to shrivel. We also found that
‘African DelightTM’, ‘Laetitia’ and
‘Sapphire’ have open lenticels
(pores in the fruit peel) which could
also contribute towards postharvest moisture loss (Figure 4). What
is very interesting, is that ‘Songold’,
which has a relatively high peel permeability to moisture, have no micro-cracks or open lenticels in its
peel – this explains why this cultivar
is not prone to postharvest shrivel.
From what we have found in the
preharvest part of our study, it is
clear that, if extended cold-storage
is intended, fruit at the higher end
of the optimum picking window
(with a higher flesh firmness)
should be selected, since these fruit
have a lower peel permeability to
moisture and will be less susceptible to shrivel. Conversely, over mature fruit are more susceptible to
shrivel – these fruit must be sold as
soon as possible after harvest and
must be sorted from less mature
fruit to prevent mixed maturities in
the same carton. Be careful not to
pack fruit with excessive concentric
rings at their stem-end for longterm storage. These concentric
rings aid moisture loss, and hence
shrivel manifestation, from the
fruit. We also recommend that
fruit should be harvested during
the cooler time of day when the
difference in temperature between
the fruit and the outside air is
small. Since this is not always possible in a commercial set-up, the
best practice to reduce moisture
loss from the fruit after harvest will
be to keep fruit in bins in the shade
and to cover the bins with wet
blankets (Figure 5) until the fruit is
packed, or until field heat removal
commences. Once harvested, fruit
should be cooled as soon as possible in order to reduce moisture loss
and minimize the risk for shrivel
manifestation. A good rule of
thumb is that the fruit pulp temperature should always decrease
after harvest (never increase) to
prevent moisture loss.
What is happening after harvest
regarding moisture loss?
Currently a lot of focus is placed on
packaging solutions and technology to increase the relative humidity
during cold-storage to prevent
moisture loss, which is not wrong.
However, we found that a substantial amount of moisture loss also
occur between harvest and the
commencement of forced air cool-
Figure 3. Fluorescent microscope image of the
hairline cracks, seen as concentric rings by the
naked eye, at the stem-end of African Delight
plums
Photo Adriaan Theron
ing – a step in the handling chain
which is easily overlooked regarding the management of moisture
loss from the fruit. Ignoring this
part of the handling chain regarding moisture loss will only incur
cost, as no packaging or high humidity applied later on in the handling chain will reduce the effect of
moisture loss that occurred between harvest and the commencement of forced air cooling.
In practice it is not always possible
to pack and cool fruit as soon as
possible after harvest due to labour
and / or infrastructure constraints.
Fruit is then either stored at 0 °C,
just above the dew point of the
pack house or left at ambient (in
the shade / well ventilated area of
the pack house facility / farm) until
there is capacity to pack the fruit.
In our study we found that a tenth
of the fruit’s mass is lost due to
moisture loss in the period between
harvest and arrival at the pack
house. It is, therefore, strongly recommended that harvesting protocols, such as harvesting during
cooler times of the day, covering
fruit in bins with wet blankets and
keeping fruit in the shade, must be
followed stringently for shrivel sus-
Figure 4. Open lenticels in the peel of Sapphire
plums as seen under a fluorescent microscope
Photo Adriaan Theron