Download Proteins - Food Science & Human Nutrition

Proteins
1
Proteins – basic concepts

Role of proteins
1.
Nutrition
 Energy and essential amino acids
 Can possess anti-nutritional properties
 Trypsin inhibitors in soy = reduced digestibility
 Allergens – IgE mediated food allergy attributed to naturally occurring food
proteins (negative immunological response to a protein)
 Toxins – α-amanitin a cyclic peptide found in a poisonous mushroom
species
2.
Structure
 Provide structure in living organisms and also foods
 Collagen – main component of connective tissue
 Gelatin – hydrolyzed collagen – eg. Jello
 Proteins roughly contain 5-50 % C, 6-7 % H, 20-23 % O, 12-19 % N, and 0-3 %
S (Barret, 1985 –Chemistry and Biochem of Amino Acids)
 Measuring N content is often used to estimate the protein content in foods
3.
Catalysts
 Enzymes (which are proteins) catalyze chemical reactions in living tissue
and foods
2
Proteins – basic concepts

Role of proteins
4.
Functional properties





5.
Gelation
Emulsifiers
Water bonding
Increase viscosity
Texture
Browning
 Have amino acids that can react with reducing sugars
 Maillard Browning
 Acrylamide (produced by asparagine rxn with reducing sugars)
 Some enzymes can also cause browning
 Polyphenol oxidase - Apples
3
Typical protein contents of the edible portion of various foods
Food or Beverage
Apples, raw, with skin (09003)
Beer, regular (14003)
Milk, human, mature (01107)
Bananas, raw (09040)
Cabbage, raw (11109)
Potatoes, white, flesh and skin, raw (11354)
Potatoes, microwaved, flesh and skin (11675)
Corn, sweet, yellow, canned, whole kernel, drained solids (11172)
Rice, brown, long-grain, cooked (20037)
Soy milk, original and vanilla, with added Ca, Vitamins A & D (16139)
Milk, whole, 3.25% milk fat, with added vitamin D (01077)
Ice creams, vanilla (19095)
Yogurt, plain, low fat (01117)
Tofu, soft (nigari) (16127)
Cereals, ready-to-eat, cornflakes (08020)
Chocolate, dark, 70-85% cacao solids (19904)
Rice, brown, long-grain, raw (20036)
Lentils, mature seeds, boiled (16070)
Bread, white (18069)
Pasta, fresh-refrigerated, plain (20093)
Egg, whole, cooked, hard boiled (01129)
Cod, Pacific, raw (15019)
Cod, Pacific, cooked, dry heat (15192)
Almonds, dry roasted (12063)
Chicken, breast meat only, raw (05062)
Cheese, cheddar (01009)
Tuna, light, canned in water, drained solids (15121)
Lentils, raw (16069)
Chicken, breast meat only, roasted (05064)
Cheese, Parmesan, hard (01033)
Protein g/100g
0.26
0.46
1.03
1.09
1.28
1.68
2.44
2.46
2.58
2.60
3.15
3.50
5.25
6.55
6.61
7.79
7.94
9.02
9.15
11.31
12.58
15.27
18.73
21.06
21.23
24.90
25.51
25.80
31.02
35.75
Values obtained from the USDA National Nutrient Database, numbers in parentheses are the USDA 5 digit identifier
4
Proteins – basic concepts


Proteins are biological polymers that fold into a 3D
structure with amino acids being their basic structural unit
20 amino acids common to proteins (L-amino acids =
natural form)
◦ There are 20 common amino acids that are genetically coded – book has 21, includes
selenol (contains Selenium) which was discovered in 2002
◦ More (100s) amino acids exist in nature but are not genetically coded

Differ by their side chains (R-groups)
◦ All have central α C, basic amino group, and a carboxyl group

Amino acid charge behavior
◦ Neutral
◦ Acidic
◦ Basic
α
α
5
Proteins – basic concepts

Amino acids are generally grouped into 3
classes
1. Charged and polar
2. Uncharged and polar

