Principles of Freezing at Low Temperature
*I. Introduction*
1. *Definition*: Freezing is the process of separating water from other food components and freezing.
2. *Purpose*: Preservation of food quality by slowing down chemical reactions and removing water from activity.
*II. Freezing Process*
1. *Stage 1*: Initial falling rate period (temperature falls rapidly to just below 0°C).
2. *Stage 2*: Thermal arrest period (temperature remains constant at about –1°C as bulk of water freezes).
3. *Stage 3*: Final falling rate period (temperature drops again, freezing remaining water).
*III. Critical Range and Thermal Arrest*
1. *Critical range*: –1.1°C to –5°C (freezing begins and 80% of water is frozen).
2. *Thermal arrest*: Ideal duration is 30 minutes.
*IV. Energy Requirements*
1. *Latent heat*: Energy required to change liquid to solid (80 calories/g for water).
2. *Specific heat*: Energy required to lower temperature of 1g of water by 1°C (1 calorie for liquid water, 0.5 calories for ice).
3. *Total energy*: 120.5 kcal required to freeze 1 kg fish from 25°C to –30°C.
*V. Importance of Quick Freezing*
1. *Prevents large ice crystals*: Slow freezing can cause large ice crystals to form, potentially damaging cell walls.
2. *Prevents protein denaturation*: Slow freezing can cause denaturation of protein, leading to inferior quality product.
3. *Prevents bacterial spoilage*: Certain bacteria are still active at temperatures around 0°C, causing spoilage.
*VI. Key Principles of Freezing*
1. *Principle of Latent Heat*: Energy required to change state from liquid to solid.
2. *Principle of Specific Heat*: Energy required to change temperature of a substance.
3. *Principle of Thermal Conductivity*: Rate of heat transfer through a substance.
*VII. Key Points*
1. *Freezing time*: Ideally, fish should be frozen to –30°C in 2 hours.
2. *Freezing completion*: Process is complete when equilibrium temperature reaches –18°C.
3. *Unfrozen water*: Even at –30°C, some water remains unfrozen in fish muscle.
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