Cold Requirement of Apples and Pears
Certain pears have a cold requirement that must be met during storage. For most pear cultivars, direct cooling and packing provide adequate cold, as exported fruit typically has a minimum journey of 4 weeks at –0.5°C. The riper the fruit is picked, the less cold it needs to ripen normally. When this cold requirement is met, fruit will ripen as expected.
- Summer pears (e.g., Bon Chretien): cold requirement as little as 3 weeks.
- Red D’Anjou pears: up to 8 weeks.
- Forelle pears: as much as 12 weeks.
Special arrangements in the South African industry manage Forelle’s requirement. Fruit is sampled and analyzed in various regions to identify harvest release dates. First shipping dates are set eight weeks after release; the remaining four weeks of cold requirement are achieved during shipping and marketing.
Methods of Cooling
1. Pre-cooling in Forced-Air Tunnels
- Fruit is cooled rapidly in tunnels, reducing cooling time from 5 days to 48 hours.
- Rapid heat removal minimizes moisture loss and slows respiration.
- Once the correct temperature is reached, fruit can be moved to a normal cold room.
2. Room Cooling
- Fruit is placed in a cold room and stacked in front of cooling fans so bins form a barrier to air flow.
- Air is forced through the barrier.
Cooling to –0.5°C should be accomplished in:
- 24–48 hours for Summer pears
- Up to 144 hours for apples
Longer cooling periods increase moisture loss and risk of wilted fruit. All Summer pears should be cooled within 48 hours.
Cold Room Preparation
- Cold rooms should be disinfected and switched on before harvesting to ensure walls and floors are cold before loading fruit.
- Disinfection: Use a chlorine solution (75–100 ppm) or non-corrosive disinfectants like quaternary ammonium compounds.
- Do not exceed the cooling capacity of the room.
- Keep doors closed or use plastic/automatic air curtains to prevent warm air entry.
- Once the recommended temperature is reached, reduce fan speed to minimize moisture loss.
Temperature, Relative Humidity (%RH) Control, and the Psychrometric Chart
- Fruit contains 85–95% water. After picking, it cannot replenish lost water.
- Water loss is driven by temperature and humidity differences.
- Cooling causes drying out: Keep cooling periods short and air speed low (except during initial cooling).
- Moisture loss is minimized by reducing the vapour pressure deficit (VPD), not just by increasing relative humidity.
Key Concepts
- Saturation Vapour Pressure (SVP): Maximum water vapour air can hold.
- Vapour Pressure Deficit (VPD): Difference between actual vapour pressure and SVP; this is the “water sucking power” of air.
- VPD increases exponentially with temperature.
Example: Moisture Loss Scenarios
| Scenario | VPD Calculation | VPD (mbar) |
|---|---|---|
| Fruit at 30°C, 30% RH, in 0°C, 90% RH room | 43 – 14 = 29 | 29 |
| Warm fruit at 30°C, 30% RH, in 0°C, 90% RH | 43 – 6 = 37 | 37 |
| Fruit cooled to 0°C, in 0°C, 90% RH | 6.5 – 6 = 0.5 | 0.5 |
Conclusion: More moisture is lost when VPD differences are large. Minimize periods of high VPD.
Cooling Best Practices
- Move fruit to cold rooms as quickly as possible to minimize VPD exposure.
- Forced air-cooling is more effective than room cooling, especially for high-risk cultivars.
- Moisture loss is cumulative and greatest on day one of cooling.
- When fruit and room temperatures equalize, VPD stays low and RH becomes more important for moisture loss.
Use the Psychrometric Chart to calculate relative moisture loss under different conditions.
Purpose of Cooling
- Significantly slows down respiration, moisture loss, ethylene production, and pathogenic decay.
Defects After Poor Cooling
- Over-ripe fruit (low firmness)
- Wilted fruit (especially pears)
- Yellowing (Bon Chretien, Royal Gala, Golden Delicious)
- Bitter pit (Golden Delicious, Braeburn)
- Reduced storage potential and shelf-life
- Mealiness (Red Delicious types)
- Greasiness (Granny Smith, Cripps’ Pink/Pink Lady®) linked to over-maturity