PROTEIN

B.Sc. in Nutrition and Food Engineering

Course Code: 0721-2103
Course : Food Chemistry
Daffodil International University
 PROTEIN

• Proteins are nitrogenous organic compounds composed of one or more long chains of amino acids. It
contains C,H,O,N,S. Protein is the 50% dry weight of cells.
• The word “protein” was coined by Swedish chemist Jöns Jacob Berzelius in 1838. It derives from the Greek
word “prōteios,” meaning "primary" or "in the lead".
• After water, protein is the most plentiful substance in the body. Proteins grow, maintain, and replace the tissues in
our bodies. Therefore our muscles, organs, and immune systems are mostly made of protein. Once protein is
digested it is broken down into its amino acid.
• Protein is a long chain-like molecule that is made up of small units known as amino acids, joined together by
peptide bonds. There are 20 amino acids. The specific order of amino acids determines the structure and function
of each protein.
• Out of 20 amino acids 9 are essential that need to take everyday from food and 11 are non essential that body can
make themselves
 PROTEIN

• Proteins grow, maintain, and replace the tissues in our bodies. Therefore our muscles, organs, and
immune systems are mostly made of protein.
• Complete protein that has all amino acids including essential ones are meat, poultry, fish, dairy
products, eggs, and soy. Incomplete protein sources include nuts, grains, fruits, and vegetables.
Therefore it is important for vegetarians to pair meals wisely in order to get all essential amino
acids in their daily diet.
• Like carbohydrates and fats, proteins are considered a major nutrient for the body due to the
energy (calories) they provide.
• Protein can be found in foods like eggs, nuts, beans, fish, meat, milk etc.
 FORMATION OF PROTEIN

α-Amino acids are the basic structural units of proteins.

These amino acids consist of an α-carbon atom
covalently attached to a hydrogen atom, an amino group,
a carboxyl group, and an R group, which is commonly
referred to as the side chain. The structures of amino
acids (shown in Figure) differ only in the chemical nature
of the side chain R group. The physicochemical
properties, such as net charge, solubility, chemical
reactivity, and hydrogen bonding potential, of the amino
acids are dependent on the chemical nature of the R
group. α-Amino acids
 FORMATION OF PROTEIN

Amino acids can be covalently linked

together by the formation of an amide
bond between the α-amino group of
one amino acid and the α-carboxyl
group of another amino acid. The
resulting amide bond is known as
"peptide bond". The formation of a
dipeptide from two amino acids in a
condensation reaction is accompanied
by the removal of a water molecule. A
majority of natural proteins usually
contain up to 20 different amino acids
linked together via amide bonds.
 AMINO ACIDS

➢ Amino acids are organic or carbon-based compounds that have two

functional groups: an amino group and a carboxylate group.
➢ The amino group is slightly basic, while the carboxylate group is acidic.
See the illustration below for an example. The R group or side chain
mainly determines the amino acid's chemical and physical properties. It
can be of various sizes, shapes, and reactivities.
➢ So far, more than 500 amino acids have been identified, but only 20 of
these are commonly found in the human body. These are the ones that
have direct correspondence to our genetic codes. Out of these 20, Essential amino acids
there are just nine essential amino acids. ▪ Histidine
➢ These are all important in many functions of the human body, but they ▪ Isoleucine
cannot be synthesised by the body; we have to get them from food. ▪ Leucine
➢ Amino acids are organic compounds that combine to form proteins. ▪ Lysine
Amino acids and proteins are the building blocks of life. ▪ Methionine
➢ When proteins are digested or broken down, amino acids are left. The ▪ Phenylalanine
human body uses amino acids to make proteins to help the body. ▪ Threonine
▪ Tryptophan
Break down food.
▪ Valine
 How is protein used? How much protein does the body need?
The body breaks down consumed protein The daily maximum amount the body can use for
into amino acids, and absorbs it. It is used to protein synthesis is said to be around 2 grams per
build muscles and organs, to make 1 kilogram of body weight. Consuming more
hormones and antibodies, to be stored as protein will not increase synthesis, but increase the
fat, and to be burned as energy. amount consumed as energy, and lead to an
increase in body fat. Too much protein can also
burden the liver and kidneys.

