Good Temperature Management Improves Fruit Quality
The key to the best quality tree fruit is to keep it cold. Start cooling as quickly after harvest as possible and recool after packing.
Delaying cooling after picking reduces storage life. For example, Red Delicious apples lose 4 days of storage life for every day they are kept at 58 °F, and lose 9 days for every day they are kept at 70 °F. Fruit stored in direct sunlight is especially prone to quality loss. In the orchard, accumulate fruit bins in the shade or cover with reflective material. Additionally, make frequent trips to the cold storage to minimize time between picking and the start of cooling.
Hydrocooling is a fast way to initially cool apples or pears. Bins of fruit can also be air cooled with a tunnel cooler or serpentine type airflow systems. Pictures and complete design details and use recommendations for these cooling systems can be found in the 65-page "Commercial Cooling of Fruits, Vegetables, and Flowers." Call 530-752-6941 or visit our web site at postharvest.ucdavis.edu for ordering details.
Cool after Packing
After packaging, fruit must be recooled to maintain its best quality. Transport vehicles, particularly trucks, are not very effective in cooling product in transport. If a product leaves the pack house warm, it will likely be warm at destination and will have poor quality. This can be serious problem in long marine shipments. For example, one study showed that apples stored for 6 months and then held for 2 weeks at 70 °F had a 40% firmness loss and 20% increase in scald compared with the same stored fruit held at 32 °F.
Forced air cooling is the most common method of cooling packed fruit. It works well with most existing packaging systems but does require that corrugated boxes have about 5% vent area and vents on side and end panels must align if boxes are cross-stacked.
Protect the fruit from temperature damage (high temperatures in the summer and freezing damage in the winter) by loading only trailers that are in good repair. Check to see that:
- Trailer has a front bulkhead.
- Ceiling chute is in good repair.
- Door seals and walls are in good repair.
- Floor is clean.
- Thermostat is calibrated.
- Trailer is odor free.
Air from the refrigeration unit must be able to flow around the product to prevent heat flow in or out of the load. Ensure the following steps are used when loading:
- Precool trailer before loading.
- Turn off refrigeration unit when loading.
- Load away from smooth sidewalls.
- Do not block the air chute or load up to the ceiling.
- Load away from the rear doors and brace the load.
- Keep iced products away from dry products but do not allow divider to block floor air flow.
- Check the thermostat setting.
Refrigerated containers supply conditioned air through the floor. The air will only reach the rear of the unit if it is completely covered with pallets or other solid material. Uncovered floors allow air to bypass the rear of the load, causing poor temperature management in that part of the vehicle. The following few points summarize proper use of containers:
- Precool the container before loading (turn on generator and refrigerator units).
- Turn off refrigeration unit while the doors are open for loading.
- Cover the entire floor with produce or other solid material.
- Block exposed pallet openings to prevent air from bypassing around product.
- Brace the load to prevent shifting.
- Do not load above the limit line.
- Select supply-air temperature control and set thermostat.
A new 36-page publication on container use, "Marine Container Transport of Chilled Perishable Products," is available through our web site at postharvest.ucdavis.edu.
University of California, Davis
3022 Banier Hall
Davis, CA 95616
16th Annual Postharvest Conference, Yakima, WA
March 14-15, 2000