These two classes of amino acids are found on
the surface of proteins
3. Non-polar and hydrophobic
 These are found more in the interiors of proteins
where there is little or no access to water
◦ You are expected to be able to identify which
amino acids are polar or non-polar
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Proteins – basic concepts
Polar Amino Acids - Hydrophilic
7
Proteins – basic concepts
Non-polar Amino Acids – Hydrophobic/Amphophilic
8
Proteins – basic concepts
Four levels of protein structure
Primary  Secondary  Tertiary  Quaternary
1. Primary structure
◦ Linear sequence of amino acids of the protein
molecule (backbone)
◦ Described by the amino acid sequence
that make up a polypeptide chain
 Amino acids are linked to each
other in a chain via a peptide bond
 A covalent bond
◦ Sequence always described N-terminal to C-terminal
◦
N-H partial (+), carbonyl oxygen (C=O) partial (-)
◦ This backbone structure dictates
rest of the structure (2°, 3°, etc. structure)
Condensation reaction
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Proteins – basic concepts
2. Secondary structure
 Refers to arrangement of the polypeptide
backbone
◦ Random coil
◦ Helical and sheet

Predictable arrangement of two main
secondary structures (regular spatial
arrangement)
◦ -helix
◦ -sheet
a) -helix
◦ A coiled structure formed with internal H bonds
(between C=O and N-H)
◦ Amphiphilic – both polar and non-polar surfaces
◦ Is the main structure in fibrous proteins (myosin is
an ex.) – more often in hydrophilic proteins
◦ Less in globular proteins
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Proteins – basic concepts
b) -sheet
◦ “Flat sheets” parallel or antiparallel
structure
◦ These sheets are stabilized with regular
bonding of C=O with NH (via H-bonds)
between -sheets
 Antiparallel are more stable due to better
alignment of hydrogen bonding atoms
◦ More stable than α-helix
◦ High amount in insoluble (hydrophobic)
proteins, but more stable to
denaturation
-sheets
c) Random coils
◦ Absence of secondary structure (order)
◦ Irregular random arrangement of a
polypeptide chain
http://www.youtube.com/watch?v=wM2LWCTWl
rE
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Proteins – basic concepts
3.Tertiary structure



Represents the secondary structure
folding into a 3D conformation/structure
This is the highest degree structure
of many proteins
The type of tertiary structure
formed is dictated by
◦ Amino acid sequence
◦ -helix/-sheet
◦ Proline content
 α-helix breaker
◦ Stabilizing forces
 H bonding
◦ Solvent conditions

Dictates where amino acid residues are
located
◦ Surface – interact w/ solvent
◦ Interior – interact w/ side chains (effects
stability)
β-lactoglobulin
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Proteins – basic concepts
4. Quaternary structure
 A complex of two or more
tertiary structures
 The units are linked together
through non-covalent bonds
◦ β-lactoglobulin
 Milk (pH 6.8)– 37 kDa dimer
 Cheese (pH 4.5) – 144 kDa octamer

Some proteins will not
become functional unless
they form this structure.
◦ Examples:
 Hemoglobin
 Myosin
 2 heavy chains, 4 light chains (475 kDa)
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Proteins – basic concepts
Types of forces/bonds that stabilize the protein structure
Solvent-solute interactions
14
Molecular forces involved in protein
structure
Type
Bond Energy
(KJ/mol)
Functional groups from
amino acid side chains
involved
Van der Waals interactions
(dipole)
1-9
Permanent, induced and
instantaneous dipoles
Hydrophobic
interactions
4-12
Aliphatic and aromatic side
chains
Hydrogen bond
8-40
Carboxyl, amide, imidazole,
guanidino, amino, hydroxyl and
phenolic groups
Electrostatic interactions
42-84
Carboxyl and amino groups
Covalent bond
330-380
Disulfide moiety
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Proteins – basic concepts
Proteins exist in two main states
NATIVE STATE
DENATURED STATE
 Loss of native conformation
Usually most stable
 Usually most soluble
 Polar groups usually on the outside
 Hydrophobic groups on inside

◦ Altered secondary, tertiary or
quaternary structure
◦ May be reversible or irreversible,
partial or complete

