Postharvest Handling of Lychees
Postharvest handling operations for lychees (Litchi chinensis Sonn.) are
relatively simple and result in high-quality fruits that can be exported.
Harvest maturity, prevention of mechanical damage, cooling, and temperature
maintenance are critical factors that, when optimized, can allow for storage
and shelf-life of as long as six weeks.
Quality Criteria
Import markets require lychees to be fresh in appearance, free from blemishes
and mechanical damage to the skin, with uniform red color. Minimum size
should be a diameter of 2.5 centimeters, length of 3.0 centimeters, and
weight of 20 grams. The pulp (or aril) should be easily removable from the
seed coat and make up at least 75 percent of the weight. Cultivars that
have little aril or aril that remains stuck to the seed are not normally
acceptable. The aril should be translucent and sweet with a Brix level of
around 17 °.
Cultivars
 Acceptable cultivars
are those that meet market quality criteria and offer growers good yields,
adaptability, and disease resistance. There are about 100 lychee cultivars,
but only a few are produced commercially. Cultivar names can cause confusion
because the same cultivar may have different names in different regions.
Cultivar selection has been based on performance under specific production
conditions. The main cultivars planted for export are H.L.H. Mauritius and
Red McLean. Cultivars can be divided into three groups:
Mauritius Group. Cultivars
in this, the most common group, produce satisfactory yields of quality fruit.
These include H.L.H. Mauritius, Muzafarpur, Late Large Red, Hazipur, Saharanpur,
and Rose- Scented.
Chinese Group. These
cultivars have poor production yields, but the fruit is of high quality.
These include Haak Yip, Shang Shou Huai, Kontand, Glutinour Rice, and Three
Months Red.
Madras Group. These
cultivars are prized for their color (very red), but their fruit is of poor
quality, with soft flesh and large seeds. Cultivars in this group include
Kafri, Shorts Seedless, Johnstone's Favourite, Emmerson, Durbhanga, Maries,
Mooragusha, Red McLean, Brewster, and Bedana.
Cultivar selection is also based on the fruiting and harvesting period.
H.L.H. Mauritius is a midseason cultivar. Three Months Red and Haak Yip
ripen one to three weeks before H.L.H. Mauritius. Several Mauritius selections
and the Madras cultivars ripen between one and three weeks after H.L.H.
Mauritius.
Selection may also be
based on the growing region because different cultivars have different temperature
requirements. Kai Mana and Groff are tropical cultivars requiring minimum
temperatures of 15 ° C (night) and 25 ° C (day). Kwai May Pink,
Salathiel, and Mauritius are subtropical cultivars requiring minimum temperatures
of 10 ° C (night) and 20 ° C (day).
The typical colors are
bright red (Bengal), dull red (Wai Chee), purple red (Haak Yip), and pinkish
red (Brewster) with wide-ranging skin thickness. Mature skin segments vary
from smooth (Haak Yip) to swelling (Wai Chee) to sharp pointed (Kwai May
Red).
New cultivars being
developed and introduced include the following:
- Souey Tung - An early cultivar planted in the cool, dry areas of Australia;
- Haak Yip ‹ A cultivar with a long harvesting period, suitable mainly
for processing;
- Kwai May Pink ‹ A late cultivar with a long harvesting period;
- Salathiel ‹ A small tree with attractive skin color and small seeds;
and
- Wai Chee ‹ A cultivar with a long harvesting period but poor tree
structure (the most common cultivar in China).
Harvest Maturity 
Lychees achieve maturity
between 100 and 110 days after pollination, at which point the fruits weigh
around 20 grams. Although climatic factors influence the time of flowering,
they do not necessarily affect fruit development or maturity. Harvesting
takes place at five to eight weeks, depending on the cultivar, production
area, and conditions.
