Now reading The Science of Ice Cream

The Science of Ice Cream

The answers to all of your questions, from the Big Gay Science Club.

How many of you remember your science teachers’ names, let alone the contributions to science made by folks like Maillard, Gay-Lussac (hehe, we said “gay”), and Boyle? Not us. That was the dilemma we faced when we chose a yearbook concept for our first cookbook. We couldn’t have a yearbook without geeky science club–style contributions. We remembered the lessons we learned from courses at the Department of Health—but stuff from twenty-five years ago? That was another story.

For that, we turned to our dear friend Kim Brisack. Kim is an honest-to-goodness, goggles-and-lab-coat-wearing, laboratory-working, globetrotting, foreign-language-speaking scientist. For all we know, she could be up on the International Space Station this very moment, curing cancer in zero Gs. She helped us understand the basic scientific principles happening in the kitchen that the home cook may know nothing about. They became our book’s Science Club sidebars, and we’re sharing them here because, as the saying goes, the more you know!

Why Does Salt Make Ice Colder?

0144_Quin_9780385345606_art_r1Water normally freezes at 0°C (32°F). Salt water freezes at about -21°C (around -6°F), so it stays in liquid form at lower temperatures than plain water. The addition of salt means ice that was solid, at, say, 25°F, will become a liquid at that same temperature. You see how salting icy roads in winter makes sense, right?

With an old-fashioned ice cream maker, the ice you put into it will melt at the normal ice/liquid transition temperature, 32°F. At this temperature, the ice is melting and refreezing at the same rate. But when you add salt, the ice that melts won’t refreeze until the temperature gets a lot lower. So, as the ice/liquid temperature gets lower, it will draw heat from anywhere it can—namely, your soon-to-be ice cream. If you think that’s cool, skip the sodium chloride (regular salt) and go to your local hardware store for some calcium chloride. It will lower the freezing point even more, down to -29°C (-20°F). That would ice your cream a lot faster!

Why Do Sauces Go Crazy When I Add Alcohol to Them? 

0147_Quin_9780385345606_art_r1-pg55Doesn’t everything go a little crazy when you add alcohol to it? Remember that time we made out that night at that bar after those shots? Yeah, I knew you were thinking the same thing.

Alcohol has a lower boiling point than most other liquids. That means it will transform from liquid into gas at a lower temperature. If you mix everything at room temperature and then heat it slowly, you won’t see the same reaction as you would if you added the alcohol to a simmering sauce. If the sauce is just simmering (below the boiling point) and you add alcohol, it immediately turns into a gas, and the result can be quite violent. The taste will be left behind, but the alcohol itself usually evaporates within 20 to 30 seconds.

Baking Soda in Dulce de Leche—What’s the Point?

0120_Quin_9780385345606_art_r1Baking soda isn’t really necessary to make a delicious dulce de leche, but it will produce a fuller, darker, smoother, more fabulous sauce, and that’s what really matters. How does it do that? Baking soda is sodium bicarbonate—it’s the same stuff they drank in Mad Men when they had indigestion. When you have too much acid in your stomach, adding something that is alkaline (the opposite of acidic), will even everything out by balancing the pH level.

Adding baking soda to the condensed milk prevents the milk from curdling when heated. Without it, you may end up with a somewhat grainy texture after the mixture cools, because the milk proteins will stick together more. And with it, the dulce de leche will be a few shades darker, a result of the Maillard reaction. This is what causes your steak to turn brown when you grill it, and it’s also responsible for that nice golden crust on your bagel. It’s similar to caramelization but involves proteins instead of sugars.

Why Does Carbonated Soda Go Apeshit When Ice Cream Is Added to It?

0112_Quin_9780385345606_art_r1Let’s first discuss why carbonated soda foams at all. “Carbonated” means that carbon dioxide gas has been dissolved in the soda. “Duh, everyone knows that!” you say. “We can see the bubbles, after all.” Well, in reality, if the gas were actually dissolved, you wouldn’t see any bubbles. It’s like when sugar is dissolved in a liquid—you can no longer see the sugar.

So, then, why can you see bubbles? Carbon dioxide gas actually likes being a gas and prefers that to being dissolved in a liquid. It doesn’t take much encouragement: any little thing that attracts its attention will cause it to leave the liquid and turn into bubbles. The change in pressure when you open a can of soda does this. Pouring the soda into a glass does this. Adding ice does this. Adding sugar really does this (don’t try that at home unless you have help to clean up a sticky mess). If you’ve ever seen the Mentos/Diet Coke YouTube videos, you know that the combination can be like detonating a bomb!

