Packaging Frozen Foods

Introduction

Frozen food packaging serves as the primary barrier between the product and the storage environment, directly impacting refrigeration load calculations, moisture migration, and product quality preservation. Packaging material selection influences heat transfer rates, sublimation losses, and the effectiveness of the refrigeration system.

The packaging system must maintain structural integrity and barrier properties across temperature ranges from ambient (+70°F) through blast freezing (-40°F to -100°F) to long-term storage (-10°F to 0°F).

Moisture Vapor Barrier Requirements

Water Vapor Transmission Rate (WVTR)

WVTR defines the rate at which water vapor passes through a packaging material under specified conditions:

WVTR = (m × t) / (A × Δp)

Where:

• WVTR = water vapor transmission rate (g/m²·day)
• m = mass of water vapor transmitted (g)
• t = thickness of material (mil or μm)
• A = transmission area (m²)
• Δp = vapor pressure differential (Pa)

WVTR Requirements by Product Type

Temperature Effect on WVTR

Permeability increases with temperature following the Arrhenius relationship:

P = P₀ × e^(-Ea/RT)

Where:

• P = permeability at temperature T
• P₀ = pre-exponential factor
• Ea = activation energy (J/mol)
• R = universal gas constant (8.314 J/mol·K)
• T = absolute temperature (K)

WVTR typically doubles for every 18°F increase in temperature. Packaging must be tested at actual storage temperatures, not ambient conditions.

Oxygen Barrier Properties

Oxygen Transmission Rate (OTR)

Oxygen permeation causes oxidative rancidity in fats, color degradation, and vitamin loss:

OTR = (V × t) / (A × Δp × τ)

Where:

• OTR = oxygen transmission rate (cm³/m²·day·atm)
• V = volume of oxygen transmitted (cm³)
• t = material thickness (μm)
• A = area (m²)
• Δp = partial pressure differential (atm)
• τ = time (days)

Oxygen Barrier Classifications

Critical Oxygen Levels

Freezer Burn Prevention

Sublimation Mechanics

Freezer burn results from ice crystal sublimation at the product surface when vapor pressure gradients exceed packaging barrier capacity:

Sublimation Rate = k × A × (Pₛ - Pₐ) / d

Where:

• k = mass transfer coefficient (g/m²·Pa·s)
• A = exposed surface area (m²)
• Pₛ = vapor pressure at product surface (Pa)
• Pₐ = vapor pressure in storage environment (Pa)
• d = boundary layer thickness (m)

Vapor Pressure at Frozen Temperatures

Lower storage temperatures reduce the driving force for sublimation, but packaging must compensate for temperature fluctuations during storage.

Package Design for Freezer Burn Prevention

1. Minimize Headspace

• Reduce air volume between product and package
• Maximum recommended headspace: 5-10% of package volume
• Use vacuum or nitrogen flushing for irregular shapes

2. Eliminate Air Pockets

• Conform packaging tightly to product surface
• Use shrink films heated to 180-240°F
• Apply vacuum: -0.85 to -0.95 bar gauge

3. Multi-Layer Barrier Construction

• Outer layer: mechanical protection
• Middle layer: gas/moisture barrier (EVOH, PVDC)
• Inner layer: heat seal integrity (polyethylene)

Package Integrity at Low Temperatures

Material Brittleness and Glass Transition

Polymers exhibit glass transition temperature (Tg) below which they become brittle:

Impact Strength at Frozen Temperatures

Impact resistance decreases with temperature:

Impact Strength Ratio = IS(T) / IS(73°F)

Heat Seal Integrity

Heat seal strength must be verified at storage temperature:

Minimum Seal Strength Requirements:

• Lightweight packages (<2 lb): ≥800 g/in width
• Medium packages (2-10 lb): ≥1200 g/in width
• Heavy packages (>10 lb): ≥1800 g/in width

Seal failure modes at low temperature:

• Peeling: inadequate seal initiation temperature
• Delamination: incompatible sealant layers
• Fracture: excessive brittleness of sealant polymer

