Orange Storage

Orange cold storage requires precise environmental control to maintain fruit quality, minimize physiological disorders, and extend marketable life. HVAC system design must account for variety-specific temperature sensitivities, humidity requirements for moisture retention, and optional degreening or controlled atmosphere protocols. Proper refrigeration strategies prevent chilling injury while controlling fungal decay and physiological breakdown.

Temperature Requirements by Variety

Orange storage temperatures vary by cultivar, with Florida and California fruit requiring different strategies based on climate adaptation and peel characteristics.

Valencia Oranges

Storage temperature: 3-9°C (37-48°F)

Valencia oranges tolerate temperatures at the lower end of the orange storage range. The optimal temperature depends on storage duration and initial fruit maturity:

• Short-term storage (2-4 weeks): 3-5°C (37-41°F)
• Extended storage (8-12 weeks): 5-7°C (41-45°F)
• Maximum safe duration: 12 weeks at 5°C (41°F)

Temperature uniformity throughout the storage chamber must maintain ±0.5°C to prevent localized chilling injury. Air circulation rates of 30-60 air changes per hour ensure even cooling without causing dehydration at fruit surfaces.

Navel Oranges

Storage temperature: 5-9°C (41-48°F)

Navel oranges exhibit greater susceptibility to chilling injury than Valencia cultivars. Temperature management focuses on preventing pitting and rind disorders:

• California navels: 7-9°C (45-48°F) for 4-8 weeks
• Arizona navels: 5-7°C (41-45°F) for 6-10 weeks
• Early-season fruit: Higher temperatures (8-9°C) to prevent pitting
• Late-season fruit: Lower temperatures (5-6°C) acceptable with reduced chilling sensitivity

The commercial storage strategy often uses 7°C (45°F) as a compromise temperature providing decay control while minimizing chilling injury risk.

Blood Oranges

Storage temperature: 4-7°C (39-45°F)

Blood orange varieties (Moro, Tarocco, Sanguinello) require specific temperature management to preserve anthocyanin pigments:

• Optimal storage: 5-6°C (41-43°F)
• Color preservation: Temperatures below 6°C maintain red pigmentation
• Maximum duration: 6-8 weeks before color degradation
• Anthocyanin stability: Requires darkness in addition to temperature control

Blood oranges tolerate slightly lower temperatures than standard navels but remain susceptible to peel pitting at temperatures below 3°C.

Storage Temperature Parameters

Humidity Control Specifications

Relative humidity: 85-90%

Orange storage requires high humidity to prevent moisture loss and maintain peel turgidity. Insufficient humidity causes weight loss, peel shriveling, and reduced marketability.

Humidity Management Systems

Refrigeration systems for orange storage must incorporate humidity control:

Direct expansion systems:

• Evaporator coil TD: 2-3°C (4-5°F) maximum to minimize dehumidification
• Coil surface area: 150-200% of standard refrigeration to reduce TD
• Hot gas defrost: Preferred method to minimize humidity fluctuation

Flooded evaporator systems:

• Reduced TD capability improves humidity maintenance
• Liquid overfeed ratio: 3:1 to 4:1 for larger wetted surface
• Gravity or pumped recirculation from accumulator vessel

Humidification equipment:

• Ultrasonic foggers: 10-20 micron droplet size prevents surface wetting
• Centrifugal atomizers: Install in supply air ductwork upstream of fans
• Evaporative media: Secondary humidity boost with minimal temperature depression
• Control range: ±3% RH around setpoint

Moisture Loss Prevention

Weight loss calculation:

WL = (Vi - Vf) / Vi × 100

Where:

• WL = weight loss percentage
• Vi = initial weight
• Vf = final weight

Acceptable weight loss: 3-5% maximum over storage period

Moisture loss rate depends on vapor pressure deficit (VPD) between fruit surface and surrounding air:

VPD = es(Tfruit) - ea

Where:

• es(Tfruit) = saturation vapor pressure at fruit surface temperature
• ea = actual vapor pressure of air (RH × es at air temperature)

Target VPD: <0.3 kPa for minimal moisture loss

Degreening Process Parameters

Ethylene degreening converts chlorophyll in orange peel to reveal underlying carotenoid pigments. This process improves visual appeal for fruit with mature internal quality but green external appearance.

