Right; A fluorescent microscope image of an open
lenticel (a tiny pore that
makes gas exchange possible
between the fruit and its
surrounding atmosphere).
This one was spotted on
a ‘Sapphire’ plum (200 x
magnification).
PLUM SHRIVELLING
Hairline fractures and tiny openings in the skin of some plum cultivars are causing
producers to export more than sweetness in a box. Research shows some plum peels
are also letting through huge amounts of the fruits’ precious water content into the
surrounding air, causing it to shrivel up due to moisture loss.
Above: An ‘African Delight’
plum with concentric hairline
cracks extending over its
shoulders. The exact reason
why this cultivar develops
these cracks is not clear,
but it might be linked
to environmental conditions
like temperature or humidity.
JORISNA
BONTHUYS
T
he peel of some plum cultivars,
especially the popular ‘African
Delight’, is like a kitchen sieve. It lets
through so much water through tiny
lens-like openings in its skin that this
dehydrates the fruit on its way to the market.
This causes some fruit to look old, long before
its sell-by date.
These are some of the research findings in a study
done by Adriaan Theron, an MSc student in the
Department of Horticultural Science at Stellenbosch University. Theron looked at moisture loss
in and shriveling of Japanese plums produced
in orchards in the Franschhoek and Stellenbosch region. His two year long study, funded
by HORTGRO Science, shed new light on this
problem which causes major headaches for local
producers. Theron explains: “Exporting plums
to overseas markets can be very challenging as
fruit should tolerate cold storage at -0.5°C for up
to eight weeks. Irrespective of the long storage
period, the consumers expect fruit to look fresh
when it arrives on supermarket shelves.”
68 TEGNOLOGIE HORTGRO | APRIL/MEI 2016
It is known that prolonged storage can cause
some cultivars to develop a shrivelled appearance due to moisture loss. This is problematic
given that three quarters of locally produced
plums are exported. This is exacerbated by the
fact that plums have a higher respiration rate
than most fruit and therefore ages quickly.
Theron studied four cultivars, namely ‘African
Delight’ (which is highly susceptible to shrivelling), ‘Laetitia’ (susceptible to shrivelling),
‘Sapphire’ (also susceptible to it) and ‘Songold’
(not susceptible to it). He used a variety of
techniques - including fluorescent microscopy - to find tiny cracks and openings in the
fruit peels. Theron also considered different
handling techniques used by and available to
local farmers to determine which actions pose
the least risk of moisture loss.
Theron says he was surprised by the results.
He found that ‘African Delight’ has open
hairline cracks in its peel. These cracks, visible
to the naked eye as concentric circles at the
stem (pedicel) end on the fruit, creates direct
DID YOU KNOW?
• In South Africa plums are a valuable commodity. The total value of plums exported during
the 2013/2014 season was R849.4 million.
• During the same season 74% (or 55 192 tons)
of the plums were shipped overseas, 23% were
sold locally and 3% used for processing.
• Exporting requires plums to last as long
as eight weeks in cold storage. Prolonged
storage can cause some cultivars to develop a
shriveled appearance due to moisture loss.
• Plums ‘exhale’ water vapour through openings, wounds and cracks in its skin. These tiny
openings help to cool down the fruit while
still on the tree and can add to the problem of
(post-harvest) moisture loss as it creates tiny
‘windows’ through which water molecules
from inside the plant escapes into the
surrounding environment.
A fluorescent microscopic image of a hairline crack in the stem area
of an ‘African Delight’ plum. This crack, magnified 200 times under a
microscope, causes huge moisture loss from the fruit.
pathway for water vapour to escape into the
fruit’s surrounding environment.
is probably why this cultivar is not susceptible
to post harvest shrivelling, says Theron.
Says Dr. Marian Jooste, Theron’s study leader,
“We now know that these rings are not only
cosmetic. It is like the Grand Canyon where
water molecules are escaping through it. No
wonder this cultivar is shrivelling.”
It was also found that a mass loss of only 2% in
plums due to moisture loss caused shrivelling
in ‘African Delight’ plums. “This finding underlines that farmers can reduce moisture loss by
managing the way fruit are handled from the
tree to the consumer,” says Theron.
This is because water vapour always moves
along a gradient; from a high to a lower
concentration, she explains. Says Jooste: “Fresh
fruit are alive, meaning they will keep respiring
even if they have been removed from the tree.
For some cultivars this poses a real problem
as they develop a shrivelled appearance
during cold storage due to moisture loss in the
handling chain.”
