Post Harvest Handling and Storage

Summary

• Berries are highly perishable and high value crops with high respiration rates.
• Sensitive to bruising and other mechanical damage.
• Precooling with forced air cooling is important for getting to storage temperature (~32^oF) quickly.
• Need to maintain high humidity (>90% RH, but avoid liquid water from, e.g., condensation.)
• Postharvest is a hotel, not a hospital: Nothing improves with storage and distribution, especially not berries.
• Consider intentional air movement and exchange for ethylene management.

Introduction

This will focus on strawberries, blueberries, and raspberries. A key principle when considering postharvest operations is that berries are still alive even after they’ve been harvested. The main process of life is respiration; converting glucose to carbon dioxide and water vapor. This results in heat production as well. Meanwhile, berries are dying. Ethylene, a naturally occurring ripening hormone, is being produced and along with bruising, other physiological disorders and postharvest pathogens, will eventually lead to decay.

The storage of berries demands precooling to storage temperature as quickly as possible. There is a need for high humidity, but not liquid water or condensation. It may also be possible to achieve higher quality for longer periods by providing air exchange in storage to remove ethylene.

Packing

Postharvest handling of berries aims to minimize the number of times the fruit is handled. Fruit is generally field packed in pint or quart punnets or even clam shell containers when destined for retail markets.  Wholesale or processing harvest may be handled differently. Packing containers are ideally new, clean, sturdy and ventilated to allow for precooling and air exchange. Bruising is controlled by keeping the stacked level of fruit to a minimum (some say no more than 3 berries high). Flats and lugs can be used to safely move and stack multiple punnets for efficient handling and cooling.

Precooling

Forced air cooling (FAC) is the industry standard for rapid reduction of temperature following harvest.  The lower the temperature the lower the respiration rate.  For every 10^oF reduction in temperature, the respiration rate is generally reduced by 50%. The lower the respiration rate, the lower the rate of sugar conversion and longer the berry quality will be maintained. FAC draws cold room air over the product with an intentional flow path by using punnets and lugs or flats designed with ventilation holes for the purpose.  A tarp and blocking sheets are used to prevent bypass flow through pallet holes and other larger areas. The key mechanism by which FAC works is convective cooling; the rate of heat transfer from the berries to the air is increased due to air velocity. Air distribution is important which is the reason for drawing air through the stacks by suction instead of pushing air through by pressure. Larger enterprises may have need for a dedicated precooling room, although a shared cooler can be used for the purpose by employing a dedicated air plenum and fan. Designs of forced air coolers for 1-3 cartons, half pallets, or full pallets can be found at the following link: http://blog.uvm.edu/forced-air-cooling-on-the-farm/.

Control of humidity in the precooling environment is important.  Humidity is the amount of water vapor in air. If humidity is too low, berries will suffer from weight loss, shrivel and discoloration. If humidity is too high, condensation can occur on colder surfaces leading to liquid water and postharvest diseases.

Storage

Berries are ideally stored near 32^oF with high humidity >90% RH. They tend to not be sensitive to chilling injury or freezing injury and can tolerate even 30-31^oF air temperatures due to their high respiration rates and sugar content. Many northeast growers avoid berry storage by growing for immediate sale and picking directly for market. However, storage for multiple days is possible under the right conditions (see Table 1).

Ethylene is a naturally occurring ripening hormone that is an especially important consideration. Berries have relatively high respiration rates (heat producing) and also generally high ethylene production rates.  In berries, the production of ethylene has been linked with Botrytis cinerea. The easiest way to reduce ethylene is to provide air exchanges in the storage area.  This does present a challenge for maintaining high humidity in drier conditions.

The control of humidity is best handled by ensuring the refrigeration system is designed with a high evaporator temperature (relatively high refrigeration temperature).  The temperature of the refrigerant determines the absolute minimum temperature in the room which determines the amount of water removed from the air when it is cooled.