1. Correct Harvest Maturity for Long-Term Storage
Harvest maturity is the most important factor contributing to the development of diffuse browning. Do not store fruit harvested after optimal maturity for long periods.
Criteria for correct harvest maturity:
- Starch breakdown: Optimal 20%–30%, maximum 40%. Fruit can be released at 15% depending on other parameters.
- Firmness: Measured with an 11.1-mm tip, must be more than 7.8 kg (to ensure arrival in the UK at 6.8 kg).
- Total soluble solids: Preferably above 12.5%; must be 13.0% at reception.
- Titratable acids: 0.75%–0.55%.
2. Orchard Factors Affecting Storability
The history of progressive defects and ripening rate for each orchard provides an indication of the risk of post-harvest problems.
Factors increasing the likelihood of defects after long-term storage:
- Tree age: Preferably do not store fruit from young trees for more than 12 weeks (including shipping).
- Crop load: Smaller harvests can be more susceptible to post-harvest defects due to earlier ripening.
- Irrigation status: Both too little and too much irrigation can lead to faster ripening and poor storability.
- Soil type: Sandy soils tend to increase the rate of ripening.
- Mineral nutrition: Important for both the rate of ripening and storability.
- Topworked trees: May have a greater risk of internal browning due to higher stress levels.
- Stress: Fruit from stressed trees should be packed and sold within one week—maximum six weeks including all handling and shipping.
3. Ways to Reduce Variation
- Sort orchards according to harvest maturity three weeks before harvest.
- Take samples to assess the rate of ripening and storability before delivery to the pack house.
- Market fruit from orchards with more rapid ripening first, rather than attempting to store it under controlled-atmosphere (CA) conditions.
- Determine the maturity of fruit from both inside and outside the canopy:
- Fruit from the outside will be harvested first (meets block colour standard of 40%–60% for some markets earlier).
- Fruit from the inside is often left longer to develop better red colour, but may be more mature even at the first picking. Preferably do not store inner canopy fruit under CA or for longer than 12 weeks (including shipping).
- Avoid a protracted picking window and long intervals between harvesting fruit.
- The colour of fruit from the inner canopy can be improved by using reflective mulch. Leaves may also be removed after the first pick if the risk of sunburn is not too high.
- Fruit from the second pick can be stored in long-term CA depending on starch breakdown levels and the extent of the first pick.
- Fruit with starch breakdown >50% should not be stored in CA for longer than 12 weeks. Such fruit may be stored in regular atmosphere with or without 1-MCP or in short-term CA.
4. Handling After Harvest and During Storage
- Do not leave bins of fruit in the sun. Accumulate them in the shade before transporting to the pack house as soon as possible (same day as harvested).
- Load fruit into the cold room as soon as possible—within six hours after delivery to the pack house. Do not leave fruit outside the pack house in the shade or overnight to cool.
- Cold-room temperatures must not fluctuate above 3°C–4°C while warm fruit is added. Excessive fluctuation can impair cooling. Use curtains to separate warm and cold fruit inside the cold room. Ensure cooling capacity is sufficient for the volume of fruit—monitor this. Load two to three cold rooms simultaneously to achieve more even cooling.
- Pulp temperature must reach 4°C after 48 hours. Fruit should be cooled stepwise thereafter (see protocols below). Stepwise cooling yields better results than single-temperature storage at 1°C for radial and diffuse browning after long-term (>7 months) CA storage.
- Monitor CO₂ levels when warm fruit is closed in a cold room. CO₂ should never exceed 1%. Use additional lime or new-generation scrubbers to reduce CO₂ to 0.5%.
- Ethylene inhibitors (1-MCP): Benefit fruit by reducing the rate of ripening, greasiness, superficial scald, and internal browning. Treat fruit within three (max seven) days of harvest, especially if stored in regular atmosphere for up to three months, or CA for longer than three months. Use in combination with stepwise cooling, especially for longer storage.
5. Stepwise Cooling: Experimental Results
All protocols reduced the occurrence of internal browning, but stepwise cooling starting at 3°C (protocol 2) yielded the best results.
- 1-MCP (SmartFresh) was applied within seven days of harvest in all cases.
- CA conditions: O₂ levels of 1.5%, CO₂ levels of 0.5%.