Results
◦
◦
◦
◦
◦
Decrease solubility
Increase viscosity
Altered functional properties
Loss of enzymatic activity
Sometimes increased
digestibility
•
•
•
•
•
Heat
pH
Pressure
Oxidation
Salts
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Proteins – basic concepts
Factors causing protein
denaturation
 pH
 phenolic
 Alkyl etc.
100
%Denatured
◦ Too much charge can cause high
electrostatic repulsion between
charged amino acids and the protein
structure unfolds
◦ As unfolds, hydrophobic interior is
exposed.
◦ Unfavorable because of buried
groups
0
0
pH
12
17
Proteins – basic concepts
Factors causing protein
denaturation
 Temperature
100
%Denatured
◦ High temperature destabilizes the
non-covalent interactions holding the
protein together causing it to
eventually unfold
◦ Freezing can also denature due to ice
crystals & weakening of hydrophobic
interactions (water participation less)
0 0
T (C)
100
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Proteins – basic concepts

Detergents
◦ Prefer to interact with the hydrophobic part of the protein (the
interior) thus causing it to open up (e.g. SDS)

Lipids/air (surface denaturation)
◦ The hydrophobic interior opens up and interacts with the
hydrophobic air/lipid phase (e.g. foams and emulsion)

Shear
◦ Mechanical energy (e.g. whipping) can physically rip the protein
apart or introduce the protein to a hydrophobic phase (air or
lipid – foaming and emulsification)
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Proteins – basic concepts
Important reactions of proteins and
effect on structure and quality
1.
Hydrolysis
◦ Hydrolysis of proteins also referred to as
proteolysis
 Cleaves peptide bond and adds H2O (reverse of peptide bond formation)
◦ Proteins can be hydrolyzed (the peptide bond) by
acid or enzymes to give peptides and free amino
acids (e.g. soy sauce, fish sauce etc.)
 Hydrolyzed protein usually listed as an ingredient on soy
sauce label
◦ Modifies protein functional properties
 E.g. increased solubility
◦ Increases bioavailability of amino acids
 Excessive consumption of free amino acids is not
good however (too much N)
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Proteins – basic concepts
Important reactions of proteins and
effect on structure and quality
2.
◦
◦
◦
◦
Maillard reaction (carbonyl - amino
browning)
Can change functional properties of proteins
Changes color (browning)
Changes flavor (roasted, buttery, burnt etc.)
Decreases nutritional quality (participating amino
acid lost from a nutritional standpoint)
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Proteins – basic concepts
Important reactions of proteins and
effect on structure and quality
3.
Alkaline reactions
◦ Soy protein concentrates (textured vegetable protein)
 0.1 M NaOH for 1 hr @ 60°C or greater
 Denatures proteins by hydrolysis
 Some amino acids become highly reactive
 NH3 groups in lysine
 SH groups and S-S bonds become very reactive (e.g.
cysteine)
◦ Loss of some aa as a result (cysteine, cystine, serine, and
threonine), ↓ nutritional quality (minimal)
22
Proteins – basic concepts
Important reactions of proteins and
effect on structure and quality
3.
Alkaline reactions
A. Isomerization (racemization)
 L- to D-amino acids (we cannot digest D-amino acids)
B. Lysinoalanine formation (LAL)
 Lysine becomes highly reactive at high pH and reacts with
dehydroalanine forming a cross-link = lysinolalanine
 Lysine, an essential amino acid, becomes unavailable (problem
because is limiting aa in cereal grains)
HO
O
OH
NH
NH2
O
H 2N
Lysinoalanine
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Proteins – basic concepts
Heat
4.
◦ Mild heat treatments lead to alteration in protein structure and often
beneficially effect digestibility or bioavailability (↓ solubility)
◦ However, severe (above 200 °C) heat treatment drastically reduces
protein solubility and functionality and may give decreased
digestibility/bioavailability
 Pyrolysis
 Degradation of cysteine
H
H
H
H

+
H 3C
H 3C
SH
H 2S
OH
H 2O
Leads to terrible flavor problems  H2S(g)
Amide crosslinking (isopeptide bond formation)
•
O
O
H2N
+
H2N

