Food Proteins: Structure, Types, and Functions in Nutrition

Food Proteins

Introduction to Food Proteins

Proteins are one of the major macronutrients found in food, essential for human nutrition and food structure. They are complex molecules made up of amino acids and play diverse roles in both biological systems and food products.

• Major food components: proteins/enzymes, carbohydrates, lipids, water

• Minor constituents: micronutrients (vitamins, minerals), which do not provide calories

Different Proteins in Foods

Types and Sources of Food Proteins

Proteins in foods come from various sources, each with unique properties and nutritional values.

• Milk proteins: casein, whey

• Egg protein: albumin

• Tofu: a soy protein

• Wheat/barley protein: gluten

• Meat proteins: collagen, gelatin

Definitions and Basic Structure

What Are Proteins?

Proteins are highly complex polymers constructed from a pool of 20 different amino acids. They are essential macromolecules containing nitrogen, carbon, and oxygen, and may also contain sulfur.

• Amino acids: The building blocks of proteins, each with a unique side chain (R-group)

• General structure of an amino acid:

• Central (α) carbon atom

• Amino group ($\ce{H2N-}$)

• Carboxyl group ($\ce{-COOH}$)

• Hydrogen atom

• R-group (side chain, varies for each amino acid)

Peptide Bonds

Formation of Protein Chains

Amino acids are linked together by peptide bonds to form longer chains:

• Dipeptide: 2 amino acids

• Tripeptide: 3 amino acids

• Polypeptide: More than 3 but less than 50 amino acids

The peptide bond is a covalent bond formed between the carboxyl group of one amino acid and the amino group of another, releasing a molecule of water:

$\ce{Amino ext{-}acid_1-COOH + H2N-Amino ext{-}acid_2 \rightarrow Amino ext{-}acid_1-CO-NH-Amino ext{-}acid_2 + H2O}$

Protein Structure

Levels of Protein Structure

Proteins have four levels of structural organization, each contributing to their function and stability:

• Primary structure: Linear sequence of amino acids; very hard to destroy

• Secondary structure: Periodic arrangement of the amino acid chain, such as alpha-helix and beta-sheet

• Tertiary structure: Further folding of the chain to form a three-dimensional shape

• Quaternary structure: Arrangement of two or more tertiary structures into a functional protein complex

Essential and Non-Essential Amino Acids

Classification and Nutritional Importance

Amino acids are classified based on whether the human body can synthesize them:

• Essential amino acids: Must be obtained from the diet; the body cannot synthesize them

• Non-essential amino acids: Can be synthesized by the body

Complete proteins contain all essential amino acids in adequate amounts (e.g., most animal proteins, some plant proteins like beans).

Functional Properties of Food Proteins

Roles in Food Products

Proteins contribute to the structure, texture, and acceptability of food products through their functional properties:

• Structure formation: Provide structure to foods (e.g., gluten in bread, gelatin in desserts)

• Gel formation: Create 3D networks that trap water and other ingredients (e.g., gummy bears, yogurt)

• Foaming: Stabilize whipped toppings and beer froths

• Flavor binding: Bind flavors and contribute to taste (e.g., MSG, aspartame)

• Color development: Participate in Maillard reaction, leading to browning in baked goods and meats

• Denaturation: Loss of native structure due to heat, pH, freezing, or mechanical action, altering functional properties

Example Table: Types of Food Proteins and Their Sources

Protein Type	Source	Example Food
Casein, Whey	Milk	Cheese, Yogurt
Albumin	Egg	Egg whites
Soy Protein	Soybeans	Tofu
Gluten	Wheat, Barley	Bread, Pasta
Collagen, Gelatin	Meat	Jelly, Gummy candies

Example: Gluten in Bread

Gluten is a protein found in wheat and related grains. It gives elasticity to dough, helps it rise, and contributes to the chewy texture of baked products.

Additional info:

• Proteins are also important for nutrition, providing essential amino acids required for growth and maintenance of body tissues.

• Denaturation is often irreversible, especially when caused by heat, and can affect the texture and appearance of foods.