drink on a hot summer day and what they want when the winter cold comes are two different flavors.
Reporter: "What makes it a winter beer?"
Nadeau: "I think because it's a darker beer. I think the conception is lighter in the summer, darker in the winter."
What's growing locally also factors in. The hops for the fall Vermont Hop Harvest beer came from Walden and Cabot, as well as their own brewery.
"When we receive them at the brewery, we have to open up those hops, we have to smell them, we've got to evaluate them, make a little tea with them and sort of see what kinds of flavors we're going to get," he says.
And with fall also comes pumpkin beer, a staple for almost every brewery, and one Nadeau says customers expect. To stand out, brewers have to try something different. He leaves out the pumpkin pie spices and instead turns to the trees.
Reporter: "What does the spruce do for that sort of beer?"
Nadeau: "Spruce is really neat. It's got an herbal quality to it. If you've ever smelled a spruce forest, you're getting that kind of evergreen-y pine-y type aroma."
And even if a beer is very popular, there are reasons brewers keep some seasonal. Nadeau says their springtime maple flavor would lose its allure if you could get it anytime. Plus, trying to set aside a year's worth of seasonal ingredients, like drums of syrup, takes space that many small breweries don't have.
"I don't think I could store the syrup," Nadeau says.
It all comes down to variety, he says. The seasonal beers help them switch it up so their customers don't get tapped out.