Cream Science: On Whipping, Butter, and Beyond

Cream is a remarkably versatile ingredient. Poured into coffee, spooned onto fresh berries, drizzled into soups, or stirred into risotto, it adds richness and a silky texture to any dish it's used in.

But, of course, cream isn't just special in its liquid state: it possesses unique characteristics that allow us to drastically alter its form, converting it into the stable foam structure we know as whipped cream or emulsifying it into butter. With only a small amount of brute physical force and an even smaller amount of time, you can effectively transform cream into a scoopable, spreadable, and all around easier-to-wrangle product.

Back in the day (I'm talking pre-1900), procuring fresh cream was a lengthy process. Until dairy rockstar Gustaf de Laval manufactured the first (hand-cranked) centrifugal milk-cream separator in the late 19th century, cooks were forced to wait up to a day for cream to naturally separate from milk; only then could it be skimmed off and collected for whipping. Hard to imagine if, like me, you're not particularly skilled in the arts of planning and thinking ahead.

Luckily, today, ready-to-whip, homogenized heavy cream is readily available for purchase and manipulation (and you can manipulate it into so many things).

From a chemical standpoint, it's packed with potential. Apply some work and you have a rich whipped topping. Apply more work and you've got fresh butter. Add some acid-producing bacteria and you get crème fraîche and, if you decide to whip that, full-flavored cultured butter and tangy buttermilk.

For all of these iterations, we have fat to thank. Not only is milk fat responsible for that smooth, mouth-coating quality, but it provides the framework that holds each and every one of these cream products together; if whipped cream is a body, then fat is its skeleton. But how does is get to that point? By whipping, you're changing the physical structure and chemical properties of the lipids within the cream. But what may sound simple on the macro level is actually quite complex on the microscopic. Taking the time to understand how and why cream can be transformed from a puddle of liquid into a cloud of semi-solid foam will allow you to isolate the factors that make your recipes successful.

What's in Your Cream?

Before we get into the cool stuff you can make, let's talk about the starting material. Cream is that fat-enriched portion of milk that rises (or is forced by centrifugation) to the top of milk. Milk is a "colloid," a substance in which small, insoluble particles are suspended throughout another substance. In this case, those particles are fat globules—little droplets of fat—distributed in a water-based solution. If fresh, un-homogenized milk is left undisturbed, the lighter-than-water fat globules will eventually float to the top and gather together, where they can be skimmed away from the "skim milk" left on the bottom. In the United States "heavy whipping cream" is defined by the FDA as "cream which contains not less than 36 percent milk fat." The rest is mostly water, along with a few proteins, minerals, and milk sugars. This is a slightly higher concentration than the percentage legally required by the UK and Switzerland (35%).*

*Those numbers mean that Americans are legally entitled to an extra gram of fat for every hundred grams of heavy cream, and that makes me feel very patriotic.

"Creaminess" is kind of its own sensation; somehow it's fatty without being greasy. For that, you can thank emulsion: the large amounts of tiny fat globules suspended in a small amount of liquid. These things are really, really small; we're talking micrometers, way too tiny for our clunky tongues to distinguish as individual particles. Dense crowds of these minuscule globules is what allows for that seamless, luxurious mouthfeel. If they were instead large enough to be detected by feel, cream and creamy products would cease to be smooth and velvety; it would feel like kind of like drinking a loose mixture of oil and water—not what you want in a dessert.

The processes of transforming cream into butter or whipped cream are similar, but how hard and how long you whip it have a big effect on the outcome. Length of whipping time is particularly important when making whipped cream, so let's start there.

The Chemistry of Cream

Whipped cream is a foam—a suspension of gas bubbles in another substance. Unlike egg-based foams, which are stabilized by protein, whipped cream is stabilized by its own fat. Milk fat is a complex mixture of lipids, but the most prevalent one is triglyceride, made by combining three fatty acids (that's the "tri-" part) and glycerol (that's the "glyceride" part).

Quick disclaimer: if high school chemistry classes left you sweating in your seats, you may want to jump ahead a few paragraphs!

Fatty acids are simply carboxylic acids with super long carbon chains attached. Carboxylic acid is a class of carbon containing acids in which a carbon is connected to an oxygen atom by a double bond, and an oxygen-hydrogen grouping by a single bond. It looks like this:

The R's are place holders for any number of carbon-containing chains or rings. In acetic acid (a carboxylic acid which gives vinegar its characteristic taste and smell) the "R" is this part:

Glycerol is a simple sugar alcohol which looks like this:

When a fatty acid is combined with glycerol, you get a triglyceride, and it looks like this:

Fat hates water, but these triglycerides are protected by membranes of phospholipids, special biological molecules that possess hydrophilic (water-loving) and hydrophobic (water-fearing) regions. Phospholipids look like this:

The hydrophilic head faces water molecules, forcing the hydrophobic tails to gather around the fatty triglycerides. The resulting globule looks like this:

When you whip cream with a whisk, a couple of things happen. First, air is forcibly integrated into the cream, forming bubbles of gas that pop almost as quickly as they form; the surface tension of the cream simply isn't strong enough to entrap them. But, after a few more minutes of being knocked around, the fat globules in the cream begin to destabilize as their protective phospholipid membranes are broken apart by the force of the whisk. This exposes portions of the water-fearing triglycerides, causing them seek each other out and stick together in their hour of darkness.

