7

FORGE

big block of ice, but many little ones, broken up by
bits of fat or oil. If these crystals are too large, then
the ice cream will feel gritty before it melts.
There are two ways of preventing the crystals
getting too large, and ideally you should do them
both. Firstly, you should mix the liquid as it’s freezing,
as this breaks up the crystals as they form. This
also helps to trap some air into the mixture, which
improves the texture. Secondly, you should freeze
the liquid as fast as possible, as this gives the crystals
as little time as possible to form. It’s this second
process that can be challenging for people making
ice cream at home. Most domestic freezers hold the
temperature at -18 degrees C, but may go as low
as -25. While this is cold, and easily cold enough to
freeze the ice cream, it will do so quite slowly. This is
compounded by the fact that air conducts heat quite
slowly, so if you put a bowl of unfrozen ice cream in
the freezer, the air around it won’t conduct the heat
away very quickly. There are some ways of mitigating
this to some extent (such as putting a heavy bowl in
the freezer to cool down, which will conduct the heat
out of the ice cream faster than the air in the freezer).
However, ultimately, the temperature just isn’t cold
enough to make really great ice cream.
So, we need something that can cool down a liquid
quicker than a freezer. Enter dry ice, or frozen carbon

Above

Mix the flavours into

the base gently to

avoid knocking out

too much air

dioxide, which is at least -78 degrees C. Obviously,

this is much colder than the freezer, but dry ice has

another trick up its sleeve that helps it make great

ice cream – it’s a gas at normal freezer temperature.

This means that you can add the dry ice directly to

the ice cream mixture and it will sublimate away,

leaving only bubbles of trapped gas behind. Because

of some curious quirks of chemistry, carbon dioxide

doesn’t form a liquid at atmospheric pressure, so it

goes directly from being a solid to a gas. Both the

temperature and the chemical properties of dry ice

make it excellent for freezing ice cream very quickly

(liquid nitrogen can also work – see box on page 107).

Those are the basics, so let’s dive in and start

making. There’s nothing special about a recipe for

dry ice ice cream, and any ice cream recipe should

work but, in our experience, some work better than

others. Broadly, ice cream recipes tend to fall into one

of two forms: custard-based recipes that start with

heating milk and adding egg yolk very carefully so that

it doesn’t fully cook, and cream-based recipes that are

based on whipped cream. The former can work, but

will work better if you have an ice cream churn to mix

the liquid viciously while it freezes.

Above

We had better results starting with whipped

cream than with an egg custard

There are two ways of preventing the crystals

getting too large, and ideally you should do

them both

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