Because lychees are
nonclimacteric and stop maturing when harvested, the quality, color, and
flavor are determined at harvest time. The stage of maturity at harvest
is therefore critical in determining consumer acceptability. Immature lychees
have poor skin color and are acidic, not sweet. Lychees should be harvested
in the morning or late afternoon when ambient temperatures are lower; harvesting
during hot periods can speed up the rate of water loss. Harvesting is carried
out every two to three days and is based on fruit maturity, especially during
the early part of the season. Yet not all fruits on a tree mature at the
same time; even fruits on the same panicle do not mature uniformly. Optimum
maturity can be judged by the following:
- Fruit weight. Ripe fruits average 21 grams to 25 grams in weight.
- Fruit color. Ripe fruits are red on both the outer and inner surface of
the skin.
- Time from pollenization. Normally 100 to 110 days elapse before maturity.
- Sugar and acid contents. During ripening on the tree, the acid content
of the aril decreases, and the sugar content increases: acid ranges from
5.5 percent to 0.5 percent, and sugar ranges from 10 percent to 17 percent.
Fruits with an acid content of 0.9 percent taste sour, whereas those below
0.4 percent tend to be bland because of a sugar and acid imbalance.
Skin texture. With some cultivars, maturation can be detected from the skin,
which becomes smoother as the small protrusions become flatter.
Harvesting
Harvesting can be done
using secateurs or pruning shears to remove panicles, each containing 4
to 20 fruits, from the tree. Where harvesting is carried out from the ground,
the bunches are placed into plastic field crates. Where ladders are used,
the picker places the bunches into a cotton bag that is regularly lowered
to the ground and transferred to field crates.
When picking tools are
used to cut the panicles, a bag should be attached to the pole so the fruits
do not fall to the ground. A blade, knife, or pruning shears form a part
of the tool to cut the main stem of the panicle; to avoid breaking the fruits'
skin, the fruit should not be pulled from the bunch.
For transportation from
the field to the packing facilities, plastic field crates are preferred
to sacks, bags, or baskets. Field crates should be shallow, light colored,
ventilated, and clean. Damage to fruits frequently occurs during these handling
operations from dropping, excess weight on top of the fruits, and rubbing
of the fruits against one another or against the container. Damage results
in increased water loss, which causes fruit to turn brown or black, thereby
reducing shelf-life, appearance, and marketability. To minimize increases
in fruit temperature, crates should not be exposed to direct sunlight but
be kept in the shade and loaded onto covered or protected vehicles for transportation.
Fruits wet from the rain should not be picked.
Packhouse Operations
The simple operations for lychee grading and packing include trimming, preclassifying,
cleaning or brushing, fumigating (sometimes), packing, precooling, storing,
and transporting. The time between harvesting and packing should be kept
to a minimum. The sooner the fruits are packed and cooled (or shipped) after
harvest, the better their quality on arrival in the market. Delays between
harvesting and packing are frequently the cause of water loss and diminished
quality.
Grading and Packing
Packing facilities should
be arranged linearly and include a delivery table, brush rollers, and grading
and packing tables. Fungicides are not normally used. Lychees must be allowed
to dry completely before packing and storage because moisture from either
washing or condensation increases the fruits' chance of developing disease.
On arrival at the packing
facility, fruits should be cleanly clipped from bunches using secateurs,
with 0.5 centimeters of the stem left attached. Fruits that are brown or
not uniform in color, are undersized, show any damage or disease, or have
cracking skin should be rejected at this point. Fruits infested with fruit
fly or lychee moth begin fermenting within a day as a result of secondary
fungal infection.
Acceptable fruits can
be packed directly into the export carton or placed on the delivery table
and moved to roller brushes to be cleaned; the brushes should be soft and
revolve slowly. The fruit are then placed on grading tables, which should
be slightly sloped and protected with a sponge layer and a clear plastic
covering. If a conveyor belt is used, the tables can be located on one or
both sides of the belt. Some countries use postharvest treatments to reduce
disease development‹for example, 0.5 grams benlate per liter of hot
water (52Y´C) for two minutes. Shelf-life studies show that treating
lychees in a low-pH solution helped maintain full skin color, or intensified
the color without turning fruits brown. Importing-country legislation should
always be checked before fungicide is used in postharvest treatments.