But some of these naked areas of fat may not find another triglyceride to glom onto and, because they would rather face anything but the dreaded water molecule, they align themselves with fairly neutral pockets of air. A network of fat globule-surrounded air bubbles develops and the stable, somewhat solid structure known as whipped cream is born. Because this whole house of cards is held up by these exposed and frightened triglycerides, it only works if there are a large amount of them in there—you can't really whip anything with lower than a 30% fat content.

That means that in the U.S., you'll need to reach for either a carton of whipping cream (which runs between 30 and 35% fat) or heavy cream (at least 36% fat). The former will whip up into soft, tender peaks, while the latter, because of its higher fat content, tends to form stiffer, more spoonable or pipe-able peaks.

When to Whip

But how do you know when to stop whipping? Since eyes are not microscopes, and it's impossible to see the little triglycerides clumping together while surrounding pockets of air, we have to zoom out to the macro level and look for larger, visual cues.

At first, you'll see trails in the cream that don't immediately disappear; you have partially damaged some of those protective membranes and are beginning to trap a very small amount of air. Next, you'll start to see some soft peaks that sit on top of the cream's surface, but no real change in volume. Pay close attention, because once you notice an increase in volume, accompanied by firmer peaks that hold their shape, you've made it. Shut it down and get that stuff on some strawberries.

If you decide to whisk boldly on, you'll continue to wreck the phospholipid membrane, exposing even larger portions of fat. These newly exposed regions are now free to clump with their fatty friends. The air—no longer surrounded and stabilized by the network of globules—escapes and your foam deflates, leaving you with a greasy and granular product. Your whipped cream will appear stiff and slightly yellow, and you may even be able to see little clumps.

If this happens, don't freak out. Your whipped cream may be ruined, but you're well on your way to something equally delicious...

When Cream Becomes Butter

Once you've gone past that pillowy, firm-but-not-stiff whipped cream stage, and you begin to see evidence of dense globule gatherings, you're making butter. Butter can be made in a food processor, stand mixer, or even a jar. The key is agitation.

Shaking cream in a jar until it turns into butter can be exhausting (it's kind of like a culinary Shake Weight) but whether you are whipping, shaking, or thrashing the cream around in the food processor, what you're ultimately doing is smashing those little globules of fat into each other, damaging their walls and causing the hydrophobic (water-fearing) regions to clump together. The cream will become thicker and thicker as more and more fatty triglycerides gather into one mass. Eventually, enough fat is exposed and there's room for everyone to get together, eliminating the need for triglycerides to partner up with air. In other words, fat was just stringing air along until other fat became available.

Once air leaves (feeling humiliated and used) the network collapses, and the water that was being held in suddenly and dramatically separates from the solid mass of butterfat.

The solid portion is butter, now ready to be drained and washed. Scoop it out, letting the watery milk drain off, and place your solid butter in a bowl of clean ice water. Fold it and press it around the bowl a few times, dumping and replacing the water until it rinses clear. Dispose of the last bit of rinse water and continue to knead the butter a little while longer, expelling excess liquid. Water promotes microbial growth, and failure to remove the watery skim milk can result in it souring, which would spoil all of your beautiful butter.

Once you've squeezed out as much liquid as humanly possible, pack it tightly together, wrap it up with plastic wrap, and refrigerate or freeze. (Or maybe spread directly onto some good bread and get it into your mouth pronto.)

Let's Get Cultured

Remember way back when, before Gustaf de Laval busted out his centrifuge? Remember those dark times? Back then, the only way to separate delicious, fatty cream from milk was to let gravity do the work. Raw milk would just sit there, and someone would have to skim the cream off of the top. Well it wasn't actually that bad. All that sitting around meant that bacteria had time to grow, something that sounds gross but is actually awesome.

Bacteria is what gives cultured cream, butter, and buttermilk their delightfully acidic tang. These "cream cultures" are a group of various bacteria that allow us to create wonders such as cheese and sour cream. One of these guys is Lactococcus lactis ("lacto" meaning "milk" and "coccus" meaning "sphere"), a microbe that is informally classified as the lactic acid bacterium, due to its ability to transform lactose into lactic acid through fermentation.