The body can't store protein

so once needs are met, any extra is
used for energy or stored as fat
 STRUCTURE OF PROTEIN
Proteins are macromolecules with different levels of structural organization. The
primary structure of proteins relates to the peptide bonds between component
amino acids and also to the amino acid sequence in the molecule.
Primary Structure
The sequence of amino acid in a poly peptide chain is called primary structure.
Each protein has a unique sequence of amino acids linked together by peptide
bonds
Secondary Structure
The secondary structure is the shape of the polypeptide chain through twisting
and folding. There are two types of secondary structures:
1. α-helix (alfa-helix)
2. β-sheet (beta-sheet)
α-helix It is the most common spiral structure of the protein in which the amino
acids are tightly packed and coiled.
The formation of alfa-helix requires the lowest energy.
β-sheet In this, hydrogen bonds are formed between the neighbouring segments
of polypeptides.
The peptide is held together giving a sheet-like structure. Can be parallel (same
direction) or antiparallel (opposite direction).
 STRUCTURE OF PROTEIN
Tertiary Structure
It is the three-dimensional arrangement of protein structure. In this, the hydrophobic side chain is
held inside and hydrophilic groups are held outside (surface). The above arrangement gives stability
to the molecule.
Quaternary Structure
Quaternary means four.
• It is the arrangement of multiple folded protein or coiling protein molecules in a multi-subunit
complex.
A variety of bonding interactions including Hydrogen bonding, salt bridges and disulfide bonds
holds the various chains into a particular geometry.

Tertiary Structure
Quaternary Structure
STRUCTURE OF PROTEIN
 CLASSIFIC ATION

Protein can be classified from different aspect. Generally they are classified on the basis of
composition, shape of molecules, function etc.

On the basis of shape of molecules

1. Fibrous proteins
Fibrous proteins are long and thread or ribbon like and tend to lie side by side to form fibers. They are
generally insoluble in water as the intermolecular forces in these proteins are rather strong. They serve
as the chief structural material of animal tissues. Examples are keratin, myosin, collagen etc.
2. Globular proteins
Globular proteins are spheroidal in shape. They are generally soluble in water or aqueous solution
of acids, bases or salts as intermolecular forces in these proteins are relatively weaker. These
proteins are generally involved in physiological processes of the animal body. Examples are
enzymes, some hormones, haemoglobin, etc.
 CLASSIFIC ATION
On the basis of composition
On the basis of composition proteins are classified into three groups viz. simple proteins,
conjugated proteins
and derived proteins.
1. Simple proteins
These are the proteins which consist of only amino acids – They do not contain other
class of compounds.
2. Conjugated proteins
These are the proteins which consist of amino acids as well as other class of compounds
3. Derived proteins
They represent various stages of hydrolytic cleavage of simple or conjugated proteins. e.g.
proteoses, peptones, peptides, etc.
Classification of protein based on function

