Effects of postharvest treatments on storage quality of lime (Citrus latifolia Tanaka) fruit : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, New Zealand
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Date
2009
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Massey University
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Abstract
Limes (Citrus latifolia Tanaka) are an attractive fruit crop but generally suffer a loss in
value as their colour changes from green to yellow. Various approaches were taken to
slow degreening including low temperature storage, use of controlled atmosphere (CA)
environments, and treatment of fruit with physiologically active agents such as gibberellic
acid (GA3). However, the cold storage life of lime fruit can also be restricted by a
number of factors including chilling injury (CI) and rots. Various pretreatments such as
the use of fungicide (thiabendazole, TBZ) and hot water dipping (HWD) and several
postharvest regimes based on temperature conditioning (step down technique) and
intermittent warming (IW) regimes were further investigated to protect the fruit against
rots and CI during cold storage. The objective of this study was to determine what
storage conditions and pretreatments would permit long term storage of NZ limes with
minimal loss of quality.
CA storage (10% O2 with 0 or 3% CO2) was compared to regular air storage (RA) and IW
(varying durations) treatments across a range of temperatures. Although some CA
storage regimes could assist in delaying degreening, none of the treatments provided
protection against CI. CA storage at 3% CO2 delayed yellowing and gave better fruit
quality than the low CO2 treatment. High CO2 CA treatments at 5 or 7°C decreased the
rate of colour change compared to other constant temperature treatments but did not
protect against CI. CI limited storage of fruit under all conditions at constant low
temperatures.
Including fungicide (TBZ) in the dip water reduced the incidence of rots and had a
secondary effect on protection against CI of lime fruit. However, fungicide use may
sometimes exacerbate stresses such as heat injury on lime peel. Hot water dipping has
been shown previously to hold potential as a storage pretreatment, but this technique may
give risk of damage on produce if it is dipped at too high a temperature. Some HWD
treatments did delay degreening, but there was no major effect on CI. HWD at > 47°C
for = 4 min caused heat injury to NZ limes. All HWD treatments showed severe CI
(>15%) after 10 weeks of cold storage; and HWD fruit stored under RA at 13°C did not show any CI but showed some pitting (= 10%) and degreened rapidly. Overall no suitable
HWD treatment for limes was identified in this trial.
This project identified the critical periods and temperature conditions for successful IW of
limes. The IW conditions successfully delayed losses in quality of lime fruit provided the
first warming period was applied within the first 20 days of storage. At least 2-cycle IW
was required to maintain lime quality during long term storage. Some benefits were found
after just one cycle of IW treatment but there were not enough to extend storage life.
IW storage benefited fruit quality and provided the highest overall fruit quality of all
postharvest treatments tested. The degreening of lime during cold storage at 5°C could be
delayed by IW treatments in which the fruit were stored at 5°C for 12, 16 or 20 days then
moved to 15°C for 2 days. Both 2- and 6-cycle IW treatments proved satisfactory for
maintaining colour on the green and yellow side of lime for 12 weeks of storage. IW
treatments in which fruit were warmed within 20 day of cold storage did not show
significant CI symptoms after 12 weeks of storage, and the 2-cycle IW treatment showed
only a low percentage of CI fruit at this time. A 2-cycle IW treatment was almost as
effective as 6 cycles, and a step down treatment also showed some promising results,
indicating that it may be possible to further optimize the time and duration of variable
temperature storage regimes to meet both quality requirements and the constraints of
temperature management in commercial coolstores. The application of these regimes to
other citrus species may also be beneficial. There are a number of physiological
explanations that may account for the effectiveness of IW including positive effects on
heat shock protein (HSP) and cell membranes. Nutritional factors such as vitamin C and
flavonoid compositions were also investigated and fruit that did not show visible CI were
found to retain at-harvest levels of these factors. Practical ways of implementing IW are
discussed.
In order to understand the effectiveness of IW on degreening, I used a logistic model to
describe degreening of lime peel. This modelling approach demonstrated that IW did not
change the mechanism of lime degreening based on the similarity between the hue values
predicted by the model and the actual hue values measured during lime storage. The
activation energy (Ea) for degreening based on either hue angle (H°) or colour score (CS)
during air storage was estimated to be ~53 and ~86 KJ.mol-1, respectively. Relationship between colour (H° and CS) and chlorophyll content, relationship between reflectance
spectra (%), chlorophyll content and H° of lime fruit stored under different conditions are
presented and discussed. This data allowed deduction to be made about the changes in
individual pigments that are driving colour change during “good” and “bad” storage.
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Keywords
Fruit colour, Fruit storage