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Green-Seeds.com
Fruits
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Postharvest Cooling and Handling of Blueberries
Freshness Facts
Preferred cooling method: ... Forced air Optimum temperature: ........
33 to 34 F Freezing temperature: ............. 28 F
Optimum humidity: .............90 to 95%
Storage life: ............. 2 to 3 weeks
 Note: Blueberries for
processing may be quick frozen and held for extended periods at a temperature
below 0 F. Blueberries are a popular early summer small fruit. Successful
blueberry marketing requires that the fruit not only be of very high quality
but that it also be properly packaged and thoroughly cooled. This publication
explains the postharvest handling procedures necessary to deliver North
Carolina blueberries of the highest quality. Most consumers purchase fresh
blue-berries on impulse and are prompted primarily by the perception of
quality. Wholesale buyers also associate appearance and firmness with fruit
quality and freshness. Successful blueberry marketing thus requires that
the fruit be of the highest quality and appearance. However, blueberries
are extremely perishable and easily damaged by rough handling and adverse
temperatures. Because blue-berries are often harvested and handled during
hot, humid weather, attention to proper postharvest handling is essential
to maintaining quality. Harvesting Blueberries of the highbush type (Vaccinium
corymbosum L.) and the rabbiteye type (Vaccinium ashei Read) are grown in
North Carolina. More than 90 percent of the blueberries produced commercially
are of the highbush type. Harvesting of highbush blueberries begins in late
May and often continues through late June. Rabbiteye varieties ripen in
late June, and harvesting continues into August. Blueberries, even those
growing on the same bush, do not all ripen at the same time. Under normal
growing conditions, blueberries ripen over a period of three to four weeks.
Ripe blueberries should have a completely uniform blue color. Fruit with
a red tinge are less mature and will not be as sweet as more mature berries
even though they ship well and will turn completely blue within several
days. Depending upon the crop and the weather conditions, blueberry fields
may be harvested as many as four times in intervals of five to seven days.
For rabbiteye varieties the interval between harvests may be as long as
nine days in cool dry weather. Harvesting more frequently may not make efficient
use of labor, whereas less frequent harvesting may result in a high percentage
of overripe fruit. In general, high temperatures during blueberry harvest
season promote ripening. Heavy rains during periods of high temperature
can further hasten ripening, cause splitting, and greatly reduce storage
quality. When picking labor is limited, prime fruit may easily become overripe.
Overripe blueberries are extremely easy to damage. Blueberries that don
t look too bad when picked may be totally unacceptable when they reach the
consumer. Be careful! Price, and particularly reputations, depend largely
on quality. Once damaged, your reputation and that of your marketing organization
may be very difficult to repair. Consumers long remember experiences with
a poor quality product. Whenever possible, avoid harvesting wet blueberries.
Wet berries are very susceptible to decay organisms and often resemble overripe
berries leaking juice. Waiting a few hours to begin harvesting after a rain
or heavy dew can significantly reduce the chance of infection by postharvest
disease organisms and can improve the appearance of the fruit. Manual Harvesting
Proper instruction and careful supervision of the picking crew is essential
to the success of any harvesting operation. Postharvest quality control
begins in the field. Packing house problems and buyer complaints are often
the result of poorly trained and supervised pickers. Have a responsible
person supervise the pickers at all times. If the fruit is harvested directly
into pint cups, it is a good idea to check one or two pints at random in
each flat. This check can be made easily by emptying the contents of a pint
cup into a shallow pan where each berry can be examined individually. Finding
more than 6 to 8 percent poor-quality fruit per pint indicates a need for
greater quality control. Ripe blueberries are easily removed from the bush.
Fruit should be removed with the thumb and forefinger, keeping the hand
cupped under the berry to avoid dropping it. The whitish, dusty appearance
(bloom) of the blueberry is highly desirable, and consumers regard it as
a sign of quality. Overhandling the berries will remove the bloom. In addition,
all workers should be reminded that they are handling a food product that
is eaten fresh and often not washed. Clean hands and sanitary personal habits
are required at all times. Harvested blueberries should never be allowed
to remain in the sun. The dark berries will readily absorb heat. If berries
are picked into buckets, the buckets should be no larger than 5 quarts and
of rigid construction to minimize mashing and the accumulation of respiration
heat. 
Rules for Blueberry
Pickers ------------------------------------------------------------------------
1. Keep your hands clean. Remember that you are handling a food product.
The law requires you to wash your hands after each visit to the rest station.
2. Pick all the ripe blueberries on the bush before moving on to the next
bush. 3. Harvest only well-ripened fruit. Leave immature fruit for the next
harvest. 4. Place your hand under the clusters to avoid dropping the berries.
5. Avoid overfilling your hands; do not squeeze or roll the fruit. 6. Do
not put trash or cull berries into the container. 7. Never allow harvested
fruit to remain in the sun. Mechanical Harvesting Recurring labor problems
have prompted some blueberry growers to investigate mechanical harvesting.
The major advantage is a significant reduction in management and labor costs.