Enzymic Proteins- They are the most varied & highly specialized proteins with
catalytic activity. Enzymes catalyze a variety of reactions
Structural Proteins- These proteins aid in strengthening or protecting biological
structures. Example: Keratin
Transport protein: these protein helps in transport of ions and molecoules in the
body. Example: hemoglobin
Nutrient and storage protein: These protein provide nutrition to growing
embryos and store ions
Defense Proteins- These proteins defend against other organisms. Example:
Antibodies
Regulatory Proteins- They regulate cellular or metabolic activities. Example:
Hormones:- Insulin
Toxic Proteins - These proteins hydrolyze or degrade enzymes. Example: snake
venom
 P H YSI C AL & CH E MI C AL P ROP E RT I E S
Amino Acid Composition and Sequence: Proteins are polymers of
Color and Taste: Proteins are generally colorless and tasteless in amino acids linked by peptide bonds. The sequence and
their natural state. However, some hydrolyzed proteins, like those composition of these amino acids determine the protein's
found in protein powders, can have a slightly bitter taste. structure and function
Shape and Size: Proteins come in a wide range of shapes and sizes. Peptide Bond Formation: The peptide bond, formed through a
They can be globular (spherical), fibrous (elongated), or membrane- dehydration reaction between the amino group of one amino
bound. acid and the carboxyl group of another, is fundamental to the
Molecular Weight: Proteins have large molecular weights, ranging protein structure.
from a few thousand to millions of Daltons (Da). A Dalton is a unit Ionizable Groups : Proteins contain ionizable groups, particularly
of mass equal to the mass of one proton. The size and complexity in the side chains of amino acids, which can accept or donate
of a protein determine its molecular weight. protons depending on the pH of the environment.
Solubility: Proteins have varying solubility in water, which is Isoelectric Point (pI): The isoelectric point is the pH at which a
influenced by pH, ionic strength, temperature, and the presence of protein carries no net electric charge. At this point, a protein is
other solutes. Solubility is a critical factor in protein purification, least soluble in water and may precipitate out of solution.
food processing, and formulation Enzymatic Activity: Many proteins function as enzymes,
Optical Activity: Most proteins are optically active, meaning they catalyzing biochemical reactions with high specificity.
can rotate the plane of polarized light. Hydrophobic and Hydrophilic Interactions: The amino acid side
Adsorption and Surface Activity: Proteins can adsorb at interfaces chains of proteins can be hydrophobic or hydrophilic, influencing
(air-water or oil-water), reducing surface tension and stabilizing how proteins fold and interact with their environment.
emulsions and foams. This property is exploited in food products Binding and Recognition: Proteins have the ability to bind
like ice cream and salad dressings specifically to other molecules, including substrates, inhibitors,
Colloidal Nature: Proteins are colloids, which means they exist as DNA, and signaling molecules
suspended particles in a liquid. Colloids are larger than individual Oxidation-Reduction (Redox) Reactions: Certain proteins, such as
molecules but smaller than microscopic particles. This colloidal those containing cysteine residues, can participate in redox
nature gives proteins some unique properties, such as the ability to reactions, which are important for cellular processes like
form gels and foams. metabolism and signal transduction
 FUNCTIONAL PROPERTIES
Water Absorption and Retention
Proteins that are made up of mostly hydrophilic amino acids will tend to absorb and retain
more water. For example, bakery products containing high-protein ingredients such as soy
and other pulses will be more moist and heavier due to greater water retention. This is an
important functional property for bakers because more water retention means greater
product yield and higher profits.
Solubility
Proteins that are made up of mostly hydrophilic amino acids will be more soluble. This is
particularly important when you are making beverages. For example, soybean protein and
pea proteins are found to be highly soluble in water. This makes them ideal for use in
beverages and soups.
Color
Proteins react with reducing sugars to form flavors and color compounds in a process called
Maillard reaction. The dark colors that you see on the surface of bread, and the grill marks on
