On St. Patrick’s Day tomorrow, beer will be in a lot of glasses across campus. But what exactly is in the beer?

Matt Greff, Arbor Brewing Company’s head brewer, explained recently what has to happen to go from grains to glass.

Wandering among the pub’s brewing equipment, he explained that beer starts with four essential ingredients: water, malted barley, hops, and yeast.

Malted barley – malt for short – is barley that has been allowed to partially germinate. This creates enzymes in the grain. Heating the barley in a process that Greff called “kilning” stops the germination.

This is where the brewer begins to have creative control over his product. The barley can be kilned relatively lightly to produce what is known as Pilsner malt.

Greff said that this grain makes up the base of his beers and is 70 to 95 percent of the malt that goes into any particular brew. Specialty malts, he said, are what give different beers their distinctive looks and flavors.

Greff pointed out one called a caramel malt.

“It tastes a little like Grape Nuts,” he said.

It’s the time of year for Guinness, with its distinctive black color. Stouts – Guiness among them – are loaded with specialty malts, Greff said.

Roasted malt is barley that has been treated like coffee, he said. It’s no accident then that this grain smells a little like fresh coffee beans.

Chocolate malt – which, as Greff said, has nothing to do with chocolate other than the color – and black malt are also added to the mixture to give certain beers their distinctive dark color.

What distinguishes these grains from their paler counterparts is simply the amount of kilning they get. That’s what gives Guinness and other stouts their dark color.

These malts are mixed in the proportions the brewer desires with 150-degree water in a metal vat known as a “mash tun.” ABC’s mash tun holds 7 barrels, which is 217 gallons.

The hot water causes the enzymes to break down the starches in the malt into fermentable sugars. The malt steeps for about an hour, Greff said, after which the liquid, now called the “wort,” filters into another metal tank called the “brew kettle.”

Here the brewer adds the spice of his product – hops.

Hops are flowering plants. Greff’s have been ground and formed into pellets for easier shipping and storage. In general, hops are what give beer its bitterness.

Greff said that he mostly uses a variety called “cascade” hops, which give his beer a citrusy flavor. While the wort is boiling for an hour, Greff adds his hops. Putting them in the mix earlier releases more acids from the hops, he said, creating a bitterer brew.

Adding them later, however, preserves the grapefruit-and-flower-like aroma common to beers like pale ales.

After an hour, the mixture is cooled to about 75 degrees and sent to fermenting tanks. Yeast is added, which feeds on the sugars released from the malt during the mashing process.

Greff said that as the yeast multiplies, it creates two byproducts – carbon dioxide and alcohol. The yeast creates so much carbon dioxide, in fact, that some has to be vented during fermentation.

At the end of the process, Greff closes the valves to the tank and the carbon dioxide is trapped in the beer, creating the carbonation.

After about a week in the fermenter, the now-alcoholic and bubbly mixture is sent to serving tanks in the basement, where it cools and waits for you to come order a pint.

A mixture of carbon dioxide and nitrogen creates pressure in the serving tanks, so when the bartender pulls the tap handle, the cold, bubbly beverage travels up from the basement and into your glass.

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