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Frozen or on the rocks? Let’s ask a margarita-loving scientist


Kate Biberdorf might be from Michigan, but she loves margaritas as much as any Austin native.

The chemistry lecturer at the University of Texas spends the majority of her time teaching, but even when she’s not working, friends like to pepper her with questions about science in everyday life. Nuclear power is a subject she frequently is asked about, but a few weeks ago I reached out to her on a topic that’s nearly as polemic: frozen or on the rocks?

First, a little background: Biberdorf has a personal preference for frozen. “That’s our family drink. No matter when we get together, we have margaritas. When I drink one, I think of home,” she says.

They always make frozen ones in a blender using frozen limeade (never a mix), top shelf tequila and “cheap triple sec,” she says. Salt is technically optional, but it’s requisite for Biberdorf.

When she sets aside her own nostalgia and puts on her chemistry hat, though, Biberdorf — who runs the Fun with Chemistry program to help teach the public about science — can help us look at our favorite summer drink in an entirely new way.

Let’s start with the ice. There’s no way around it: A frozen margarita, packed with a million tiny snowflakes, will melt faster than one on the rocks because each tiny piece of ice is surrounded by liquid.

“If you grind up ice, you are going to have more surface area, and when you have more surface area, the kinetic energy from the liquid component is going to go from the liquid to the ice, and it is going to melt the ice,” Biberdorf says. Ideally, frozen margaritas, which melt faster, should be served in smaller quantities.

One of the key principles of thermodynamics is that elements are always seeking a state of equilibrium, so the liquid is warming the ice, the ice is cooling the liquid, and the combined elements in the margarita are cooling the air around the drink, as the air around the drink transfers heat to the glass and what’s in the glass.

With a margarita served on the rocks, those larger ice cubes are like icebergs in an ocean, melting more slowly because only the outer layer of the ice is in contact with the liquid. “The cold in the center of those cubes is completely contained,” she says.

No matter if your ice is crushed or in cubes, you should try to keep the ice cubes or crystals as submerged as possible because the air will melt the ice faster than the liquid.

That’s why the straw that is almost always served with a margarita is so important. Even if you don’t drink out of the straw, it’s the perfect tool for stirring the drink to equally distribute the heat, as well as the ethanol in the tequila, which will sink below the other liquids. But too much stirring will cause friction and, therefore, heat, so don’t get crazy.

What can help your drink stay colder longer? Starting with cold ingredients and a cold blender and glasses.

Ethanol in its purest state, as in Everclear, has an unpleasant taste, and even though many people like the taste of tequila, which is only about 40 percent alcohol by volume, the other ingredients in a margarita — lime juice, a sweetener and sometimes an orange liqueur, such as Cointreau — are meant to help mask that boozy flavor.

Tequila isn’t pure ethanol, but it won’t freeze in commercial freezers, so as soon as it hits the ice, it will start to melt it. Many bartenders try to keep the total alcohol by volume in a frozen drink to about 10 percent because the higher the alcohol content, the faster the ice will melt.

Biberdorf says that if you’re looking for the most bang for your ethanol buck, you should order a margarita on the rocks because there’s more room for liquid between the large cubes of ice. But beware: The margarita will taste boozier, too.

Those slushy-like frozen margaritas often don’t taste as alcoholic as on the rocks because you are consuming ice with the margarita liquid, which is why a frozen margarita will taste colder than one on the rocks. “Even if you just ingest a small quantity of ice, your mouth is incredibly sensitive to ice, you’ll notice,” Biberdorf says.

Every frozen margarita fan has an ideal sipping point, where the ice is still mostly frozen but the liquid isn’t too cold to give us brain freeze. Generally speaking, however, frozen margaritas start to go downhill after they leave the spout on the machine. The ice melts so quickly, especially on these really hot afternoons on a patio, and the remaining ice crystals float to the top, leaving the watery margarita mix on the bottom.

For an ideal rocks margarita with the least dilution possible, use the biggest ice cubes you can find, Biberdorf says. Don’t worry about using one of those fancy ice sphere molds, though. Even though it has a smaller surface area than a cube, the amount of time that the ice is in a mixed drink — as opposed to in a straight spirit, such as a whiskey — won’t make much of a difference. Biberdorf recommends letting a margarita on the rocks sit for a few minutes to let the liquid mixture and the ice start to equilibrate because bartenders mix the drink knowing that it will be best enjoyed once the ice starts to melt a bit.

When making margaritas at home on a sunny day, Biberdorf says that you should watch out for what is called “lime’s disease.” This isn’t the Lyme disease that comes from ticks; this is the severe sunburn that can happen if you have lime juice on your hands and are exposed to the sun. Lime and lemon juice acts as a photosensitizer that makes the sun’s radiation much more potent, especially the closer to the equator you are.

If you’ve ever seen a beer bottle served upside down in a margarita chalice, you’re watching even more science at work. The pressure from the solution of ice and liquid keeps the beer from pouring out of the bottle and into the goblet.

Biberdorf, who has been known to travel with her blender, has a specific order in which she likes to make the frozen margaritas. “Fill the blender with ice, pulverize it, add the tequila, triple sec and then limeade at the end.” Because the limeade is so strong, she often dilutes it with water or — to make what she and her family call “jolly ritas” — a light beer.

But a week after our chat, when she was in Michigan with her family, Biberdorf reported that after all this scientific analysis, they started making them on the rocks.

Frozen Margarita

To make simple syrup, add 2 cups of sugar and 1 cup water to a saucepan. Heat and stir until the sugar has dissolved, then cool. If you don’t have a cocktail jigger, there are about two tablespoons in a fluid ounce.

2 oz. silver tequila

2 oz. Cointreau or Paula’s Texas Orange

2 1/2 oz. fresh lime juice

1 oz. simple syrup

Lime wedge for garnish

Combine the tequila, orange liqueur, lime juice and simple syrup in a blender cup and fill with ice until the ice is just covered by the liquid. Blend thoroughly (it may be necessary to add a little bit more ice to achieve the desired slushiness). Pour into a goblet, rimmed with coarse salt if you like, and garnish with a lime wedge. Serves 2.

— From “Tipsy Texan: Spirits and Cocktails from the Lone Star State” by David Alan (Andrew McMeel, $19.99)

On the Rocks Margarita

2 oz. silver tequila

3/4 oz. Cointreau or Paula’s Texas Orange

3/4 oz. freshly squeezed lime juice

1 tsp. (or bar spoon) simple syrup

Kosher salt, for the rim of the glass

Lime wedge, for garnish

Combine the tequila, orange liqueur, lime juice and simple syrup with ice in a cocktail shaker. Shake vigorously with the ice to properly chill and emulsify the ingredients. Strain into a salt-rimmed, chilled cocktail glass or onto fresh ice in a rocks glass or footed goblet. Garnish with the lime wedge. Serves 1.

— From “Tipsy Texan: Spirits and Cocktails from the Lone Star State” by David Alan (Andrew McMeel, $19.99)



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