steak is due to Maillard reaction. For this reason, bread containing milk or soy flour will have
a darker color.
Gelation
Some proteins have the ability to form a gel. A prime example of this type of protein is
gelatin. Gelatin is made from collagen which is a rope-like protein polymer from the bones
and tissue (skin, tendons, and ligaments) of animals (usually pigs and cows). When gelatin is
heated, it dissolves and is dispersed in solution. But, as it cools, the rope-like strands bond
together and trap water between them in the process. This results in the formation of a gel.
 FUNCTIONAL PROPERTIES
Emulsification
Emulsifiers are substances that are able to prevent the separation of oil and water in food.
They are able to do this because part of their structure allows them to interact with water
and another part with oil. Therefore they can grab onto both water and oil to form a
bridge between them. Many proteins have this property because they contain both
hydrophilic and hydrophobic amino acids. For example, milk in ice cream contributes to an
emulsification effect by helping to prevent the separation of fat and water.
Viscosity and Texture
Proteins can make foods not only more viscous (thicker), but also elastic. this property is
called visco-elasticity. The best example of this is seen in gluten proteins. As water is
added to wheat and the mixture is molded, dough is formed. We are able to stretch this
dough like an elastic which recoils when it is released. This is an important characteristic
that gives bread its texture. As the bread dough rises during fermentation, the strong
visco-elastic property of the gluten proteins prevents the bread from collapsing. Higher
protein content in bread and bakery products will produce a firmer texture. Cakes are
generally made using wheat with a low protein content (7-9%) to give a soft texture,
whereas bread flours have high protein (14-16%) for firmness.
Foam Formation
A food foam is formed when air bubbles are dispersed in water. Examples of food foams
are whipped cream, ice cream, marshmallows, and beaten egg whites. Proteins stabilize
foams by forming a protective coat around the air bubbles in the foam, which prevents
the bubbles from collapsing. They are able to form this coat because of their
hydrophilic/hydrophobic nature. The hydrophilic part of the protein will bind to water and
the hydrophobic part will bind with the air, creating a stable bridge.
 FUNCTIONAL PROPERTIES
Flavor-Binding
Proteins are generally odorless compounds on their own, but they can bind flavor
compounds and therefore impart new flavor to foods. It is observed at the flavor profile of
bean flour fractions. Flour fractions with more protein, have a more diverse range of
flavor compounds, and these are present at very high concentrations. The ability of
proteins to bind favors can have a negative impact on the flavor of end-products if off-
flavors are trapped. On the positive side, manufacturers can use this property to trap and
retain certain flavor ingredients in food.
Curdling
Proteins can coagulate with the addition of acids. The point at which proteins precipitate
or fall out of solution, is called the isoelectric point. It is a point where the charge on the
protein changes to neutral. At a neutral charge, it is no longer capable of dissolving in
water. For example, the production of yogurt and cheese.
Enzymatic Browning
One type of browning is called enzymatic browning. That is when browning is caused by
enzymes. Enzymes are proteins that speed up the rate of chemical reactions in living
systems. One type of reaction that they speed up is the browning reaction. This reaction is
caused by the action of the enzyme polyphenol oxidase on phenol compounds in foods, in
the presence of oxygen. The result is a brown compound called melanin. This reaction is
evidenced in apples and potatoes after they are cut and left exposed to oxygen.
 Why is skimmed milk
best for cappuccinos?
Proteins make the
Fat particles simply
foam stable by forming
increase the rate of
a film at the air-liquid
foam disintegration by
interfaces around the
increasing the rate of
air bubbles stabilising
coalescence.
them.
Mixing at high speed or injection of steam causes a suspension
of air bubbles in the aqueous phase to form. Foams are
inherently unstable so break down over time by :
1. air bubbles may float to the surface (creaming) whilst the
liquid drains away, 2. bubbles may join together (coalescence),
 CHANGES OF PROTEIN DURING PROCESSING