Degreening Room Conditions

Temperature: 20-30°C (68-86°F)

• Optimal: 27-29°C (81-84°F) for Florida oranges
• Optimal: 20-24°C (68-75°F) for California oranges
• Higher temperatures accelerate degreening but increase decay risk

Relative humidity: 90-95%

• Maintains peel turgidity during treatment
• Reduces moisture loss and weight shrinkage
• Prevents ethylene-induced peel injury

Ethylene concentration: 1-10 ppm (parts per million)

• Standard application: 5 ppm continuous flow
• Citrus-specific: 1-5 ppm to minimize peel damage
• Concentration uniformity: ±0.5 ppm throughout chamber

Treatment duration: 24-72 hours

• Florida oranges: 24-48 hours typical
• California oranges: 48-96 hours due to thicker peel
• Monitoring: Visual inspection every 12-24 hours

Ventilation: 10-20 air changes per hour

• Ensures ethylene distribution uniformity
• Removes CO2 and metabolic heat
• Prevents accumulation of volatile compounds

Degreening Room HVAC Design

Heating capacity:

• Design load: 150-250 W/m² floor area
• Includes metabolic heat from fruit respiration (20-40 W/tonne at 27°C)
• Infiltration load through door openings
• Outside air ventilation load

Cooling capacity:

• Required when ambient exceeds setpoint
• Oversized evaporators (TD = 3-4°C) maintain humidity
• Dehumidification minimized through coil design

Air distribution:

• Horizontal airflow across stacked fruit containers
• Velocity at fruit surface: 0.2-0.5 m/s
• Perforated duct distribution for uniformity
• Return air plenum below floor or at ceiling

Ethylene injection system:

• Flow-through method: Continuous injection with exhaust to atmosphere
• Recirculation method: Catalytic converter removes ethylene for air reuse
• Concentration monitoring: Electrochemical or infrared sensors
• Safety: Exhaust through activated carbon scrubber if required by regulations

Degreening Parameters by Variety

Controlled Atmosphere Storage Options

Controlled atmosphere (CA) storage extends orange shelf life beyond conventional refrigerated storage through modification of oxygen and carbon dioxide concentrations.

CA Parameters for Oranges

Oxygen concentration: 5-10%

• Standard protocol: 5-7% O2
• Too low (<3%): Anaerobic respiration causes off-flavors
• Too high (>10%): Minimal benefit over air storage

Carbon dioxide concentration: 0-5%

• Moderate elevation: 3-5% CO2
• Fungistatic effect: Inhibits mold growth at elevated CO2
• Peel injury threshold: >10% CO2 causes rind damage

Temperature: 5-7°C (41-45°F) during CA storage

Storage life extension: CA storage extends marketable life by 25-50% compared to conventional cold storage at equivalent temperature.

CA Storage Facility Requirements

Gas-tight construction:

• Infiltration rate: <2% room volume per day
• Door seals: Inflatable gaskets on all access doors
• Pressure relief valves: Set at 250-500 Pa differential
• Structural design: Withstand ±500 Pa pressure differential

Atmosphere control equipment:

• Nitrogen generator: PSA or membrane type for O2 reduction
• CO2 scrubber: Activated carbon or hydrated lime for CO2 removal
• CO2 injection: Liquid CO2 vaporization system if elevation required
• Mixing fans: Continuous operation (10-15 air changes/hour)

Monitoring and safety:

• O2 analyzer: Paramagnetic or electrochemical, ±0.1% accuracy
• CO2 analyzer: Infrared, ±0.1% accuracy
• Data logging: Continuous recording at 15-minute intervals
• O2 deficiency alarm: Personnel safety for room entry

Decay Control Through Environmental Management

Fungal decay represents the primary cause of orange storage losses. Environmental control strategies reduce infection and slow disease progression.

Temperature Effects on Decay

Lower storage temperatures directly suppress fungal growth rates:

• Penicillium digitatum (green mold): Growth rate reduced 50% for each 5°C temperature decrease
• Penicillium italicum (blue mold): Minimum growth temperature ~5°C
• Geotrichum candidum (sour rot): Growth inhibited below 10°C

Decay control requires maintaining fruit temperature uniformly throughout storage. Warm spots in corners or near doors accelerate fungal growth.

