2. Proteins: Amino Acids and Peptides
Chapter 11
Proteins: Amino Acids and Peptides

3. Objectives
Identify amino acid classifications based on nutritional use and chemical properties of the side chains.
Describe the primary, secondary, and tertiary structures of proteins.
List at least six factors that denature proteins.
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4. Objectives State the functions of protein in food production.
Apply basic principles of the chemistry of protein to cooking eggs, milk, and meat products.
Compare the nutritional functions of proteins with the functions of carbohydrates and fats.

5. The Structure of Protein
Protein is composed of carbon, hydrogen, oxygen, nitrogen, and usually sulfur
may also contain iron, copper, phosphorus, or zinc
Made up of subunits that are organic acids called amino acids
found in the human body, animals, plants, and single-celled organisms
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6. The Structure of Protein
Amino acids have 3 basic parts
a side chain of carbon and hydrogen atoms
a carboxyl group (-COOH)
an amine group (-NH2) of one nitrogen atom and 2 hydrogen atoms
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7. The Structure of Protein
The carboxyl group acts as an acid and the amine group as a base
When 2 amino acids combine, a peptide bond is formed and water is released
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8. The Structure of Protein
A polypeptide is a chain of amino acids bound together by peptide bonds
Most proteins have a chain from 100 to 500 amino acids
Amino acids are classified by
their nutritional use
the chemical nature of their side chains
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9. The Structure of Protein
Classification by nutritional use
Out of the 20 amino acids needed by the body for growth and body functions, many are dispensable (nonessential) amino acids that are made by the body
Indispensable (essential) amino acids are the 9 amino acids that are not made by the body and must be supplied by the diet
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10. The Structure of Protein
Classification by nutritional use
Certain conditions prevent the body from producing enough dispensable amino acids and they have to be obtained from the diet
They become known as conditionally indispensable (conditionally essential) amino acids
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11. The Structure of Protein
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12. The Structure of Protein
Classification by nutritional use
Eggs, milk, fish, poultry, and meats are called complete proteins because they contain all the indispensable amino acids
Incomplete proteins from grains and vegetables are short one or more of the essential amino acids
Combining incomplete proteins, such as red beans and rice, can form a complete source
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13. The Structure of Protein
Classification by side chains
The shape and function of protein depend on the polarity of the side chains
Side chains can be nonpolar, uncharged polar, positively charged, or negatively charged
Nonpolar side chains are less soluble in water and are attracted to other nonpolar compounds, such as lipids and cholesterol
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14. The Structure of Protein
Classification by side chains
Amino acids with neutral polar side chains will form hydrogen bonds and are attracted to other polar molecules, such as water
Positively and negatively charged side chains enable some proteins to act as buffers

15. Protein Structures Proteins are complex molecules because of the
number of amino acids
order in which they combine
interaction of the side chains
Primary structure is the order the amino acids occur in the chain
results from the chain of peptide bonds
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16. Protein Structures
Secondary structure refers to the shape of sections of the amino acid
A helix is a repeating coil
A random coil is tangled and twisted
A pleated sheet is like a paper fan
Tertiary structure refers to the 3-dimensional structure of an entire amino acid chain
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17. Protein Structures Tertiary Structure
Globular proteins do not form networks
Fibrous proteins are usually made from helix-shaped strands, are strong and elastic, and form networks
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18. Molecular Interactions of Proteins
A hydrogen bond can form between the hydrogen atom of one side chain and the hydroxyl group of another
Hydrogen bond formation is basic to the stability of secondary and tertiary structures and makes some proteins water soluble
Disulfide cross-links are covalent bonds between 2 protein molecules at side chains containing sulfur
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19. Molecular Interactions of Proteins
The more disulfide bonds there are, the more stable the molecule is
Hydrophobic, or water repelling, interactions occur between side chains that are nonpolar
Hydrophobic proteins include
caseins in milk that form cheese curds
whey, a by-product of cheese production

20. Color Changes of Protein Pigments
Myoglobin is the iron-building protein pigment in muscle that provides color
is bright red when oxygen molecule is attached
is purplish if oxygen molecule is not attached
is brown after prolonged exposure to oxygen
Oxidation and reduction is the reversible process of adding and removing oxygen
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21. Color Changes of Protein Pigments
Nitrites are added during the curing process to preserve meats
Cured meats such as ham and bacon
are very stable
have pink-red color
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22. Denaturation of Proteins
Any change in the shape of a protein molecule without breaking the peptide bonds is called denaturation
a loosening or unfolding of the tertiary and sometimes secondary structure
is sometimes reversible
involves only hydrogen bonds
Breaking disulfide cross-links, however, is not reversible
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23. Denaturation of Proteins
Coagulation is a permanent denaturation
results when liquid or semi-liquid proteins form solid or semisoft clots
The coagulation of a protein
changes its physical characteristics
alters the ability to bind with water
interferes with biological interactions of enzymes
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24. Denaturation of Proteins
Some denatured proteins like beaten egg whites can return to their original state
Coagulated proteins such as cooked eggs hold their new shape.
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25. Physical Methods of Denaturing Protein
Temperature changes
Heat speeds up denaturation
Colder temperatures may cause curdling
Mechanical actions
Beating, rolling, and kneading disrupt protein structures
Gluten strengthens during kneading
Sound waves and irradiation
Prolonged exposure at high levels is needed

