HEAT
TRANSFER
In order for food to be cooked, heat
must be transferred from a heat source (such as a gas flame or an electric
element) to and through the food. Understanding the ways in which heat is
transferred and the speed at which it is transferred helps the cook control the
cooking process.
Heat is transferred in three ways:
conduction, convection, and radiation. It is important to remember that, during
a cooking process, more than one of these methods of transfer may be happening
at the same time. For example,food on a grill may be heated by conduction from
the hot metal grill, by convection from hot air rising from the burner or
charcoal, and by radiation from the glowing burner or coals.
CONDUCTION
Conduction occurs in two ways:
1. When
heat moves directly from one item to something touching it—for example, from the top of the range to a soup pot
placed on it, from the pot to the broth inside, and from the broth to the solid
food items in it.
2. When
heat moves from one part of something to an adjacent part of the same item—for
example, from the exterior of a roast to the interior, or from a sauté pan to
its handle.
Different materials conduct heat at
different speeds. Heat moves rapidly through copper and aluminum, more slowly
in stainless steel, more slowly yet in glass and porcelain. Air is a poor
conductor of heat.
CONVECTION
Convection occurs when heat is spread by the movement of air, steam, or liquid
(including hot fat).There are two kinds of convection:
1. Natural.
Hot liquids and gases rise while cooler
ones sink.Thus, in any oven, kettle of liquid, or deep-fat fryer a
constant,natural circulation distributes heat.
2. Mechanical.
In convection ovens and convection
steamers, fans speed the circulation of heat. Thus,heat is transferred more quickly
to the food,and the food cooks faster. Stirring is a form of mechanical
convection. Thick liquids cannot circulate as quickly as thin ones, so the rate
of natural convection is slower.This explains, in part, why it is so easy to
scorch thick soups and sauces.The heat is not carried away from the bottom of
the pan quickly enough, so it stays concentrated on the bottom and scorches the
food. Stirring redistributes the heat and helps prevent this. (Using heavy pots
made of a material that conducts heat well also helps prevent scorching because
the pot conducts the heat more quickly and evenly across the bottom and up the
sides.) Convection is the process that carries the heat from the heat source to
the food. Once the carrier of the heat (air or liquid) comes in contact with
the food, the heat is transferred from the carrier to the food by conduction.
RADIATION
Radiation occurs when energy is transferred by waves from a source to the
food. The waves themselves are not actually heat energy but are changed into
heat energy when they strike the food being cooked.(Light waves, radio waves, and
x-rays are examples of radiation not used for cooking.)
Two kinds of radiation are used in the
kitchen:
1. Infrared.
Broiling is the most familiar example of
infrared cooking. In a broiler, an electric element or a ceramic element heated
by a gas flame becomes so hot that it gives off infrared radiation, which cooks
the food.High-intensity infrared ovens are designed to heat food rapidly.
2. Microwave.
In microwave cooking, the radiation
generated by the oven penetrates partway into the food, where it agitates the
molecules of water. The friction this agitation causes creates heat, which
cooks the food.
• Because microwave radiation affects
only water molecules, a completely waterless material will not heat in a
microwave oven. Plates become hot only when heat is conducted to them by
hot foods.
• Because most microwaves penetrate no
more than about 2 inches (50 mm) into foods, heat is transferred to the center
of large pieces of food by conduction, just as in roasting. Cooking with
microwaves is discussed in more detail later in this chapter.
COOKERY PROCESSES
(COOKING METHODS)
A lot of cooking methods are used in
catering and hotel industry. Each is specific and has its advantages and
disadvantages.
The cookery processes or cooking methods
are:
a) Boiling
b) Poaching
c) Stewing
d) Braising
e) Steaming
f) Baking
g) Roasting
h) Pot roasting
i) Grilling
j) Shallow Frying
k) Deep Frying
l) Microwaving
1. Boiling
1.1 Definition
Boiling is
cooking prepared foods in a liquid (water, bouillon, stock, milk) at boiling
point.