Humidity and Surface Moisture Management

Free water elimination:

• Remove surface moisture before storage entry
• Air circulation without direct impingement on wet fruit
• Condensate drainage prevents dripping on fruit
• Evaporator defrost scheduling to prevent moisture release

Equilibrium relative humidity:

• 85-90% RH prevents excessive moisture loss

• 95% RH promotes surface condensation and mold germination

• Condensation on fruit surface creates infection sites for fungal spores

Air Exchange and Sanitation

Ventilation strategies:

• Outside air exchange: 1-2 air changes per day minimum
• Removes ethylene produced by fruit respiration
• Dilutes fungal spores and volatile decay metabolites
• Infiltration through door openings often provides adequate exchange

Ozone application:

• Concentration: 0.1-0.3 ppm continuous or 1-3 ppm intermittent (1 hour per day)
• Fungistatic effect reduces surface mold growth
• Material compatibility: Ozone degrades certain rubber compounds in gaskets
• Worker exposure: Ensure dissipation before room entry (0.1 ppm PEL)

UV-C irradiation:

• Wavelength: 254 nm germicidal effectiveness
• Surface treatment: Conveyors or in-room fixtures for air sterilization
• Dose: 1000-2000 µW·s/cm² for surface treatment
• Shielding: Prevents direct UV exposure to workers

Chilling Injury Symptoms and Thresholds

Chilling injury occurs when oranges experience temperatures below critical thresholds for extended periods. Symptoms develop during storage or after transfer to ambient conditions.

Pitting

Appearance: Sunken areas on peel surface, 1-5 mm diameter, brown discoloration

Mechanism: Membrane damage in peel oil glands causes localized cell collapse

Temperature threshold:

• Valencia oranges: <3°C for >4 weeks
• Navel oranges: <5°C for >2 weeks (early season), <4°C for >4 weeks (late season)

Prevention:

• Maintain temperature above variety-specific threshold
• Gradual temperature reduction (1°C per day) improves tolerance
• Conditioning treatment: 15-20°C for 3-7 days before storage

Rind Breakdown

Appearance: Large irregular brown areas on peel, tissue collapse, water-soaked appearance

Development time: 2-8 weeks at chilling temperatures, symptoms worsen after removal to ambient conditions

Contributing factors:

• Excessive moisture loss combined with chilling stress
• Fruit maturity: Overmature fruit more susceptible
• Storage duration: Extended exposure increases severity

Stem-End Rots

Causative organisms: Diplodia natalensis, Phomopsis citri

Temperature influence: Chilling stress weakens natural resistance, allowing latent infections to progress

Prevention strategies:

• Proper storage temperature prevents stress-related susceptibility
• Fungicide application (thiabendazole, imazalil) at postharvest
• Careful handling to prevent stem injuries

Storage Life by Condition

HVAC System Design Considerations

Orange storage facilities require refrigeration systems designed for narrow temperature control, high humidity maintenance, and optional CA capability.

Refrigeration load components:

• Transmission load through insulated envelope
• Product cooling load from field heat removal
• Respiration heat: 30-50 W/tonne at 5-7°C storage temperature
• Infiltration through door openings
• Internal heat gains (lights, fork trucks, workers)

Evaporator selection:

• Unit cooler TD: 2-3°C maximum for humidity control
• Fin spacing: 6-8 mm for reduced frosting in high humidity
• Defrost strategy: Hot gas or electric with cycle scheduling to minimize temperature fluctuation
• Air throw: Low-velocity horizontal discharge across fruit pallets

Control system requirements:

• Temperature control: ±0.5°C around setpoint
• Humidity monitoring and control capability
• Data logging for temperature, humidity, O2, CO2 (if CA)
• High/low temperature alarms
• Refrigeration system fault detection
• Door-open alarms and refrigeration delay

Structural envelope:

• Insulation R-value: RSI 5.3-7.0 (R-30 to R-40)
• Vapor retarder: Continuous on warm side of insulation
• Thermal bridging minimization through structure
• Pressure relief venting for CA rooms