Citrus Fruit Cold Storage

Citrus fruits require precise environmental control during cold storage to maintain quality, prevent physiological disorders, and extend shelf life. Each citrus variety exhibits distinct temperature tolerances and storage characteristics that dictate refrigeration system design and operational parameters.

Temperature Requirements by Variety

Oranges store optimally at 0-9°C (32-48°F) depending on variety and maturity. Early-season varieties tolerate lower temperatures (3-5°C), while late-harvest fruit requires warmer storage (5-9°C) to avoid chilling injury. Valencia oranges store best at 3-6°C, while Navel oranges require 5-7°C to prevent peel pitting.

Grapefruit requires warmer storage conditions at 10-15°C (50-59°F) due to high chilling sensitivity. Storage below 10°C causes surface pitting, watery breakdown, and increased susceptibility to decay organisms. White grapefruit tolerates slightly lower temperatures (10-12°C) compared to red varieties (12-15°C).

Lemons maintain quality at 10-13°C (50-55°F) with strict upper limits. Temperatures above 13°C accelerate senescence and color loss, while storage below 10°C induces chilling injury manifested as peel browning and oil gland breakdown. Green lemons destined for extended storage require 13-14°C initially, then gradual reduction to 10°C.

Chilling Injury Prevention

Chilling injury represents the primary physiological disorder affecting citrus during cold storage. The injury threshold varies by species, cultivar, growing region, and harvest maturity. Symptoms include surface pitting, internal browning, watery breakdown, and increased decay susceptibility.

Critical temperature thresholds below which chilling injury occurs:

• Grapefruit: 10°C
• Lemons: 10°C
• Limes: 7°C
• Early oranges: 3°C
• Late oranges: 5°C
• Tangerines: 4°C

Temperature fluctuations accelerate chilling injury development. Maintain variation within ±0.5°C of setpoint throughout the storage chamber. Rapid cooling from field temperature to storage temperature increases injury risk. Implement precooling protocols that reduce fruit temperature at 2-3°C per hour rather than immediate exposure to final storage conditions.

Fruit harvested from coastal growing regions exhibits greater chilling tolerance compared to desert-grown citrus due to thicker peel structure and higher oil content. Adjust storage temperatures based on production region and pre-harvest climate conditions.

Degreening Process Integration

Degreening chambers require precise environmental control to convert chlorophyll to carotenoids in mature but green-skinned fruit. The process uses ethylene exposure at elevated temperatures before cold storage.

Degreening parameters:

• Temperature: 20-29°C (68-84°F)
• Relative humidity: 90-95%
• Ethylene concentration: 1-5 ppm
• Duration: 24-72 hours
• Ventilation rate: 1-2 air changes per hour

Higher temperatures (27-29°C) accelerate color development but increase decay risk. Lower temperatures (20-24°C) provide slower, more uniform color change with reduced fungal growth. Maintain humidity above 90% to prevent weight loss and peel desiccation during the process.

Following degreening, fruit requires immediate transfer to cold storage. The thermal load from warm degreening chambers demands refrigeration capacity 30-40% greater than standard cold storage systems. Design degreening room cooling systems with rapid pulldown capability to transition fruit from 29°C to storage temperature within 12 hours.

Wax Coating Application

Wax coatings reduce moisture loss, enhance appearance, and provide fungicide delivery for decay control. Application occurs after washing and before storage, creating additional HVAC considerations for the packing facility.

Coating application environment:

• Temperature: 18-24°C
• Relative humidity: 50-60%
• Air velocity: <0.5 m/s at fruit surface

The wax application area requires temperature control warmer than cold storage to ensure proper coating adhesion and curing. Low humidity during application prevents water interference with wax bonding to the fruit surface. Excessive air movement causes uneven coating distribution and surface defects.

Wax coatings reduce water vapor transmission by 50-70%, decreasing evaporative cooling load in cold storage rooms. Calculate latent heat removal requirements using actual coated fruit transpiration rates rather than bare fruit data. Typical coated citrus exhibits moisture loss rates of 0.1-0.3% per week compared to 0.5-1.0% for uncoated fruit.

