Fish Chilling

Fish chilling immediately after harvest is critical for quality preservation, microbial control, and shelf life extension. Rapid temperature reduction from ambient or physiological temperature to near 0°C minimizes enzymatic activity and bacterial growth. The chilling method selection depends on vessel type, processing scale, product form, and target quality specifications.

Chilling Methods Comparison

Chilling Rate Calculations

The heat removal rate from fish during chilling follows transient heat transfer principles. The required cooling load accounts for sensible heat removal and the thermal mass of both product and chilling medium.

Total Heat Removal:

Q_total = m_fish × c_p × ΔT + m_ice × h_fusion

Where:

• Q_total = Total heat to be removed (kJ)
• m_fish = Mass of fish (kg)
• c_p = Specific heat of fish, typically 3.6-3.9 kJ/(kg·°C) for lean fish, 3.2-3.5 kJ/(kg·°C) for fatty fish
• ΔT = Temperature reduction (°C), typically 15-25°C depending on harvest conditions
• m_ice = Mass of ice required (kg)
• h_fusion = Latent heat of fusion for ice, 334 kJ/kg

Chilling Time Estimation:

The chilling time can be estimated using a simplified lumped capacitance model for fish with high surface area to volume ratio:

t = -(ρ × V × c_p) / (h × A) × ln[(T - T_medium) / (T_initial - T_medium)]

Where:

• t = Chilling time (s)
• ρ = Fish density, typically 1040-1070 kg/m³
• V = Volume of individual fish (m³)
• h = Convective heat transfer coefficient (W/(m²·°C))
• A = Surface area (m²)
• T = Fish temperature at time t (°C)
• T_medium = Chilling medium temperature (°C)
• T_initial = Initial fish temperature (°C)

For ice slurry: h = 200-500 W/(m²·°C) For RSW with circulation: h = 100-300 W/(m²·°C) For static ice contact: h = 50-150 W/(m²·°C)

Ice Chilling Systems

Crushed and Flake Ice

Traditional ice chilling remains prevalent due to reliability and simplicity. Ice production on vessels or shore facilities uses plate or tube ice makers with capacities from 1 to 50 tonnes/day.

Ice Production Requirements:

Ice_production = (m_fish × c_p × ΔT) / h_fusion + melting_losses

Melting losses account for:

• Ambient heat gain: 10-20% in insulated holds
• Handling losses: 5-10% during transfer operations
• Contact surface warming: 15-25% initial contact melting

Total ice requirement typically ranges from 0.8:1 to 2:1 ice-to-fish ratio by weight, with higher ratios for warm ambient conditions or extended storage periods.

Ice Storage and Distribution:

Ice storage bins require insulation of R-20 to R-30 (SI: RSI-3.5 to RSI-5.3) to maintain ice quality. Distribution systems use screw conveyors, pneumatic transfer, or elevator systems with capacities matching processing rates, typically 5-20 tonnes/hr for medium-scale operations.

Slurry Ice Systems

Slurry ice consists of ice crystals (20-40% by weight) suspended in chilled seawater or brine, creating a pumpable mixture at -1.5 to 0°C. The high surface area of microscopic ice crystals (1-5 mm diameter) provides superior heat transfer compared to conventional ice.

Slurry Ice Generation:

Common generation methods include:

• Scraped surface heat exchangers with evaporator temperatures of -8 to -12°C
• Vacuum ice generation at reduced pressures (600-800 Pa absolute)
• Fluidized bed crystallizers with refrigerant temperatures of -10 to -15°C

Refrigeration capacity for slurry ice systems:

Q_ref = (m_slurry × ice_fraction × h_fusion) / (t_production × COP)

Where:

• Q_ref = Required refrigeration capacity (kW)
• m_slurry = Slurry production rate (kg/s)
• ice_fraction = Ice concentration, typically 0.20-0.40
• t_production = Production time (s)
• COP = Coefficient of performance, typically 2.5-3.5 for -10°C evaporator

Distribution and Application:

Slurry ice pumping requires specialized pumps (centrifugal or progressive cavity) with capacities of 10-100 m³/hr. Piping systems use oversized lines (velocity <1.5 m/s) with insulation thickness of 50-100 mm polyurethane foam to prevent ice crystal melting and agglomeration.

Refrigerated Seawater (RSW) Systems

RSW systems circulate chilled seawater (-1 to 1°C) through insulated holds containing bulk fish. The system provides rapid, uniform chilling for high-volume operations, particularly pelagic species like tuna, mackerel, and herring.