Furthermore, open lenticels were also found
in the peels of ‘African Delight’, ‘Laetitia’ and
‘Sapphire’. Lenticels are tiny openings in the
fruit peel which act as ‘open windows’ between
the plum and its environment through which
water molecules escapes. For ‘Songold’ no peel
cracking or open lenticels were observed. This
Theron’s study shows that fruit should be
packed and be under forced air cooling within
six hours after harvest, or be pre-cooled to 0
°C to have the least moisture loss. “We found
that fruit left at ambient temperature had huge
amounts of moisture loss, especially if the fruit
were exposed to ambient temperatures for 48
to 72 hours. Most of the moisture loss happens
even before the fruit gets shipped, between
when the fruit arrive at the packing facility and
the end of forced air cooling.”
Overall this research found that great care
should be taken in following correct postharvest handling protocols for plums. Locally
the industry handling protocol is to remove
APRIL/MAY 2016 | TECHNOLOGY HORTGRO
69
field heat from the plums and reduce
the fruit pulp temperature to 15 °C
within three hours of harvesting.
Although it is recommended that
plums should be packed on harvest
day and be force air cooled to a pulp
temperature of -0.5 °C within 24 to 36
hours, this does not always happen.
Due to labour and infrastructure
constraints, it is not always possible
for local producers to pack and cool
fruit immediately.
Adriaan Theron.
Says Theron: “Our research shows
that the best way possible for producers to try
and control moisture loss is to minimise the
driving force behind it. They need to manage
post-harvest factors influencing water vapour
permeability to the best of their capability. For
breeders it should be important to develop
cultivars that have low peel permeability.
SOME OF THE FINDINGS:
• The popular ‘African Delight’ cultivar has open hairline cracks in its peel. These cracks, often
only just visible to the naked eye, allow water vapour to escape through the fruit’s peel. The
wider these cracks, the more moisture loss will occur. It was, therefore,
not surprising to find that the fruit peel of ‘African Delight’ also has higher water
permeability than the other cultivars studied.
• Open lenticels (tiny pores in the fruit’s skin that allows gas exchange between the fruit and
the surrounding air) were found in the peels of ‘African Delight’, ‘Laetitia’ and ‘Sapphire’.
This could be contributing to moisture loss and shrivelling in plums. For ‘Songold’ no peel
cracking or open lenticels were observed. This is probably why these plums do not shrivel.
• Variation between fruit, orchards and harvest dates contributed the most towards the
variation in how much water vapour a plum releases into the surrounding
environment (its water vapour permeance).
• As little as 2% mass loss can cause plums to develop a shriveled appearance.
A tenth of the total post-harvest mass loss observed happens between
harvesting and when the plums reach the packing facility.
• Plums should be packed and be under forced air cooling within six hours after harvest.
Alternatively they should be pre-cooled to 0°C to minimise moisture loss.
• Regardless of how plums are handled, and although the fruit might be packed in perforated bags to prevent dehydration, it will lose about 1% of its mass during cold storage.
70 TEGNOLOGIE HORTGRO | APRIL/MEI 2016
“It is strongly suggested that fruit should be
harvested in the cooler time of day, kept in the
shade after harvesting and covered with wet
blankets. Field heat removal and forced air
cooling start as soon as possible after harvest.
Plums can be stored at 0 °C or 15 °C for up
to 72 hours before it is packed and force air
cooled.”
Theron says plums should never be left at
ambient temperature for extended periods
after the harvest. “Fruit should be packed in
perforated bags or shrivel sheets, depending on the cultivar at hand. This is needed to
decrease the water vapour deficit between the
fruit and the surrounding atmosphere.
“Fruit with excessive hairline cracks at the stem
(pedicel) end should not be packed.”
Furthermore, it is suggested that the local plum
breeding programme should test cultivars with
concentric rings at the stem end to make sure
the rings are not open hairline cracks, contributing towards moisture loss. Says Theron,
“If these rings are open hairline cracks, these
cultivars should be excluded from the breeding
programme as it was shown that is contributes
to the problem of moisture loss from the fruit.”
Theron says variation between fruit (even from
the same tree) was the largest contributor to
the increase in the water vapour permeance of
the cultivars studied. “Currently fruit maturity
is not determined on the packing line,” he
explains. “Fruit is sorted by colour and size,
leading to large variation between fruit that
gets shipped in packed cartons. This could be a
contributing factor why some fruit in the same
carton develop shrivel and others not.”
The effect of open lenticels on moisture loss
in ‘African Delight’ and ‘Laetitia’ could not be
determined in his study. The composition and
thickness of the plums’ natural waxes (and
its effects on moisture loss) are not known
and Theron suggests that it should be investigated further. More research is also needed
to determine changes in plums’ water vapour
permeability during cold storage, he says.