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:
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