Thermal Properties of Packaging Materials

Thermal Conductivity

Packaging thermal resistance affects freezing time and refrigeration load:

Impact on Freezing Time

The Plank equation modified for packaging:

t = (ρL/ΔT) × [(Pa/h) + (Ra²/4k)]

Where:

• t = freezing time (hr)
• ρ = product density (lb/ft³)
• L = latent heat of fusion (Btu/lb)
• ΔT = temperature difference (°F)
• P = package thermal resistance (hr·ft²·°F/Btu)
• a = minimum product dimension (in)
• h = surface heat transfer coefficient (Btu/hr·ft²·°F)
• R = shape factor (1 for infinite slab, 2 for infinite cylinder, 3 for sphere)
• k = product thermal conductivity (Btu/hr·ft·°F)

For typical polyethylene film packaging, thermal resistance is negligible. However, corrugated outer cartons can increase freezing time by 15-30%.

Thermal Expansion Coefficient

Packaging must accommodate differential thermal expansion:

Dimensional change from 70°F to -20°F:

ΔL = L₀ × α × ΔT

For a 12-inch LDPE package: ΔL = 12 × (15×10⁻⁵) × (90°F) = 0.016 in (negligible)

Material Selection Criteria

Single-Layer Films

Polyethylene (PE)

• Types: LDPE, LLDPE, HDPE
• WVTR: 0.4-1.2 g/m²·day (2 mil)
• OTR: 5000-8000 cm³/m²·day
• Advantages: excellent low-temp flexibility, good moisture barrier, heat sealable
• Limitations: poor oxygen barrier, requires thick films for adequate protection
• Applications: overwraps, produce bags, secondary packaging

Polypropylene (PP)

• WVTR: 0.3-0.5 g/m²·day (2 mil)
• OTR: 2500-3500 cm³/m²·day
• Advantages: higher tensile strength, better clarity than PE
• Limitations: brittle below 0°F, poor low-temperature impact resistance
• Applications: lidding films, thermoformed trays (used above 0°F)

Polyester (PET)

• WVTR: 1.5-2.0 g/m²·day (0.5 mil)
• OTR: 50-100 cm³/m²·day
• Advantages: excellent tensile strength, dimensional stability, clarity
• Limitations: not heat sealable (requires coating), expensive
• Applications: barrier layer in laminates, ovenable trays

Multi-Layer Coextruded Films

Typical 5-layer structure for high-barrier frozen food packaging:

Outer Layer (20%): LDPE or LLDPE

• Mechanical protection
• Heat seal capability
• Moisture barrier

Tie Layer (5%): Adhesive polymer

• Bonds dissimilar polymers
• Maintains laminate integrity

Barrier Layer (10%): EVOH or PVDC

• Primary oxygen barrier
• Secondary moisture barrier

Tie Layer (5%): Adhesive polymer

Inner Layer (60%): LDPE or metallocene PE

• Heat seal layer
• Food contact surface
• Primary moisture barrier

Total film thickness: 2-4 mil (50-100 μm)

High-Barrier Materials

EVOH (Ethylene Vinyl Alcohol)

• OTR: 0.02-0.5 cm³/m²·day (excellent)
• WVTR: 8-15 g/m²·day (poor - must be protected)
• Advantages: outstanding oxygen barrier, transparent
• Limitations: moisture sensitive (loses barrier when wet), expensive
• Typical concentration: 5-15% of total film thickness
• Applications: red meat, seafood, high-fat products

PVDC (Polyvinylidene Chloride)

• OTR: 0.5-2.0 cm³/m²·day (excellent)
• WVTR: 0.1-0.5 g/m²·day (excellent)
• Advantages: excellent combined moisture and oxygen barrier
• Limitations: processing difficulties, environmental concerns, HCl release at high temps
• Applications: processed meats, cheese, some frozen foods
• Coating thickness: 1-3 lb/ream

