Chapter: 1. General (physicochemical)
properties of proteins and Main sources of food
Tahseen Fatima Miano
1. Physico Chemical Properties Of Proteins
a) Chemical composition of amino acid
b) Classification of amino acid based on ph
c) Ionization behavior of amino acid
d) Peptides, oligopeptides, polypeptides, proteins
e) Protein structure
f) Electrostatic interactions van der waals interactions
2.Sources of Protein
a) Major types of soybean protein products
b) Defatted cottonseed flours
c) Mycoprotein products
d) Leaf-protein concentrate
e) Microbial-biomass protein
PHYSICO CHEMICAL PROPERTIES OF PROTEINS:
Basic structural unit
Building block of proteins.
20 amino acids
Make food proteins.
CHEMICAL COMPOSITION OF AMINO ACID
Consists of a hydrogen (H),
Carboxyl (COOH) group,
Side group (R) which are covalently linked to a central α-carbon.
The structures of amino acids differ only in the different
structure of their side group (R).
Side chain are covalently linked to the central α-carbon
and to the nitrogen of the amine group forming a cyclic
Group (R) determines,
Hydrogen bonding ability
The Joint Commission of Biochemical Nomenclature (JCBN)
International Union of Pure and Applied chemistry (IUPAC)-
International Union of Biochemistry (IUB)
symbols and nomenclature for amino acids and peptides.
single hydrogen as its R group.
Based on their side group R,
CLASSIFICATION OF AMINO ACID BASED ON pH
1. Positively charged or basic amino acids include Arg, His, Lys,
2. Negatively charged or acidic are Aspartame, Glutamate.
3. Polar uncharged amino acids include Asparagine, Glutamine,
4. Aliphatic and nonpolar are Alanine, Isoleucine, Leucine, Valine,
Glycine Proline, Methionine.
5. Aromatic and nonpolar include Phe, Tyr, Trp.
Few of the amino acids particularly Gly, His, and Cys do not fit into
a particular group
Physicochemical properties of the 20 amino acids
Molecular weights of amino acids range from 75.1 to 204.2 daltons.
The MW of an amino acid residue is calculated by subtracting the
MW of water, 18 daltons.
Hydrophobicity (or hydrophilicity) of the side groups of amino acids
that make up the protein influences protein folding and thus the
physical properties of that protein.
Range amino acids vary in their degree of hydrophobicity depending
on the constituents
Amino acids display ionization behavior and can serve as hydrogen
donors or acceptors,
Behave as acids and bases.
IONIZATION BEHAVIOR OF AMINO ACID
• Hydrogen donors or acceptors,
• Behave acids and bases.
The isoelectric point (pI) of an amino acid
pH at which the amino acid has no net charge.
pH values above their pI (pH > pI) amino acids will be negatively
At pH values below their pI (pH < pI) amino acids will be
The pI for amino acids with no acidic or basic side chains range
from 5.0 to 6.3.
Dissociation of α-carboxyl group provides pKa values of 1.8–2.7.
At neutral pH molecule is a dipolar ion or a zwitterion amino acids
(Arg, Asp,Cys, Glu, His, Lys, Tyr) have ionizable side group
Aspartic acid and glutamic acid are acidic amino acids containing
ionizable carboxyl side chains with pKa of 3.9–4.0 and 4.3–4.5,
Arginine, lysine, and histidine contain ionizable side groups with
pKa values of 12.0, 10.4–11.1, and 6.0–7.0
Tyrosine and cysteine also have ionizable side chains with pKa
of 9.7 and 9.0–9.5 for the phenolic hydroxyl and the thiol groups,
The total charge of an amino acid will be the sum of the
Derived amino acids and conjugated proteins
Derivatives of primary amino acids.
Derived amino acids have covalently or non covalently bound
moieties or maybe cross-linked.
The proteins that contain derived amino acids are called
Covalently bound groups may include phosphate or
carbohydrate moieties, as in the case of phosphoproteins and
Noncovalent groups may include lipids or nucleic acids as with
lipoproteins, and nucleoproteins.
Cysteine is a common cross-linked amino acid found in foods,
others include desmosine, isodesmosine, and di- and trityrosine.
Several simple derivatives of amino acids are found in several
Derived proteins are not naturally occurring proteins instead
they are obtained by further chemical or enzymatic modification,
in the cell or derived due to food processing
Peptides, oligopeptides, polypeptides, proteins
Linked covalently through α-carboxyl group of one amino acid and
the α-amino group
Another amino acid through an amide or peptide bond to form
peptides, oligopeptides, polypeptides, and proteins.
