STORING AND COOKING
Cooking
Cooking
Heat produces chemical and physical
changes in food, making it softer, more
digestible, and causing the food to release
nutrients. However, sometimes nutrients are
degraded when certain foods are cooked.
Why do we cook food?
Some scientists think that the discovery of cooking
(see pp.8–9) was a key trigger in our evolution.
Cooking improves and generates new flavors,
aromas, and textures. One such example is a
browning reaction, in which sugars in food lose
water when heated, producing flavor. Raw foods
are often tough, fibrous, difficult to chew, and
hard for digestive processes to attack. Unless
cooked, many food components cannot be
broken down by our digestive system. Also,
cooking helps to kill or suppress pathogens
and renders many toxins inactive.
An oven transfers heat, from a gas
flame or electric element to the food
mainly by convection, as hot air circulates in
the oven. Direct infrared radiation from the
oven’s hot walls also heats the food.
Steaming transfers heat to food
through air convection (as in baking),
but also through condensation of vapor. Just
as it takes a lot of energy to convert water into
steam, so steam gives up a lot of heat energy
as it condenses back into water as it reaches and
moistens the food.
Boiling is one of the most efficient
methods of cooking, since all of the food
is in direct contact with the heat transfer
medium (water). Browning reactions are not
triggered because of the constant presence of water.
Grilling (applying dry heat from below)
is probably the earliest method of cooking,
since it can be done with an open fire. Grilling
with a heat source above the food is called
broiling. Grilling imparts very high temperatures
to foods, enabling browning reactions,
but there is a risk of charring.
A GLOWING COAL RADIATES
40 TIMES MORE HEAT ENERGY
THAN THE EQUIVALENT
AREA OF AN OVEN WALL
Convection currents in water
carry heat from source to food
Infrared light
carries heat
to food
Water gives
off steam
Infrared
radiation
Steam delivers
heat to food
Hot air
circulating
F
R
Y
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N
G
G
R
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L
L
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N
G
B
A
K
I
N
G
/
R
O
A
S
T
I
N
G
S
T
E
A
M
I
N
G
B
O
I
L
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G
Food surface
closest to
heat source
cooks first
OVEN
PAN
GRILL
PAN
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60 61
STORING AND COOKING
Cooking
Fast and slow cooking
Cooking quickly can minimize damage to
easily degraded nutrients, and can seal the
outside of meat or fish to limit moisture loss,
but it is harder to heat food evenly and the
interior is likely to remain undercooked. Slow
cooking heats through more evenly but can
degrade nutrients and dry out food.
Turning up the heat
Flame grilling and barbecuing are better for
thin foods with high surface area to volume
ratios, since this raises the likelihood that
food will be cooked through.
Oil can reach higher temperatures than
water, and in shallow frying it is used to
conduct heat directly from the source (pan base)
to food. This means browning reactions happen
quicker. In this method all of the immersed food
surface is in contact with the heat transfer medium (oil).
Deep frying uses convective heat
transfer, but since the medium (oil)
can reach a much higher temperature than
water, food can be cooked much more quickly
than frying and browning reactions happen faster.
Microwaves agitate the water in
foods, generating heat and thereby
cooking the food. It may seem as though
microwaves heat foods from the inside out but
they tend to heat all molecules at the same time.
However, microwaves will cook the wet interior
of dry-cased foods (such as pie) more quickly.
A microwave has a transmitter that sends out waves of
around 5 in (12 cm) in length. They are shorter than radio
waves, but longer than infrared waves from grills and ovens.
A turntable rotates the food to ensure all parts are cooked.
HOW MICROWAVES WORK
Oven wall during roasting
480°F/250°C
2,000°F/1,100°C
2,900°F/1,600°C
Coal in a barbeque
Gas flame in a grill
Bottom surface of food
cooks first
Oil reaches temperatures
above 212ºF (100ºC)
Microwave—a wave with a
frequency of around 2,450 MHz
Heat from flame
transferred to food
through pan
Convection currents in
oil carry heat from
source to food
Agitated water molecules
generate heat
Wave guide
Transmitter, or
“magnetron”
Turntable
Food
Microwaves
bounce off
reflective wall
M
I
C
R
O
W
A
V
I
N
G
D
E
E
P
F
R
Y
I
N
G
F
R
Y
I
N
G
PAN
PAN
MICROWAVE
US_060-061_Cooking.indd 61 23/02/2017 11:26
60 61
STORING AND COOKING
Cooking
Fast and slow cooking
Cooking quickly can minimize damage to
easily degraded nutrients, and can seal the
outside of meat or fish to limit moisture loss,
but it is harder to heat food evenly and the
interior is likely to remain undercooked. Slow
cooking heats through more evenly but can
degrade nutrients and dry out food.
Turning up the heat
Flame grilling and barbecuing are better for
thin foods with high surface area to volume
ratios, since this raises the likelihood that
food will be cooked through.
Oil can reach higher temperatures than
water, and in shallow frying it is used to
conduct heat directly from the source (pan base)
to food. This means browning reactions happen
quicker. In this method all of the immersed food
surface is in contact with the heat transfer medium (oil).
Deep frying uses convective heat
transfer, but since the medium (oil)
can reach a much higher temperature than
water, food can be cooked much more quickly
than frying and browning reactions happen faster.
Microwaves agitate the water in
foods, generating heat and thereby
cooking the food. It may seem as though
microwaves heat foods from the inside out but
they tend to heat all molecules at the same time.
However, microwaves will cook the wet interior
of dry-cased foods (such as pie) more quickly.
A microwave has a transmitter that sends out waves of
around 5 in (12 cm) in length. They are shorter than radio
waves, but longer than infrared waves from grills and ovens.
A turntable rotates the food to ensure all parts are cooked.
HOW MICROWAVES WORK
Oven wall during roasting
480°F/250°C
2,000°F/1,100°C
2,900°F/1,600°C
Coal in a barbeque
Gas flame in a grill
Bottom surface of food
cooks first
Oil reaches temperatures
above 212ºF (100ºC)
Microwave—a wave with a
frequency of around 2,450 MHz
Heat from flame
transferred to food
through pan
Convection currents in
oil carry heat from
source to food
Agitated water molecules
generate heat
Wave guide
Transmitter, or
“magnetron”
Turntable
Food
Microwaves
bounce off
reflective wall
M
I
C
R
O
W
A
V
I
N
G
D
E
E
P
F
R
Y
I
N
G
F
R
Y
I
N
G
PAN
PAN
MICROWAVE
US_060-061_Cooking.indd 61 23/02/2017 11:26
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