Metallized Films

• OTR: <0.1 cm³/m²·day (excellent)
• WVTR: <0.1 g/m²·day (excellent)
• Advantages: superior barrier, light barrier, cost-effective
• Limitations: not transparent, pin-hole sensitive, non-microwave compatible
• Metallization: aluminum vacuum deposition (200-500 Å)
• Applications: coffee, snacks, long-term storage frozen foods

Aluminum Foil

• OTR: 0 (absolute barrier)
• WVTR: 0 (absolute barrier)
• Advantages: complete barrier to gas, moisture, light, and odor
• Limitations: expensive, prone to flex cracking, not transparent
• Typical thickness: 0.35-1.0 mil
• Applications: premium frozen entrees, ice cream novelties

Rigid and Semi-Rigid Containers

Thermoformed Trays

• Materials: PP, PET, CPET, PS
• Wall thickness: 10-30 mil
• Advantages: product protection, microwave compatibility, shape retention
• Lidding: sealed with flexible film (OTR/WVTR determined by lidding)
• Applications: frozen entrees, seafood, prepared meals

Injection Molded Containers

• Materials: PP, HDPE
• Wall thickness: 20-60 mil
• Advantages: reusable, high impact resistance, stackability
• Limitations: poor oxygen barrier unless coated
• Applications: ice cream, bulk frozen vegetables

Coated Paperboard Cartons

• Base: SBS (solid bleached sulfate) or CUK (coated unbleached kraft)
• Coating: PE, wax, or polymer dispersion
• Thickness: 12-24 pt (0.012-0.024 in)
• WVTR: 5-20 g/m²·day (depends on coating)
• Advantages: printability, rigidity, consumer appeal
• Applications: frozen vegetables, prepared foods, ice cream

Vacuum Packaging

Vacuum Level Requirements

Residual oxygen content determines product stability:

Vacuum Packaging Equipment

Chamber Vacuum Machines

• Product placed inside chamber
• Chamber evacuated to set point
• Seal made under vacuum
• Air returns to chamber
• Achievable vacuum: 1-50 mbar
• Cycle time: 20-60 seconds
• Applications: high-value products, institutional packs

External (Nozzle) Vacuum Machines

• Nozzle inserted in package opening
• Air evacuated from package
• Seal made after nozzle withdrawal
• Achievable vacuum: 50-200 mbar
• Cycle time: 3-10 seconds
• Applications: high-speed production, consumer packages

Vacuum Packaging Film Requirements

Puncture Resistance

• Critical for bone-in products
• Measured by ASTM D1709 (dart drop test)
• Minimum: 200 g for boneless, 400 g for bone-in

Seal Through Contamination

• Meat juices, fats, and product particles can compromise seals
• Use textured or channeled seal bars
• Employ double or triple seals
• Minimum seal width: 3 mm for smooth products, 5 mm for contaminated

Shrink Properties

• Films heated post-packaging to conform to product
• Shrink temperature: 180-240°F (water bath or hot air tunnel)
• Free shrink: 40-60% in machine direction, 20-40% in transverse direction
• Shrink tension: must not compress delicate products

Modified Atmosphere Packaging (MAP)

Gas Composition for Frozen Foods

While freezing arrests microbial growth, MAP provides additional protection during temperature abuse and distribution:

Gas Flush Equipment

Continuous Flow Systems

• Gas introduced as product passes through
• Package sealed immediately
• Gas usage: 1.5-3× package volume
• Line speed: 30-120 packages/minute
• Residual O₂: 2-5%

Compensated Vacuum Systems

• Package evacuated (50-200 mbar)
• Gas backfilled to atmospheric pressure
• Cycle repeated 2-3 times
• Gas usage: 4-8× package volume
• Residual O₂: <1%
• Cycle time: 30-90 seconds

Gas Permeation and Package Life

Package life before O₂ reaches critical level:

t = (Vc × [O₂]crit) / (OTR × A × [O₂]atm)

Where:

• t = package life (days)
• Vc = package free volume (cm³)
• [O₂]crit = critical oxygen concentration (%)
• OTR = oxygen transmission rate (cm³/m²·day)
• A = package surface area (m²)
• [O₂]atm = atmospheric oxygen (21%)

Example: 1000 cm³ package, OTR = 1 cm³/m²·day, A = 0.05 m², [O₂]crit = 2%

t = (1000 × 0.02) / (1 × 0.05 × 0.21) = 1905 days (5.2 years)

This assumes constant storage at frozen temperatures. Temperature abuse accelerates permeation.