26. Chemical Methods of Denaturing Protein
Change in pH
Exposure to acids or alkalis can cause proteins to unfold
Sour cream, buttermilk, and yogurt are the result of acids denaturing milk proteins
Exposure to mineral salts or metals
Sodium and potassium salts and various metals denature proteins

27. Functions of Protein in Food
To determine how effectively proteins will work in a food product, food scientists analyze a protein’s
degree of water absorption
solubility
viscosity
stability in acids and alkalis
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28. Functions of Protein in Food
1. Proteins form gels
A protein gel is a mixture of fluids locked in a tangled 3-dimensional mesh of denatured and coagulated protein
Gels have 2 parts—the 3-dimensional molecular structure and liquid that is attracted to the protein
Protein gel has a narrow melting and solidifying temperature range
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29. Functions of Protein in Food
Acids soften gelatin, which may develop syneresis if stored or cooked too long
Gel stability
increases with mineral salts, hard water, more gelatin, and slow cooling rates
decreases with acids, sugar, food pieces, and rapid cooling
A protein gel is formed from muscle tissue when salt is added to destabilize proteins
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30. Functions of Protein in Food
2. Proteins can change texture
Most globular proteins can be spun into fibers under the right conditions
Texturizing is used
with soy protein to create meat substitutes by denaturation or heat-coagulation under pressure
to produce processed cheeses
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31. Functions of Protein in Food
3. Proteins emulsify
Denatured protein can act as an emulsifier since the polar side chain attracts water-based liquid and a nonpolar side chain attracts oil
Pressure used in homogenization of milk enables casein to act as an emulsifier
Protein’s ability to form an emulsion makes extraction of oils from seeds more difficult
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32. Functions of Protein in Food
4. Proteins form foams
A gas is suspended in liquid or a semi-solid
Foams are formed by bubbling gas through a mixture, beating or whipping, and depressurization
Meringue, foam cakes, marshmallows, souffles, and bread are examples
The albumin in egg whites and milk is a good foaming agent
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33. Functions of Protein in Food
5. Proteins develop gluten
Gluten is a strongly cohesive and elastic protein formed when a high-protein flour is combined with moisture and stirred or kneaded
Strength results from the disulfide cross-links formed during kneading
Trapped gases expand when heated, and the gluten structure coagulates when baked

34. Cooking High-Protein Foods
High temperatures and prolonged cooking damage high-protein foods, which include
eggs
milk products
meat, poultry, and fish
Protein molecules shrink and squeeze out water, resulting in a dry, rubbery, tough product

35. Storing and Cooking Eggs
Eggs deteriorate when carbon dioxide is lost through the shell or water moves into the egg yolk
Egg producers spray eggs to reduce loss of carbon dioxide and moisture
Cooking eggs at low temperatures and short cooking times allow the egg white to coagulate, yet remain soft and tender

36. Principles of Cooking Milk
Curdling in sauces and soups can be prevented by combining the acid with starch before adding milk
Scorching of puddings and sauces can be prevented by using constant stirring, double boilers, and lower temperatures

37. Principles of Cooking Meat
Most meat contains muscle fibers, connective tissue, and myoglobin
Goal is to soften collagen in connective tissue while making muscle tissue more tender
Moist heat methods work best on meat with much connective tissue
Dry heat methods are ideal for meats with little connective tissue

38. The Maillard Reaction
Amino acids in grains and meats at high temperatures have a chemical reaction that causes changes in food color and flavor
One step in the 3-phase reaction is oxidation or dehydrogenation of an alcohol to form an aldehyde
The Maillard reaction causes food to brown when cooked

39. Nutritional Contribution of Proteins
Support growth and repair
Provide nitrogen and amino acids for growth and repair of muscle fibers, connective tissue, cell walls, and red and white blood cells
Fight disease
Antibodies are proteins designed to attack foreign substances
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40. Nutritional Contribution of Proteins
Maintain fluid and mineral balance
Protein in cell walls control movement of fluids and minerals in and out of cells to help nerves, the brain, and muscles function properly
Maintain pH balance
Proteins in the blood are buffers that pick up or release acids and bases
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41. Nutritional Contribution of Proteins
Control bodily functions
Proteins are a part of hormones that maintain body functions and enzymes that are necessary for chemical reactions in the body
Provide energy
The body changes amino acids into an energy source when extra protein is consumed
The breakdown of excess protein produces ammonia and ketones, straining the kidneys
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42. Nutritional Contribution of Proteins
Food scientists are developing affordable proteins to meet future demands worldwide by
developing grains such as triticale and amaranth
using biotechnology
Health concerns
Food sensitivities and allergies require accurate food labeling

43. Recap
Food scientists are finding ways to meet future worldwide protein needs
Protein molecules are made of subunits called amino acids consisting of
a side chain of carbon and hydrogen atoms, a carboxyl group, and an amine group
Protein structures include
primary, secondary, and tertiary
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44. Recap Proteins are denatured by
changing temperature
beating, rolling, and kneading
sound waves and irradiation at high levels
changing pH
Protein serves many functions in food production, including to
form gels, texturize, emulsify, form foams, and develop gluten
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45. Recap
High-protein foods must be carefully cooked to preserve moisture due to the rapid denaturation of protein when heated
Nutritionally, protein functions to
support growth and repair, fight disease, maintain fluid and mineral balance, maintain pH balance, control bodily functions, provide energy, and fight disease