7.1 Protein and Dairy Introduction

Protein Basics

Proteins are made up of amino acids. Understanding the structure of proteins helps explain the functions of a variety of proteins in food.

• Primary Structure: Amino acids linked together with peptide bonds.
  • Peptide bonds are covalent (strong) bond between the acid group of one amino acid and the amino group of another amino acid.
• Secondary Structure has a three-dimensional organization, that can include a helical structure, pleated sheet, or a random coil.
  • The amino acid chain forms the 3-D structure to get to its lowest energy state.
  • Bonding forces are mainly weak and include hydrogen bonds, van der Waals forces, and disulfide bridges.

• Tertiary Structure is a more complex 3-D Structure and can form as globular proteins or fibrous proteins.
  • The orientation of amino acids for a protein in a water system would be hydrophobic on the interior and hydrophilic on the exterior.
  • Bonding forces are weak and include hydrogen bonds and van der Waals forces.
• Quaternary Structure is a large protein molecule formed by ~4 polypeptide chains.
  • Bonding forces are again weak and include hydrogen bonds and van der Waals forces. 

Changing protein structure

Protein structure can change through denaturation, coagulation, and hydrolysis.

Denaturation is a physical change altering the 3-D structure and properties of proteins.

• Breaking of hydrogen bonds
• Disruption of van der Waals forces
• Unfolding of tertiary structure
• Different side chain groups become exposed
• No change in primary structure (amino acid sequence)

Coagulation is the next step after denaturation and is a further physical change to the protein.

• Clumping together of denatured protein molecules to form an insoluble protein mass
• May be desirable or undesirable

Proteins can denature and coagulate due to heat, beating or physical manipulations (egg whites), and even freezing. Proteins can also denature and coagulate due to the addition of acid and the protein reaching its isoelectric point.

The isoelectric point is the pH at which the protein has a neutral charge and is least soluble.

• Number of positive charges = number of negative charges
• Protein molecules — unstable
• Form curds or precipitate
• Wanted with cheese, not with tomato soup!

Hydrolysis is the breaking of peptide bonds to form smaller peptides.

• Alters primary structure
• Most commonly done by enzymes such as rennet — to make cheese

Dairy Products

Most dairy products in the United States are made from cow’s milk. There are many products that are classified by fat content and if they have been cultured. See the dairy recipes for sensory evaluation of a variety of dairy products.

Milk is also used to make cheese. Here are helpful definitions to have in order to understand the cheese-making process and cheese types.

Cheese Making Definitions

• Casein Protein: curd, denatured by acid, not heat
• Whey Protein: denatured by heat, not acid
  • Gjetost: Goat cheese made from the whey, 43% Lactose
• Acid Coagulated: i.e. “souring” Acid coagulation.  Hydrogen displaces Calcium, lost in whey.  Soft curd. i.e. cottage cheese
• Rennin (or enzyme) Coagulated: Enzyme that coagulates milk, cleaving proteins at specific points enables the retention of Calcium. Tougher curd.
• Ripening (Changes): Stronger flavor profiles, less moisture, smaller protein chains.
• Process Cheese: Cheese is melted and mixed with an emulsifier to improve melting characteristics