Proteins
CH 11- AMINO ACIDS AND PEPTIDES
Unit objectives:
Identify amino acid classifications based on nutritional use and chemical
properties of side chains
Describe the primary, secondary, tertiary and quaternary structures of proteins
Identify factors that denature protein
State the functions of proteins
Apply basic protein cookery principles
Proteins
CHO plus
◦ nitrogen
◦ and usually sulfur
◦ Some also may contain:
◦ iron,
◦ Copper
◦ Phosphorus
◦ Zinc
Protein
Proteins contribute 4 cal/g
◦ Require 0.8 g protein per kg body wt ( adults)
◦ Excess is converted to energy and stored as fat
Amino Acid- three parts
Proteins- Amino Acids
20 different amino acids naturally occurring in the human body and in foods
8 are indispensable/ essential ( cannot be synthesized by humans) They must be obtained from foods
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TV TILL PM
Threonine
Valine
Tryptophan (limiting in corn)
Isoleucine
Leucine
Lysine (limiting in wheat)
Phenylalanine (used in aspartame…)
Methionine (limiting in legumes)
◦ #9 (Histidine ( essential for infants)
We cannot make these AA so often rely on animal sources to eat them and we get them from the animal meat.
There are also nonessential and conditionally indispensable amino acids pg306 text
Classifications of AA by side chains
Shape and function depends on polarity
◦ Nonpolar◦ less soluble in water
◦ Attracted to lipids and cholesterol (transport chol.)
◦ Uncharged polar◦ Forms hydrogen bonds
◦ Attracted to water
◦ Food prep functions
◦ Positively charged/Negatively charged
◦ Enables some proteins to act as buffers
◦ Polymers or long chains of amino acids linked by peptide bonds
O
ll
C-OH
l
H2N-C-H
I
R
R= side chain ( hydrophobic, charged, polar, aromatic)
Amino group (NH2) and
acidic (carboxylic COOH)
group share the same carbon atom
Peptide bonds
Amin group from AA combines
with carboxyl group of another.
When combined, water is released
The bond is called Peptide bond
Protein structures
Primary structure
◦ The order of the amino acids in the chain
◦ The result of the chain of peptide bonds
Secondary Structure
Shape of sections of the amino acid chains
The hydrogen bonds between AA chains cause bending
Three patterns may result
pleated sheet (paper fan)
helix (slinky)
random coil (tangled slinky)
Tertiary Structure
Structure of an entire amino acid chain
Globular proteins- do not form links that will create a protein network
Hemoglobin that carry oxygen
Lipoproteins that carry cholesterol
Caseins in milk
Albumen in egg whites
Fibrous proteins – made from helix-shaped strands.
Strong and part of connective tissues
Collagen
Elastin
Keratin
myosin
Found in muscle fibers, ligaments, tendons
fingernails, and hair
Quaternary Structure
Denaturing proteinchanging the shape of the protein molecule without breaking the peptide bond
Loosen or unfold the protein molecule
Shape can be changed without changing the primary structure
Changed;
◦ By physical or chemical means
◦ Physical- whipping cream
◦ Chemical- curdling milk
Coagulation
When liquid or semi-liquid protein forms solid or semisolid clots.
◦ Cheese
◦ Cooking eggs
Methods of Denaturing
Temperature changes
◦ 600x for every 10o C
◦ Cold temperatures change structures
◦ ie frozen cheese , milk
Mechanical actions
◦ Beating, rolling, kneading
◦ Bread dough, egg white foam
Other Physical actions
◦ Sound waves
◦ Irradiation
◦ Chemical
◦ Change pH –TOFU soy
◦ http://www.youtube.com/watch?v=rRZsfexVFCM
◦ Add acids
◦ Exposing to mineral salts
◦ exposing metals ( Cu , Fe, Mg, Ca)
Stop day 1
Exp:
Milk plastic
Molecular interactions of Proteins
1. Hydrogen bonding ( water soluability)
2. Disulfide cross-links (covalent bond between protein molecules at side chains that contain
sulfur)– causes odor of rotten eggs
3. Hydrophobic interactions- caseins in milk form curds in cheese making.
Color changes of protein pigments
◦ Myoglobin (red)-+ O2 Metmyoglobin (brown)
◦ Example of redox reaction (oxygen reduction)
◦ Nitrites are added in processing to preserve color
Functions of Proteins in Food
Form Gel
◦ Gelatin
◦ Too much gelatin makes a gummy product
◦ As an additive:
◦ Provides structure and support
◦ Stabilizes foam in whipped products
◦ Thickens pudding and pies
◦ Controls crystal growth in frozen foods
◦ Taken from muscle tissues used on hams, chicken hot dogs, bolognahttp://www.youtube.com/watch?v=2NzUm7UEEIY
◦ Custards
◦ Frozen
◦ Baked (flan)
◦ Puddings
Proteins functional properties- Gels
Form 3D structure that can trap WATER
Eg) gelatin gels
Yogurt
Cheese
Frankfurters
tofu
Proteins functional properties-Enzymes
Proteins that function as biological catalyst ( capable if triggering a reaction)
◦ Promote a chemical reaction that will not occur spontaneously
◦ Inherent in the foods or added in processing
◦ Desirable or undesirable reactions in foods
◦ What are examples of enzymes we have already discussed in the class?
What are examples of enzymes we have already discussed in the class?
Maltase
Lactase
Amylase
Specific enzymes are important for testing
Exp : Prepare cheese
Functions of Proteins in Food
Texturize◦ Extruded protein
◦ Texturized protein extenders
◦ Processed cheese (mixing and heating cheese proteins for a smoother texture)
Functions of Proteins in Food
Emulsify- polar and nonpolar ends inhibit separation
Eggs used in ice cream, mayo
Also in milk, cream, butter, and cheese
Casein in milk is an emulsifier
Proteins functional properties-Emulsifiers
Amphiphilic molecules
◦Hydrophylic : water loving
◦Glycerol linked to an organic acid
◦Hydrophobic/ lipophilic: water hating or lipid loving
groups ( i.e. fatty acids)
◦Reduce interfacial tension
◦Eg) egg yolk proteins in mayonaise
Proteins functional properties- Foams
Foams
◦Traps AIR bubbles and form rigid 3D structure
◦When heated or cooled  solid foams
◦Eg) meringues
◦Bread
◦Ice cream
Functions of Proteins in Food- Gluten
Development of Gluten in baked goods
Gluten – cohesive elastic protein
Other proteins may interfere with gluten formation
◦ So milk may be scalded before using in bread
◦ Cold milk may interfere
LAB Gluten ball experiment
Protein
Nutrition=
◦ 4 calories per gram
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Incomplete protein
vs
Missing 1 or more essential AA
May be combined to form complete protein
Does not have to be eaten together at same meal
Plant Sources ( except soy)
complete protein
contains all essential AA
animal sources and soy products
Protein Nutrition
Support growth and repair
Fight disease
Maintain Fluid and Mineral Balance
Maintain pH balance
Control body functions (hormones and enzymes)
Provide Energy- when you consume more Protein than needed body removes NH2 and C=O form
the peptide chains . The N2H is converted to ammonia. The C=O becomes part of a compound
known as ketones. Both strain the kidneys
Future protein needs
Proteins- Functional Properties
Amino Acid sequence and 3 D structure of protein determines the Functional Properties in
foods as well as nutritive value of the proteins
Functions in the human body:
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Synthesis of new tissue
Synthesis of DNA, RNA
Synthesis of enzymes, hormones, antibodies
Transportation of fat, iron, calcium in the blood
(allergies)
Proteins- protein quality
Different proteins do NOT have the same nutritional value
Protein quality of food protein depends on :
◦ Balance of essential amino acids in the protein ( ie. Food source)
◦ Digestibility of the protein
◦ Quantity of the protein in the particular food
Cooking high protein foods
Short low temperatures needed
Rapid denaturation will cause
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Shrinkage
Water loss
Dry
Rubbery
Tough
Principles of storing and cooking EGGS
Deterioration :
◦1. loss of CO2 through the egg shell (change from neutral to
basic)
◦2. Water moves to the yolk (stretches and weakens yolk sac)
◦3. albumen (white) cooked too long will toughen
Principles of storing and cooking MILK
Curdling◦ Caused when acids unfold and stick together
◦ Prevent by mixing acids with starch before milk is added
Scorching◦ Protein clumps formed by heat will sink and burn to the pan bottom
◦ Preventing:
◦ Stir constatntly
◦ Use a double boiler
◦LOW AND SLOW
Principles of storing and cooking MEAT
Goal is to soften connective tissue
◦ Muscle fibers ,connective, myoglobin (red pigment)
◦ Collagen is protein in the connective tissue
◦ Must cook carefully, overcooking or over heating destroys meat texture and
nutrition.
Proteins- allergies
Inability to digest certain proteins- exhibit symptoms of allergic reactions
The 8 most common food allergens:
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Milk
Eggs
Fish
Wheat
Tree nuts
Peanuts
Soybeans
Crustaceans (shrimp and crab)