Lactose is a relatively unsweet sugar (only 40% as sweet as table sugar by weight) found almost exclusively in milk. When bacteria are introduced to dairy, it makes a meal of lactose, converting it into energy and producing lactic acid as a happy byproduct. The increase in acid decreases the pH of the cream, changing the flavor and making the environment inhospitable to other, less friendly microbes.

We can harness Lactococcus lactis to make many wonderful things, but crème fraîche is by far the easiest. This nutty, buttery, soured cream has many savory and sweet applications. Use it as a dip for chips, on blinis with caviar, or as a tangy foil for sweet, ripe berries. The stuff can be frustratingly hard to find in prepared form and—if you do happen to find it—can cost you as much as a dollar an ounce.

Making it at home is much more cost effective, and only requires heavy cream, buttermilk, and patience. Simply add two tablespoons of buttermilk to a pint of cream, leave it at room temperature, and let the bacteria take it from there. The cultures will get to work, chowing down on that delicious lactose, producing not only acid, but other flavor compounds, such as the buttery diacetyl (the same molecule added to "buttered" popcorn).

Depending on the cream, and the temperature of your home, this process can take anywhere from 8 hours to days. Lactococcus lactis is happiest at around 70 degrees, but as long as your house isn't a freezing tundra or tropical rainforest, you should be okay. Eventually, the cream will thicken and the pH will reach around 4.5. Stir and refrigerate, and then put it on everything. (Check out the full recipe here.)

Can I Use Ultra-Pasteurized Cream?

A word on the cream: I had always heard that ultra-pasteurized cream should be avoided at all costs when attempting to make any type fermented dairy. In On Food and Cooking, Harold McGee states that ultra-pasteurization decreases the lactose content, effectively putting the bacteria on a diet and robbing them of their favorite meal.

Being the experimental chemist that I am, I decided to do a side-by-side comparison, and was surprised to find that the ultra-pasteurized batch actually came out perfectly. Not only did it thicken much faster than the pasteurized cream, but the resulting crème fraîche was much more homogenous, without any of the curd-like chunks that plagued the pasteurized batch.

Here is how they looked after twenty four hours of thickening:

Besides reminding me of my undergrad research—where every day was opposite day and the chemicals never did what they were supposed to—this flew in the face of everything I thought I knew about soured dairy. Once I collected the shattered pieces of my brain off the floor, I consulted with some of the Serious Eats editors and we came up with a few (as-yet untested and unproven) theories:

Theory #1: Carrageenan may be thickening the ultra-pasteurized cream.

High temperatures can change the texture and flavor of the cream, so food companies often add congealing agents such as carrageenan to return the cream to its original viscosity. These agents could work in tandem with the microbes to thicken the cream, speeding up the process. (Make sure to check your heavy cream labels!)

Theory #2: The more pasteurized the cream is, the less the added cultures have to compete with.

Ultra-pasteurization effectively wipes the microbial slate clean. With no other microbes to beat out, our cultures—added in the form of buttermilk—could be free to eat all of the lactose in sight, without having to share.

Theory #3: It's a fluke.

Cream is a biological product, and biology can vary from case to case. One instance of superior crème fraîche made from ultra-pasteurized cream doesn't mean that it will be superior in every case, only that it was in this one. What it does show is that it's not impossible to make decent crème fraîche with the ultra-pasteurized stuff, which is great news because I had to trek to the health food store to find cream that wasn't ultra-pasteurized. Eventually, the pasteurized version did thicken; it just took a lot longer and wasn't as thick as the ultra-pasteurized version.

After my experiment, I found myself with a lot of acidulated cream. Having two batches of crème fraîche isn't a "problem," but having a batch of crème fraîche and a batch of acidulated butter is a whole lot better.

So I took a batch and whipped it in my stand mixer until it separated into butter. The result was a more flavorful and complex butter, and the byproduct was a bonus supply of cultured buttermilk. This stuff is worth saving. Let it drain off of your butter and store it in a jar. Use it to make pancakes, fried chicken, or cornbread. Some say you can even use it to treat a sunburn. Whatever you do, just don't throw it away.

Does Temperature Matter?

You have probably read or heard that you should chill you equipment and cream before attempting to whip it. This is reasonable advice, given that you are building a structure that is held together by fat, and fat likes to melt. Keeping everything cold when making whipped cream or butter will help keep the fat in the solid phase.

In the case of whipped cream, this is crucial. If your fat backbone begins to soften and liquidize, the structure of globules can collapse, releasing the air bubbles and deflating. As for butter, keeping everything cold helps to keep things more manageable. What's easier to hold in your hand, refrigerated butter or melted butter? This is especially important during rinsing. If you rinse with warm (even tepid) water, you melt fat and wash it down the drain. The easiest way to avoid this tragedy is with the ice water method described above.

You now have the knowledge to make five delicious dairy products, all from just one or two ingredients. Knowledge, my friends, is power. Delicious, delicious power.