FDSCI 501 - Proteins

Proteins

Biopolymers of amino acids linked by peptide bonds

Enzymes

Proteins with catalytic activity

Protein Functions

Structure, transport, hormones, nutrition, food properties

Heat Set (Proteins)

Irreversible denaturation after heating (e.g., cooked egg)

Exception to Heat Set

Gelatin (reversible)

Amino Acid Structure

Contains amino group, carboxyl group, and R-group

R-group

Determines amino acid properties

Nonpolar Amino Acids

Hydrophobic (e.g., valine)

Polar Amino Acids

Hydrophilic (e.g., serine)

Acidic Amino Acids

Negatively charged (e.g., aspartic acid)

Basic Amino Acids

Positively charged (e.g., lysine)

Peptide Bond

Strong bond linking amino acids with partial double bond character

Primary Structure

Amino acid sequence

Secondary Structure

Local folding (α-helix, β-sheet, random coil)

Tertiary Structure

3D folding of a single polypeptide

Quaternary Structure

Association of multiple subunits

Example of Quaternary Structure

Hemoglobin (4 subunits)

Alpha Helix

Right-handed coil stabilized by hydrogen bonds

Beta Sheet

Sheet-like structure with hydrogen bonding

Parallel Beta Sheet

Chains run in same direction

Antiparallel Beta Sheet

Chains run in opposite directions

Random Coil

Irregular protein structure

Disulfide Bonds

Strong covalent bonds between cysteines (S–S)

Hydrophobic Interactions

Nonpolar groups cluster inside protein

Hydrogen Bonds

Weak but numerous stabilizing forces

Electrostatic Interactions

Ionic interactions between charged groups

van der Waals Forces

Weak attractions between molecules

Maillard Reaction

Non-enzymatic browning between reducing sugars and amino groups

Products of Maillard Reaction

Brown pigments and flavor compounds

Conditions for Maillard Reaction

High temp, pH > 6, intermediate water activity

Stage 1 Maillard

Sugar-amine condensation and Amadori rearrangement

Stage 2 Maillard

Formation of reductones and Strecker degradation

Stage 3 Maillard

Formation of brown pigments (melanoidins)

Pyrazines

Flavor compounds from Maillard reaction

HMF (Hydroxymethylfurfural)

Aroma compound from sugars

Acrylamide

Potential carcinogen formed in heated foods

Control of Maillard Reaction

Lower temp, reduce pH, control water, remove sugars

AGEs (Advanced Glycation Endproducts)

Harmful compounds linked to aging and disease

Casein

Major milk protein (80%), forms micelles

Isoelectric Point of Casein

pH 4.6

Whey Proteins

Heat-sensitive milk proteins (20%)

Sarcoplasmic Proteins

Water-soluble muscle proteins (enzymes, myoglobin)

Myofibrillar Proteins

Salt-soluble proteins (myosin, actin)

Collagen

Connective tissue protein that forms gelatin when heated

Elastin

Tough, non-soluble connective tissue protein

Gluten

Wheat protein complex (glutenin + gliadin)

Function of Gluten

Provides elasticity and traps gas in dough

Celiac Disease

Autoimmune response to gluten

Gluten-Free Standard

Egg White Proteins

Ovalbumin, lysozyme, avidin

Egg Yolk Proteins

Lipoproteins (emulsifiers)

Soy Protein

Complete plant protein

Soy Protein Isolate

~90% protein

Soy Protein Concentrate

~70% protein

Pea Protein

Non-allergenic plant protein used in meat alternatives

Chickpea Protein

Used in foods like hummus and plant-based products

Proteins bonds stabilizing structure

Disulfide (S–S), hydrogen bonds, hydrophobic interactions, electrostatic (ionic), van der Waals forces

Disulfide bonds

Covalent bond between cysteine residues (S–S); strong, stabilizes tertiary structure; broken by reduction (S–H)

Hydrophobic interactions

Nonpolar R-groups cluster inside protein away from water; includes van der Waals and London dispersion forces

Hydrogen bonds

Weak bonds between H and electronegative atoms (O, N); numerous and stabilize structure

Electrostatic interactions

Ionic attraction between charged groups (NH3+ and COO−); affected by pH

Divalent cation bridges

Metal ions (e.g., Ca2+) link negatively charged groups in acidic proteins

Secondary structure

Local folding due to hydrogen bonding; α-helix, β-sheet, random coil

Tertiary structure

Overall 3D shape of a polypeptide stabilized by multiple interactions (S–S, hydrophobic, H-bonds, ionic)

Alpha helix

Right-handed coiled structure stabilized by hydrogen bonds

Beta sheet

Sheet-like structure stabilized by hydrogen bonds; parallel or antiparallel

Random coil

Irregular, disordered protein region

Protein denaturation

Unfolding of protein structure without breaking peptide bonds

Heat denaturation

Breaks hydrogen bonds → unfolding (e.g., cooked egg)

pH denaturation

Changes charge → disrupts ionic bonds

Mechanical denaturation

Physical disruption (e.g., whipping egg whites)

Gelation

Protein network traps water to form a gel (e.g., gelatin, eggs)

Emulsification

Proteins stabilize oil-water mixtures (e.g., egg yolk in mayonnaise)

Foaming

Proteins trap air bubbles (e.g., whipped egg whites)

Protein emulsification mechanism

Amphiphilic proteins align at oil-water interface forming protective layer around droplets

Protein foam stabilization

Proteins unfold at air-water interface forming films that trap air and increase viscosity

Milk proteins

Casein (micelles, major), whey (β-lactoglobulin, α-lactalbumin)

Muscle proteins

Myosin and actin (contractile), collagen (connective tissue)

Wheat proteins

Gluten = glutenin + gliadin

Egg proteins

Ovalbumin (egg white), lipoproteins (yolk)

Soy proteins

Globulins (major), albumins

Protein concentrate

~60–70% protein; contains carbohydrates and fat

Protein isolate ~90% protein; more purified

Maillard reaction

Non-enzymatic browning between reducing sugars and amino groups

Maillard reaction components

Reducing sugar + amino group (protein or amino acid)

Maillard reaction conditions

High temperature, pH > 6 (optimal 8–9), intermediate water activity

Initial Maillard reaction

Sugar + amine → glycosylamine (Schiff base) → Amadori rearrangement product

Strecker degradation

Amino acid reacts with dicarbonyls → aldehydes, CO2, amines; contributes to flavor

Pyrazines

Flavor compounds giving roasted/nutty aromas

Foods with pyrazines

Roasted coffee (methylpyrazine), bread crust (dimethylpyrazine), roasted nuts (trimethylpyrazine), cooked meat (alkylpyrazines)

Nonpolar amino acids

Hydrocarbon R-groups; hydrophobic (e.g., valine)

Polar amino acids

Contain O or N; hydrophilic (e.g., serine)

Acidic amino acids

Contain COO− group; negatively charged (e.g., aspartic acid)

Control of Maillard browning

Lower temperature, lower pH (<6), control water activity, remove reducing sugars

Enhancing Maillard browning

Increase temperature, increase pH, add reducing sugars, optimize moisture

Acrylamide

Toxic compound formed during high-temperature cooking; potential carcinogen

Acrylamide formation

Asparagine + reducing sugar → Schiff base → decarboxylation/deamination → acrylamide

Foods high in acrylamide

French fries, potato chips, bread crust

Reducing acrylamide formation

Lower temperature, reduce reducing sugars, blanch potatoes, use low-asparagine varieties, shorten cooking time