1. Proteins in Foods General functionalities of proteins in foods:
Source of nutritionally essential amino acids
Provide structure & texture to foods

2. Protein content of common foods
Food (as eaten) Amount yielding 25 g protein*
Meats, fish oz
Cheddar cheese 4 oz
Black beans oz (1 ½ cups)
Tofu (soybean gel) cups
White bread 9 slices
Egg eggs
Milk cups
Spaghetti cups
Lettuce cups
Apples apples
Celery stalks
Coffee oz (94 cups)
* about 50% of RDA for adults

3. Structure of Proteins Proteins are polymers of amino acids:
(“R group”)
A generic amino acid:
20 common amino acids
9 are essential (required in the diet)
Amino acids are distinguished by their side chain, or “R” group :
Polar vs non-polar
Ionizable vs non-ionizable
R group :

4. Amino acids with polar (hydrophilic) R groups :
serine
Amino acids with non-polar (hydrophobic) R groups :
alanine
Note: these R groups are
hydrocarbons (C, H only)
leucine
phenylalanine

5. Amino acids with ionizable R groups :
Those with carboxyl (COOH) or amine (NH2) groups
Mostly Neg charged above pH 4
What is the
pH range
of foods ?
Mostly Pos charged below pH 9
“ pKa ” pH at which [charged] = [uncharged]
pKa
Carboxyl group
Amine group

6. Amino acids with ionizable R groups :
Examples:
lysine
glutamic acid
( Note: these are also polar )

7. glutamic acid, an amino acid
Free amino acids are relatively rare in foods
Exception: monosodium glutamate (MSG):
sodium salt of
glutamic acid, an amino acid
Flavor enhancer (“umami” taste)

8. Amino acids are linked together by peptide bonds
(strong covalent C--N bond) to form proteins (“polypeptides”)
Peptide bonds can be hydrolyzed by protease enzymes:
Upper GI tract (secreted, stomach &pancreas)
Enzymes in foods

9. Spatial conformation of proteins
Fibrous proteins
(often extended helices)
Globular proteins
Myosin
muscle protein
tend to be smaller,
more soluble than
fibrous proteins
Connective tissue
protein, from which
gelatin is made

10. Protein Conformation and Denaturation
"Native" Conformation:
The preferred, or most stable conformation of a protein (its "natural" shape or arrangement)
Each different protein has its own native conformation, determined largely by its
amino acid sequence and its folding pattern

11. A globular protein in aqueous solution (native conformation)
water
molecules
hydrophobic amino acids ( )
on interior (away from H2O)

12. Chemical bonds involved in the 3-D conformation of proteins

13. Protein “denaturation” : Change in conformation
Unfolding, uncoiling
energy
Hydrophobic amino acid
(e.g. alanine, leucine)
Complete denaturation is often irreversible

14. Physical consequences of protein denaturation:
Exposes hydrophobic amino acid residues normally "hidden" on the interior.
* Increases hydrophobic character of the protein
* Decreases water solubility
* Causes proteins to spontaneously aggregate via hydrophobic interactions

15. Causes of protein denaturation
Unfavorable physical or chemical environment
1. Heat
Heat energy disrupts bonds responsible for native 3-D structure
2. Changes in pH
Can disrupt ionic or disulfide bonding responsible
for native 3-D structure
"Isoelectric pH" (“pI”)
pH at which the number of negative charges (COO- )
equals the number of positive charges (NH3 +) on a protein molecule
At this pH, the protein, as a whole molecule, is net neutral
At the isoelectric pH:
* Charge repulsion between proteins is minimal
* Many proteins exhibit minimal solubility; clump and precipitate from solution

16. 3. Surface or interfacial tension
Causes of protein denaturation (con’t)
3. Surface or interfacial tension
High energy water at surfaces (between water and air), or interfaces (between oil and water)
4. Mechanical shear
High speed mixing, pumping under high pressure, etc.
Protein denaturation can be desirable or undesirable,
depending on the product and circumstances

17. Functional Properties of Proteins in Foods
(as natural components or ingredients)
Texturizing
Responsible for texture of meats, dough-based products (pasta):
Characteristics: Chewy Elastic
Crispy Tough
Thickeners (water binding): hydrophilic proteins only; often
used with CHO thickeners
Soups, sauces, gravies
Beverages
Gelling
Some proteins can form gels under certain circumstances
* Gelatin: dessert, snack gels, etc.
* Casein (milk protein) : cheeses, yogurt, other dairy products
* Egg albumin: custards, etc.
* Tofu (soybean protein)

18. Surfactants (surface-active agents):
Foaming agents: (air cell dispersal)
Baked products (cakes, cookies, breads)
Toppings
Beverages
Emulsifying agents (fat and water dispersal)
Sauces
Mayonnaise, salad dressings, etc.
Sausages, meat products