1.2 Methods
Food is boiled in two ways:
a) food is placed into boiling liquid,
reboiled, then the heat is reduced, so that the liquid boils gently –
simmering;
b) food is covered with cold liquid,
brought to the boil, then the heat is reduced, so that the food simmers.
1.3 Advantages
a) older, tougher joints of meat can be
made palatable and digestible
b) appropriate for large-scale cookery
- 2 -
c) Economic on fuel
d) Nutritious, well flavoured stock is
produced
e) labor saving, requires little
attention
f) Safe and simple
g) Maximum colour and nutritive value
are retained with green vegetables – but the boiling time must be kept to the
minimum
1.4 Disadvantages
a) foods can look unattractive
b) it can be slow
c) loss of soluble vitamins in the water
1.5 Examples of foods which might be
cooked by boiling
- stocks (beef, mutton, chicken, fish)
- sauces (brown, white, curry)
- glazes (fish, meat)
- soup (tomato, lentil)
- farinaceous (pasta)
- fish (cod, salmon)
- meat (beef, leg of mutton)
- vegetables (carrots, cabbage, potatoes).
2. Poaching
2.1 Definition
Poaching is cooking food in the required
amount of liquid at just below boiling point.
2.2 Methods
a) Shallow poaching: foods (fish,
chicken) are cooking in the minimum of liquid (water, stock, milk or wine). The
liquid is not allowed to boil; it is kept at a temperature close to boiling.
b) Deep poaching: foods are cooked in
enough water to cover them, brought to the boil and then simmered (eggs) or
placed into simmering liquid and cooked gently.
2.3 Advantages
Poached food is easily digestible.
2.4 Disadvantages
- Skill is required for poaching food
- It is not a suitable method for many
foods
2.5 Examples of foods which might be
cooked by poaching
- farinaceous (gnocchi)
- eggs
- fish (sole)
- fruits (fresh and dried)
- poultry (chicken)
3. Stewing
3.1 Definition
Stewing is a long, slow cooking method
where food is cut into pieces and cooked in the minimum amount of liquid,
water, stock or sauce. The food and the cooking liquid are served together.
3.2 Methods
All stews have a thickened consistency.
Stewed foods may be cooked
- in a covered pan on the stove;
- in a covered pan in the oven.
3.3. Advantages
a) Meat juices are retained as part of
the stew
b) Correct slow cooking results and very
little evaporation
c) Economic on fuel
d) Nutrients are conserved
e) Tough foods are tenderised
f) Economical in labour because the
foods can be bulk cooked
3.4 Disadvantages
Stewing is a slow cooking method.
3.5 Examples of foods which might be
cooked by stewing:
- fish (e.g. bouillabaisse – French fish
soup / stew)
- meat (goulash, minced beef, Irish
stew, white stew of veal)
- poultry (chicken fricassee, curried
chicken)
- vegetables (ratatouille)
4. Braising
4.1 Definition
Braising is a method of cooking in the
oven. The food is cooked in liquid in a covered pan or casserole. It is a
combination of stewing and pot roasting.
4.2 Methods
a) Brown braising: joints and portion
cuts of meat are marinaded and may be larded and then sealed quickly by
browning on all sides in a hot oven or in a pan on the stove. Sealing the
joints helps retain flavour and nutritive value and gives a nice brown colour.
Joints are then placed on a bed of root vegetables in a braising pan, with the
liquid and other flavourings, covered with a lid and cooked slowly in the oven.
b) White braising: vegetables and
sweetbreads are blanched, refreshed and cooked on a bed of root vegetables with
white stock in a covered container in the oven.
4.3 Advantages
a) Older, tougher joints of meat and
poultry can be used
b) maximum flavour and nutritive value
are retained
c) variety of presentation and flavour
is given to the menu
4.4 Examples of foods which might be
cooked by braising:
- farinaceous (rice)
- meat (lamb, beef)
- poultry (duck)
- vegetables (celery, onions)
5. Steaming
5.1 Definition
Steaming is cooking prepared foods by
steam (moist heat) under varying degrees of pressure.