Peptides with 10–20 residues are called oligopeptides;
more called polypeptides.
Polypeptides of more than 50 residues are referred to as proteins
The free α-amino group of the first amino acid is the N-terminal,
The free α-carboxyl of the last amino acid is the C-terminal end of
a peptide or protein.
Peptide bond (Cα – CO-NH-Cα) lie in a single plane.
Polypeptide can be represented as a series of Cα – CO-NH-
Cα planes connected by Cα, with only N-Cα and Cα-Cφ and ψ
Respectively, having rotational freedom. However, the
rotations of N-Cα and Cα-C bonds are also restricted due to
stearic hindrances from side chain groups. Due to these
restrictions, the flexibility of a peptide and protein is also
When proline is involved in the peptide bond, the peptide bond
is normally in a trans configuration.
Peptides, oligopeptides, and proteins only have one free α-
amino and α-carboxyl group at either end of the molecule.
Which ionize similar to a free amino acid, and contribute to the
acid–base properties of a protein, thus the total charge of the
Primary, secondary, tertiary, and quaternary structure.
Primary structure of a protein is its specific amino acid sequence
which in turn determines the secondary, tertiary, and quaternary
structures of a protein.
The primary structure is the linear sequence of the amino acids
that make-up the peptide or protein.
The primary structure is determined by its genetic code and
post-translational covalent modifications.
The primary structure of a protein ultimately determines the
physicochemical, and thus the functional properties of that food
The biological function of a protein is determined by its
secondary, tertiary, and quaternary structures.
Secondary structure of a protein is the local three-dimensional
arrangement of a protein which is determined by the amino acid
sequence of the polypeptide.
Secondary structure may result from Aperiodic or periodic
Random coil is an Aperiodic structures and regions in the protein
where consecutive amino acids residues possess different sets
In food proteins, although α-, 310- and β-helix, are found, the right-
handed α-helix is the most common and is also the most stable.
Φ AND Ψ
The rotations of the polypeptide backbone around the bonds
between N-Cα (called Phi, φ) and Cα-C (called Psi, ψ).
The 310- and β-helix are not so common.
In α-helix each helical rotation has 3.6 amino acid residues.
Hydrogen bonding at N-H and C O groups gives structure.
α-helical structure is due to the helix surface being made up of
hydrophobic (or nonpolar) residues as well as hydrophilic (or polar)
Quaternary structure in a protein is determined by the
stabilized by noncovalent interactions such as hydrogen bonding,
hydrophobic, and electrostatic interactions protein structure.
Higher levels of structures food proteins may be classified as
globular or fibrous proteins.
Globular proteins: are those that the polypeptide chain folds to
form a globular shape.
Forces involved in stability of proteins
• Three-dimensional structure is complex.
• Intra- and intermolecular interactions.
• Intramolecular interactions forces intrinsic.(i.e., stearic, van der
• Intermolecular interactions result surrounding solvent (i.e.,
hydrogen bonding, electrostatic, and hydrophobic interactions).
• During protein folding all the thermodynamically favorable
interactions are optimized
• Thermodynamically unfavorable interactions are minimized.
Thermodynamically unfavorable interaction of nonpolar
groups with water
Minimizing their association with water.
Hydrophobic and hydrophilic tendencies of amino acid
Greater the hydropathy of an amino acid residue interior of
the protein molecule.
Hydrophobic interactions are the main reason proteins fold
into their tertiary structures,
Hydrophobic free energy provides stability to this structure.
Polar molecule if it has a permanent dipole. A permanent dipole is due to a
difference in electronegativity between the atoms involved in a covalent bond.
Non polar molecule
Electrostatic interactions Van der Waals interactions
polarization of the electron
neutral atoms in protein molecules.
van der Waals interactions are relatively weak
The strength of these forces decreases rapidly, with increase in
Hydrogen bonds are ionic interactions attached electronegative
atom such as O, N, or S
Hydrogen bonds are essential to protein structure
Ionic interactions involve negative and a positive charge.
Covalent bonds or chemical cross-links
Covalent cross-linking of side groups found in proteins are
disulfide bonds (S S).
Sulfhydryl (thiol) groups of two cysteine molecules
Disulfide bonds may be inter- or intramolecular,
Folding of a polypeptide and protein
Stability : Stearic hindrances from side chain atoms.
Interactions: noncovalent and covalent
2.Sources of Protein
1. Animal-protein Products
Egg proteins= methionine bcystine+ lactalbumin
Milk protein isolate,
Whey-protein concentrate+ lysine and tryptophan
Beef, tuna+ Threonine.
