Protein Changes in Food: Heat, Denaturation, and Functionality

Effects of Heat on Proteins in Food Processing

Functional Properties of Proteins in Foods

• Mass Hydration: Formation
• Solubility: Emulsification
• Viscosity: Foam formation
• Gelification: Flavoring capture
• Texture: Interaction with other food components

Denaturation of Proteins

Physics

Heating, cooling, mechanical treatment, hydrostatic pressure, radiation.

Chemistry

Acids, bases, metals, organic solvents.

Effects of Protein Denaturation

• Changes in solubility by exposure of hydrophilic or hydrophobic peptide units
• Changes in water absorption capacity
• Loss of biological activity
• Increased risk of chemical attack by exposure of other peptide bonds
• Changes in viscosity of solutions
• Decreased ability of crystallization

Color Changes

Red purple myoglobin converts to metmyoglobin (brown). Chlorophyll (bright green) converts to pheophytin (green/brown).

Functions of Oils and Fats in Foods

• Lubricant
• Heat transfer medium
• Absorbed into fried food
• Enhance flavor and texture of food
• Improve the appearance of food
• Form the crust by chemical interaction with food (e.g., crust and tenderness in bread)
• Low vapor pressure prevents volatility
• Improve conservation (fresh bread)

Effects of Heat on Meat Proteins

Functional Properties of Meat Proteins

• Solubility
• Water retention capacity
• Liquid emulsifying capacity
• Coagulability
• Ability to link
• Color
• Flavor absorption

Water Holding Capacity

This is defined as the ability of meat to retain its water when external forces are applied: cutting, pressure, grinding, heat treatment.

Many of the physical properties of the flesh (color, texture, firmness, juiciness) depend largely on the water holding capacity.

Much of the water in the muscle is firmly attached to proteins.

During the conversion of muscle into meat, a decrease in pH and subsequent denaturation of proteins decrease the water retention capacity.

The heat treatment denatures the protein and loses water.

Emulsifying Capacity

If the amount of protein is low in relation to fat, all fat globules are not covered, breaking the emulsion by applying heat.

In emulsions poor in flesh and excess fat and collagen (pig skin), fat is removed during heating.

Coagulability

Meat proteins are partially denatured (coagulate) when meat is acidic (low pH). Meat proteins are partially denatured when alkaline (high pH). Meat proteins are partially denatured by heat. Meat proteins are partially denatured by the effect of low temperatures (freezing).

Clotting can maintain a stable emulsion in a finished product (sausages, mortadella, salami, etc.).

Ability to Link

The contractile proteins and other solubilized components, going from sol to gel (thick substance), help unite the pieces of meat during heating or change in pH, which strongly bind to clot components, obtaining a homogeneous, compact product.

Color

The color of meat is given by the pigment myoglobin. It acquires a rust color to brown (metmyoglobin), as Fe++ changes its valence to Fe+++, commercially undesirable.

The nitrosomyoglobin, formed by the addition of curing salts in cured meats, becomes nitrosomyochromogen with heat, a more stable pigment (e.g., the pink of ham).

Digestibility of Protein

The protein quality of food depends on its digestibility and its ability to provide all essential amino acids needed to meet human requirements.

Solubility

The muscle contractile proteins (actin, myosin, troponin, and others) are released from the muscle fiber when subjected to salt (ionic strength) and breaking the membrane during mechanical pitting (cutter). By adding salt, brine, and/or polyphosphates, proteins are solubilized, from sol to gel state, forming a sticky substance that flows.