Freezing curve:
Here's a simplified explanation of the three stages of the freezing curve:# Stage 1: Initial Falling Rate Period
1. *Temperature drops rapidly*: The temperature falls quickly until it reaches just below 0°C.
2. *Specific heat removal*: Heat is removed from the fish, cooling it down.
3. *Supercooling*: In some cases, the temperature may drop below the freezing point without ice forming.
4. *Temperature rise*: When ice starts forming, the temperature rises due to the release of heat.
# Stage 2: Thermal Arrest Period
1. *Temperature remains constant*: The temperature stays around -1°C as most of the water (75%) freezes into ice.
2. *Latent heat removal*: Heat is removed as the water changes to ice.
3. *Ice crystal formation*: Pure ice crystals form during this stage.
# Stage 3: Final Falling Rate Period
1. *Temperature drops again*: The temperature falls as the remaining water (25%) freezes.
2. *Sensible heat removal*: The last bit of heat is removed from the frozen fish.
3. *Final freezing*: The fish is now completely frozen.
These stages are important to understand to produce high-quality frozen fish products.
# Critical Range and Freezing Process
1. *Critical Range*: The temperature range between -1.1°C and -5°C, where freezing begins and about 80% of water is frozen.
2. *Thermal Arrest*: The period where the temperature remains constant, ideally lasting only 30 minutes.
3. *Complete Freezing*: Freezing is complete when the product temperature reaches -18°C at the thermal center.
# Freezing of Water in Fish
1. *Pure Ice Crystals*: Water in fish freezes out as pure ice crystals.
2. *Unfrozen Water*: Remaining unfrozen water contains higher concentrations of salts and other compounds.
3. *Depressed Freezing Point*: The freezing point of unfrozen water is depressed due to increasing concentration of salts and compounds.
# Importance of Quick Freezing
1. *Energy Extraction*: More than 50% of energy extraction occurs during Stage 2 (thermal arrest period).
2. *Critical Period*: The thermal arrest period is critical for producing good frozen products.
3. *Quick Passage*: Fish should pass through the thermal arrest period quickly to prevent quality loss.
In summary, the critical range and thermal arrest period are crucial for proper freezing of fish. Quick freezing helps preserve quality and prevents energy loss.
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