System Components

Heat Exchangers:

Shell-and-tube or plate heat exchangers remove heat from circulating seawater. Evaporator temperatures operate at -5 to -8°C to achieve seawater outlet temperatures near -1°C without freezing.

Heat exchanger duty:

Q_HX = m_sw × c_p,sw × (T_in - T_out)

Where:

• Q_HX = Heat exchanger capacity (kW)
• m_sw = Seawater flow rate (kg/s), typically 0.5-2.0 kg/s per tonne of fish
• c_p,sw = Specific heat of seawater, 3.93 kJ/(kg·°C)
• T_in = Return seawater temperature, 2-5°C
• T_out = Supply seawater temperature, -1 to 1°C

Circulation Pumps:

Centrifugal pumps with corrosion-resistant materials (bronze, stainless steel 316, or nickel-aluminum bronze) provide circulation rates of 1-4 hold volumes per hour. Flow distribution systems ensure uniform coverage across the entire fish mass.

Refrigeration Systems:

R-717 (ammonia) or R-404A systems with capacities from 50 kW to 1000 kW cool the seawater. Screw or reciprocating compressors operate with evaporator temperatures of -5 to -8°C and condensing temperatures of 30-40°C in seawater-cooled condensers.

Refrigeration capacity calculation:

Q_RSW = (m_fish × c_p,fish × ΔT_fish) / t_chill + Q_ambient + Q_seawater

Where:

• Q_RSW = Required refrigeration capacity (kW)
• t_chill = Target chilling time (s), typically 4-12 hours
• Q_ambient = Heat gain through insulation, typically 5-15% of fish load
• Q_seawater = Sensible heat of seawater, approximately 10-20% of fish load

RSW Operational Considerations

Temperature Control:

Seawater temperature maintenance at -1 to 0°C prevents ice formation while maximizing chilling rate. Temperature stratification in holds creates warm zones requiring adequate mixing (minimum 1.5 hold volumes/hr circulation).

Fish Quality Factors:

Extended RSW storage (>24-48 hours) can cause:

• Water absorption: 2-8% weight gain affecting product quality
• Salt penetration: Altered flavor profile in some species
• Protein degradation: Enhanced in fatty fish species
• Bacterial proliferation: If temperature exceeds 2°C

Energy Consumption:

RSW systems consume 0.8-1.5 kWh per kg of fish chilled, depending on initial temperature differential and system efficiency. Optimization through variable speed drives on pumps and compressors reduces energy consumption by 15-30%.

Temperature Requirements by Species

Equipment Selection Criteria

Vessel-Based Systems

Small Vessels (<20m):

• Ice capacity: 1-5 tonnes
• Hold volume: 5-20 m³
• Insulation: 100-150 mm polyurethane
• Power: 10-30 kW ice production
• Method: Crushed or flake ice with manual distribution

Medium Vessels (20-40m):

• Ice capacity: 5-20 tonnes or RSW 50-200 m³
• Hold volume: 20-100 m³
• Insulation: 150-200 mm polyurethane
• Power: 50-150 kW refrigeration
• Method: Flake ice automated or RSW for bulk species

Large Vessels (>40m):

• RSW capacity: 200-2000 m³
• Hold volume: 100-1000 m³
• Insulation: 200-250 mm polyurethane
• Power: 200-1000 kW refrigeration
• Method: RSW with automated monitoring

Shore-Based Processing

Processing plants select equipment based on throughput (tonnes/hr), product form, and quality requirements:

• High-volume bulk processing: RSW receiving tanks with 50-500 m³ capacity
• Value-added products: Plate chillers or slurry ice for controlled cooling
• Mixed operations: Combination of ice production (10-100 tonnes/day) and mechanical refrigeration

Insulated Storage:

Chilled fish storage rooms maintain 0-2°C with:

• Insulation: R-25 to R-35 (RSI-4.4 to RSI-6.2)
• Air changes: 0.5-2.0 per hour
• Humidity control: 90-95% RH to prevent surface drying
• Refrigeration load: 100-200 W/m² floor area

Monitoring and Control Systems

Temperature monitoring uses distributed sensors (RTDs or thermocouples) with accuracy of ±0.2°C positioned throughout the fish mass. Data logging systems record temperatures at 5-15 minute intervals for traceability and HACCP compliance.

Automated control systems regulate:

• Refrigeration capacity through compressor staging or VFD control
• Circulation pump flow rates maintaining target velocity
• Ice production rates matching consumption
• Alarm conditions for temperature excursions >1°C above setpoint

Critical control points include initial product temperature, chilling rate (°C/hr), final core temperature, and maximum time to target temperature, all documented for quality assurance protocols.

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