Savings in labor costs of 30 to 45 percent have been reported. One mechanical
harvester can replace as many as 100 manual pickers. Over-the-row mechanical
blueberry harvesters have been available since the mid-sixties. These machines
have been primarily of the shake-and-catch design, which detach the blueberries
by shaking or vibrating the bush. The fruit drops onto a capturing device
and conveyor, on which it is moved past an air-blast cleaner and finally
into field lugs. Even if carefully adjusted and operated, mechanical harvesters
will harvest a significant amount of unacceptable fruit. These machines
have no means of detecting quality and will harvest immature, overripe,
diseased, and damaged fruit that would ordinarily be discarded by a human
picker. The elimination of this unacceptable fruit and foreign matter is
difficult and expensive, and it requires that the berries be handled excessively.
A mechanical harvester and the required sorting machines often subject the
fruit to excessive damage. Overripe fruit, in particular, is easily damaged
by these machines. Any additional handling adversely affects the shelf life
and quality of the blueberries. Although a significant percentage of the
blue-berries grown worldwide are harvested by machine, most North Carolina
growers strive to produce fruit for the higher-valued fresh market. Mechanically
harvest-ed blueberries are generally considered unfit for the fresh market
and are therefore usually frozen for processing.
Rules for Minimizing
Damage to Fruit by Mechanical Harvesting ------------------------------------------------------------------------
1. Harvest only dry fruit in the coolest part of the day.
2. Minimize the drop heights on the machine and pad the impact surfaces.
3. Eliminate as much vibration as possible.
4. Limit the depth of harvested fruit in the field containers to 4 or
5 inches.
5. Handle and dump field containers very gently. 6. Cool the fruit as
soon as possible after harvest. Sorting and Packing Sorting harvested
blueberries intended for the fresh market is time consuming, expensive,
and damaging to the fruit. Therefore, this practice is generally not recommended.
The best time to remove defective berries is during manual harvesting,
thus minimizing handling. For machine-harvested blueberries, there may
be no alternative but to remove trash and defects on a grading line. When
a grading line must be used, it should be operated in such a way as to
minimize mechanical damage. Whether blueberries are mechanically or manually
harvested, every effort should be made to keep the percentage of defective
fruit to an absolute minimum. Seriously defective fruit may include those
with insect damage, broken skin, decay or mold, attached stems, fruit
remaining in clusters, and fruit that is overripe, underripe, shriveled,
or off-color. U.S. Number 1 is the only specified grade for harvested
blueberries. Blueberries grading U.S. Number 1 may have no more than 1
percent serious defects and no more than 13 percent total defects. However,
in practice and especially with fresh-market blueberries buyers may require
less than 5 percent total defective fruit. Most buyers prefer large fruit
over small fruit if they are of good quality. The U.S. Number 1 grade
specifies that the count of berries per 1/2-pint container must not exceed
250. Within the U.S. Number 1 classification there are four size classifications,
as listed in Table 1. Table 1. USDA Standard Blueberry Size Classifications
- U.S. Number 1 ------------------------------------------------------------------------
Extra large ........... Fewer than 90 berries per 1/2-pint cup Large .....................
90 to 129 berries per 1/2-pint cup Medium ................... 130 to 189
berries per 1/2-pint cup Small ..................... 90 to 250 berries
per 1/2-pint cup ------------------------------------------------------------------------
For the complete USDA standard classifications for blueberries, refer
to 46 FR 63203 or the latest revision. Almost all North Carolina fresh-market
blueberries are marketed in cellophane-covered pint containers shipped
in 12-pint master containers known as flats. Flats may be conveniently
stacked, six to a layer, on a standard 40-by-48- inch shipping pallet
(Figure 1). A pallet of blueberries customarily consists of 96 flats (1,152
pints) with a gross weight of approximately 1,200 pounds. Figure 1. Packaged
and palletized blueberries being received at a central cooling and shipping
facility. Cooling At warm field temperatures, harvested blueberries are
extremely perishable and decline rapidly in quality. Visible signs of
decay organisms can develop on warm, wet blueberries in less than 12 hours.
The most common blueberry postharvest decay organisms in North Carolina
are Alternaria tenuissima (Alternaria rot) and Botrytis cinerea (gray
mold). In situations where blueberries are wet and the risk of decay is
high, a rinse in chlorinated water (75 to 125 ppm available chlorine)
has been successful in controlling these diseases. However, chlorination
is a preventive measure only. It will not salvage fruit already infected
because it cannot kill microorganisms that are already below the surface.
