New Scientist - USA (2022-02-19)

19 February 2022 | New Scientist | 51

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What you need
400 grams of ice cream
2 egg whites
75 grams crushed cornflakes
Half a teaspoon of cinnamon
2 litres vegetable oil, for frying

WHEN cooking food, we need heat

to diffuse from the outside to its

centre. If we want food to be evenly

cooked throughout, this can be a

problem: by the time heat reaches

the centre, the outside may be

overcooked. But in some cases, we

can use the slow diffusion of heat

to our advantage, to create foods

with a surprise in the middle.

One example is a molten

chocolate cake, aka a chocolate

fondant. Essentially, this is an

undercooked cake. The key is

to bake it just long enough so

that the outside is firm while

the centre remains liquid.

But exactly how long should

that be? An equation can give us

an idea. The teachers at Harvard

University’s science and cooking

class use the molten chocolate

dessert to demonstrate how

we can model the diffusion of

heat using the equation L = √4 Dt,

where L^ is the distance heat travels

in centimetres, t is the time

in seconds and D is the heat

diffusion coefficient, a measure

of how fast heat travels through

a given substance.

For water, D is 0.0014 cm^2 per

second. Since most foods are

largely made of water, we can use

this value to approximate how

quickly heat will move through

many types of food.

Let’s say that a chocolate cake

is cooked for about 12 minutes

(720 seconds) in a ramekin 8 cm

wide. The equation tells us that the

heat will travel about 2 cm through

the batter in this time, giving us a

nice thickness of solid cake on the

outside and a decent molten core.

If you want to create some crowd-pleasing culinary surprises,

bring some fundamental physics to the table, says Sam Wong

The science of cooking

Great balls of frozen fire

In their book Science and

Cooking – touted as a place

where “physics meets food” – the

Harvard trio of Michael Brenner,

Pia Sörensen and David Weitz

explain how to make an even

more impressive dessert, similar

to the one pictured, by covering a

ball of ice cream in a thin coating

and deep-frying it for just long

enough to cook the outside while

keeping the ice cream frozen.

Their recipe has a cooking time

of about 15 seconds, during which

our equation predicts that heat

will travel 0.3 cm through water.

But this recipe uses whipped egg

whites to coat the ice cream. Like

other foams, it is a great insulator

and so should give the ice cream

excellent, if brief, protection

from the heat.

To give this recipe a try, scoop

your ice cream into four 100-gram
balls, place on a tray and put them
back into the freezer for an hour.
Whisk the egg whites until foamy.
Then, separately, mix the crushed
cornflakes and cinnamon. Roll the
ice cream balls in the egg whites,
then in the cornflakes.
In this time, the outside of the
ice cream balls will already have
started to warm up, so they need
to go back into the freezer for a
couple of hours to ensure they are
completely frozen before frying.
After this, heat your oil to 190°C
in a large saucepan or deep fryer.
Fry the balls, one or two at a time,
for 15 to 20 seconds, then drain
The science of cooking on paper towels before serving. ❚
appears every four weeks

Next week
Stargazing at home

Sam Wong is assistant news
editor and self-appointed
chief gourmand at
New Scientist. Follow
him @ samwong1

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