Chicken and meat products.
High In Lysine, Methionineþcystine, Threonine, And Tryptophan
Gelatin Or Collagen Is Animal Protein + Tryptophan
Protein digestibility-corrected amino acid score (PDCAAS)
Codex Committee on Vegetable Proteins (CCVP) has developed an
international general standard for vegetable protein products
Dehulled and solvent-extracted remove the oil.
Flakes dried, resulting in defatted soy flakes.
• Bakery Products, breakfast cereals, beverages, infant formulas,
dairy, meat, and dairy and meat analogs.
Including gelling, emulsifying, water-holding, and fat absorbing
MAJOR TYPES OF SOYBEAN PROTEIN PRODUCTS
Including flour, concentrates, isolates and textured soybean
Full fat, low-fat and defatted, as well as toasted and textured.
Defatted soybean flour
Produced by grinding the defatted flakes.
Protein content of about 50%.
USE : In bakery products.
FULL-FAT SOYBEAN FLOUR
Soy milk and tofu
SOYBEAN PROTEIN CONCENTRATE
Aqueous alcohol extraction of defatted soy flakes.
Protein content 70%
Remaining portion mainly insoluble carbohydrates.
Prepared by the wet extraction from wheat or wheat flours of
Certain non-protein constituents (starch and other
Protein content of 80%on a dry-weight basis.
Although high in protein digestibility,
Wheat gluten is drastically limiting in lysine,
Other Vegetable-protein Products
Pea protein concentrate 92%.
Cottonseed-protein products 60%.
Sunflower-protein isolate. 94%.
Peanut flour 37% to 50%.
Canola (rapeseed)-protein concentrate
DEFATTED COTTONSEED FLOURS
60% protein, N6.25
Fractionation: cellular proteins during isolate preparation also
produces both favorable and unfavorable effects.
Storage protein isolates
(99–100% protein) are low in lysine and the sulfur amino acids,
low in protein quality.
Non storage protein isolates
Fractionation of the seed proteins
Quality DFCSP varies physiologically active or ‘free‘ gossypol.
Gossypol is a binaphthyl, dialdehyde, polyhydroxyl pigment which is
highly reactive and deleterious to monogastric animals, including
Gossypol in the seed is in discrete extracellular glands.
Used only since 1985.
Protein is made from a type of fungus, Fusarium venenatum
Mycoprotein production steps.
Organism is first grown in an aerobic, axenic, continuous
Using food-grade carbohydrate substrates and other components
needed for growth.
The mycelium of the fungus is then heat treated to reduce the
ribonucleic acid content to safe levels.
Single-cell protein sources tend to be high in nucleic acids, which
can lead to excessive blood uric acid levels.
Binders, flavorings, and other ingredients are added to achieve
desired organoleptic and Physical properties.
Considerable toxicity and nutritional testing
Protein quality was evaluated.
Protein quality of mycoprotein is similar to that of soybeans.
Brand name, QuornTM, in the UK and Europe for some 10–15
years and has secured considerable consumer acceptance.
USA Mycoprotein is typically 12% water, 3% fat, 3% available
carbohydrates, 6% fiber, and 2% ash. B complex vitamins and
mineral nutrients are also present in small amounts.
Green leaves are protein factories producing good quality protein.
Low concentration, high fiber content, and palatability problems,
vegetables, such as lettuce, spinach, and cabbage. However,
some green forage crops produce several times more protein per
1960s and 1970s many product development
LPC obtained by harvesting leaves by first pressing out the juice.
Serum drained away. Solid material dried and powdered.
‘whole‘ leaf protein concentrate (WLPC),
about 45–60% crude protein,
or 34–45% true protein.
There are different types of protein in leaves that can be separated by coagulating at
ALFALFA the protein digestibility of the WLPC is about 88%,
WLPC also contains significant amounts of vitamin A
precursors, iron, and other nutrients which may be
reduced in the white-protein fraction.
The term microbial-biomass protein was coined to permit the
inclusion of protein from multicellular fungi.
Possible way to produce useful food
Reducing waste products and surplus raw materials
Industrial and agricultural activities such as pulp mill waste liquor.
100 plants producing microbial biomass protein,
Economical availability of a suitable substrate,
Assurance for safe food materials
Technology and facilities to produce attractive food products.
Physicochemical properties of food proteins, peptides, and
oligopeptides and understanding the forces that contribute to their
stability will provide us the tools to better utilize food proteins in
Addition of beef to vegetable protein sources proves significantly
the protein quality of soy, pea,peanut, sunflower and wheat-