Secondary Packaging

Cartons and Overwraps

Functions:

1. Mechanical protection during handling
2. Additional moisture barrier
3. Marketing and information display
4. Unitization for palletization

Carton Design Considerations:

Material Basis Weight:

• Lightweight products (<1 lb): 18-24 pt SBS
• Medium weight (1-5 lb): 24-28 pt SBS
• Heavy products (>5 lb): 28-36 pt SBS or E-flute corrugated

Moisture Resistance:

• PE coating: 10-20 lb/ream per side
• Wax coating (legacy): 15-25 lb/ream
• Water vapor barrier: WVTR 2-8 g/m²·day

Thermal Properties:

• Carton thermal resistance: 0.02-0.04 hr·ft²·°F/Btu per 20 pt
• Limits heat transfer during freezing by 5-15%
• Provides insulation during temporary temperature excursions

Master Cartons (Corrugated)

Flute Selection:

Burst Strength Requirements:

Burst Strength (psi) = (Box Weight × Stack Height × 5.5) / Box Area

For 25 lb box, 12 ft stack height, 400 in² area: Burst Strength = (25 × 144 × 5.5) / 400 = 49.5 psi (use 275 lb/in² board)

Moisture Degradation:

• Corrugated loses 50% strength when saturated
• Wax or polymer coating required for frozen storage
• Coating adds 10-20% to cost but essential for freezer environments

Labeling and Traceability

Label Adhesive Selection

Frozen storage presents challenges for pressure-sensitive labels:

Application Requirements:

• Surface must be clean, dry, and above dew point
• Label applied at room temperature before freezing
• Minimum dwell time: 24 hours at 70°F before freezing
• For direct application to frozen products: freezer-grade adhesive mandatory

Required Label Information

Regulatory (US):

• Product name and identity
• Net weight or volume
• Ingredient list (descending order)
• Allergen declaration
• Manufacturer/distributor name and address
• Country of origin
• Safe handling instructions

Storage and Handling:

• “Keep Frozen” or “Store at 0°F or Below”
• “Do Not Refreeze” after thawing
• Use-by or best-by date
• Preparation instructions
• Storage time after thawing

Traceability:

• Lot code or batch number
• Production date (Julian or calendar)
• Plant code
• Time stamp (for critical products)
• 2D barcode or QR code for supply chain tracking

HVAC and Refrigeration System Implications

Moisture Load from Packaging

Inadequate packaging increases refrigeration load through sublimation:

Sublimation Load Calculation:

Q_sublimation = ṁ_vapor × h_sublimation

Where:

• Q_sublimation = heat load (Btu/hr)
• ṁ_vapor = mass flow rate of sublimed vapor (lb/hr)
• h_sublimation = latent heat of sublimation (1220 Btu/lb at 0°F)

For 10,000 lb of product with poor packaging losing 0.5% weight/month:

ṁ_vapor = (10,000 × 0.005) / (30 × 24) = 0.0694 lb/hr

Q_sublimation = 0.0694 × 1220 = 84.7 Btu/hr (0.007 tons refrigeration)

While small per package, this accumulates across large cold storage facilities. A 1 million ft³ freezer with 20 million lb capacity experiencing 0.5% monthly weight loss requires an additional 169 tons of refrigeration capacity.

