Peach and Nectarine Handling

Overview

Peach and nectarine handling requires precise environmental control to manage their high respiration rates, climacteric ethylene production, and susceptibility to physiological disorders. These stone fruits demand rapid cooling after harvest, controlled ripening protocols, and specific temperature-humidity regimes to prevent chilling injury while extending marketable storage life from 2 to 4 weeks depending on cultivar maturity.

Harvest Maturity Determination

Firmness Testing

Flesh firmness measured with penetrometer equipped with 8 mm diameter tip serves as primary maturity indicator:

Soluble Solids Content

Minimum soluble solids content (SSC) measured by refractometer indicates adequate sugar development:

• Fresh market peaches: 10% minimum Brix
• Fresh market nectarines: 11% minimum Brix
• Processing varieties: 8-9% minimum Brix
• Premium cultivars: 12-14% Brix at harvest

Background color change from green to yellow (peaches) or red blush development (nectarines) provides visual maturity confirmation.

Precooling Requirements

Cooling Rate Necessity

High respiration rates at harvest temperature (20-25°C) generate substantial metabolic heat requiring removal within 4-6 hours:

• Respiration rate at 20°C: 40-80 mg CO₂/kg·h
• Respiration rate at 0°C: 8-15 mg CO₂/kg·h
• Heat of respiration: 1,910 kJ/tonne per day at 20°C
• Target pulp temperature: 0-2°C for storage cultivars

Forced Air Cooling

Forced air cooling provides most effective precooling method for stone fruits in shipping containers:

System Design Parameters:

• Airflow rate: 1.5-2.5 L/s per kg fruit
• Air temperature: -0.5 to 0°C
• Cooling time to 7/8 cooling: 2-4 hours
• Static pressure: 125-250 Pa across pallet
• Air velocity through container: 1.5-2.5 m/s

Cooling Chamber Configuration:

Vertical or horizontal forced air tunnels with plastic curtains creating plenum separation. Exhaust fans pull air through fruit containers with baffled openings aligned to cooler supply air. Monitor pulp temperature with thermocouples in geometric center of pallet load.

Hydrocooling Limitations

Hydrocooling proves less suitable for peaches and nectarines due to:

• Increased decay susceptibility from extended wet periods
• Skin damage from water absorption
• Removal of protective wax bloom
• Contamination risk from recycled water

Hydrocooling may be used for processing fruit when immediate cooling required, but fresh market fruit relies on forced air methods.

Room Cooling Inadequacy

Conventional room cooling requires 24-48 hours to achieve 7/8 cooling due to:

• Low air velocity (0.1-0.3 m/s) provides minimal convective heat transfer
• Boundary layer resistance dominates heat transfer
• Extended exposure to elevated temperatures accelerates ripening
• Increased water loss from prolonged cooling period

Storage Temperature Management

Optimal Storage Conditions

Temperature uniformity throughout storage space must remain within ±0.5°C to prevent localized chilling injury or accelerated ripening.

Chilling Injury Considerations

Temperature-Time Relationship:

Chilling injury (CI) develops as cumulative exposure below critical temperature:

• Critical temperature: 2-5°C (cultivar dependent)
• Symptom development: 7-21 days at 2°C
• Manifestation: Upon transfer to 20°C ripening conditions
• Severity increases: With extended storage duration

Chilling Injury Symptoms:

• Flesh mealiness (wooliness): Loss of juice upon mastication
• Flesh browning: Internal oxidative discoloration
• Leatheriness: Rubbery texture without flavor release
• Failure to ripen: Arrested softening at room temperature
• Surface pitting: Depression development on skin
• Red pigment bleeding: Anthocyanin diffusion into flesh

Cultivar Sensitivity:

Early season cultivars exhibit greater CI susceptibility than mid-season or late-season varieties. White-flesh peaches typically more sensitive than yellow-flesh cultivars. Nectarines generally show less CI than peaches at equivalent storage duration.

Internal Breakdown Prevention

Internal breakdown (IB) represents physiological disorder distinct from chilling injury:

Causative Factors:

• Extended storage beyond cultivar-specific limits
• Temperature fluctuations during storage
• Advanced maturity at harvest
• Physical impact during handling
• Inadequate calcium nutrition during fruit development

Symptoms:

• Central cavity browning radiating from pit
• Flesh translucency followed by browning
• Soft texture without mealiness
• Fermented off-flavor development
• No external symptoms until advanced stages

Prevention Strategies:

• Harvest at proper maturity stage
• Limit storage duration to 3 weeks maximum for susceptible cultivars
• Maintain strict temperature control (±0.5°C)
• Minimize physical impacts during packing
• Pre-storage calcium dip treatment (0.5-1.0% CaCl₂)

