versity, and funded by the South
African Stone Fruit Producers’ Association (SASPA), took a closer look
at the plum peel’s permeability to
moisture as well as at the handling
chain from orchard until the end of
cold-storage.
What is happening in the orchard
regarding the fruit’s susceptibility
to moisture loss?
It is known that pre-harvest factors
such as differences between cultivars, farms, orchards on the same
farm, trees in the same orchard,
and fruit on the same tree as well as
harvest date can play an important
role in fruit peel moisture permeability.
In our study we found that differences between fruit from the same
tree explained more than 45% of
the differences in fruit peel permeability during fruit development on
the tree. Factors such as position of
the fruit on the tree, exposure to
sunlight, split pit and shape of the
fruit could possibly contribute to
the variation in peel permeability of
the different fruit on the same three
that were of the same maturity.
Commercial harvesting is done by
hand and fruit are picked according
to visual appearance. Fruit maturity
is also not determined on the packing line, but fruit is sorted by their
visual appearance (colour) and size.
This leads to large variation between fruit in packed cartons, and
could be a contributing factor why
some fruit in the same carton develop shrivel and others not.
Additionally, harvest date contributes >20% to the total variance in
water vapour permeance of plum
cultivars. We found that plum peels
were the least permeable to moisture approx. 2 weeks before their
respective optimum harvest dates,
after which permeability increased
two-fold until the fruit was overripe. This drastic increase in peel
permeability as the fruit matures is
probably caused by changes in the
cuticle thickness and composition
over time. It is, therefore, strongly
recommended that mixed maturity
in the carton must be prevented by
stricter sorting to prevent over mature fruit, which are more prone to
moisture loss and, hence shrivel, to
end up in the carton.
Cultivar differences contributed >
5% of the differences in fruit peel
permeability in this study. We
found that ‘Sapphire’, which is
highly susceptible to shrivel, have
the highest peel permeability to
water vapour of the cultivars tested
in our study – which explains why it
shrivels so easily. ‘African DelightTM’
plums had the second highest peel
permeability followed by ‘Songold’.
Surprisingly, we found that
‘Laetitia’, which is highly susceptible
to shrivel, had the lowest peel permeability to water vapour of the
cultivars we tested. ‘African DelightTM’ plums are known for the
concentric rings at its pedicel end
(Figure 2). We found that these
concentric rings are actually open,
hairline cracks in the peel of the
plums – these act as easy escape
routes for moisture from the fruit
(Figure 3) and probably is the main
reason why this cultivar is so prone
Figure 1. A shrivelled plum after 7 weeks of coldstorage
Fi gure 2. Concentric rings found commonly at the
stem-end of African Delight plums
Photo Adriaan Theron
Photo Adriaan Theron