Humidity Control Requirements

All citrus varieties require relative humidity between 85-90% during cold storage. Lower humidity accelerates weight loss, causes peel shrivel, and reduces market value. Higher humidity promotes fungal growth on fruit surfaces and packaging materials.

Humidity management strategies:

• Evaporator coil selection for minimal dehumidification
• Humidification systems for rooms with high air change rates
• Vapor barriers on floors and walls to reduce moisture migration
• Package design that maintains microenvironment humidity

Refrigeration system design significantly impacts humidity control. Large temperature differentials between evaporator coils and room air cause excessive dehumidification. Select evaporator coils with approach temperatures of 2-4°C rather than conventional 8-10°C differentials. This requires increased coil surface area but maintains target humidity levels without supplemental humidification.

Humidity measurement locations affect control accuracy. Position sensors in representative air streams away from direct airflow from evaporator coils or doorways. Calibrate humidity sensors quarterly using reference standards to prevent drift-related storage problems.

Storage Duration by Variety

Maximum storage life varies substantially among citrus types based on physiological activity, peel thickness, and decay susceptibility:

Storage duration assumes optimal temperature, humidity, and atmospheric conditions with minimal pre-storage damage and effective decay control measures. Fruit quality at harvest determines actual achievable storage life more than environmental control during storage.

Refrigeration Load Calculations

Calculate refrigeration loads accounting for citrus-specific heat generation and cooling requirements:

Respiration heat generation rates at optimal storage temperatures:

• Oranges: 8-15 mW/kg
• Grapefruit: 10-18 mW/kg
• Lemons: 12-20 mW/kg

Field heat removal represents the largest initial cooling load. Citrus harvested at 25-30°C ambient temperature requires removal of sensible heat before reaching storage temperature. Use specific heat capacity of 3.8 kJ/kg·K for citrus fruit in precooling calculations.

Product load = Mass × Specific heat × Temperature change × Safety factor

Design refrigeration systems for peak harvest loads when rooms receive maximum daily fruit intake. Include adequate reserve capacity (15-20%) for abnormal weather conditions that elevate field temperatures or delay harvest schedules.

Air Distribution Design

Proper air circulation prevents temperature stratification and maintains uniform conditions throughout the storage chamber. Citrus storage requires different air distribution approaches compared to other commodities due to package design and stacking patterns.

Target air velocity through citrus bins: 0.5-1.0 m/s

Lower velocities cause inadequate heat removal from bin centers. Higher velocities increase moisture loss from exposed fruit surfaces despite high room humidity. Design duct systems and fan speeds to maintain velocity within this range at all points in the storage chamber.

Vertical air distribution systems work effectively for citrus storage when bins stack 4-6 high. Horizontal airflow systems require careful design to prevent short-circuiting between supply and return points. Use computational fluid dynamics modeling or scale testing to verify air distribution patterns before construction.

Decay Control and Air Quality

Fungal decay during storage originates from infections occurring at harvest or during postharvest handling. HVAC system design influences decay development through temperature control, humidity management, and air quality maintenance.

Maintain storage rooms at precise temperature setpoints without fluctuations. Each 1°C increase above optimal storage temperature doubles fungal growth rates. Temperature variations during defrost cycles or door openings create conditions favorable for decay organism proliferation.

Carbon dioxide accumulation from fruit respiration reaches 0.3-0.5% in poorly ventilated storage rooms. While citrus tolerates moderate CO₂ levels, concentrations above 1% cause off-flavors and peel damage. Design ventilation systems providing 1-2 air changes per day using outside air or CO₂ scrubbing systems in controlled atmosphere applications.

Ethylene accumulation affects citrus quality during extended storage. Oranges and grapefruit exhibit low ethylene production (0.1-0.4 μL/kg·h) but sensitivity to external ethylene varies. Lemons show increased senescence and peel yellowing at ethylene concentrations above 1 ppm. Remove ethylene through ventilation or catalytic/permanganate scrubbing systems when storing citrus with high ethylene-producing commodities.