Frost Buildup on Evaporators

Sublimated moisture from poorly packaged products deposits on evaporator coils:

Frost Accumulation Rate:

ṁ_frost = (ṁ_vapor × A_storage) / V_storage

For facility with high product turnover and poor packaging:

• Vapor generation: 0.05 lb/hr per 1000 ft³
• 100,000 ft³ facility: 5 lb/hr frost accumulation
• Monthly frost: 3,600 lb (assumes no defrost)

Defrost Cycle Impact:

• Increased defrost frequency: 6 to 8 cycles/day
• Extended defrost duration: 20 to 30 minutes per cycle
• Energy penalty: 50-100% increase in defrost energy
• Product temperature rise during defrost: additional 2-4°F

Packaging Thermal Mass Effect

Packaging contributes to the thermal mass requiring cooling during freezing:

Q_packaging = m_pkg × c_p,pkg × ΔT

For 2 lb product in 0.1 lb polyethylene packaging:

Q_packaging = 0.1 × 0.55 × (70 - (-10)) = 4.4 Btu

This represents 2-5% additional load compared to the product itself. For high-throughput blast freezers processing 10,000 lb/hr:

Additional load = 0.03 × 10,000 lb/hr × (ΔH_product + ΔH_packaging)

This amounts to 3-10 additional tons of refrigeration capacity depending on product and packaging type.

Package Condensation During Distribution

When frozen products move through loading docks and distribution:

Condensation Onset:

Condensation forms when package surface temperature is below dew point of ambient air:

T_surface < T_dew = T_ambient - ((100 - RH) / 5)

At 80°F and 60% RH: T_dew = 80 - ((100-60)/5) = 72°F

Frozen package at 0°F immediately experiences condensation upon exposure to ambient air. Moisture accumulates on outer cartons, degrading structure and potentially causing label failure.

Solutions:

1. Minimize exposure time: <5 minutes in transition zones
2. Use vapor-tight shrink wrap for pallet loads
3. Maintain loading docks at 45-55°F and <60% RH
4. Pre-cool shipping containers before loading

Storage Environment Interaction

Package permeability affects equilibrium moisture content in storage:

For permeable packaging in -10°F storage at 90% RH:

Equilibrium Relative Humidity Inside Package:

RH_internal = RH_external × (P_sat,storage / P_sat,product)

If product surface is at -8°F and storage air is -10°F:

RH_internal = 0.90 × (80 Pa / 88 Pa) = 0.82 (82% RH inside package)

This creates a moisture gradient driving sublimation from product to package to storage environment.

Quality Preservation Through Packaging

Oxidative Stability

The relationship between oxygen exposure and quality degradation:

Q_remaining = Q_initial × e^(-k×[O₂]×t)

Where:

• Q_remaining = remaining quality parameter (color, flavor, vitamin content)
• k = reaction rate constant (varies by product and temperature)
• [O₂] = oxygen concentration (%)
• t = time (days)

For frozen ground beef at 0°F:

• k_color = 0.15 day⁻¹·%⁻¹ (color degradation)
• k_flavor = 0.08 day⁻¹·%⁻¹ (rancidity development)

With 5% residual oxygen after 6 months: Color retention = e^(-0.15×5×180) = 0.08 (8% remaining, severely degraded)

With 0.5% residual oxygen (high-barrier packaging): Color retention = e^(-0.15×0.5×180) = 0.23 (23% remaining, acceptable)

Vitamin Retention

Oxygen-sensitive vitamins (A, C, E) degrade according to first-order kinetics:

High-barrier packaging (OTR <1 cm³/m²·day) preserves vitamins 3-5× longer than low-barrier alternatives.

Color Stability

Myoglobin oxidation in meat products:

Oxymyoglobin (bright red) → Metmyoglobin (brown)

Rate proportional to oxygen availability and temperature:

Rate = k₀ × [O₂] × e^(-Ea/RT)

At -10°F, color deterioration rate is 10-20× slower than at 32°F, but packaging oxygen barrier remains critical for storage beyond 6 months.