OH
OH
NH2
O
O
NH
OH
NH3
H3C
NH2
Need severe heat for this reaction - not very common
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Proteins – basic concepts
5.
Oxidation
◦ Lipid oxidation
 Aldehyde, ketones as a result of lipid oxidation react
with lysine making it unavailable
 Usually not a major problem
◦ Methionine oxidation (no major concern)
 Produces sulfoxide, sulfone also possible
 Oxidized by; H2O2, ROOH etc.
O
S
H3C
OH
NH2
O
O
H3C
S
CH3
Met Sulfoxide
H3C
S
CH3
O
Met Sulfone
 Met sulfoxide still active as an essential amino acid
 Met sulfone – no or little amino acid activity
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Proteins – functional properties

Functional properties defined as:
◦ “physical and chemical properties of proteins that
affect the behavior of molecular constituents in food
systems. Relates to:
◦ Preparation
◦ Storage
◦ Quality
Processing
Consumption
Organoleptic (sensory) attributes
Many food products have functionality because
of food proteins
 Protein functionality plays a key role in the
(1)improvement of existing products (2) new
product development (3) protein waste
products utilized as new ingredients

26
Proteins – functional properties
Example of protein functional properties in different food
systems
Functional
Property
Solubility
Food System
Beverages, Protein concentrates/isolates
Water-holding ability Muscle foods, cheese, yogurt, surimi
Gelation
Muscle foods, custards, eggs, yogurt, gelatin,
tofu, baked goods, surimi
Emulsification
Salad dressing, mayonnaise, ice cream,
gravy, frozen desserts
Foaming
Meringues, whipped toppings, angel cake,
sponge cake, marshmallows, yeast-leavened
breads
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Example functional proteins
Product
Major functional
protein(s)
Representative
protein ingredient
Cereals
Glutenin, gliadin
Wheat gluten
Legumes
11S Globulin, 7S
Globulin
Soy protein
concentrates or isolates
Meat, Poultry
Myosin
Surimi
Fish
Collagen
Gelatin
Eggs
Ovalbumin
Dried egg white
Milk
Casein
Caseinates
Milk
α-lactalbumin, βlactoglobulin
Whey protein
concentrates (50-80 %
protein) or isolates (90
% protein)
28
The properties of food proteins are altered by environmental
conditions, processing treatments and interactions with other
ingredients
29
Proteins – functional properties
I.


Solubility
Functional properties of proteins
depend on their solubility
Affected by the balance of
hydrophobic and hydrophilic amino
acids on its surface
◦ Hydrophilic surface = good water solubility

Charged amino acids play the most
important role in keeping the protein
soluble
◦ The proteins are least soluble at their
isoelectric point (no net charge)
◦ The protein become increasingly
soluble as pH is increased or decreased
away from the pI
30
Proteins – functional properties

Solubility
Salt concentration (ionic
strength) is also very important
for protein solubility
◦ At low salt concentrations protein
solubility increases (salting-in)
◦ At high salt concentrations protein
solubility decreases (salting-out)
%Solubility
1.
Salt concentration
31
Proteins – functional properties


Denaturation of the protein can both increase or
decrease solubility of proteins – condition dependent
pH - very high and low pH denature but the protein is
soluble since there is much repulsion
Low pH
+

+
+
+
+
+
+
+
+
+
+
+
Temperature (very high or very low) on the other
hand will lead to loss in solubility since exposed
hydrophobic groups of the denatured protein lead to
aggregation (may be desirable or undesirable in food
products)
Insoluble complex
32
Proteins – functional properties

How do we measure solubility?
◦ Most methods are highly empirical as results vary greatly with
protein concentration, pH, salt, mixing conditions, temperature
etc.
◦ Generally, the assay consists of putting the protein in samples of
different pH and centrifuging
 The more protein that stays in solution (supernatant), the more soluble the
protein is
 The bigger the pellet the less soluble the protein is
Centrifuge at 20,000g for 30 min
Protein samples at different pH’s
at 0.1M NaCl
Solubility (%) =
More
soluble
Less
soluble
pellet
protein left in supernatant
𝐱 𝟏𝟎𝟎
total protein
33
Proteins – functional properties
II.