In other words, anything that is not already a liquid will cause carbon dioxide to leave the liquid and reclaim its preferred status. This is called nucleation. It requires a little something to start the reaction, and air is the best way to do that.

Ever blow bubbles into your fountain drink? It causes a lot more bubbles! Ice cream has a lot of air in it—that’s what gives it a soft, light texture, and that’s what causes the explosion of bubbles when it’s combined with soda. If you add soda to your ice cream, you will get a much grander eruption than if you add ice cream to your soda. This is because if you pour the soda into the glass first, a lot of the carbon dioxide will have formed bubbles and popped by the time you add the ice cream. If you add the ice cream to the glass first, there will actually be more bubbles when you add the soda.

Ice cream also has thickeners and fats in it—that’s what causes the foam in a float. Basically, the outside of the bubbles are more stable, so they don’t pop like normal bubbles. Cheers!

Why Do I Need to Cover the Blender When Blending a Hot Liquid?

0164_Quin_9780385345606_art_r1The laws of physics can be quite dangerous when you’re not paying attention, and doing things like taking a sharp curve at high speed, falling down the stairs, or putting hot things in a blender. While we don’t have to worry about any of those here, the last is courtesy of the Pressure-Temperature Law, which is sort of a composite of Boyle’s law, Charles’ law, and Gay-Lussac’s law. All of these guys studied different facets of the same phenomenon—that gas expands when heated and contracts when cooled.

Have you ever bought balloons for your sweetheart on Valentine’s Day, then walked outside into the snow and had all of the balloons deflate?

You walk back into the store to demand a refund, and as you are waiting in line, the balloons magically begin to inflate again. Same thing. When you put a hot liquid in the blender, the air above the liquid is heated. As it heats, it expands.

As it expands, the pressure will build up. As the pressure builds up, the top of the blender will pop off, and the next thing you know, you’re cleaning purée from the ceiling.

To prevent this, use a towel and your hand to cover the top of the blender. You can exert more pressure than that measly expanding gas.

Additionally, the center of the blender lid does more than allow you to add ingredients while blending. Used properly, it also lets steam (or gas) escape to avoid a buildup of pressure.

What’s the Difference Between All These Sugars?

Most people are familiar with white (granulated) sugar and powdered (confectioners’) sugar, but there are lots of other ways to sweeten your ice cream.

Granulated sugar is usually made from sugarcane and is highly processed to remove the natural brown color. Removing the natural brown color also changes the taste and texture (see below). The chemical that makes up common sugar is sucrose. Sucrose is a disaccharide, because it is composed of two sugar subunits, and is digested as glucose and fructose.

Powdered sugar is white sugar that has been milled to a finer grain and has cornstarch added to prevent clumping.

Molasses is usually made from sugarcane juice that has been boiled and had most of the sugar (in the form of sucrose) removed. It’s somewhat sweet, but it is usually used more for its rich flavor rather than for sweetness.

Brown sugar is refined white sugar that has had molasses added to it. The amount of molasses (usually between 3 percent and 10 percent) determines whether it is “light brown” or “dark brown.” Adding molasses to white sugar means the color and taste can be tightly controlled. Turbinado, demerara, and muscovado sugars (also called raw sugars) are made from sugarcane, but are less refined than white and brown sugars. Muscovado is the darkest of these sugars and has the most residual minerals, but the mineral content is quite negligible, so there aren’t really health benefits to using it.

Corn syrup, as its name implies, is made from corn. Because it is a liquid instead of a solid, and won’t crystallize, it can often produce smoother ice cream. Light corn syrup and dark corn syrup can usually be used interchangeably. Light corn syrup has a more delicate flavor and a slight vanilla taste, while dark corn syrup has a touch of molasses flavor and is better for baked goods.

