The Curious Cook
IT lasted only a moment, but it was the most refreshed I’ve ever felt at the dining table. All of a sudden my mouth was shockingly cold, so cold that I could see my breath. As the cold dissipated I could sense acidity, astringency, the aroma of lime. Meanwhile, there was the sight of my companions, eyes wide open and vapor jets shooting from their lips and nostrils. Each of them looked like Yosemite Sam blowing his stack.
The morsels that had cleansed our palates and minds were a mixture of lime juice, green tea, vodka, sugar and egg white that was whipped into a light foam, portioned into spoonfuls, and frozen. At 320 degrees below zero. In liquid nitrogen.
The nitro-poached mousse was invented in 2001 at the Fat Duck, near London, and has been much emulated since. These days there is less talk in cutting-edge kitchens about burners and B.T.U.’s, and more about the Antigriddle, a boxy flat-top appliance that keeps its surface at minus 30 degrees Fahrenheit. With it you could, for example, freeze puddles of crème anglaise and flip them into soft-center ice cream flapjacks. Cold is the new heat.
In the summer kitchen, cold is welcome relief from heat. And it’s an underexploited asset. You can do many useful and delicious things with an ordinary refrigerator and ice cubes, but you need to know a few tricks to make the most of them.
When we put food in the refrigerator or the freezer, we’re not adding cold to the food, we’re subtracting heat. Heat is essentially commotion, and temperature is a sign of how energetically a material’s atoms and molecules are jostling around. Hot molecules really hop, and cold ones plod. Cooling a food sucks energy out of it so that its molecules move more slowly.
What’s the good of plodding molecules?
For one thing, stability. Cold things don’t change as fast as warm or hot things, and cold living things are less lively. So cold foods go stale and spoil more slowly.
With the exception of a few sensitive items like tomatoes and basil, the colder we keep our foods, the longer they stay in good shape. It’s worth getting a point-and-shoot thermometer and regularly checking the temperature in various parts of your refrigerator. It will cost you a little more in electricity, but try to get the colder back corners down to 35 degrees, and the back of the freezer below 10 degrees. Put fish and meats in the coldest spots. Keep the compartments filled so that they’ll cool again quickly after you open the door and let warm air in.
Cold, slow-moving molecules also give foods a more solid consistency. Thin soups and sauces thicken and cling. Gelatin-rich meat juices and stocks turn into jellies that then melt into a cooling sauce in the mouth. Meats, fish and breads are all firmer and easier to slice cleanly when they’re on the verge of freezing for making tartares and carpaccio, sushi and delicate one-bite sandwiches.
Soak lettuce leaves and cucumber slices in ice water to replenish their moisture and chill their solid cell walls, and chill the vinaigrette in the freezer, and you’ll end up with the crispest possible salad and the clingiest dressing. The same ice water treatment turns shredded cabbage into the crunchiest coleslaw.
Unlocking the Power of Ice
In summer, the sensation of coldness is refreshing in itself. But it can take hours to subtract enough heat to chill a pitcher of tea or freeze ice cubes or ice cream. You can chill out faster and easier if you appreciate the tricky nature of ice and know how to unlock its impressive power to absorb heat.
When we remove enough heat from water that its molecules stop moving from one point to another and just jiggle in place, they begin to bond to one another and form solid crystals of ice. In plain water, this freezing process happens at 32 degrees Fahrenheit. And the freezer has to extract a lot of heat to make it happen. To turn a tray of 32-degree water into 32-degree ice, it removes an amount of heat that would bring the same tray of water all the way from 180 degrees down to 32 degrees.
That’s why ice cubes are so effective at cooling drinks. Ice doesn’t just suck out a degree’s worth of water heat when it melts, it sucks out about 80 times that much.
The disadvantage of using ice to cool a drink is that it dilutes the drink as it melts. Bartenders have various strategies for mitigating dilution in cocktails. The thing to remember is that large cubes have less surface area than the same weight of crushed ice and will dilute a drink more slowly, though they’ll also cool it more slowly. To keep a strong drink cold and strong, prechill it and the glass, and use large cubes.