5.2 Methods
a) Atmospheric or low pressure steaming:
food may be cooked by direct or indirect contact with the steam:
- direct: in a steamer or in a pan of
boiling water (steak);
- indirect between two plates over a pan
of boiling water.
b) High pressure steaming: in
purpose-built equipment which does not allow steam to escape; steam pressure
builds up, the temperature increases and cooking time is reduced.
5.3 Advantages
a) Retention of nutritional value
b) Some foods become lighter and easier
to digest
c) Low pressure steaming reduces the
risk of overcooking
d) High pressure steaming enables food
to be cooked quickly, because steam is forced through the food, cooking it
rapidly
e) Labour-saving and suitable for
large-scale cookery
f) High speed steamers enable frequent
cooking of small quantities of vegetables. Vegetables are freshly cooked,
retaining colour, flavour and nutritive value.
g) With steamed fish, natural juices can
be retained; they are served with the fish or used to make an accompanying
sauce.
h) Economical on fuel (low heat is
needed and a multi-tiered steamer can be used).
5.4 Disadvantages
a) Foods can look unattractive
b) It can be a slow method
5.5 Examples of foods which might be
cooked by steaming:
- fish (sole)
- meat (tongue, ham, bacon)
- vegetables (almost all)
6. Baking
6.1 Definition
Baking is cooking food by dry heat in an
oven (the action is modified by steam).
6.2. Methods
a) Dry baking: during the baking process
steam rises from the water content of the food; it combines with the dry heat
of the oven to cook the food (cakes, pastry, baked jacket potatoes).
b) Increased humidity baking: during the
baking process with some foods (bread) the oven humidity is increased by
placing a bowl of water into the oven to increase the water content of the food
and to improve eating quality.
c) Bain marie: during the baking process
food is placed in a container of water (bain marie) to cook the food more
slowly.
6.3 Advantages
a) A wide range of savoury and sweet
foods can be produced.
b) Bakery products are appealing to the
eye and to the mouth.
c) Bulk cooking can be achieved with
uniformity of colour and degree of cooking.
d) Baking ovens have effective manual or
automatic controls
e) Straightforward access for loading
and removal of items.
6.4 Disadvantages
a) Requires regular attention
b) Expensive energy
6.5 Examples of foods which might be
cooked by baking:
- eggs
- meat (steak, cornish pastries)
- chicken
- vegetables (potatoes)
- apples
- fruit flans
- cakes
- bread.
7. Roasting
7.1 Definition
Roasting is cooking in dry heat in an
oven or on a spit with the aid of fat or oil.
7.2 Methods
a) Placing prepared foods (meat,
poultry) on a roosting spit over/in front of radiating heat;
b) Placing prepared foods in an oven
with:
- dry heat
- forced air convected heat
- convected heat combined with microwave
energy
7.3 Advantages
a) Good quality meat and poultry is
tender when roasted
b) Meat juices from the joint are used
for gravy
c) Use of energy and oven temperature
can be controlled
d) Cooking can be observed (transparent
oven doors)
e) Straightforward access, adjustment or
removal of items
f) Continual basting with meat juices
adds to a distinctive flavour
7.4 Disadvantages
a) Requires regular attention
b) Expensive energy
7.5 Examples of foods which might be
cooked by roasting:
- meat (lamb, beef, pork, veal)
- poultry and game (duck, chicken,
pheasant)
- vegetables (potatoes, parsnips)
8. Pot roasting
8.1 Definition
Pot roasting is cooking on a bed of root
vegetables in a covered pan. This method retains maximum flavour of all
ingredients.
8.2 Method
Food is placed on a bed of root
vegetables and herbs, coated with butter or oil, covered with a lid and cooked
in an oven.
8.3 Advantages
a) Maximum flavour is retained
b) Nutrients are retained
c) Cheaper cuts of meat can be used
d) Vegetables used in pot roasting can
be served as an accompaniment
8.4 Disadvantages
Pot roasting is a slow cooking method.
8.5 Example of food which might be
cooked by pot roasting:
- meat (cheaper cuts)
9. Grilling
9.1 Definition
Grilling is a fast method of cooking by
radiant heat (also called broiling).