Lychees exported from
Israel to the United States are subject to quarantine treatment for fruit
fly. The treatment entails low-temperature storage as follows:
|
Days
|
Maximum Temperature (Y´C)
|
|
10
|
0
|
|
11
|
0.55
|
|
12
|
1.11
|
|
14
|
1.65
|
|
16
|
2.22
|
Packing
Fruits should be completely
dry before packing. Some markets require packing by size (normally medium
and large), but others allow mixed sizes as long as the minimum size is
met. It is essential to check with buyers to determine size and grading
requirements.
Lychees are placed one
at a time into cartons, ensuring maximum use of space; forcing them into
tight spaces will damage the skin. A second layer of fruits is placed on
top of the first, which should remain flush with or slightly lower than
the top of the carton so that the fruits are not forced down when the lid
is closed. This method of packing prevents excessive movement and settling
of the fruits during handling and transportation. Carton net weights are
2 kilograms to 4 kilograms, depending on the requirements of the importing
country and the buyer. Some markets prefer lychees prepacked in punnets,
clamshells, or trays wrapped with plastic film. The weight and number of
fruits per clamshell depend on the market; one- quart, half-pint, or one-pint
clamshells may be used. Packing into these smaller units aids in presentation;
reduces mechanical damage; and, in most cases, extends shelf-life.
Packaging
Packaging is done in
one- or two-piece telescopic cartons that are self-locking and have a minimum
breaking force of 175 pounds per square inch. Ventilation is required in
all four walls. Narrow vertical ventilation is preferred because round vent
holes become blocked by the fruits. If one-piece cartons are used, there
should be a maximum of 2 centimeters between the lid and the fruit. Carton
dimensions vary depending on the net weight and the market. The most common
package size is 2 kilograms with external dimensions that are compatible
with ISO pallets:
| Dimensions (cm) |
| Length |
Width |
Height |
| 40 |
20 |
9 |
| 30 |
20 |
9 |
If clamshells are
used, then compatible flats are required; these are usually standard sizes
and fit with the ISO pallets. Packaging should meet all labeling
requirements of the importing country, including:
- Country of origin
- Product, variety
- Net weight; and
- Exporter and importer names.
Research indicates
that color retention and shelf-life can be increased with modified-atmosphere
packaging using plastic bags or wax treatments, although such packaging
methods have not necessarily been adopted by commercial operations.
Cooling and Storage
If a cold chain can
be maintained, the faster the fruits are cooled, the longer they will
retain the quality they had when harvested. Unlike other tropical and
subtropical fruits, lychees are not chilling sensitive and should be stored
at temperatures of 0 ° C to 1 ° C, with 85 percent to 95 percent
relative humidity. Fungal growth at this temperature is minimized, and
the fruits retain their color, texture, and flavor. The probability of
browning increases when fruits are stored in low humidity. Hydrocooling
can be used to precool the fruits, although normally it is preferable
to use forced-air precooling that will remove the field heat within two
to three hours if vertical column palletization is used and carton ventilation
is appropriate. After cooling, the pallets should be placed in a standard
cold room. If normal cold-room cooling is used, the cartons should be
stacked to enable air movement around each carton. Cooling in this manner
can take between 8 and 12 hours.
Some countries allow
a fumigation treatment that will increase storage life and improve skin
color by preventing browning. Fruits are placed in a sealed room with
1 kilogram of pure sulphur per 1,600 kilograms of fruit. The sulphur is
burned, and the sulphur dioxide fumes bleach the lychees' skin to a cream
color with a slight red tinge. The skin turns red when it is exposed to
air and light. If it fails to turn red or green during fumigation, it
will not turn red later. Fumigation also makes the skin less rigid and
less prone to mechanical damage. Packing the fruits in plastic bags immediately
after fumigation can reduce drastically the level of disease development
during long-term storage and shipping. Not all markets allow this treatment,
however.
