Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1996.Litchi fruit are non-climacteric, and are able to endure relatively low storage temperatures
compared to other subtropical fruits. Unfortunately however, the litchi rind is relatively thin
and lacks a thick, durable cuticle. Consequently, post-harvest desiccation is a major factor, and
rind colour changes rapidly from red to brown, unless counter measures are taken immediately
after harvest. Presently, the South African industry uses sulphur fumigation to prevent
browning, but sulphur treatment is undesirable in many respects, only partially successful, and
some overseas markets have lowered the permissible level of sulphur to 10 mg.kg-1 in the fruit
flesh. Alternatives to sulphur fumigation were accordingly researched.
The author tested the hypothesis that, in order to preserve the desirable red rind colour, it was
necessary to break down rind cell membrane integrity, so that the vacuole-bound anthocyanin
pigments can be exposed to zero pH solution, which effects rind colour preservation.
Thereafter, rind desiccation must be reduced.
A 2 s steam (95°C) treatment followed by 4 min immersion in zero pH solution resulted in
fruit which retained excellent red rind colour, with normal pulp characteristics and tasted
similar to control fruit after 28 days storage at 1°C. Ultrastructural studies showed that 2 s
steam (95°C) treatment resulted in rind cell membrane breakdown, and this was enhanced
when used in conjunction with 4 min in zero pH solution. In addition, electrolyte leakage
studies showed that rinds of untreated control fruit had lowest electrolyte leakage, while those
of fruit subjected to 2 s steam (95°C) had highest electrolyte leakage, making the previously
compartmentalized and vacuole-bound pigments available for preservation in the desirable red
colour. Polyphenol oxidase in litchi rinds was strongly inhibited by 2 s steam (95°C), but even
more so when fruit were subjected to 2 s steam (95°C) followed by 4 min in zero pH solution.
Energy dispersive x-ray microanalysis studies found that chlorine concentrations were
relatively high on both the inner and outer surfaces of fruit subjected to 2 s steam (95°C)
followed by 4 min in zero pH solution. Similarly, sulphur concentrations were high in rinds
of sulphur-fumigated fruit, but this element was also present at low concentrations in nonsulphur-
fumigated fruit.
Rind colour of untreated control fruit lightened when stored at 30°C and hue changed from
red to reddish orange. Rinds of fruit subjected to 2 s steam (95°C) only, lost colour rapidly
and were a pale yellow hue 24 hr after treatment. The hue of fruit rinds subjected to 2 s steam
(95°C) followed by 4 min in zero pH solution changed from reddish orange to red within 4
hr and then darkened up to 24 hr after treatment. Red colour was preserved in fruit held at
30°C for 72 hr, but lightened after 24 hr. HPLC of anthocyanin pigments found that the
presumed cyanidin-3-rutinoside, pelargonidin-3-glucoside and pelargonidin-3,5-diglucoside all
decreased in untreated fruit over 5 days storage at 30°C. Concentrations of presumed cyanidin-
3-rutinoside in fruit subjected to 2 s steam (95°C) followed by 4 min in zero pH solution
increased immediately after treatment, peaked 24 hr later, but then decreased to about double
the concentration of fruit treated on the day of harvest after 4 days at 30°C. Furthermore, no
copigmentation or self-associations of anthocyanins took place in rinds of fruit subjected to 2
s steam (95°C) followed by 4 min immersion in zero pH solution.
Semi-commercial trials showed that the steam: acid dip treatment is feasible, and has the
potential to replace sulphuring as a fungicidal treatment. It also has the advantage of more
permanently preserving the desirable rind colour, and in a more intense red colour
