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Membrane studies in Japanese plums (Prunus salicina Lindl.)
ENGLISH ABSTRACT: The export of Japanese plums from South Africa is challenging, since most cultivars are prone to develop chilling injury (CI) when stored at low temperatures. This injury manifests as gel breakdown or internal browning in the mesocarp tissue of the fruit on removal from low stor...
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| Format: | Thesis |
| Language: | en_ZA |
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Stellenbosch : Stellenbosch University
2012
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| Summary: | ENGLISH ABSTRACT: The export of Japanese plums from South Africa is challenging, since most cultivars are prone to
develop chilling injury (CI) when stored at low temperatures. This injury manifests as gel
breakdown or internal browning in the mesocarp tissue of the fruit on removal from low storage
temperature conditions, i.e. in the consumer’s fruit basket, who subsequently does not buy plums
again.
Loss of cell membrane integrity and oxidative stress are, respectively, the primary and secondary
physiological responses to CI. The main aim of this study was to investigate changes in cell
membrane composition and levels of antioxidants in plums throughout fruit development and
maturation, during forced air cooling (FAC) and storage under different temperature regimes.
‘Sapphire’ (a chilling susceptible cultivar) accumulated high levels of glutathione and
polyunsaturated fatty acids (PUFAs) during fruit development. Therefore, the cultivar is protected
against lipid peroxidation while developing on the tree, but the high levels of PUFAs, which are
easily oxidised, may cause this cultivar to be chilling susceptible when stored at low temperatures.
It is suggested that the high levels of monounsaturated fatty acids (MUFAs), which are not easily
oxidised, and ascorbic acid that accumulated in ‘Angeleno’ (a chilling resistant cultivar) during fruit
development, render this cultivar CI resistant during long-term cold-storage.
When stored at -0.5 °C, CI development increased at a higher rate, ethylene evolution rates were
higher and water soluble antioxidant activity (HAA), ascorbic acid and glutathione levels, and the
MUFA:PUFA ratio were lower in H2 (more mature) ‘Sapphire’ plums than H1 fruit (less mature).
Therefore, concurrent with H2 fruit having lower levels of antioxidants to quench free radicals
caused by chilling stress, their cell membranes were more vulnerable to oxidation due to their
phospholipid fatty acid composition. H2 fruit also had higher levels of saturated fatty acids, and
hence less fluid cell membranes than H1 fruit when stored at -0.5 °C.
An intermittent warming (IW) regime delayed symptom appearance and reduced CI severity in
plums significantly compared to storage at -0.5 °C. Fruit stored under the IW regime had a more optimal phospholipid fatty acid composition and lower membrane sterol levels under shelf-life
conditions to keep the membranes fluid. It also had higher levels of HAA and lipid soluble
antioxidant activity, ascorbic acid and glutathione, which rendered fruit better protected against
oxidation.
Elevated storage temperatures (2.5 °C to 7.5 °C) caused higher levels of lipid peroxidation or low
ascorbic acid levels and poor fruit quality compared to the IW regime in ‘Sapphire’ plums. ‘Laetitia’
plums stored at 5 °C and 7.5 °C had significantly less CI than under the IW regime, but softened
quicker due to higher ethylene evolution rates.
‘Sapphire’ tolerated both long and short FAC durations, but a slower initial FAC rate prevented CI
manifestation and caused a higher HAA after cold-storage in this fruit. ‘Laetitia’ cooled with a
slower initial FAC rate and for a longer duration resulted in the best fruit quality and had higher
HAA, total phenolic, phospholipid and saturated phospholipid fatty acid concentrations during
storage. |
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