Protein

• The chemical composition of protein includes nitrogen, carbon, hydrogen, oxygen, phosphorus and sulphur.
• Denaturation of proteins happens when the protein experiences changes in acids, alkali, temperature, high salt concentrations, or force, causing changes in the polypeptide chain and changing the protein's properties.
• Denaturation of protein can be physical (whisking or heat) or chemical (acid).
• Hydrophobic parts of the protein become exposed on the outside of the unfolded polypeptide chain the more that denaturation occurs.
• Exposed hydrophilic parts attract water molecules.
• Exposed hydrophobic (blue) molecules are attracted to each other, forming a random complex shape with water molecules trapped inside.
• Heat = denaturation = solid gel formed, water molecules trapped inside = coagulation – irreversible change in structure.
• Partial denaturation of protein can be reversed if the influence (e.g. heat, mechanical whisking etc) is removed.
• If egg whites are beaten only until they form soft peaks, the proteins are only partially denatured and retain some of their elasticity.
• The partially denatured protein strands surround the air bubbles and when heated, these proteins fully denature and solidify, creating a protective wall so that the air bubbles don't burst.
• Other protein-rich foods undergo denaturation as well.
• It’s part of the process of milk becoming cheese, and you can watch it happen when cooking a fish fillet: The meat changes from translucent to opaque as the proteins uncoil and re-bond.
• If a food is missing one or more of the indispensable/essential amino acids, it has a low biological value (LBV).
• Protein complementation is when two LBV proteins are eaten together.
• Amino acids are organic compounds that contain a carboxyl group and an amino group. They join together in long chains (known as polymers) to form protein molecules. Amino acids are known as the building blocks for protein molecules.
• Amino acids are joined to more amino acids through a peptide bond. This polymer chain of amino acids creates a polypeptide.
• A polymer is the general name for a substance that is made up of many similar units bonded together. They include polymers, polysaccharides and proteins.
• Polypeptide chains are held together by bonds (such as hydrogen bonds) to prevent the protein molecules from coming apart. They are formed in a series of structures.
• Primary structure: A polypeptide chain composed of lots of amino acids.
• Secondary structure: The polypeptide either twists into a helix shape or folds into “concertina” (corrugated) shaped sheet. Some of the amino acids are HYDROPHILIC (attracted to water) and some are HYDROPHOBIC (repelled by water) so this affects how the chain folds.
• Tertiary structure: The chain continues to fold so the protein molecules become compact and more bonds are formed to hold it together.
• Quaternary structure: Several polypeptide chains join and fold together to form compact bundles. It is now a protein consisting of one of more amino acids chains.
• Denaturation happens when the protein

  experiences changes in: Acids (lemon, vinegar), Alkali (baking soda), Temperature (heating or freezing), High salt concentrations and Force (such as whisking).
• 1: Quaternary structure of protein

  unravels back to a primary polypeptide

  chain.

  2: Hydrophobic parts of the protein become

  exposed on the outside of the unfolded

  polypeptide chain the more that denaturation

  occurs.

  3: Exposed hydrophilic parts attract water

  molecules.

  4: Exposed hydrophobic (blue) molecules are attracted to each other, forming a random complex shape with water molecules trapped inside.
• Partial denaturation of protein can be reversed if the influence, such as heat or mechanical whisking, is removed.
• Once the protein structure is fully denatured, it cannot be reversed.
• If egg whites are beaten until they are stiff, they are fully denatured and have no elasticity, losing their original properties and not being able to return to their former state.
• If egg whites are beaten only until they form soft peaks, the proteins are only partially denatured and retain some of their elasticity.
• The partially denatured protein strands surround the air bubbles and when heated, these proteins fully denature and solidify, creating a protective wall so that the air bubbles don't burst.
• This is why such dishes as meringues and soufflés are light and fluffy.
• Other protein-rich foods undergo denaturation as well.
• Denaturation is part of the process of milk becoming cheese, and you can watch it happen when cooking a fish fillet: The meat changes from translucent to opaque as the proteins uncoil and re-bond.
• The biological value relates to how many amino acids are present in a protein. If a food is missing one or more of the indispensable/essential

  amino acids, it has a low biological value (LBV). For example, baked beans have an LBV. If a food has all the indispensable/essential amino acids, it has a high biological value (HBV). For example, steak has an HBV.
• Protein complementation is when two LBV proteins are eaten together. By eating two LBV proteins in the same meal, you can make up

  for the lacking amino acids in each, therefore giving yourself a meal with a high biological value (HBV).
• Proteins have many functions including structural support, enzymes, hormones, antibodies and transport.
• 1: Protein digestion begins in the stomach by hydrochloric acid and the enzyme, pepsin.

  2: Protein-digesting enzymes are secreted by the pancreas into the small intestine.

  3: The small intestine is the major site of protein digestion; final digestion occurs here.

  4: Absorbed amino acids enter the blood and travel to the liver.

  5: The liver regulates the distribution of amino acids to the rest of the body.

  6: A small amount of dietary protein is lost in the faeces.