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PROTEIN
What are proteins?
 Proteins are complex substances that are essential for the structure and function of all
living cells.
 Our muscles, cartilage, ligaments, skin and hair are made up
of protein.
 Proteins are very important for body building.
Functions of proteins
1. For ______________________ of body cells to form ______________________.
E.g. muscles, bones, teeth, skin, lungs, blood.
2. For ___________________ and _________________ of worn-out or damaged cells
E.g. to heal a cut, bruise or broken bones, to replace worn-out skin
3. To form _______________________________in the body.
E.g.
Antibodies – to protect against diseases.
Enzymes – to digest food
Hormones – to regulate growth and other body functions
Haemoglobin – to carry oxygen in the blood
Myoglobin – in the muscles for storing oxygen.
Fibrinogen – to help blood clot
4. To regulate the amount of body fluid in and around the tissues.
5. To supply heat and energy if there are not enough of other energy food in the diet
(carbohydrate and fat).
What is the elemental composition of protein?
Protein is made up of:
 _____________________________
 _____________________________
 _____________________________
 _____________________________
It also contains small amounts of:
 _____________________________
 _____________________________
What is the chemical composition of protein?
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Proteins are made up of basic units called
__________________ ________________.
There are at least 22 naturally occurring amino
acids.
Different proteins consist of a varying number and
combination of these amino acids.
When two amino acids combine, they form a
structure known as a ______________.
A strong bond known as a peptide bond forms
between the amino acids.
If three or more amino acids are linked by peptide
bonds, a _________________ is formed.
The amino acids link with one another to form long
coiled chains.
A protein molecule may be made up of one or more of such chains.
Types of amino acids
There are two types of amino acids - ___________________ _______________
________________
and
__________________________
_________________
__________________.
Essential amino acids
Certain amino acids cannot be manufactured by the body and must be obtained from food.
They are known as ____________________ __________________ _____________.
In children, ________________ essential amino acids are required for growth and repair.
In adults, ___________________essential amino acids are required for repair and
maintenance.
Non-essential amino acids
Non-essential amino acids can be made by the body. Hence, it is not necessary to obtain
these amino acids from food.
Biological Value of Protein
Biological value of protein is a measure of how nourishing a protein food is.
The biological value of a protein food is measured by the number of essential amino acids
it contains in proportion to the body requirements.
Types of protein
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High Biological Value (HBV) Protein
 High biological value (HBV) protein contains _______ the ________________
___________ _______________ in the ____________ proportion needed by the body.
 They are known as ______________ protein and most of them are from ___________
sources.
 Examples eggs, human milk have the highest biological value followed by meat, fish,
cheese and cow’s milk.
Low Biological Value (LBV) Protein
 Low biological value (LBV) protein _______ _______ contain ______ the
________________ ________________. They are lacking in one or more of the
essential amino acids.
 They are called ______________________protein.
 Most of them are from _________________________sources.
 Examples: cereals, pulses (peas, beans, lentils), nuts and vegetables.
Exception
Soya beans from plants contain proteins of high biological value.
Gelatine from animal source contains no essential amino acids.
What are complementary proteins?
LBV proteins are also important. They can be combined in such a way that the essential
amino acids lacking in one type of food present in other. In other words, the protein
content of one food compensates for the other’s deficiencies. This is called
complementary protein.
Examples: eating bread with cheese, spaghetti and meatballs, rice and fish curry.
Sources of Protein
Meat
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Beef
Mutton
Veal
Venison
Pork
Poultry
Offal
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Animal Proteins
Seafood
Fish
Prawns
Crabs
Lobster
Crayfish
Mussels
Oysters
Plant Protein
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Farm Products
Milk
Dairy products such as
cheese and yoghurt
Eggs
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Pulses
Soya beans
Lentils
Peas
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Legumes
Long beans
Broad beans
French beans
Nuts
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Peanuts
Almond nuts
Cashew nuts
Requirements of Protein
The recommended intake of protein each day is 1gram for each kilogram of body weight.
For example, if a person weighs 70kg, he will require 70g protein each day. More is
required for children, teenagers, pregnant and nursing mothers.
Babies and children require a lot of protein as they are growing rapidly. An average child
requires 30-50g per day.
Adolescents require protein for their rapid spurt of growth, about 60-75g per day.
Pregnant women require more than normal to cater for the growing baby (90g per day).
Nursing mothers require more than normal for milk production during breast-feeding
(lactation).
Extra protein is also required during illness, convalescence and after surgery.
Effects of Deficiency
1. Degeneration (wasting) of the body as worn-out cells are not replaced.
2. Retardation of growth in children.
3. Malfunctions of various organs due to hormone and enzyme deficiency.
4. Lack of anti-bodies, which makes the body more susceptible to disease.
5. Possible obesity, due to overindulgence in high-calorie foods.
6. Severe deficiency (e.g. in underdeveloped countries) results in weight loss and
disease.
Two protein deficiency diseases are Marasmus, caused by a lack of protein and calories
and Kwashiorkor, a protein deficiency which affects children.
Proteins in Food Preparation
Denaturation
When proteins undergo a change in their structure, the process is known as
denaturation. When denaturation occurs, the bonds that hold the shape of the coiled
polypeptide chains break. The original structure of the protein is replaced by a looser, less
compact structure. Denaturation often affects the original properties of the protein.
Heat is the most common means of denaturing proteins. The extent and speed of protein
denaturation depends on the structure of the protein and the temperature applied. Proteins
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can also be denatured by pressure, presence of chemicals or by mechanical means such
as beating, whipping and kneading.
Coagulation
Coagulation is the reaction in which a protein changes from a liquid state to a semisolid or solid state in the presence of heat, pressure or chemicals. In coagulation,
protein molecules unfold, collide with each other and clump together to form a solid.
Coagulation is an irreversible reaction. Examples of coagulation in food are:
 Cheese, when heated to about 70°C becomes rubbery and tough.
 Milk proteins coagulate as milk is heated and form a ‘skin; on its surface.
 The protein albumin in egg white denatures and coagulates at about 60°C and the egg
white turns solid and opaque. The egg yolk coagulates at 70°C and on further heating
becomes hard and crumbly.
Maillard Browning
Maillard browning is a complex chemical reaction that occurs when foods
containing sugars and proteins are heated. Upon heating, the proteins in the food
denature and react with the sugars to form several compounds, giving rise to a brown
colouration, usually seen on the surface of the food. As a result, the flavour and aroma of
the food are greatly enhanced. This reaction generally occurs at temperatures of 150°C
and above. This is why productions of Maillard browning are typically found on the surface
of the food being cooked, where the temperature is high. Effects of Maillard browning can
be seen on bread crust and the surface of roasted meat.
Foaming
Foam is formed when air is incorporated into a protein such as egg white. When egg
white is whisked, the agitation causes the protein albumin to denature. The molecules of
the protein stretch and join together loosely. Air bubbles beaten into the egg white are
trapped between these protein molecules. This makes the foam stable and elastic. The
trapped air bubbles also increase the volume of the mixture. Examples of foods that
incorporate foam are whipped cream and meringues.
Protein
Written Exercises
Rewrite the questions on lined paper. Write the answer after each question.
1. Name the elemental composition of protein.
2. What is meant by high biological value protein?
3. What is meant by low biological value protein?
4. What are complementary protein? Give examples.
5. Give three functions of protein. Give examples.
6. What is denaturation?
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