Ripening Room Design

Environmental Control Requirements

Ripening rooms facilitate controlled softening and color development after cold storage:

HVAC System Configuration

Heating and Cooling:

• Modulating hot gas reheat for precise temperature control
• Electric resistance or hot water coils as alternative
• DX cooling coil with electronic expansion valve
• Discharge air temperature control ±0.5°C setpoint

Humidity Control:

• Steam injection humidification to 85-90% RH
• Prevents moisture loss causing flesh dehydration
• Ultrasonic or compressed air atomization acceptable
• Water quality: TDS < 100 ppm to prevent mineral deposition

Air Distribution:

• Overhead supply diffusers with adjustable pattern
• Multiple zones for large rooms (> 200 m²)
• Return air plenums at floor level
• Circulation fans separate from refrigeration system

Ethylene Management

Peaches and nectarines produce high ethylene levels during ripening:

• Production rate: 10-100 μL/kg·h at 20°C
• Autocatalytic response: Ethylene stimulates further production
• Ripening acceleration: 100 ppm ethylene for uniform ripening
• Ventilation requirement: 20-40 air changes per hour removes excess ethylene

External ethylene application (100-150 ppm for 24 hours at 20°C) initiates uniform ripening for fruit from cold storage. Catalytic converters or ventilation prevents excessive accumulation.

Humidity Management

Moisture Loss Control

Weight loss exceeding 3-5% causes noticeable shrivel and loss of turgor. High humidity (90-95% RH) essential to maintain quality during storage and ripening.

Condensation Prevention

Surface condensation promotes decay organism growth:

• Maintain discharge air temperature 1-2°C above room air temperature
• Use hot gas reheat during defrost cycles
• Prevent cold air stratification near floor
• Eliminate cold surfaces (insulated walls, doors, ceilings)

Controlled Atmosphere Storage

Controlled atmosphere (CA) provides limited benefits for peaches and nectarines compared to other fruit:

Low oxygen atmospheres (< 1% O₂) induce fermentation and off-flavor development. Elevated CO₂ (> 5%) causes internal browning. CA storage not economically justified for most commercial operations.

Refrigeration Load Calculations

Heat Load Components

Respiration Heat:

Q_resp = m × R × CF

Where:

• m = fruit mass (kg)
• R = respiration rate at storage temperature (mg CO₂/kg·h)
• CF = conversion factor (13.6 kJ per g CO₂)

At 0°C with respiration rate 12 mg CO₂/kg·h: Q_resp = 1,000 kg × 0.012 g/kg·h × 13.6 kJ/g = 163 kJ/h = 45 W per tonne

Field Heat Removal:

Q_field = m × cp × ΔT / t_cool

Where:

• m = fruit mass (kg)
• cp = specific heat (3.6 kJ/kg·K for peaches)
• ΔT = temperature reduction (typically 25°C to 0°C)
• t_cool = cooling time (typically 3 hours)

For 10,000 kg cooled from 25°C to 0°C in 3 hours: Q_field = 10,000 × 3.6 × 25 / 3 = 300,000 kJ/h = 83.3 kW

Total Cooling Load:

Sum of field heat, respiration, infiltration, transmission, and equipment loads with appropriate safety factor (1.2-1.3) for design capacity.

Air Distribution Design

Storage Room Air Circulation

• Supply air: Overhead distribution along ceiling
• Return air: Floor-level grilles or ducted returns
• Air velocity at product: 0.5-1.0 m/s maximum
• Temperature stratification: < 1.0°C floor to ceiling
• Air changes: 40-60 per hour for occupied storage rooms

Forced Air Cooling Plenum

• Differential pressure: 125-250 Pa across fruit containers
• Fan location: Downstream (pull configuration preferred)
• Airflow uniformity: ±10% across tunnel cross-section
• Plenum air temperature: -0.5 to 0°C maintained

Packaging Considerations

Container ventilation area affects cooling efficiency and storage air circulation:

Align vent openings horizontally when stacking to create continuous air channels through pallet loads. Maintain minimum 10 cm clearance between pallet and wall surfaces.

Quality Monitoring

Temperature Monitoring:

• Wireless sensors in multiple pallet locations
• Data logging at 15-minute intervals
• Alarm thresholds: ±1.0°C from setpoint
• Pulp temperature verification: Daily during first week

Quality Assessments:

• Firmness measurements: Weekly during storage
• Soluble solids: At storage and removal
• Visual inspection: Decay, CI symptoms, IB
• Ripening trials: Every 2 weeks from stored lots

Postharvest Disorders Summary