Protein Structure, Function, and Food Processing Effects

Protein Characteristics and Classification

Polar Functional Groups

• AA Rating: Polar functional groups present, polar, neutral, and able to build hydrogen (H) bonds between molecules.
• Polar Basic Groups: Positively charged side chain.
• Polar Acidic Groups: Side chain is negatively charged.

Hydrodynamic Properties

These properties depend on the interactions of proteins with water (H₂O), such as absorption and retention of H₂O.

Surface Characteristic Properties

Properties related to surface characteristics, e.g., foaming and emulsions.

Changes in Protein During Food Processing

Processing causes changes in:

• Texture
• Loss of function
• Loss of nutritional quality
• Development of flavor
• Formation of toxic compounds

Factors influencing these changes include: Heating (in the presence or absence of carbohydrates), extreme pH conditions, and exposure to oxidative conditions.

Protein Denaturation

Denaturation: A change in the composition of all parts of the molecule, resulting in the loss of its biological activity and functionality. It involves changes in its structure, but not the primary structure.

Treatments causing denaturation: Heat, surface change, change of pH and salt concentration, dehydration, and additives.

Alteration of Technological Properties

Denaturation leads to changes in:

• Solubility
• Viscosity
• Reactivity of side groups
• Digestibility

Pasteurization and heating can affect several toxic proteins (toxins and natural microbial inhibitors).

Hydration Properties

Texture and rheological properties of foods depend on the interaction of H₂O with other food components, especially proteins and polysaccharides.

Factors Affecting Hydration

• Intrinsic Factors (Molecule): Amino acid composition (hydrophobic/hydrophilic ratio) and conformation (secondary and tertiary structure).
• External Factors: Changes in temperature, protein concentration, ionic strength, pH, and the presence of solvents and other denaturing agents.

Alteration of Solubility

Protein solubility is defined as the percentage (%) of protein that remains in solution or colloidal dispersion under specific conditions (i.e., does not sediment with moderate centrifugal forces).

Solubility depends on three factors:

1. Degree of hydration.
2. Density and distribution of charges along the chain.
3. Presence of non-protein components such as phosphates, carbohydrates, and lipids.

Factors Affecting Solubility

General Solubility

Good solubility allows for complete dissolution of molecules (homogeneous state), which is crucial for the degree of extraction and purification of the protein.

External Factors

• pH
• Ionic strength
• Temperature
• Solvent type

Neutral salts increase solubility up to a certain limit. Solubility generally increases from 0 to 40 °C, but above 40 °C, protein molecules denature.

Classification by Solubility Degree

• Albumins: Soluble in H₂O at pH 6.6.
• Globulins: Soluble in diluted salt solutions at pH 7.
• Prolamins: Soluble in 70% ethanol.
• Glutelins: Soluble only in very acidic or very alkaline solutions.

Effect of Salt Addition

• Affects hydration and the ability of proteins to retain H₂O.
• Polyphosphates used in meat, poultry, and fish processing control fluid loss and juiciness. The use of polyphosphates increases pH, increases ionic strength, and complexes with metal ions (Ca²⁺ and Mg²⁺).