By air the same day,
cooling and low-temperature storage are not recommended. The condensation
that develops on removal from cold storage to hot ambient conditions is
ideal for fungal growth, which exporters should not risk with fruit that
will be shipped right away.
Transport
Lychees are generally
exported by air, although they may be shipped by sea if the cold chain
is maintained. Transportation from the packing facility to the airport
should be in cool trucks if the fruits have already been cooled. In all
cases, trucks should be covered to prevent contact with wind, rain, and
sun.
Air shipments are
made in aircraft containers or aircraft pallets. Heat build-up in containers
is possible, especially when left exposed to the sun in the airport while
waiting for arrival and loading. Because heat is detrimental to the fruits,
aircraft containers and pallets should be kept in the shade or in cool
conditions until loading.
Potential Postharvest Problems
Fruit Cracking and
Fruit Browning. A principal cause of losses in lychees is cracking during
the ripening period. Cracking is caused by a range of factors, including
lack of moisture or a nutrient deficiency during the initial stage of
fruit growth, followed by high rainfall in the later stages. Insects and
sunburn may also cause cracking.
Fruit browning is
believed to be a process of early cell aging and death. If it occurs during
growth, the skin may crack where the cells are dead. Browning is not caused
by insect damage or fungal diseases, but the following factors may be
involved:
Nutrient imbalance
- Tension in the pericarp causing cell damage; and
- Exposure to high temperature and low humidity during growth.
- Diseases. The postharvest diseases of lychees are predominantly secondary
infections that develop as a result of mechanical damage or through the
cut stem of the fruits. These may include Aspergillus rot, Pestalotiopsis
rot, Peronophythora rot, sour rot, Botryodiplodia, Colletotrichum, and
Rhizopus. Disease control begins in the orchard, with measures against
piercing insects. Careful handling, prevention of mechanical damage, and
rapid cooling minimize secondary infections.
Additional Information
"The cultivation of litchis." Bulletin 425. 1992. Institute
for Tropical and Subtropical Crops. Agricultural Research Council, Nelspruit,
South Africa. 61 pp.
Galán Saúco, V. and U.G. Menini. 1984. "Litchi cultivation."
FAO Plant Production and Protection Paper No. 83. Food and Agriculture
Organization, Rome.
"Lychees," Horticultural Products Newsletter. 1989. International
Trade Centre, Market News Service, Rome. 29 pp.
Snowdon, A.L. 1990. "General introduction to fruits." A color
atlas of postharvest diseases and disorders of fruits and vegetables.
Volume 1. Wolfe Scientific Publications, London.
Underhill. S.J.R., J. Bagshaw, A. Prasad, G. Zauberman, R. Ronen, and
Y. Fuchs. 1992. "The control of lychee (Litchi chinensis Sonn.) postharvest
skin browning using sulphur dioxide and low pH." Acta Horticulturae.
2 (321): 73241.
Underhill. S.J.R., D.H. Simons, and C. Critchley. 1992. "Postharvest
pericarp browning of lychee (Litchichinensis Sonn.) fruit." Acta
Horticulturae. 2 (321): 71825.
Wong, L.S., K.K. Jacobi, and J.E. Giles. 1991. "The influence of
hot benomyl dips on the appearance of cool stored lychee (Litchi chinensis
Sonn.)." Scientia Horticulturae. 46 (34): 245541.
Zauberman, G., R. Ronen, M. Akerman, S. Weksler, I. Rot, and Y. Fuchs.
1991. "Postharvest retention of the red color of litchi fruit pericarp."
Scientia Horticulturae. 47 (12): 8997.
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