Comment & Analysis
BAKING SCIENCE
he biscuit tin in my office is an
essential tool for getting science
done. For pondering the thorniest
scientific problems, tea alone is not enough
and a chocolate biscuit or two is necessary
to help the process along. But last week
I didn’t put the lid back on properly and
by Monday morning my stash of biscuits
was soft and spongy. Their weedy nature
certainly didn’t hit the biscuit spot. At the
same time, leftover cake in my kitchen at
home was getting closer and closer to the
texture of a dry loofah, hard and rigid. Both
cake and biscuits were going stale, but what
makes one go hard and the other go soft?
This distinction played a role in the famous
tax decision on Jaffa Cakes in 1991, because
the court had to decide whether to classify
them as cakes or biscuits. It was decided
that they are cakes, but it turns out that the
innards of cakes are a bit more mobile than
most people suspect.
I like to think of both cakes and biscuits
as food architecture – a structure made of
different interlocking components. Baking
is a process of construction, and the texture
of cakes and biscuits reflects their structural
integrity. The framework of the cake is
provided by the flour. Flour is about 75 per
cent starch, and it’s the starch that provides
the strength. Working with the starch
is gluten, an elastic protein that forms a
network holding everything together with
enough stretchiness to let the dough rise.
Sugar, fat and eggs mostly help make and
hold bubbles of gas that expand in the oven.
So when it comes to whether the cake is
hard or soft, the place to look is the major
structural support: the starch.
When I bake a cake, the time in the oven
is a part of the building process. The heat
forces the starch granules in the mixture to
absorb water and expand. Water can slip
into the gaps between the huge molecules
that make up the starch grains, pushing
them apart. The final baked cake is soft,
because the starch arrangement is soft. But
the second you take it out of the oven, the
process of going stale starts. It’s got nothing
to do with bacteria or mould. It’s all about
the position of the starch molecules.Even as the fresh-baked smell is wafting
across the kitchen, small rearrangements are
happening inside the cake. Water molecules
are small and mobile, and when they’re parked
between the starch molecules, the giant starch
chains can shift. As time goes on, the starch
chains slowly shunt to line up, forming regular
crystalline regions instead of messy amorphous
ones. This is why the cake goes hard – its
main structural component is getting more
rigid. Sugar and fat slow this process, but they
won’t stop it. The cake drying out isn’t the
major player here.
Biscuits are different because they start off
with much less water. Baking a biscuit dries
it out pretty thoroughly, so the starch isimmobile and the structure rigid. But sugar
absorbs water from the air and will pass it
on to the starch, softening the structure. So
biscuits go soft at first, although they will
eventually go hard if the starch crystallises.
The details depend on the exact mixture
of ingredients and the conditions in which
you store your cakes or biscuits. The lesson
here is clear – put the lid on the biscuit tin
and eat your cake sooner rather than later!
And that’s the sort of thought that really
cheers up a tea break... ßT
DR HELEN CZERSKI is a physicist and BBC presenter whose
MAIN ILLUSTRATION: MATT CLOUGH PORTRAIT: KATE COPELAND most recent series was Colour: The Spectrum Of Science
“Cakes and biscuits both go stale, but what makes
one go hard and the other go soft?”