A Fickle Phenomenon
It takes hours in an ordinary freezer to turn water into ice cubes or sweetened cream into ice cream. So on top of the fact that it sounds completely impossible, there’s a practical reason for looking into what is known as the Mpemba effect, the observation that liquids sometimes freeze faster when they start out hot.
In 1963 a Tanzanian high school student named Erasto Mpemba noticed that his hastily prepared, still hot ice cream mix froze faster than a friend’s properly precooled batch. His observation has stimulated much learned discussion, in print and on the Internet.
The consensus is that hot liquids sometimes do freeze faster than cold ones, possibly because evaporation leaves less liquid to freeze, and heating drives dissolved gases that can interfere with freezing out of the liquid. But the Mpemba effect is fickle. I’ve experimented with ice cubes and found that hot and cold trays end up freezing in about the same time. Even that fact would help you win bets, but it doesn’t win you faster ice.
Building a Better Ice Bucket
Once frozen solid, ice cubes can get as cold as the freezer, which might be 10 degrees or even less. But when they’re covered with any liquid water — sloshing around with beer cans in a cooler, for example, or melting in direct contact with a can — that water can’t be any colder than 32 degrees. If it were any colder, it would be solid ice. So no matter how cold the cubes are inside, their effective cooling temperature is their surface temperature, which is 32 degrees. And that limits how fast they can work for us to cool things down.
How can we get quicker access to the inner coldness of our ice cubes? Simply by sprinkling them with salt, as the highway department does to icy winter roads.
Whenever a substance is dissolved in water, its molecules get in the way of the water molecules bonding with one another into solid crystals, and the water has to get colder before it starts to crystallize. So dissolved salt lowers the freezing point of water. And this means that when an ice cube at 10 degrees is salted and begins to melt at its surface, salt dissolves in the liquid water. The solid cube can then cool that salty water colder than 32 degrees. With enough salt, the brine can get close to 10 degrees.
Ice and salt are big timesavers. Say you want to chill a warm bottle of white wine. You can air-cool it to 50 degrees in a couple of hours in the fridge, and maybe an hour in the freezer. In a plain ice bath it’ll take just half an hour, because the water is a thousand times denser than the air in your freezer and conducts heat away more efficiently. And in a bucket of ice sprinkled with a half-cup of salt, the wine will be ready in just 15 minutes. If you want a truly icy swallow of beer or soda, and you want it fast, salted ice will deliver a 35-degree can in 15 minutes. (Bottles take longer to chill because metal conducts energy better than glass.)
No-Tech Ice Cream
Nor is there any need to wait hours freezing the canister of an electric ice cream maker or break a sweat cranking an old manual machine. Salted ice is old hat in this area — it’s what made the invention of frozen desserts possible 400 years ago. But I recently heard about a clever gadget-free method for making ice cream that seems to be used mainly in schools and at 4-H fairs, and came up with a timesaving version of it.
The basic method is to put sweetened cream and milk, or any other ice cream mix, into a plastic freezer bag, and the bag into another bag containing salted ice.
My idea was to use the bag’s flexibility to maximize the contact of mix and ice.
If you put a pint of mix in a gallon bag, you can spread it into a thin layer around a quarter of an inch deep. This exposes much of the mix directly to the surrounding cold, so it freezes very quickly. And quick freezing means smaller ice crystals and a smoother texture than you usually get in unstirred ice cream.
It took me five minutes to put together an eggless Philadelphia-style ice cream mix, and later on, about the same time to sweeten a purée of fresh strawberries for a sorbet. Both times I spread out the bag of freshly made, unchilled mix in a plastic bowl and surrounded it with salted ice cubes. Thirty minutes and a few vigorous shakes later, the mixes were firm enough to serve.
According to Guinness World Records, the fastest time for making a batch of ice cream is a shade over 18 seconds. It was set on June 22, 2005, when Peter Barham, an English physicist, poured liquid nitrogen onto a liter of mix and stirred fast. I’m sure the vapors were flying.
Ice cubes and salt and freezer bags aren’t that fast or theatrical. But they’re handy and easy, and they make a refreshing dessert while I clear the dinner table. That’s cool enough for me