Dr Mariana Jooste
Research Fellow, Department of Horticultural Science,
STELLENBOSCH UNIVERSITY
I
n South Africa > 74 000 tons
of Japanese plums (Prunus salicina Lindl.) are produced annually on 5000 ha for fresh consumption (Hortgro, 2014). Of this,
74% (the equivalent of > 10.5 million 5.25 kg cartons) is exported
primarily to the EU, Russia, the UK,
Asia and Middle and Far East. The
total 2013/2014 season’s total exported plum fruit value was R895.2
million CIF (Cost, Insurance and
Freight).
However, the export of fresh plums
from South Africa comes with challenges. The markets are distant approximately 17 days by ship - and
fruit must be able to store for between 5 to 8 weeks. Irrespective of
the long storage period expected of
this highly perishable product, consumers expect fruit to look fresh
when it arrives on supermarket
shelves. Unfortunately, most plum
cultivars develop a shrivelled ap-
pearance after extended storage at
low temperatures (Figure 1). Moisture loss causes a loss in the turgidity of the surface cells of the fruit
which, in turn, manifests either as
shrivel on the fruit surface or as
mass loss. If shrivel incidence is
high, fruit has to be repacked at
high expense overseas, consumers
are discouraged from purchasing
the fruit, or the fruit may be rejected. Although packaging solutions
to reduce shrivel in plums exist,
shrivel incidence is still high for
most cultivars.
Fruit loses water in the form of water vapour, which always moves
from a high to a lower concentration. In fruit, water vapour moves
from the intercellular airspaces and
cell walls, where the water vapour
pressure is usually close to saturation, through the fruit cuticle (the
fruit peel) into the surrounding atmosphere, where the concentration
of water vapour is usually lower.
The main driving forces for moisture loss from the fruit are the fruit
pulp temperature, air temperature
and the relative humidity of the air
surrounding the fruit.
The cuticle of the fruit is a very effective barrier to moisture loss and
acts as a protective layer between
the plum and its surrounding environment. Water vapour exits the
fruit at various openings (either
natural or caused by injury) in the
fruit peel. Generally there are four
exit routes through which moisture
can escape from the fruit peel,
namely: wounds, stomata or lenticels (pores in the fruit peel),
through the cuticle and cracks in
the cuticle.
To investigate when and where
moisture loss occurs from plums, a
recently completed study (Theron,
2015) at the Department of Horticultural Science at Stellenbosch Uni-