LPT 321: Meat Science (1+1)

METHODS OF FREEZING

• Freezing undertaken in a cabinet freezer is essentially a slow process and is referred to as slow freezing, or in blast freezers in which case, it is quicker and the method is often referred to as quick-freezing.
• Quick freezing can also be achieved by Freezing by Liquid Immersion and Liquid Sprays and also by Cryogenic Freezing.
  Slow freezing generally takes about 72 hours while quick freezing is completed in 30 minutes.
• The delay involved in slow freezing usually results in greater drip loss, during thawing than that associated with quick- freezing.
• The reasons for this difference is summarised below:
• Slow freezing
• The temperature of the product being frozen remains near the initial freezing point for an extended time in case of slow freezing. This results in the formation of a continuous freezing boundary and freezing proceeds slowly from the exterior to the interior.
• Extracellular water freezes more rapidly than intracellular water due to its lesser solute concentration.
• Slow freezing favours the formation of pure ice crystals and concentration of solutes in unfrozen condition.
  Moreover, intracellular solutions may be deficient in nucleation sites (suspended microscopic particles) necessary for small ice crystals.These conditions favour gradual migration of water out of muscle fibres, resulting in collection of large extracellular pools at sites of ice crystal formation, and intra cellular concentration of solutes.
• Consequently, there is even further reduction in the freezing point of intracellular water. This process of depressing the freezing point by increasing concentration of solutes is called eutectic formation.
• The total contribution of this process to freezing damage is not fully understood, but it apparently causes chemical alterations, including losses in protein solubility and elasticity of thawed muscle tissue.
• Long periods of crystallisation exist in slow freezing, producing numerous large extracellular masses of ice crystals that are easily lost as drip during thawing.
• Slow freezing also might result in mechanical damage to muscles, due to volume changes, associated with formation of large ice crystals and concomitant shrinkage of muscle fibres that have lost water to extracellular pools.Such muscle tissue has a distorted structure in frozen form that completely obliterates normal striations.
• Quick freezing
• The temperature of the product being frozen falls below the initial freezing point rapidly in case of quick freezing.
• Numerous small ice crystals with filament like appearance are formed both intra- and extracellularly at approximately the same speed.
• Small ice crystals formed, have very little opportunity to grow in size.Hence, quick freezing causes spontaneous formation of numerous small ice crystals, resulting in a discontinuous freezing boundary and very little translocation of water.
• Most of the water inside the muscle fibre freezes intracellularly, so drip losses during thawing are considerably lower than in slow frozen meat.
• Muscle fibre shrinkage and distortion are minimised than in slow frozen meat.
• Volume changes are less and periods of crystallisation are shorter than in slow frozen meat and consequently, mechanical damage is correspondingly less.
• Filament like ice crystals entrap solutes and thus minimise the ion concentration effect.
• In addition, smaller and numerous ice crystals formed in quick freezing reflect more light from meat surfaces, resulting in lighter colour than in slow frozen meat.