High-fructose corn syrup is different from the corn syrup you buy at the grocery store because it is chemically modified to convert glucose to fructose, tastes sweeter on the tongue, and bypasses part of the digestive process, triggering the creation of fats and insulin. It is used in highly processed foods. Agave nectar is made from the agave plant, which is also used for making tequila. It has been touted as a more healthful alternative to sugar or corn syrup, but in reality it is not healthy at all. Although it won’t give you the blood-sugar spike that regular sugar does (thus the “low-glycemic” claim), the high fructose content can cause other problems because it is processed in the liver into fat deposits. Agave nectar is a highly processed sweetener and contains even more fructose than high-fructose corn syrup.

Honey and maple syrup are the most “natural” sweeteners because they are simply harvested and bottled with minimal changes. Make sure you get a natural maple syrup without additives.

If you must avoid sugars altogether, there are numerous substitutes. Most of them contain a mélange of chemicals, so long-term usage may cause more problems than just using white sugar. If you must use something other than what’s listed above, consider stevia. It’s made from a natural herb and contains no actual sugar molecules. The stevia you can buy off the shelf is highly processed, but it’s still better for you than aspartame, sucralose, or xylitol.

What is Homogenization, and Why Should I Care?

Have you ever seen fresh raw milk, where the cream separates into a thick layer at the top? It’s almost like butter. You can spoon it into your coffee like cream or shake up the bottle to reincorporate it into the milk. If you remove it, the remaining milk is actually pretty low in fat.

Modern milk manufacturers decided that consumers didn’t like that plug at the top. It looked too much like spoiled milk. So homogenization was born. When milk is homogenized, the fat particles are broken into very small bits that stay throughout the milk and don’t rise to the top. Homogenization also allows large milk manufacturers (who combine milk from several different dairies and types of cow) to make a more uniform and consistent final product—ultimately so they can control the final taste and fat content.

So why does homogenization matter? Well, there are a lot of people who think they are lactose intolerant when they really are just drinking the wrong kind of milk. When the fat particles are mechanically broken down, they can pass too quickly through the digestive process and not be digested. Proteins that should be digested are allowed to pass into the bloodstream. This can cause a host of belly problems that we won’t get into. Suffice it to say, every time you eat processed food—homogenized milk included—you are ingesting something your body was not designed to digest.

Now, don’t confuse this with pasteurization. This is a process where milk is heated in order to kill bacteria and is usually required for any milk sold commercially. Pasteurized milk also has a longer shelf life.

So: homogenization = bad, pasteurization = good. One other note: Different types of cows produce different kinds of milk. Jersey cows (the brown ones) have a much higher “solids content” than Holstein cows (the black-and-white ones). This means that Jersey milk is much richer, and that will affect the taste and texture of everything you make with the milk, from ice cream to yogurt or cheese to your morning cappuccino. Ronnybrook Farm Dairy, in New York’s Hudson Valley—who provide us with our East Coast dairy—produces a line of nonhomogenized milk called Creamline. If you can find nonhomogenized Jersey milk near you, thank us later, after you’ve had a mouthful of that deliciousness!

What is an Emulsifier? Why Do I Need One to Make Ice Cream?

We all know that oil and water don’t mix. That’s because oil is hydrophobic, which is a fancy word meaning that it doesn’t like water. So it tends to hang out in its own little clique, as separate as possible from any water. Think about salad dressing—the oil and vinegar usually separate into two very distinct layers. You can shake the container of dressing hard (or whisk it) and break up the oil into lots and lots of smaller droplets, which will be suspended in the vinegar. But if you let it sit, eventually the oil separates out again, and you have two distinct layers. If you look at the variety of salad dressings at the grocery store, you may see some vinaigrettes that don’t look like two layers. Check the ingredient list—you will find an emulsifier. With ice cream, the ice plays the part of the water and the cream plays the part of the oil.

You wouldn’t want ice cream that is separated into ice and cream; you want everybody to get along. An emulsifier plays the role of peacekeeper. It’s made up of molecules that have one end that likes fat and one end that likes water. It holds these two together, so everything stays well mixed and doesn’t separate. Common emulsifiers are egg yolks, some dairy products, seaweed extracts (such as carrageenan), lecithin, xanthan gum, cellulose gum, and various types of glycerides. This kumbaya moment stays around after the ice cream base is frozen, allowing the ice cream ingredients to stay solid enough to bind together, but not so strongly that they become an ice cream brick.

Reprinted from Big Gay Ice Cream. Copyright © 2015 by Bryan Petroff and Douglas Quint. Illustrations by Jason O’Malley. Published by Clarkson Potter, an imprint of Penguin Random House, LLC.