Management of avocado postharvest physiology.
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Date
2011
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Abstract
Avocados are an important horticultural crop in South Africa, especially in the provinces of
KwaZulu-Natal, Mpumalanga and Limpopo. The distance to traditional export markets,
phytosanitary restrictions to lucrative markets such as China, the USA and Japan and
increased competition in the European market have challenged the South African avocado
industry. The industry has responded with improved logistics and shipping, a co-ordinated
market access program and a global system to co-ordinate exports of avocados to the
European market. To remain competitive on the global market, further improvements and
innovations are required to improve the efficiency of postharvest operations. These
improvements and innovations should be guided by a greater understanding of postharvest
physiology. Avocados are a relatively new export crop, so there is still much to be learnt about
avocado postharvest physiology and the optimisation of postharvest management. In this
regard, reduced temperature storage (1°C) and modified humidity packaging (MHP) were
investigated for their effect on fruit physiology and quality, the effect of a water- and ABA-infusion
on ripening was examined and the effect of a cold chain break on fruit physiology and
quality determined; near-infrared spectroscopy was also examined for its potential for its use
in the avocado industry.
As an initial study, the relationships between individual sugars, protein and oil were
studied to understand the changes in avocado fruit during ripening. It was found that
mannoheptulose and perseitol were the predominant sugars at harvest, but declined to very
low levels during the first 10 days postharvest. The concentrations of glucose and fructose
increased, while sucrose declined slightly during ripening. The concentration of protein
increased sigmoidally during ripening, reflecting the increase in the ripening enzymes,
particularly cellulase and polygalacturonase. The oil content fluctuated slightly during
ripening. It is suggested that mannoheptulose and perseitol are important carbon and energy
sources during ripening. Glucose concentration was also found to increase earlier in fast
ripening fruit compared to slow ripening fruit, which is related to increased cellulase activity
and may be related to the ABA functioning.
Thereafter, storage and ripening trials in two consecutive seasons showed that 1°C
storage and the use of MHP for 28 days reduced mass loss, water loss, ethylene production,
respiration, softening and heptose consumption, without appreciably affecting fungal rots,
physiological disorders or external chilling injury, compared to fruit stored at 5.5°C and
regular atmosphere respectively. Also, the storage of fruit in MHP delayed the rise in the
activity of cellulase during ripening, compared to fruit not stored in MHP, but there was no
significant difference in the peak activity of cellulase, polygalacturonase or pectin
methylesterase.
In a separate experiment, fruit ripening was significantly affected by the infusion of ABA
in an aqueous solution. Water slightly reduced the variation in ripening while ABA reduced
the time to ripening and the variation; it is suggested that water stress and ABA are
intrinsically involved in the ripening processes and may act as a ripening trigger. The water
concentration in fruit was measured non-destructively using reflectance NIR; this model was
used to determine the maturity of fruit and the loss of water during cold storage.
In the cold chain break experiment, it was found that although fruit recovered after a
cold chain break, in terms of ethylene production and respiration, there was a loss in quality
because of severe shrivelling as a result of increased water loss. Fruit that were stored at 1°C
were generally of a better quality at ripeness, if the cold chain was broken, compared to fruit
stored at 5.5°C. In a follow-up experiment, it was found that significant changes occurred in
avocado physiology over a 6h period. The respiration rate of fruit significantly increased after
4h at room temperature and mannoheptulose declined by 32% in control fruit and by 16% in
ethephon-treated fruit after 6h. This demonstrates the potential for quality loss in a short
amount of time.
Furthermore, a model of avocado ripening is proposed, outlining the role of water, ABA,
ethylene, respiration, ripening enzymes and individual sugars. This study has contributed to
the understanding of avocado postharvest physiology and should aid in better management of
avocados for improved fruit quality and consumer satisfaction.
Description
Thesis (Ph.D.Agric.)-University of KwaZulu-Natal, Pietermaritzburg, 2011.
Keywords
Avocado., Avocado--Postharvest physiology., Avocado--Ripening., Avocado--Packaging., Avocado--Storage., Avocado--Postharvest technology., Avocado--Quality., Theses--Horticultural science.