Postharvest Cooling and Handling of Strawberries
Freshness Facts
Preferred cooling method: ... Forced air
Optimum temperature: .............. 32 F
Freezing temperature: ............. 31 F
Optimum humidity: ............ 90 to 95%
Storage life: .............. 5 to 7 days
 Why is Postharvest
Management Required?
In North Carolina most strawberries are grown for the pick-your-own market.
When strawberries are marketed in this way, growers need not be concerned
about handling and storage because these functions are performed by the
consumer. Recently, however, strong competition and a desire to expand
their operations have led many growers to consider other markets. Among
these are grocery stores, restaurants, and produce terminals. To penetrate
these markets, growers need the equipment and expertise to deliver a high-quality
product. The first step is to grow and harvest high-quality strawberries.
Equally important, however, is maintaining product quality after harvest
with the right cooling, handling, and storage methods.
Why is Postharvest
Cooling Important? Strawberries are extremely perishable and have unusually
demanding postharvest handling requirements. Even under ideal conditions
they can rarely be kept for more than 7 days after harvest. They must
be cooled immediately to their lowest safe temperature (32 to 34 F) to
prevent overripening and decay, and they must not be allowed to rewarm.
Even after picking, strawberries remain alive and produce heat as a natural
consequence of respiration. The amount of heat they produce depends on
the storage temperature. At 32 F a ton of strawberries will produce approximately
3,300 Btu per day, whereas at 80 F, a ton will produce 41,800 Btu! Strawberries
intended for storage must be free of bruises and other injuries, as bruised
berries are very susceptible to decay.
Very careful handling
is essential to maintaining quality. Remember that while proper postharvest
cooling and handling techniques can help maintain product quality, they
can never improve it. Berries without stem caps are particularly perishable
and must be eliminated from any fruit to be stored. The two most common
types of decay are gray mold, Botrytis cinerea, and rhizopus rot. Even
a small amount of infestation can quickly spread throughout an entire
package. Berries that have been cooled and then allowed to rewarm (causing
moisture to condense on them) are extremely susceptible to decay and must
be processed or consumed immediately.
Cooling and Handling
Methods Strawberries must be cooled immediately after harvest by forced-air
cooling to a temperature of 40 F or lower. Hydrocooling (flooding them
with chilled water) is not recommended because wet berries are much more
susceptible to decay. Cooling with crushed or "liquid" ice is
even worse because the berries are likely to sustain physical damage.
The most common carton for strawberries is an open-top, single-layer tray
containing 8 1-quart or 12 1-pint containers. Trays are stacked in layers
of six to form a pallet load of 60 or 84 trays, respectively, weighing
approximately 1,000 pounds. The cartons have enough open area to allow
for passage of cooling air. To prevent severe bruising and a reduction
in quality and appearance, care must be taken not to overfill the trays.
It is never sufficient to simply place the packaged strawberries inside
a cooling room and allow them to cool gradually. For palletized loads,
the cooling process would take much too long.
The fruits in the
center of the pallet would not be adequately cooled and would begin to
decay. Without forced movement of the cooling air, the heat from natural
respiration can destroy the fruit. Normally, air is forced through the
packages with a fan, which produces a difference in air pressure between
opposite sides of the load. Through proper placement of the load and the
use of baffles, cooling air is directed to flow through the containers
of fruit. The air should be pulled, never blown, through the packages.
The fans and cooling system must be sized to cool the entire load in 2
hours or less. For more information on this subject, refer to Agricultural
Extension Service publication AG-414-3, Maintaining the Quality of North
Carolina Fresh Produce: Forced-Air Cooling.
Humidity as well
as temperature must be controlled in storage facilities. If the air inside
the storage room is too dry, water will evaporate from the strawberries
and they will become soft and shriveled. At a storage room temperature
of 32 F, the relative humidity should be from 90 to 95 percent. Much of
the water that evaporates from the fruit condenses on the inside surfaces
of the room or is absorbed into packing materials. Under certain atmospheric
conditions, it may be necessary to add moisture with a humidification
system. Modified atmospheres containing as much as 15 percent carbon dioxide
(CO2), which retards respiration, can be obtained using plastic pallet
covers. However, this technique may not be applicable or cost-effective
on a small scale. Designing a Cooling Facility Satisfactory cooling facilities
can be built by the owner or by a contractor. In either case, make certain
that sound engineering principles are followed to ensure proper performance
and energy-efficient operation. For more information, see Agricultural
Extension Service publication AG-414-2, Design of Room Cooling Facilities:
Structural and Energy Requirements, or contact your county Extension agent.
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