Ice Cream Class

ChefSteps

A Lick of Science

The name ice cream—made up of two simple words, ice and cream—belies the many complexities that work together to create a soft, lickable scoop of the stuff. In fact, frozen cream on its own is rock hard. Add sugar to soften and sweeten it, and you lower the freezing point. That means it won’t get cold enough to freeze when you put it in solid ice, because the freezing point of water is higher than that of sweetened cream. Thirteenth-century Arabs cracked the code on that conundrum, adding salt to the water to lower its freezing point. But when you put sweetened cream directly into the salted ice, you wind up with salty, grainy, diluted cream. Put the sweetened cream in a vessel surrounded by salted ice, however, and then quickly churn it to develop tiny ice crystals? Now that’s what we call ice cream.

On a molecular level, ice cream is made up of three basic components: ice crystals, concentrated sweetened cream (made of liquid water, milk fat globules, milk proteins, and sugar), and air. <a href="http://www.curiouscook.com/site/">Harold McGee</a> outlines these components very clearly in his book <a href="http://curiouscook.typepad.com/site/on-food-and-cooking.html">On Food and Cooking</a>, and we paraphrase them below. To delve even deeper into the science of ice cream, check out world-renowned ice cream expert <a href="http://vimeo.com/65780089">César Vega’s video</a>, Knowledge Map of Ice Cream. 

The ice crystals form from water molecules as the ice cream mix freezes. The size of the crystals determines whether the ice cream is fine and smooth or coarse and grainy, and it also determines the temperature you perceive as it hits your tongue: grainy ice cream will feel colder than a more smooth-textured mixture. (Why? Because the larger crystals require more heat to melt. That heat comes from your mouth, and thus large ice crystals cool your mouth more, making the ice cream feel colder overall.) Though ice crystals account for much of the ice cream’s solidity, they make up only a fraction of its volume. 

The concentrated sweetened cream is what is left of the mix when ice crystals form. Thanks to all the dissolved sugar, about a fifth of the water in the mix remains unfrozen even at [f 0]. The result is a very thick fluid made of about equal portions of liquid water, milk fat globules (both liquid and semicrystalline), milk proteins (casein micelles and whey), and sugar. This fluid coats each of the many millions of ice crystals and sticks them together. When you churn the ice cream, the semicrystallized globules of milk fat knock together and form long, pearl-like strands that wrap around the air bubbles, yielding a stable foam and giving ice cream most of its volume.

Air cells are trapped in the ice cream mix when it’s churned. They interrupt and weaken the matrix of ice crystals and cream, making that matrix lighter and easier to scoop and giving your ice cream a melty, luscious mouthfeel. The air cells also inflate the volume of the original mix. The increase is called overrun, and in a fluffy ice cream, it can be as much as 100 percent: that is, the final ice cream volume is half mix and half air. The lower the overrun, the denser the ice cream.

What Is Ice Cream?

If you’ve ever mined a six-month-old pint of ice cream from the depths of your home freezer, you’ve unwittingly become acquainted with the perils of the freeze-thaw cycle. Ice cream stored in a busy home kitchen will coarsen over time, developing large ice crystals that ruin the smooth, creamy texture that’s so important in frozen desserts. 

Here’s why: As you fill up and empty your freezer or open and close the door, the fine ice crystals originally formed in the ice cream begin to melt and then refreeze into a smaller number of larger crystals. The result is a field of ever-fewer, ever-larger ice crystals that make up the body of the ice cream, giving it that unpleasantly coarse and crispy texture.

Freeze-Thaw Cycles

To delve even deeper into the science of ice cream, check out world-renowned ice cream expert <a href="http://vimeo.com/65780089">César Vega’s video</a>, entitled “Knowledge Map of Ice Cream.”

To delve even deeper into the science of ice cream, check…

Making ice cream involves five simple steps, two of which are optional: preparing the mix, prechilling, aging (optional, but recommended), freezing, and hardening (optional, depending on how you make the ice cream). Modifying any of these steps will affect your outcome, but with some basic knowledge and the proper tools, the process is pretty hard to screw up. 

Preparing the mix: Every frozen dairy dessert has a different combination of ingredients, but these desserts are always made up at least of cream, milk, and sugar, and sometimes egg yolks, condensed milk or milk powders, and other sweeteners. Sorbet is the notable exception: it’s made up of just sugar and water, usually from fruit purées or juices.

When you prepare an ice cream mix, there are a few simple principles to keep in mind:

- Adding more fat in the form of cream will make your ice cream richer. Add too much fat, and it will have a slick mouthfeel and can harden completely in your freezer. High fat slows flavor release, making for a long, lingering flavor, while lean ice cream has a crisp, bursting flavor that doesn’t linger as long.
- Adding more sugar will improve flavor (to a point) and soften the final product. Add too much, and your ice cream could be cloying or runny, or it won’t freeze at all. 
- Milk will make your ice cream icier and harder because milk is made up mostly of water, which will freeze into ice crystals. In moderation, milk is very important to the molecular makeup of your frozen treat (and it will help to reduce the richness added by cream), but it is possible to overdo it.
- Eggs will make your ice cream dense (by adding solids that thicken the mix), custardy (eggs will, of course, add egg flavor), and smooth. Ice cream made with eggs is much less likely to be coarse or icy because the proteins in the eggs help control ice crystal growth. The hotter you cook the eggs, the more “eggy” flavor you’ll get in the final product—French ice creams tend to include a high ratio of eggs that are cooked at high temperatures before the mix is chilled and churned. 

