Fruits Storage

Fruit storage refrigeration systems must account for the complex interplay of temperature, humidity, air movement, and atmospheric composition to maintain product quality while minimizing physiological disorders and decay. Unlike processed commodities, fruits remain living tissues with ongoing metabolic processes that generate heat, consume oxygen, produce carbon dioxide, and release ethylene gas.

Precooling Methods

Precooling removes field heat rapidly after harvest, reducing respiratory activity and extending storage life. The method selection depends on fruit type, packaging, and facility constraints.

Room Cooling: Simplest method placing product in refrigerated space. Cooling rate follows Newton’s law of cooling with seven-tenths cooling time typically 8-24 hours depending on fruit size and airflow. Suitable for apples, pears, and citrus in bulk bins. Heat removal rate limited by poor heat transfer coefficient (5-15 W/m²·K).

Forced-Air Cooling: Directs refrigerated air through stacked containers using pressure differential. Achieves seven-tenths cooling time of 2-8 hours. Pressure drop across product load typically 25-75 Pa. Requires 1-2 cfm per pound of product. Critical for stone fruits, berries, and grapes where rapid temperature reduction prevents deterioration.

Hydrocooling: Immersion or spray application of chilled water (0.5-2°C). Provides heat transfer coefficient of 500-1500 W/m²·K, achieving seven-tenths cooling in 10-30 minutes. Essential for peaches, nectarines, and cherries. Water chlorination (50-100 ppm free chlorine) prevents cross-contamination. Not suitable for fruits susceptible to water damage or those with natural bloom.

Vacuum Cooling: Limited application for fruits due to moisture loss concerns. Pressure reduction to 4-5 mmHg absolute. Primarily used for lettuce; occasionally for berries in specialized applications.

Storage Temperature Requirements

Optimal storage temperature represents a balance between reducing metabolic rate and avoiding chilling injury for sensitive species.

Temperature uniformity within ±0.5°C is critical. Spatial temperature variation causes condensation, uneven ripening, and accelerated deterioration in warm zones.

Chilling Injury Mechanisms

Chilling injury occurs when chilling-sensitive fruits are exposed to temperatures below their critical threshold but above freezing. Subtropical and tropical fruits are particularly susceptible.

Physiological Mechanisms: Membrane phase transition from liquid-crystalline to gel state disrupts cellular compartmentation. Temperature below 10-13°C for citrus, 7-10°C for stone fruits (cultivar dependent), and 12-15°C for bananas causes metabolic dysfunction. Symptom development may occur during storage or post-storage at higher temperatures.

Manifestations: Surface pitting, internal browning, failure to ripen normally, off-flavor development, and increased susceptibility to decay organisms. Grapefruit develops scald below 10°C. Peaches and nectarines exhibit woolly breakdown (mealiness) from pectin metabolism disruption.

Prevention Strategies: Intermittent warming cycles (1 day at 15-20°C per week of storage), conditioning treatments (gradual temperature reduction), and pre-storage heat treatments (38-42°C for 24-72 hours) can mitigate chilling injury in some commodities.

Respiration Rates and Heat Load

Respiration rate directly determines refrigeration load and storage life. Heat of respiration varies with temperature according to Q₁₀ relationship (rate doubles per 10°C increase).

Respiratory Heat Generation: Calculated as: q = m × R × h_fg

Where q = heat generation (W), m = mass (kg), R = respiration rate (mg CO₂/kg·h), h_fg = conversion factor (220 J/mg CO₂ equivalent).

High respiration rate correlates with short storage life. Strawberries and raspberries (climacteric peak 60-100 mg CO₂/kg·h at 20°C) require immediate cooling and have limited storage potential compared to apples (15-30 mg CO₂/kg·h at 20°C).

Ethylene Management

Ethylene (C₂H₄) is a plant hormone triggering ripening in climacteric fruits (apples, pears, stone fruits, bananas). Concentration as low as 0.1-1.0 ppm accelerates ripening and senescence.

Ethylene Production Rates: Vary by commodity and maturity stage. Apples produce 10-100 μL/kg·h during storage. Pears generate 5-50 μL/kg·h. Stone fruits at climacteric peak produce 50-200 μL/kg·h.

Control Methods:

• Ventilation with outside air (minimum 1-2 air changes per day) dilutes ethylene concentration
• Catalytic oxidation systems (heated platinum or palladium catalyst at 200-500°C) destroy ethylene at rates of 50-200 cfm per 1000 ft³ storage volume
• Potassium permanganate scrubbers provide passive adsorption and oxidation
• 1-methylcyclopropene (1-MCP) treatment blocks ethylene receptors, extending storage life 50-100% for apples and pears

Segregate high ethylene producers (apples, pears, stone fruits) from ethylene-sensitive commodities (kiwifruit, persimmons) to prevent premature ripening.

Controlled Atmosphere (CA) Storage

CA storage modifies oxygen and carbon dioxide concentrations beyond normal atmospheric levels (20.9% O₂, 0.03% CO₂) to reduce respiration rate and extend storage life.

Standard CA Conditions for Apples:

• Oxygen: 1-3%
• Carbon dioxide: 1-3%
• Temperature: 0-2°C
• Achieves 2-3× storage life extension versus refrigerated air storage

Low-Oxygen CA: Ultra-low oxygen (0.5-1.0% O₂) reduces respiration rate an additional 20-40%. Dynamic controlled atmosphere (DCA) adjusts O₂ based on chlorophyll fluorescence monitoring to approach anaerobic compensation point without inducing fermentation.

Equipment Requirements: Gas-tight room construction (leakage rate <2% room volume per day at 250 Pa test pressure), oxygen scrubbers (catalytic burners or pressure swing adsorption), carbon dioxide scrubbers (hydrated lime or molecular sieve), and continuous gas monitoring (paramagnetic O₂ analyzers ±0.1%, infrared CO₂ analyzers ±0.1%).

Establishment Protocol: Reduce oxygen concentration gradually over 7-14 days to prevent low-oxygen injury. Pull-down rate of 2-3% O₂ per day. CO₂ accumulation controlled to target level through scrubbing or venting.

Physiological Effects: Oxygen reduction below 8% inhibits ethylene synthesis by limiting 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase activity. Elevated CO₂ (5-10%) inhibits ethylene action and reduces respiration through competitive enzyme inhibition. Excessive CO₂ (>15%) causes internal browning and off-flavors.

Design Considerations

HVAC system design for fruit storage must address specific requirements beyond basic refrigeration:

Air Distribution: Velocity over product surfaces 50-100 fpm (0.25-0.5 m/s) provides adequate convective heat transfer without excessive moisture loss. Overhead or underfloor distribution with return air plenums. Avoid dead air zones where ethylene and CO₂ accumulate.

Humidity Control: Maintain 85-95% RH depending on commodity. Evaporator temperature difference (ETD) limited to 2-4°C to prevent excessive dehumidification. Humidity deficit exceeding 1.5-2.0 kPa causes weight loss >0.5% per week. Ultrasonic foggers or wetted media humidifiers supplement moisture.

Refrigeration Capacity: Size for field heat removal (50-70% of total load), respiration heat (10-20%), transmission load through walls (10-15%), and infiltration (5-10%). Include safety factor of 10-20% for warm product loading and door openings.

Defrost Strategy: Scheduled defrost cycles (3-4 times daily) prevent frost accumulation that reduces heat transfer and increases evaporator pressure drop. Electric, hot gas, or water defrost methods. Drain pan heaters prevent freeze-up.

Sections