9.2 Methods
Grilled foods can be cooked:
a) over heat (charcoal, barbecues, gas
or electric grills)
b) under heat (gas or electric grills,
gas or electric salamanders over fired grills)
c) between heat (electrically heated
grill bars or plates)
9.3 Advantages
a) Food can be quickly cooked to order
b) Charring foods gives them a pleasing
appearance and better flavour
c) Better control as food is visible
during cooking
9.4 Disadvantages
a) More suitable for expensive cuts of
meat
b) Requires skill
9.5 Examples of foods which might be
cooked by grilling:
- fish (cod, herring, mackerel, plaice)
- meat (chops, steak)
- vegetables (mushrooms, tomatoes)
- savouries
- toasted items (bread, muffins, tea
cakes)
10. Shallow frying
10.1 Definition
Shallow frying is cooking food in a
small quantity of pre-heated fat or oil in a shallow pan or on a flat surface.
10.2 Methods
a) Shallow fry: cooking food in a small
amount of fat or oil in a frying pan or saute pan. The food is first fried on
the presentation side, then turned, so that both sides are cooked and coloured.
b) Saute:
- cooking tender cuts of meat and
poultry in a saute or frying pan. After cooking the fat is discarded and the
pan is deglazed with stock or wine as a part of the finished sauce.
- cooking potatoes or onions which are
cut into slices and tossed into hot shallow fat or oil in a frying pan till
golden brown.
c) Griddle: Hamburgers, sausages, sliced
onions are placed on a lightly oiled, pre-heated griddle (solid metal plate)
and turned frequently during cooking.
d) Stir Fry: fast frying in a wok or a
frying pan in a little fat or oil (vegetables, strips of beef or chicken).
10.3 Advantages
a) Quick cooking method
b) No loss of soluble nutrients
c) Good colour
10.4 Disadvantages
a) Suitable for expensive cuts of meat
b) Not easily digested
c) Requires constant supervision
10.5 Examples of foods which might be
cooked by shallow frying:
- eggs (omelettes)
- fish (sole, plaice, trout, cod,
herring)
- meat (lamb, kidneys, beef Stroganoff)
- poultry (chicken)
- vegetables (potatoes, onions,
cauliflower, chicory)
- sweets and pastries (pancakes)
- savouries.
11. Deep frying
11.1 Definition
Deep frying is cooking food in
pre-heated deep oil or fat.
11.2 Methods
Conventional deep-fried foods (except
potatoes) are coated with mil kand flour, egg and crumbs, batter or pastry to
protect the surface of the food from the intense heat, to prevent the escape of
moisture and nutrients ad to modify the rapid penetration of the intense heat.
The food is placed into deep pre-heated
oil or fat, fried until cooked and golden brown, drained and served.
11.3 Advantages
a) Quick cooking method
b) No loss of soluble nutrients
c) Ensures good colour
11.4 Disadvantages
a) Not easily digested
b) Safety hazard
11.5 Examples of foods which might be
cooked by deep frying:
- eggs (Scotch eggs)
- fish (haddock)
- chicken (cutlets)
- savoury (cheese fritters)
- potatoes (croquettes, chips)
- vegetables (onions)
- pastry and sweets (pineapple fritters,
doughnuts)
12. Microwaving
12.1 Definition
Microwave cookery is cooking or
reheating food using electromagnetic waves in a microwave oven powered by
electricity. Microwaves activate the water molecules or particles of food,
causing heat by friction which cooks or reheats the food.
12.2 Advantages
a) Very fast method of cooking
b) Fast method of defrosting
c) Economical on electricity and labour
d) Food is cooked in its own juices, so
its flavour is retained
e) Minimises food shrinkage and
drying-out
12.3 Disadvantages
a) Not suitable for all foods
b) Limited space
c) Can only penetrate 5 cm into food
(from all sides)
12.4 Examples of foods which might be
cooked by microwave:
- farinaceous (pasta)
- fish (trout, salmon)
- vegetables (potatoes)
- pre-cooked meals