Gelation
Gel; a continuous 3D network of
proteins that entraps water
Solution
◦ Works by protein - protein interaction and
protein - water (non-covalent)

Texture, quality and sensory
attributes of many foods depend on
protein gelation on processing
◦ Sausages, cheese, yogurt, custard

A gel can form when proteins are
denatured by
◦ Heat, pH, pressure, shearing, solvent
Gel
34
Proteins – functional properties

Thermally induced food gels (the most common)
◦ Involves unfolding of the protein structure by heat which exposes its
hydrophobic regions which leads to protein aggregation, which forms
a cross-linked network
◦ This aggregation can be irreversible or reversible
& usually cooling too
35
Proteins – functional properties
Thermally irreversible gels (also known as thermoset)
A.
◦
Thermoset gels form chemical bonds that will not break during
reheating of the gel (remains rigid even if reheated)

Examples - Muscle proteins (myosin), egg white proteins (ovalbumin)
◦ Balancing act of forces is critical in gel formation:
cooling
heating
heating
T
Gel strength/Viscosity
Denaturation (%)
 If the attractive forces between the proteins are too weak they will not form gels
 If the attractive forces are too strong the proteins will precipitate
36
Proteins – functional properties
Thermally reversible gels (thermoplastic)
Gels form on cooling (after heating) and then revert fully or
partially back to solution on reheating (“melt”)

Collagen breakdown product gelatin is this type of gel
cooling
heating
heating
T
Gel strength/Viscosity

Denaturation (%)
B.
37
Proteins – functional properties

Factors influencing gel properties
1.
2.
3.
4.
pH
Salts (ionic strength)
Temperature (final)
heating/cooling scheme
38
Proteins – functional properties

Factors influencing gel properties
1.
pH
◦ Highly protein dependent
◦ Some protein form better gels at pI
 No repulsion, get aggregate type of gels
 Soft and opaque
◦ Others give better gels away from pI
 More repulsion, string-like gels
 Stronger, more elastic and transparent
 Too far away from pI you may get no gel
 too much repulsion (stays soluble)
◦ By playing with pH one can therefore play
with the texture of food gels producing
different textures for different foods
39
Proteins – functional properties
2.
Salt concentration (ionic
strength)
 Again, highly protein
dependent
 Some proteins “need” to be
solubilized with salt before
being able to form gels, e.g.
muscle proteins (myosin)
 Some proteins do not form
good gels in salt because salt
will minimize necessary
electrostatic interactions
between the proteins
+
+
Cl+
+
+
NaCl
+Cl- Cl-+
Cl-
+
Loss of repulsion
Loss of gel strength
Loss of water-holding
40
Proteins – functional properties

How do we measure gel quality?
◦ Many different methods available
◦ Gel texture and gel water-holding capacity most commonly used
◦ One of the better texture methods is to twist a gel in a modified
viscometer (torsion meter) and measure its response (stress and strain)
until it breaks – called a “torsion test”
The results can be related to the
sensory properties of the gel
41
Proteins – functional properties
Water binding
◦ The ability of foods to take up and/or hold water is of
paramount importance to the food industry
◦ More H2O = higher weight = More $$
◦ Product quality may also be better, more juiciness
III.
42
Proteins – functional properties
III.
◦
Water binding
Water is associated with protein at several levels (Back to Water)
◦ Surface monolayer
 Very small amount of water tightly bound to charged groups on
proteins
◦ Vicinal water
 Several water layers that interact with the monolayer, slightly
more mobile
◦ Bulk phase water
 Mobile water like free water but...
 Trapped mostly by capillary action

Freely flowing water in a food product
 This is the water we are interested in when it comes to water
binding
43
Proteins – functional properties