Protocol 1:
- 30 days at 4°C
- 30 days at 3°C
- 30 days at 2°C
- Then maintain at 1°C
Protocol 2:
- 30 days at 3°C
- 30 days at 2°C
- Then maintain at 1°C
Protocol 3:
- 14 days at 2°C in a commercial CA store
- 14 days at 1°C in a commercial CA store
- Thereafter 0.5°C was maintained until fruit was moved to the Stellenbosch University experimental store in September
- After this, 1°C was maintained
6. Guidelines for Controlled-Atmosphere O₂ and CO₂ Levels
O₂ levels of 3% and CO₂ levels of less than 1% are achieved by displacement with N₂ during the first 48 hours after the room is sealed. Levels are thereafter maintained as set out below:
| %O₂ | %CO₂ | |
|---|---|---|
| Minimum | 1.0 | 0.0 |
| Optimum | 1.5 | 0.5 |
| Maximum | 2.0 | 1.0 |
7. Guidelines for Dynamic Controlled-Atmosphere (DCA) Storage
Van Amerongen ACR System (DCA-RQ)
This system relies on measuring the ratio of CO₂ production to O₂ consumption (respiratory quotient, RQ). Consult the service provider for specific guidelines. It is good practice to monitor ethanol levels in fruit in DCA storage.
| Period | %O₂ | %CO₂ |
|---|---|---|
| Room being filled (5–7 days) | – | Below 1%. Preferably 0.5%. |
| Room full, fruit at temperature (3 d) | – | Below 1%. Preferably 0.5%. |
| Reduce O₂ to 6% (2 days) | Max 6% | Below 1%. Preferably 0.5%. |
| Respiration reduces O₂ to 1.2% (7 d) | Max 1.2% | Below 1%. Preferably 0.5%. |
| Controlled atmosphere (14–28 days) | 1.2% | Below 1%. Preferably 0.5%. |
| ACR system in use (from 28 days) | Min 0.6% | Below 1%. Preferably 0.5%. |
Protocol supplied by Van Amerongen. For more information, visit van-amerongen.com or a-r-e.co.za.
Isolcell or Gas At Site DCA-CF System
These systems measure chlorophyll fluorescence (CF) as an indicator of O₂ demand. Consult the service provider for their specific guidelines. It is good practice to monitor ethanol levels in fruit in DCA storage.
| Period | %O₂ | %CO₂ |
|---|---|---|
| Room filled at 4°C (5–7 days) | – | Below 1%. Preferably 0.5%. |
| Room full, fruit temp 2.5°C (11–14 days) | – | Below 1%. Preferably 0.5%. |
| Reduce O₂ to 4% (2 days) | Max 4% | Below 1%. Preferably 0.5%. |
| O₂ consumed by respiration (3 days) | Max 1% | Below 1%. Preferably 0.5%. |
| O₂ at 1% (7 days) | 1% | Below 1%. Preferably 0.5%. |
| Remove O₂ by respiration/displacement (3 d) | Min 0.4% | Depends on O₂. <1%, pref. 0.5% |
| Monitor chlorophyll fluorescence (remaining) | Min 0.4% | Depends on O₂. <1%, pref. 0.5% |
Protocol supplied by Zanella for the 2019–2020 season in Northern Italy. For more information, contact Gas At Site at info@GasAtSite.com.
8. Specific Post-Harvest Defects: Risk Factors
Diffuse Browning
- Starch breakdown 15%–40%: Low risk during storage for three months. Risk increases with longer storage.
- Starch breakdown 40%–50%: Moderate risk during storage for three months. Risk increases with longer storage.
- Starch breakdown >50%: High risk during storage for three months.
Radial Browning
- Starch breakdown 15%–20%: Higher risk after long-term storage, especially if a single storage temperature of 1°C is applied.
- Linked to preharvest temperatures: More common in seasons with lower temperatures and fewer growing degree days during cell division (0–50 days after full bloom) and cell enlargement (50–100 days after full bloom). Take season into account when deciding about long-term storage.
Both Diffuse and Radial Browning
- Low storage temperatures (-0.5°C–1°C) are associated with both types of browning, even after only three months of storage.
- All stepwise-cooling protocols reduce both diffuse and radial browning after long-term (>7 months) CA storage, compared to storage at a single temperature of 1°C.
CO₂ Browning
- Cripps Pink is sensitive to CO₂ and will readily develop CO₂ browning when CO₂ levels rise above 1%.
- Unperforated bags can lead to CO₂ accumulation, especially with temperature fluctuations during shipping and distribution. Do not pack medium- to high-risk fruit in closed bags in boxes.
- Consider leaving the vents on shipping containers open at 15 m³ per hour.
Soft Scald
- The risk of soft scald is reduced by stepwise cooling.
- Cripps Pink should never be shipped at temperatures below 0°C.