Testing and Quality Assurance

Package Integrity Testing

Burst Test (ASTM D3078):

• Measures seal strength and package resistance to internal pressure
• Air inflation to failure point
• Minimum: 15 psi for flexible packages, 30 psi for semi-rigid

Seal Peel Test (ASTM F88):

• Quantifies heat seal strength
• Seal strip width: 1 inch, test speed: 10 in/min
• Minimum values listed earlier in heat seal section

Dye Penetration Test:

• Package filled with colored solution (methylene blue)
• Observed for leakage over 24 hours
• Pass/fail: no visible dye outside package

Vacuum Leak Detection:

• Package submerged in water under vacuum
• Air bubbles indicate leaks
• Sensitivity: detects holes >10 μm

Permeation Testing Standards

ASTM F1249: Water vapor transmission rate

• Modulated infrared sensor method
• Temperature controlled: 73°F, 100°F
• Humidity controlled: 0% to 100% RH
• Accuracy: ±2% of reading

ASTM D3985: Oxygen transmission rate

• Coulometric sensor method
• Temperature range: 50-104°F
• Relative humidity: 0-100%
• Detection limit: 0.001 cm³/m²·day

ASTM F1307: Oxygen transmission rate (alternative)

• Gas chromatography method
• Higher throughput than D3985
• Detection limit: 0.01 cm³/m²·day

Accelerated Shelf Life Testing

Temperature abuse simulation:

Q10 Method: For every 10°C (18°F) increase, reaction rates double:

ASF = Q₁₀^((T_test - T_storage)/10°C)

Where ASF = acceleration factor

Testing at 10°F instead of -10°F: ASF = 2^((10-(-10))/18) = 2^1.11 = 2.16

6 months at 10°F simulates 13 months at -10°F

Caution: Physical changes (recrystallization, moisture migration) may not follow Q10 relationship. Validate accelerated testing against real-time studies.

Economic Considerations

Cost-Benefit Analysis

Packaging cost vs. product loss trade-off:

Break-even calculation:

For 2 lb package of $5/lb product:

• Basic to mid-barrier upgrade: $0.07 additional cost saves $0.18 in product loss
• Payback: immediate
• Mid to high-barrier upgrade: $0.13 additional cost saves $0.08 in product loss
• Payback: depends on storage duration and product value

High-barrier packaging justified for:

• High-value products (>$8/lb)
• Long storage periods (>12 months)
• High-fat content (prone to oxidation)
• Premium market positioning

Best Practices Summary

Packaging Selection Guidelines

1. Match barrier to product requirements:

   • High-fat products: OTR <5 cm³/m²·day, WVTR <0.5 g/m²·day
   • Lean proteins: OTR <50 cm³/m²·day, WVTR <1.0 g/m²·day
   • Vegetables: OTR <200 cm³/m²·day, WVTR <2.0 g/m²·day

2. Verify low-temperature performance:

   • Test impact strength at storage temperature
   • Confirm seal integrity after thermal cycling
   • Validate flexibility after 24 hours at -20°F

3. Minimize headspace:

   • Target <10% free volume
   • Use vacuum or gas flush for irregular products
   • Consider shrink films for conformability

4. Multi-hurdle approach:

   • Combine high-barrier packaging with low storage temperature
   • Use MAP or vacuum in addition to barrier films
   • Apply secondary packaging for mechanical protection

5. Validate under actual conditions:

   • Test at actual storage temperature, not ambient
   • Include freeze-thaw cycling in testing protocol
   • Monitor throughout intended shelf life

Design for HVAC System Efficiency

1. Reduce sublimation load:

   • Specify WVTR appropriate to storage duration
   • Calculate sublimation contribution to refrigeration load
   • Account for packaging permeability in system sizing

2. Minimize thermal mass:

   • Use thinnest packaging consistent with protection requirements
   • Avoid excessive secondary packaging
   • Consider packaging heat capacity in blast freezer sizing

3. Prevent condensation during handling:

   • Design packaging to shed condensation
   • Use vapor-tight pallet wrapping
   • Coordinate with facility temperature management

4. Support efficient storage layout:

   • Standardize package dimensions for optimal space utilization
   • Design for airflow around packages (avoid complete blockage)
   • Enable barcode scanning without package handling

The packaging system is integral to frozen food storage system design, directly impacting refrigeration load, product quality, and operational efficiency. Proper material selection based on quantitative barrier requirements and low-temperature performance ensures both product preservation and optimized HVAC system operation.