What factors influence water binding in a
food system?
1. Protein type
 More hydrophobic = less water uptake/binding
 More hydrophilic = more water uptake/binding
2. Protein concentration
 More concentrated = more water uptake
3. Protein denaturation
 Temperature - if you form a gel on heating (which
denatures the proteins) then you would get more water
binding
 Salt type & concentration
44
Example how thermal denaturation may have an effect on
water binding
SPS = Soy protein isolate  forms gel on heating
Caseinate = Milk proteins (casein)  does not gel on heating
WPC = Whey protein concentrate  forms gel on heating
45
Proteins – functional properties
I.
Salts/ionic
strength
 This is highly
protein dependent
 muscle proteins
NaCl
Na+
Na+
Na+
Cl-
Na+
Cl-
Cl-
Na+
Cl-
46
Phosphate salts (in combination with NaCl) are frequently used in
food processing to make food proteins bind and hold more water
Na Hexametaphosphate
Na Tripolyphosphate
O
O
HO
P
O
P
O
O
O
Na O
P
OH
OH HO
O
P
Na
O
OH
Na
Na Salt
13
Salt brine
Salt brine
some phosphate
Cook
10% reduction
 phosphate
Cook
Cook
30% reduction
100% reduction
47
Proteins – functional properties
II.
pH (protein
charge)
◦ Great influence on the water
uptake and binding of
proteins
◦ Water binding lowest at pI
since there is no effective
charge and proteins typically
aggregate (i.e. don’t like to be
in contact with water)
◦ Water binding increases
greatly away from pI
◦ Muscle proteins and
protein gels are a good
example – direct interaction
with H2O or form gel
network
pI
48
Proteins – functional properties
How do we measure water binding and uptake?

◦
Usually designed for a specific product or application,
most common methods are:


Water-uptake (sorption) - Measuring water uptake of a
protein or protein food (e.g. protein gel) by adding it to a
sorbent (usually a dry powder), then remove and measure the
change in water content of the sorbent
Water-binding (also called water-holding capacity or
expressible moisture) - Subject your sample to an external
force (centrifuge or pressure) and then measure how much
water is squeezed out

Test needs to be carefully designed so that the actual internal structure
of the gel or food is not destroyed when the pressure is applied
49
Proteins – functional properties
IV.

Emulsification
Proteins can be excellent
emulsifiers because they
contain both hydrophobic and
hydrophilic groups that
decrease the interfacial
tension which allows for
stability
OIL
+
ENERGY
LOOP
TRAIN
50
Proteins – functional properties
IV.
Emulsification
Whey protein stabilized emulsion
Both phases
Whey protein stabilized emulsion
Lipid phase removed
(protein matrix showing)
51
Proteins – functional properties

Factors that affect protein-based
emulsions
◦ Type of protein
 To form a good emulsion the protein must be able
to:
1.
2.
3.
Rapidly migrate to the oil-water interface
Rapidly and readily open up and orient polar and
non-polar side chains into the proper phases
Form a stable film around the oil droplet
52
Proteins – functional properties

Factors that affect protein-based emulsions
 The following are important for the protein emulsifiers
1. Solubility of protein
 Insoluble will not form a good emulsion (can’t migrate well)
 If at pI is not good (poor solubility)
 Increasing solubility increase emulsification ability (up to a point)
2. Distribution of hydrophobic vs. hydrophilic amino acids
 Need a proper balance
 Generally increased surface hydrophobicity will increase
emulsifying properties
3. Overall shape of protein
 Globular is better than fibrous
4. Flexibility of protein
 More flexible it is, easier it opens up
53
Proteins – functional properties

How do we measure emulsifying
properties?
◦ Most are highly empirical
 Two common methods
Emulsification capacity - Oil titrated into a emulsion that
is using protein as the emulsifier with mixing and the max
amount of oil that can be added to the protein solution is
measured
Emulsification stability - Emulsion formed, then monitor
breakdown (separation into water and oil phases) with time
54
Proteins – functional properties
Foaming
 Foams are very similar to emulsion where air is the
hydrophobic phase instead of oil
 Principle of foam formation is similar to that of
emulsion formation (most of the same factors are
important)
 Foams are typically formed by
V.
◦ Injecting gas/air into a solution through small orifices producing
bubbles (sparging)
◦ Mechanically agitate a protein solution (whipping)
◦ Gas release in food, e.g. leavened breads (a special case), beer
(http://www.youtube.com/watch?v=lnjkd8z4CVI)
55
Proteins – functional properties
V.
Foaming
FOAM FORMATION
FOAM BREAKDOWN
56
Proteins – functional properties

Factors that affect foam formation and
stability
1. Type of protein is important
 Good foaming proteins exhibit:
 High rates of diffusion/adsorption at the interface
 Ability to unfold/denature at the interface
 Ability to form intermolecular associations with other
molecules (that results in film formation)
 Increased surface hydrophobicity is good
 Partially denaturing the protein often produces
better foams
 Globular is better than fibrous
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Proteins – functional properties