1. Denaturation:
- Heat exposure during cooking causes proteins to denature. Weak chemical forces
that hold tertiary and secondary protein structures together break down. As a result,
proteins lose their original shape and functionality. For instance, when you cook an
egg, the egg white proteins denature, leading to the solidification of the egg white.
2. Aggregation and Gelation:
- Aggregation occurs when proteins clump together due to changes in pH,
temperature, or other factors. This can affect the texture of foods. For example,
cheese curds form due to protein aggregation during cheese-making.
- Gelation involves proteins forming a network-like structure, resulting in gels.
Gelatin desserts (like Jello-O) are classic examples. Gelation impacts the texture and
mouthfeel of foods.
3. Enzymatic Reactions:
Enzymes can modify proteins during food processing. For instance:
- Proteases break down proteins into smaller peptides during fermentation (e.g.,
soy sauce production).
- Transglutaminase cross-links proteins, enhancing their functionality (used in meat
products).
- Amylases modify starch-protein interactions, affecting dough properties in baking.
 CHANGES OF PROTEIN DURING PROCESSING

4. Hydrolysis:
- Hydrolytic enzymes break peptide bonds, leading to protein
breakdown. This process occurs during aging (e.g., tenderizing meat) or in
the production of hydrolyzed protein ingredients (used in soups, sauces,
and snacks).
5. Emulsification and Foaming:
- Proteins can act as emulsifiers, helping oil and water mix (e.g., in salad
dressings).
- Whipped egg whites create foams due to protein denaturation and air
incorporation (think cream/pastry).
6. Dehydration and Rehydration
- During food drying (dehydration), proteins may undergo structural
changes. Upon rehydration (adding water back), proteins regain some
functionality.

these protein modifications are essential for creating diverse and

delicious foods!
 DENATURATION

Denaturation is a process that changes the molecular structure without breaking any of
the peptide bonds of a protein. The process is peculiar to proteins and affects different
proteins to different degrees, depending on the structure of a protein. Denaturation can
be brought about by a variety of agents, of which the most important are heat, pH, salts,
and surface effects. Considering the complexity of many food systems, it is not
surprising that denaturation is a complex process that cannot easily be described in
simple terms. Denaturation usually involves loss of biological activity and significant
changes in some physical or functional properties such as solubility. The destruction of
enzyme activity by heat is an important operation in food processing. In most cases,
denaturation is nonreversible; however, there are some exceptions.
 DETERMINATION OF PROTEIN

Basic principles of determination of

protein content
Numerous methods have been developed to
measure protein content.
The basic principles of these methods include:
a) Determinations of nitrogen,
b) Peptide bonds, Methods of protein analysis
c) Aromatic amino acids, There are several methods of protein analysis
d) Dye-binding capacity, a) Kjeldhal method
e) Ultraviolet absorptivity of proteins, b) Dumas method c) Infrared spectroscopy
f) Light scattering properties d) Biuret method
e) Lowry method
f) Bradford method
g) Bicinchoninic acid (BCA)
h) Ultraviolet absorptivity of proteins,
 Kjeldahl Method
The protein content is determined from the organic Nitrogen content by Kjeldahl method. The various nitrogenous
compounds are converted into ammonium sulphate by boiling with concentrated sulphuric acid. The ammonium
sulphate formed is decomposed with an alkali (NaOH) and the ammonia liberated is absorbed in excess of standard
solution of acid and then back titrated with standard alkali.

Advantages:
1) Applicable to all types of foods
2) Inexpensive (if not using an automated system)
3) Accurate; an official method for crude protein
content
4) Has been modified (micro Kjeldahl method) to
measure microgram quantities of proteins

Disadvantages:
1) Measures total organic nitrogen, not just protein
nitrogen
2) Time consuming (at least 2 h to complete)
3) Poorer precision
4) Corrosive reagent
 FOOD PROTEIN & THEIR SOURCES

Animal Source Plant source

Protein Name Source
Albumin Egg whites, milk, and blood serum Protein Name Source

Keratin Hair, nails, and the outer layer of skin Leghemoglobin Soybeans (used in plant-based meat substitutes)
Derived from collagen in animal connective tissues; used in
Gelatin
desserts and gummy candies Lectins Legumes (e.g., beans, lentils) and grains
Myosin Muscle protein found in meat, especially in skeletal muscles
Phytohemagglutinin Raw kidney beans
Muscle protein that works alongside myosin in muscle
Actin
contraction Globulins Legumes, nuts, and seeds
Fibrinogen Involved in blood clotting; found in blood plasma
Glutelin Rice and other grains
Collagen Abundant in skin, tendons, and bones
Hemoglobin Carries oxygen in red blood cells Zein Corn (maize)
Casein Main protein in milk
Vicilin Peas and other legumes
Fibroin Makes up silk fibers produced by silkworms
Elastin Provides elasticity to skin, blood vessels, and other tissues Phaseolin Common beans (e.g., kidney beans)

Lactoferrin Found in milk; has antimicrobial properties Avenalin Oats

Tubulin Component of microtubules in cells
Gliadin Component of gluten in wheat
Histones Proteins that help package DNA in the cell nucleus
Gluten Present in wheat, barley, and rye
Caseinogens Precursors to casein in milk
Ovalbumin Major protein in egg whites prolamin, glutelin,
Rice
globulin, and albumin
Question?