Moreover, chlorination cannot be used for fresh-market fruit because the
chlorine completely removes the bloom and bleaches the fruit. Decay organisms
are very active when the fruit is warm but become much less active at
lower temperatures. Warm, overripe blueberries, leaking juice from rough
handling, provide an ideal site for various postharvest decays. Shelf-
life experiments with packaged blueberries have demonstrated the dramatic
reductions in decay possible with rapid and thorough cooling. Figure 2
illustrates the effect of cooling in reducing decay. Figure 2. Percentage
of decay in packaged blueberries stored at various temperature. Harvested
blueberries should be cooled as soon as possible but always within 4 hours
of harvest, if possible. This may require more than one pickup or trip
to the cooling facility during a day of harvesting. However, the increase
in shelf life and overall quality will make it worthwhile. No fruit, particularly
well-ripened fruit, should ever be held all day without cooling. Blueberries
remain alive after harvest, respiring and producing heat. At a temperature
of 80 F, blueberries can produce as much as 22,000 Btu per ton per day
from the heat of respiration. Unless this heat is removed by cooling,
it can cause a rise in temperature of as much as 6 F. Cooling lowers the
respiration rate, slowing the ripening process and the accompanying inevitable
decline in quality. The respiration rate of blueberries at 80 F is nearly
20 times the rate at 40 F. In other words, blueberries held at 40 F have
nearly 20 times the shelf life of those held at 80 F. Softening is the
most visible physiological damage resulting from overripening. Most North
Carolina blueberries in wholesale channels receive some cooling. However,
a portion may continue to reach the consumer in poor condition as a result
of insufficient cooling. Often the effectiveness of cooling, not its availability,
is the deciding factor in quality. Cooling must be thoroughly and consistently
applied to be effective. The optimum temperature that will ensure the
longest shelf life is just above freezing: 33 or 34 F. A pallet of packaged
blueberries, initially at 80 F, allowed to remain for an hour or so in
a cooler without forced ventilation may appear to have cooled somewhat.
However, only the blueberries on the outside will cool appreciably in
that length of time. Temperature measurements taken inside cellophane-wrapped
cups near the center of pallets standing in still air at 44 F have actually
shown a slight temperature increase during the first hour of cooling.
Calculations suggest that the center cups of pallets of 80 F blueberries
would require more than 36 hours to cool to below 50 F. Pallets of hot
blueberries allowed to stand in a cooler for several hours before shipment
without forced ventilation will not be cooled significantly nor uniformly.
Furthermore, numerous tests have shown that refrigerated transports provide
little or no additional cooling. In still air, the average cooling rate
of pallets of blueberries is slow because heat is transferred from the
interior only by conduction. The cellophane wrappers, the fiber cups,
the paperboard flats, and especially the air gaps between the cups act
as effective insulation, slowing the movement of heat. This fact has prompted
shippers of many commodities to install equipment inside the cooler to
force the cold air through the containers, greatly increasing the cooling
rate. In existing refrigerated rooms not equipped with fans, the addition
of fans (Figure 3) to move the air is the single most cost-effective change
possible, provided that the refrigeration system has sufficient capacity
to remove heat rapidly. Figure 3. Fan arrangement for forced-air cooling
of blueberries Blueberries cooled with forced air reach the desired storage
temperature significantly faster than those in still air. In addition,
they cool more uniformly. Figure 4 shows typical average pulp temperatures
for a pallet of blueberries in still air and for a pallet in forced air.
Forced-air cooling is clearly much more rapid than cooling in still air.
Depending on the circumstances, the rate of cooling may be 16 to 20 times
faster. Figure 4. Cooling rates for blueberries in forced air and still
air. (Room air temperature of 44 F). Forced-air cooling gives more consistent
and thorough cooling through the entire pallet load. In situations where
a considerable investment has already been made in refrigeration equipment
and buildings, an additional nominal investment in one or more forced-air
cooling fans can dramatically reduce the time required to cool blueberries
satisfactorily, resulting in the delivery of a consistently higher quality
product to the consumer. For additional information, see Cooperative Extension
Service publication AG-414-3, Forced-Air Cooling. Marketing Although blueberries
are grown commercially in about a dozen states, the major supply areas
are Michigan, New Jersey, North Carolina, and the Pacific Northwest states.
Over the last decade, domestic per capita consumption of blueberries has
continued to expand at an average annual rate of 4 percent. In 1991, consumption
of fresh blueberries reached 1/4 pound per person; consumption of processing
blueberries (in canned and frozen form) was nearly 1/2 pound per person.
Domestic blueberry supplies are most readily available during June, July,
and August, with July being the peak harvest month. In the past, North
Carolina producers have enjoyed a timely marketing advantage because they
often harvest their berries in mid-May and June, earlier than many other
competitive supply regions. Recently, however, increased plantings of
highbush varieties in Florida, Georgia, Mississippi and Texas have intensified
early-season marketing pressures. In addition, more plentiful late-summer
low-bush supplies are available from Canada. Local grower prices for blueberries
have varied considerably over the past decade, with sharp price declines
observed in 1985 and 1990. As the supply of high-quality berries continues
to increase, local growers should continue their efforts to provide high-quality,
properly cooled blueberries to buyers in order to maintain and expand
their market share.
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Green Co., Ltd.
299K7 SONG GIONG RESIDENTIAL QUARTER, NGUYEN THI DINH ST., WARD AN PHU, DISTRICT 2, HO CHI MINH CITY, VIETNAM
Tel : (84.8) 54026784/86/87/88 - FAX : (84.8) 54026790 - Mobile : (84)(0)908002930 - DIRECT PHONE LINE : + (84)1696988825
Email: GreenSeeds@Green-Genetics.coms.com
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