Factors that affect foam formation and
stability
2. pH
 Foam formation is often better slightly away from pI
 Foam stability is often better at pI
 The farther from pI the more repulsion and the
foam breaks down
 Example; Egg foams (meringue) addition of cream of
tartar  increases stability
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Proteins – functional properties

Factors that affect foam formation and
stability
3. Salt
 Very protein dependent
 Egg albumins, soy proteins, gluten
 Increasing salt usually improves foaming (stability)
since the net charge is decreased (proteins lose
solubility  salting-out)
 Whey proteins
 Increased salt negatively affect foaming (they get
more soluble  salting in)
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Proteins – functional properties

Factors that affect foam formation and
stability
4. Lipids
 Lipids in food foams usually inhibit foaming by
adsorbing to the air-water interface and thinning it
 Only 0.03% egg yolk (which has about 33% lipids)
completely inhibits foaming of egg white!
 Cream an exception where very high level of
saturated fat stabilizes foam
 Cold coalesced fat droplets surround protein
encapsulated air bubbles
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Proteins – functional properties

Factors that affect foam formation and
stability
5. Stabilizing ingredients
 Ingredients that increase viscosity of the liquid phase
stabilize the foam (sucrose, gums, polyols, etc.)
 We add sugar to egg white foams at the later stages of
foam formation to stabilize
 Addition of flour (protein, starch and fiber) to foamed
egg white to produce angel cake (a very stable cooked
foam)
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Proteins – functional properties

Factors that affect foam formation and
stability
6. Energy input
 The amount of energy (e.g. speed of whipping) and
the time used to foam a protein is very important
 To much energy or too long whipping time can
produce a poor foam
 Proteins become too denatured
 The foam structure breaks down
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Proteins – functional properties

How do we measure foam formation and
stability?
1. Overrun (foam formation) – Start with a known
volume of protein solution (e.g. 100 mL) foam it
(usually by whipping), then measure the volume of
foam vs. that of the liquid:
% Overrun =
foam volume - initial liquid volume
 100%
initial liquid volume
2. Foam stability (drainage) – Using a special cylinder
measure the amount of liquid that drains from the
foam on storage to get a mL/min or mL/hr drain
value (the smaller the value, the more stable the
foam)
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Proteins – functional properties

Protein modification to improve function
◦ Some proteins don’t exhibit good functional properties
and must be modified
◦ Other proteins are excellent in one functional aspect but
poor in another but can be modified to have a broader
range of function
1. Chemical modification
◦ Reactive amino acids are chemically modified by adding a group to
them
 Lysine, tyrosine and cysteine
 Increases solubility and gel-forming abilities
 Modified protein has to be non-toxic and digestible
 Retain 50-100% of original biological/nutritive value
 Often used in very small amounts due to possible toxicity
 Not the method of choice for food proteins
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Proteins – functional properties

Protein modification to improve function
1. Chemical modification

Example of types of chemical groups that can be added
to proteins
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Proteins – functional properties

Protein modification to improve function
2. Enzymatic modification
a) Protein hydrolysis
 Proteins broken down by enzymes to peptides (smaller)
 Improved solubility and biological value
b) Protein cross-linking
 Some enzymes (transglutaminase) can covalently link proteins
together
 Great improvement in gel strength
c) Amino acid modification
 Peptidoglutamiase converts
 Glutamine  glutamic acid (negatively charged)
 Asparagine  aspartic acid (negatively charged)
 Can convert an insoluble protein to a soluble protein
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Proteins – functional properties
3. Physical modification
◦ Most of the methods involve heat to partly denature the
proteins
 Texturized vegetable proteins – TVP (e.g. soy meat)
 A combination of heat (above 60C), pressure, high pH (11) and
ionic strength used to solubilize and denature the proteins which
rearrange into 3D gel structures with meat like texture
 Good water and fat holding capacity
 Cheaper than muscle proteins  often used in meat products
 Protein based fat substitutes (e.g. SimplesseTM by
CPKelco former NutraSweet subsidiary)
 Milk or egg proteins heat denatured and mechanically sheared and
on cooling they form small globular particles that have the same
mouthfeel and juiciness as fat
 SimplesseTM is very sensitive to high heat (protein based so can
denature) – limits its use in processing
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