It's a step in every egg-based ice cream recipe: "chill base overnight and churn the next day." Every pastry chef worth their gram scale will tell you how important it is to age ice cream bases—i.e chill them down in the refrigerator—before churning them.
But as we've seen before, ice cream recipes are full of procedures and biases that don't pan out when put to the test. You don't need to scald dairy or temper eggs. Fancier dairy doesn't necessarily make better ice cream. Corn syrup isn't evil.
So if you're pressed for time and want to churn your ice cream the same day you make your base, do you really need to age it overnight, or just chill it down until it's cold enough to churn?*
* From this point on we're only talking about custard ice creams made with eggs, not Philly-style or other egg-free bases, which don't usually include big claims about the importance of aging.
There's a bevy of conventional wisdom about why you should follow this step. Aging bases cools the base down, and the colder a base is before it goes into the churn, the creamier the resulting ice cream will be. Aged bases are noticeably thicker than freshly made, un-aged bases, and thicker bases tend to churn faster and creamier. Plenty of flavored liquids taste better the next day, or so it's said about every stew you've ever made.
More detailed science on the question is hard to come by, and there's plenty of pseudoscience, from "water evaporates overnight" [false] to "the milk proteins need to hydrate" [they're already in water!]. This ice cream blog—that cites scientific papers and technical books on ice cream—is the most convincing version of the scientific argument I've seen for aging your base:
First, the emulsifiers (lecithin from the egg yolks) absorb [sic; I believe he means 'adsorb'] to the surface of the fat droplets....These fat globule clumps are responsible for stabilsing the air cells and creating a semi-continuous network of fat throughout the product resulting in a smooth texture and resistance to meltdown.
Second, cooling the mix to below 4°C [just under 40°F] causes the fat inside the droplets to begin to crystallise. Nearly complete crystallization is needed to promote coalescence of fat globules when the mix is frozen in an ice cream machine.
In essence, the argument goes that fat droplets in dairy need time and low temperatures to bind with emulsifiers in egg yolks and form themselves into crystals, which makes for smoother, creamier ice cream that doesn't melt as quickly. This would certainly explain why bases are thicker the next day.
But it doesn't account for any difference in ice cream's flavor, and it doesn't clarify if a home cook, with regular home equipment, would notice a difference between an aged and un-aged ice cream base once the ice cream is churned.
Early last week I made a big batch of standard vanilla bean ice cream base (in this case minus the whisky) and divided it into four containers. Three of them went into the refrigerator to age. The fourth was chilled down in a simple ice bath—a small bowl nestled in a larger bowl full of cold water and ice—until it cooled down to a refrigerated-base temperature: 39°F, which for a pint of base took about 2 1/2 hours. It was then churned immediately and left to harden in the freezer.
The next day I churned another batch of ice cream from one of the aged bases. Curious to see if even longer aging produced different results, I churned the other two batches 24 hours apart for a total of four ice cream samples aged zero to three days before eating. Once all the ice creams were firmed up to the same scoopable texture, the Serious Eats tasting panel sampled the ice creams in a blind taste test, scoring them for their vanilla flavor, creamy texture, and overall preference.*
* Astute readers may notice that this isn't a totally fair test, since some of the ice creams were sitting in the freezer for longer than others. But homemade ice cream lasts about a week before any real decline in quality, and if a little freezer time was enough to override any impact of aging on flavor or texture, well, the effect isn't very strong to begin with. Without a time machine that can bring the exact same batch of ice cream base into the future, this is the best we can do.
Going in, tasters didn't know what, if any, differences there were between the ice creams. In fact, I got more than one question asking if there was any difference between the samples. After scoring their results I understood why. See for yourself:
The ice creams' flavors don't track clearly with the other ratings, but you'll notice a correlation between creamy texture and overall preference. But take a look at the highest-rated ice cream—the one that aged just a few hours. There are some patterns here, but nothing you could call conclusive.
So does aging an ice cream base make any difference once the ice cream is churned? If it does, we can't taste it. Should you want to churn your ice cream the same day you make your base, chill it down in an ice bath until it drops below 40°F, then go right ahead.
Reasons You Might Want To Age Your Base
Aging your ice cream base may not necessary, but our tasting shows there's no downside to it either—we saw no trend between longer aging and decreased quality. And there are some reasons to consider it.
First and foremost, aging overnight ensures the base is very cold, and deep-chilled ice cream bases churn better than warmer ones. If you want to churn a base the same day you make it, you'll need to set up an ice bath to cool down your hot custard. Doing so is a big pain. You need lots of ice and two clean nesting bowls, there's the risk of you slipping your base into the bowl of ice water, and it still takes appreciable time to chill a base down to refrigerator temperatures—upwards of a couple hours. Honestly, I'd rather cool my ice cream down in the fridge.
After tabulating the results of our tasting I reached out to some pastry chefs who make ice cream regularly. Though they all age their bases overnight, none were surprised by our results. They did, however, offer some other thoughts on aging bases.
For Stella Parks of Table Three Ten (you may know her as Bravetart), ice cream bases aged overnight spin up a little lighter and fluffier than when they're un-aged, giving her an extra scoop or so per [large] batch—handy when you're a pro pastry chef with a bottom line. This lends further empirical credence to the idea that aging bases allows fat droplets to firm up and network to form a more stable ice cream that holds air better.
Ryan Butler of the Highlands thinks that most flavors intensify overnight (though some, like alcohol, dissipate), enough so that he withholds final seasoning of the base until it's aged. While I'd have to taste an aged and un-aged base side by side to see if that pans out for our vanilla, I'd definitely agree that aging a base gives you a chance to revisit an ice cream base after taking a break. Your palate is refreshed, the flavor has had time to chill down, and you can make any fine-tuning adjustments you'd like.
Tracy Obolsky of North End Grill offers another compelling reason to age your base: if you're steeping flavors into an ice cream, continuing to steep them overnight intensifies the flavor all the more. "With my toasted coconut ice cream, I used to steep the coconut and then strain it out. Now I leave the coconut in overnight and it's like a punch in the face of coconut." But take note that this is about direct contact between a base and a flavoring agent, not aging a base on its own; Obolsky imagines that "in a side by side comparison, the average person probably can't taste the difference between aged and un-aged bases."
Which brings us back to where we started. Do you need to age ice cream base before you churn it? Not really, or rather no longer than it takes to cool it down. Does it hurt? No way. So go forth, ice cream makers, unimpeded by science or tradition. Do what your schedule allows, keep cool, and your ice cream will turn out creamy and delicious.
More Ice Cream Science
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Do I Need to Use Eggs in Ice Cream (and How Many?) »
Taste Test: Does Premium Dairy Make Better Homemade Ice Cream? »
Use Invert Sugars Like Corn Syrup for Smoother, Less Icy Sorbet »
What's the Difference Between Gelato and Ice Cream? »
Our Tasting Methodology: All taste tests are conducted completely blind and without discussion. Tasters taste samples in random order. For example, taster A may taste sample 1 first, while taster B will taste sample 6 first. This is to prevent palate fatigue from unfairly giving any one sample an advantage. Tasters are asked to fill out tasting sheets ranking the samples for various criteria that vary from sample to sample. All data is tabulated and results are calculated with no editorial input in order to give us the most impartial representation of actual results possible.