19. Enzyme activity a. Endogenous enzymes (pre-existing in foods
or produced by contaminating microorganisms) :
* can adversely affect color, taste, odor, texture
Denaturation of enzymes can maintain quality by preventing enzyme-induced changes
ex.: pasteurization (heating) of milk denatures lipases and proteases, enzymes which degrade lipids and proteins to products that adversely affect taste and odor of milk
(free fatty acids and free amino acids)
b. Added desirable enzymes:
* texturizing : ex.: meat tenderizers (proteases)
* gelling ex.: rennin, a protease used in cheese-making

20. Proteins as surfactants : Use in stabilization of foams and emulsions
Foams and emulsions are dispersions of two physically incompatible phases:
Foam: Dispersion of air cells in liquid (water) or a semi-solid
Emulsion: Dispersion of oil in water
Dispersions are thermodynamically unstable due to high
surface tension (interfacial tension) at the boundary between
the two phases
Surfactants stabilize dispersions by :
* lowering surface tension
* providing a physical barrier between phases
Phase dispersal Incr. total surface area Incr. system energy
(instability)

21. Phase dispersal Incr. total surface area Incr. system energy
(instability)
Phase
dispersal
“dispersed phase”
(energy)
“continuous
phase”
Phase
separation
Surfactants stabilize dispersions by :
* lowering surface tension
* providing a physical barrier between phases

22. A stable dispersion of water in oil :
Proteins from egg
and other ingredients
act as the surfactants

23. A B C Oil phase (hydrophobic) high energy water zone (surface tension)
interface between
oil and water
high energy
water zone
(surface tension) A
globular protein
in native conformation
in aqueous solution
bulk water
(lower energy) B
unfolded (hydrophobic)
proteins tend to
“stick” at the interface
partial denaturation
of protein caused
by surface tension C
high energy water
zone no longer exists;
unfolded protein provides
barrier between water
and air
Unfolded protein
spread out at
surface

24. Stabilization of foams and emulsions by denatured proteins
air cell or
oil droplet
air cell or
oil droplet
charge repulsion
(except at pI * )
Unfolded proteins
coat surface of
air cells or oil
droplets, stabilizing
the dispersion
Good protein surfactants: flexible, easily denatured by surface tension;
amphipathic: possess both hydrophobic and
hydrophilic regions within the same protein molecule

25. Stability of an emulsion or foam depends upon:
Interfacial surface forces Size of the disperse phase droplets Viscous properties of the continuous phase Density difference between the two phases
Food dispersions in which proteins
function as surfactants
egg white foam
emulsions

26. Other Proteins as Gelling, Texturizing Agents
Gelatin
Product of hydrolysis of collagen, a structural protein
found in connective tissues of meats
Collagen gelatin
Transparent, thermo-reversible gels (reverse when heated/cooled)
> 140 F
H20
Tofu
Cooked, gelled soy proteins
Not thermo-reversible
Surimi
Texturized fish protein (white, high water content species)
Pigmented with natural or artificial colors
Imitation crab, shrimp

27. Proteins as Pigment Precursors
[ Louis Camille Maillard, 1916 ]
aldehyde
functional
group
aldehydes are notoriously reactive

28. Maltose Sucrose A reducing sugar NOT a reducing sugar
(the O atom at carbon 1
constitutes the ether bond)

29. Maillard reactants (con’t)
lysine

30. Initial reaction in Maillard browning
protein
protein
H2O
(neutral form)
(aldehyde form)
brown hues

31. Maillard browning reaction
[ A non-enzymatic process ]
primary
amine
reducing
sugar
Schiff base
(brown)
Reactive carbonyl
compounds: aldehydes
ketones (flavor, odor)
+ other proteins
“Melanoidin” pigments
CHO-protein complexes
Grey-brown-black hues

33. 4. Carbohydrate, type of sweetener
b - amylase
Flour starches
glucose, maltose
H20, heat
Sucrose
glucose, fructose
(partial)
Note: Artificial sweeteners (aspartame, saccharine, sucralose)
will not react

34. Grains also contain proteins that react
with maltose to produce brown pigments

35. Protein A “complete” protein :
One which contains all essential amino acids in amounts
and relative proportions needed to maintain life and support
growth when consumed as the sole source of protein
The biological value of individual proteins is commonly assessed
using animal growth studies or nitrogen retention
Measures of protein quality:
Protein Efficiency Ratio
Net Protein Utilization
Biological Value

36. Animal proteins Plant proteins Generally high biological value
Exception: Gelatin
Limiting amounts of 4 amino acids
(lacks tryptophan entirely)
Plant proteins
Generally lower biological value than animal proteins
Reason: limited amounts of some amino acids:
Wheat, rice: lysine
Corn (maize): lysine, tryptophan
Legumes: methionine

37. Complementary proteins Ex.: corn (maize) & black beans 50% beans
[ Opaque-2 maize: incr.
lysine, tryptophan content ]
100% beans