protein

Chemistry of Food: Proteins

• Proteins are the building blocks of cells
• Major difference between lipids and protein: Protein contains nitrogen
• Made up of long chains of amino acids
• We can only synthesize about half of the 20 amino acids needed to produce proteins. 9 are essential.

Protein Structure

• Primary protein structure is a sequence of a chain of amino acids
• Secondary protein structure occurs when the sequence of amino acids are linked by hydrogen bonds
• Tertiary protein structure occurs when certain attractions are present between alpha helices and pleated sheets
• Quaternary protein structure is a protein consisting of more than one amino acid chain.

Amino Acids

• The Essential Amino Acids
• Leucine, Isoleucine, Valine, Histidine, Lysine, Methionine, Phenylalanine, Theronine, Tryptophan
• Obtained via diet
• The Non-essential Amino Acids
  • Alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, tyrosine
  • Synthesized by the body

Main Functions of Proteins in Foods

Proteins allow the following to occur during preparation

• Hydration
• Denaturation + Coagulation
• Enzymatic reactions
• Browning

Hydration

Ability of protein to dissolve in and attract water

• Roles: capability to form a gel and aid in dough formation

Denaturation + Coagulation

• Denaturation:
• The process in which the structure of a protein is disrupted, resulting in partial or complete loss of function.
• Coagulation:
• The clotting or precipitation of protein in a liquid into a semi-solid compound.
• Both denaturation and coagulation are irreversible processes.
• Denaturation leads to coagulation

Enzymatic Reaction

• Enzymes are proteins
• Enzymes are catalysts that speed up chemical reactions
• Most enzyme names end in -ase

Browning

• Enzymatic Browning
• An enzyme acts on a phenolic compound in the presence of oxygen to produce brown-colored products
• Non-Enzymatic browning AKA Maillard Reaction
• The reaction between a sugar and a protein leads to the formation of brown complexes

Enzymatic Browning

• Certain fruits and vegetables are susceptible to browning when peeled, cut, bruised.
• Requirements:
• Phenolic Compounds
• Polyphenol Oxidase Enzymes
• Oxygen
• The polyphenol oxidase enzymes turn the color of the phenolic componds from clear to brown.
• The brown compounds known as melanins are safe to eat.

Maillard Reaction (non-enzymatic browning)

• Browning that results from chemical changes facilitated by heat
• Chemical Reaction between an amino acid and a reducing sugar with heat
• Reducing Sugar
• A reducing sugar is a sugar that has or is capable of forming an aldehyde or ketone group
• Has a free carbonyl group when the sugar is in open chain form
• Glucose, Fructose, Galactose, Maltose, & Lactose are all reducing sugars
• Sucrose is NOT a reducing sugar because it browns via caramellization
• 3 Steps involved in Maillard Reactions (series of many reactions)
• Initial Reaction. Reaction between a reducing sugar and a primary amino acid. Loss of water results in formation of glycosamine.
• Isomerisation results in amadori compounds. Rearranged glycoslamines. Rearrangement of atoms results in a different molecule.
• Amadori compounds undergo further dehydration and deamination. Hundreds of different flavor compounds are created. And they can keep breaking down to form new compounds.
• Caramelization and Maillard Reaction are different processes/reactions that have similar results.
• Both can cause browning in the same food.
• Both require low moisture environments.
• Non-enzymatic browning reactions happen noticeably at 310F. Water boils at 212F. This means that water will have vaporized by the time browning occurs. If there is moisture present, the temperature is not close to the 300+ degrees needed for browning.