Agriculture Reference
In-Depth Information
degree Brix) to titratable acidity can be used (Batten 1989).
Although recommendations vary, a TSS:TA value of 30
(Kader 2000b), 35 (Menzel & McConchie 1998) or 40
(Menzel 2002) or higher is suggested for commercial fruit.
Maturity standards based on the TSS:TA value, fruit weight
and size and peel colour have been established for most of
the commercial cultivars of Australia, China, India, Taiwan
and Thailand (Mitra 2002).
Fruit quality declines after harvest, with concentrations
of ascorbic acid, phenols, sugars and organic acids decreas-
ing. However, dipping fruit in the ethylene precursor,
ethephon, can significantly achieve ripening. Thus, Sadhu
and Chattopadhyay (1989) reported that a five-minute
dip in a 2.5 g l
−1
ethephon solution gave rise to a 50%
increase in total sugars, a 20% increase in ascorbic acid
and an increase in the TSS:TA from 20 to 30-40 over three
days. Nonetheless, ethephon has not been commercialized
and so the focus of current research and development
activities in southern Asia is to maintain rather than
improve the quality of harvested lychee (Menzel 2002).
Once lychee are harvested they begin to dry out and turn
from red to brown. This is the first sign of fruit decline.
The mechanisms of pericarp browning, colour retention
and pulp quality maintenance have been the main focus
of post-harvest research (Mitra 2002). After harvest no
further ripening occurs (Joubert 1986) and the respiration
rate declines during storage (Akamine & Goo 1973).
The colour of mature lychee results from anthocyanins in
the mid- to upper mesocarp (Underhill & Critchley 1993).
These are stable below pH 3 but are converted to colourless
chromenols as the pH rises, when polyphenol oxidases
become active at pH values between 4.1 and 4.6. This colour
change may be reversed depending on pericarp pH,
anthocyanin and brown pigment contents. Thus the bright
red colour could be maintained if the pericarp pH was
kept at 4. If the pericarp pH was above 4, reversing the col-
our change depended on the storage time (Chu
et al
. 2004).
However, Olesen
et al
. (2003) reported that although
treatment of lychee with acid alone changed the fruit colour
it then took on a patchy appearance. The acid effect could
be made more uniform if the fruit was pre-treated with hot
water. Unfortunately, acid and heat led to artificially brightly
coloured fruit that was softer and more prone to rots, nor
was the colour fixed and so browning reappeared. It was of
concern that treatment of lychee in this way disconnected
the appearance of the fruit from its eating quality. The
treatment was therefore not recommended because it could
easily lead to attractive fruit that would not taste as it should.
However, workers in Israel have suggested a hot-water
brush/acid/prochloraz treatment (Lichter
et al
. 2000).
Pre-harvest influences on fruit quality
Menzel (2002) noted that fruit quality at harvest
determines its later shelf life and market price. Damage
present at the time of harvest is worsened as the fruit
moves through the supply chain. A good supply of
nutrients and water to the trees will produce sound fruit.
Thus uneven watering during fruit development may
cause fruit splitting (Kumcha 1998), as do low concen-
trations of pericarp calcium (Huang
et al
. 2002; Li
et al
.
2001).
Damage by insects also needs to be minimised. The use
of pesticides or bags in the field limits harm and increases
the proportion of sound, marketable fruit (Menzel 2002). It
was suggested by Tyas
et al
. (1998) that bagging of the
fruit bunches may enhance fruit colour.
The risk of rots during storage can be reduced by good
hygiene in the orchard. Judicious pruning keeps canopies
open, skirting lowers the risk of infection from the soil
level and collecting dead wood removes pathogenic
propagules (Menzel 2002).
Harvesting methods
Lychee are delicate fruit and so handling should be kept to
a minimum. Given that they have a short shelf life resulting
from their perishability, a rapid turn-around is needed to
provide the best quality fruit for consumers. Menzel (2002)
suggested that fruit should ideally be shipped on the day
of harvest.
After harvest lychee keep their colour and quality for
only three to five days, though this period can be extended
by treatment with 0.5% copper sulphate and packing in
perforated polythene bags. Other post-harvest treatments
vary with the country of production. Thus, in the United
States fruits are destemmed during grading and packed in
shallow, ventilated cartoons cushioned with shredded
paper. In South Africa, lychee may be placed on trays
and dusted with sulphur, and then left in ventilated
sheds overnight. They are then allowed to wilt in lugs for
one or two days - this allows damaged fruit to become
conspicuous so that they can be removed before grading
and packing (Morton 1987).
Fruits for local consumption and markets are harvested
fully coloured whereas those for export destinations are
picked when the pericarp colour has not fully developed.
During the later stages of maturation swelling of the skin
reduces the density of the skin tubercules and causes them
to flatten out. Experienced pickers can detect these changes
and thus recognise the stage of full maturity (Morton
1987), rather than relying on measurements of TSS:TA.
Mechanized methods for harvesting lychee are not