Prechilling the mix: This is a quick, simple step that will improve results when you churn because freezing will happen more quickly and thus the ice crystals that form will be finer. We usually place our ice cream mix in a bowl over crushed ice for a few minutes to chill and thicken it before churning. 

Aging (optional): Aging allows time for fat globules to partially crystallize and for other casein proteins coating their surfaces to be displaced, leaving naked patches that will stick together during churning. This helps trap air bubbles and results in a softer, more scoopable texture. About six hours of refrigerated aging is optimal.

Freezing/churning the mix: A key objective when making any style of ice cream is to build a network of ice crystals that are as small as possible—this happens when the mix is frozen quickly while in motion; that is, during churning. Whether the final product is airy or dense, eggy or not, or high or low in fat, the freezing process should be optimized for the formation of small crystals, meaning the ice cream will be smooth, creamy, and not too cold on the palate. This is why using super-cold <a href="liquid-nitrogen-churned-ice-cream?context=ice-cream">liquid nitrogen</a> and the more-easily-sourced <a href="making-ice-cream-with-dry-ice?context=ice-cream">dry ice</a> result in much better ice cream than it’s possible to make with most home ice cream machines, which simply cannot remove enough heat energy fast enough to quickly freeze the mix.

CAUTION: Please make sure you’ve read our page on<a href="https://www.chefsteps.com/activities/frozen-cryogenic-poaching-with-liquid-nitrogen"> how to safely handle liquid nitrogen</a>.

Hardening the mix (optional): After the churning step, only about half of the water in your ice cream mix has turned to ice. You may need to freeze the rest of the water in order to have a smooth, storable block of ice cream. But not always: when preparing your treat using liquid nitrogen or dry ice—or in a Pacojet machine—you can control how hard or soft it gets. Churning in an electric home churner, however, yields an ice cream with a soft serve–like consistency. Freezing that kind of ice cream will help make it more scoopable.

If you choose to use this step, it should be done quickly—if hardening happens too slowly, drops of liquid water will bind to existing ice crystals and coarsen the ice cream. You can optimize this step by dividing your ice cream into smaller containers and placing them in the freezer, so more surface area is exposed to the cold air. About two hours in the freezer should do the trick.

Making Ice Cream

Buying ice cream is hard, we know. You’re in the freezer aisle, and there’s a gallon of rocky road for $5, sitting right next to a pint of rocky road for the same price. Which one do you buy? It’s a first-world problem of extraordinary proportions. If you’re hopelessly thrifty, you go for the gallon. But if you’re smart, you pick up both containers and consider the difference. 

Overrun is an industry term for the amount of air mixed into ice cream when it’s made. Denser ice creams have lower overrun and tend to be more expensive by volume simply because you get more actual ice cream and ingredients per cubic centimeter. Beware of larger, cheaper volumes of ice cream—they’re probably made up mostly of air. Nothing wrong with that—sometimes you just need 20 gallons of fluffy vanilla ice cream. But if you’re going for the good stuff, drop the extra buck. It’s worth it.

Bigger isn’t always better: Decoding commercial ice cream options

You’d think food researchers would have nailed down the lineage of this ubiquitous treat by now, but it remains largely a mystery. Some claim Greek heritage, some say the Chinese invented ice cream, and others give credit to India. Ice cream’s modern history is believed to have begun with the seventeenth-century Italians, who popularized a version of the frozen dairy treat that then became fashionable in France, though dairy-free “water ices” were far more popular in both countries than the cream-based version the English later adopted as their own. Thanks to British influence, the US had fully embraced frozen dairy products by the early nineteenth century and were consuming them in “enormous quantities” by 1900, as one English writer noted. (Americans claim ownership of the ice cream cone; they reportedly introduced the world to frozen dairy served in a rolled wafer at the 1904 St. Louis World’s Fair, but historians argue about which vendor originally peddled it.) These days, Australians consume the most ice cream worldwide—about 18 liters per person per year as of 2010—with New Zealand and the US close behind.

Other ice cream histories have developed in different parts of the world: India, Afghanistan, China, the Philippines, Turkey, and others all have their own versions of the frozen treat, and each has its own culinary chronicle to match. And though each frozen confection has slightly different properties, the science remains the same: they’re made up of ice crystals, sugar, fat, and air, and they’re churned and frozen to develop a desirable texture.

A Bit of History

During your undoubtedly delightful adventures in ice cream consumption—running for the ice cream truck, perhaps, or taking the kids for a cone after Little League—you may have never stopped to consider the culinary science behind this frozen favorite. (Of course, if you had, the truck would’ve gotten away, so good work.) But we bet you’ve noticed the difference between thick, creamy gelato from Italy and a light, fluffy scoop from Baskin-Robbins. We bet you’ve noticed how ice cream crystallizes after a long stint in your home freezer. And—since clearly we’re prone to gambling—we bet you’ve stood in the grocery store and scratched your head trying to choose between a whole gallon of Nestlé ice cream for $5 and a single pint of Ben &amp; Jerry’s for the same price. Well, scratch no more. Culinary science provides fascinating explanations for all of the above.So next time your young ones want to go out for ice cream, you can ask, “Would you like ice cream with lots of overrun today, kids? Or ice cream with a higher concentration of egg yolks? Ooh, how about some ice cream with less than the minimum specified fat content?” Lucky kids, right? They’re going to love the way you drop knowledge all over their icy treats.

Ice Cream Geekery