Abstract: Pericarp browning and loss in pulp quality are major problems of harvested litchi fruit. The effects of adenosine triphosphate (ATP) on the browning and quality of harvested litchi fruit were investigated. Litchi fruit were dipped into a solution of 0 or 1.0 mM ATP for 3 min and then stored for 5 days at 25°C and 80-90% relative humidity. ATP treatment effectively reduced skin browning, inhibited disease development and delayed increase in membrane permeability. Furthermore, the fruit treated with 1 mM ATP had higher concentrations of total soluble solids, titratable acidity and ascorbic acid of the flesh than the non-ATP-treated fruit, particularly in ascorbic acid. Application of ATP exhibited potential for browning control and quality maintenance of harvested litchi fruit.
INTRODUCTION
Litchi (Litchi chinensis Sonn.) is a subtropical to tropical fruit of high commercial value in the international market due to its sweet, acidic and crisp pulp (Tindall, 1994; Nakasone and Paull, 1998). However, the fruit after harvest are very perishable and rapidly lose quality (Nip, 1988; Ray, 1998). Pericarp browning and loss in flesh quality are major problems of harvested litchi fruit.
Litchi pericarp browning is generally thought to be the responsible for breakdown of cellular membranes, leading to loss of compartmentation between enzymes and substrates, which produced brown by-products (Huang et al., 1990; Jiang and Fu, 1998; Jiang, 2000). Jiang and Chen (1995a, b) reported that membrane permeability increased in aging litchi fruit while membrane fluidity deceased with increasing storage. Thus, the membrane deterioration of litchi fruit after harvest may lead to the browning.
Investigations showed that energy lack was associated with membrane deterioration during browning or senescence tissues of harvested horticultural crops (Saquet et al., 2000; Veltman et al., 2003). Trippi and Paulin (1984) found that increases in membrane permeability and proteolysis during carnation flower senescence in association with low ATP production. Duan et al. (2004) reported that membrane permeability increased while levels of ATP, adenosine diphosphate (ADP) and Adenylate Energy Charge (AEC) decreased during browning of litchi fruits. A direct relation between energy metabolism and membrane integrity was demonstrated in potato cell cultures by Rawyler et al. (1999) who found that potato cells, subjected to anoxia, showed an ATP synthesis threshold, below which membrane lipids hydrolysed. Investigations suggested that ATP might influence cellular decompartmentation, due to changes in membrane lipids of fatty acids, which may lead to altered biophysical or biochemical membrane properties (Marangoni et al., 1996; Harwood, 1988). Thus, a lack of energy supply may contribute to browning of harvested litchi fruit.
There are no published data about the role of ATP in harvested litchi fruit during storage. The objective of this study was to investigate the effects of ATP on browning of harvested litchi fruit in association with membrane integrity, disease development and sensory traits.
Materials and Methods
Plant Materials and Treatments
Fruits of litchi (Litchi chinensis Sonn. cv. Huaizhi) at an 80% mature
stage were obtained from a commercial orchard in Guangzhou, China. Fruit were
selected for uniformity of shape and colour and blemished or diseased fruit
were discarded. The fruit were dipped for 3 min in a solution of 0.1% TBZ with
0 or 1 mM ATP and air-dried for 2 h at 25°C. After treatments, fruit were
packed into 0.02 mm polyethylene bags (30 fruits/bag) and then stored at 25°C
and 80-90% relative humidity.
Skin Browning Measurement
Skin appearance was assessed by measuring the extent of the total browned
area on each fruit pericarp of 90 fruits, using the following scale (Jiang and
Chen, 1995a): 1, no browning (excellent quality); 2, slight browning; 3, <1/4
browning; 4, 1/4-1/2 browning; 5, >1/2 browning (poor quality). The browning
index was calculated as Σ (browning scale x percentage of corresponding
fruit within each class). Fruit at higher than 3.0 was considered unacceptable
for marketing. The subjective evaluation of skin browning index was correlated
well with the objective determination of the value of absorbance at 410 nm of
the skin extract (Jiang, 2000).
Disease Incidence Evaluation
The development of disease resulting from natural infection was monitored
by randomly collecting 90 fruits and then recorded as the percentage of fungal
growth or bacterial lesions on the surface.
Measurement of Membrane Permeability
Membrane permeability, expressed by relative electrolyte leakage rate, was
determined by the method of Jiang and Chen (1995a). Discs were removed with
a cork borer (10 nm in diameter) from the equatorial region of 30 fruits. Thirty
discs were rinsed twice and incubated in 25 mL of 0.3 M mannitol solution at
25°C, followed by shaking for 30 min. Electrolyte leakage was determined
with a conductivity meter (Model DDS-11A, Shanghai Scientific Instruments).
Total electrolyte leakage was determined after boiling another batch of 30 discs
for 15 min and cooling to 25°C (total electrolytes). Relative leakage was
expressed as a percentage of total electrolyte leakage.
Concentrations of Total Soluble Solids, Titratable Acidity and Ascorbic
Acid
Total soluble solids, titratable acidity and ascorbic acid of control and
ATP-treated fruit were analysed during postharvest life evaluation. Pulp (20
g) from 15 fruits were homogenised in a grinder and the supernatant phase was
collected to analyze concentrations of total soluble solids, titratable acidity
and ascorbic acid. Total soluble solids were assayed by using a hand refractometer
(J1-3A, Guangdong Scientific Instruments); titratable acidity as % citric acid
determined by titration with 0.1 M NaOH and ascorbic acid by 2,6-dichlorophenolindophenol,
respectively.
Data Handling
Experiments were arranged in a completely randomized design. There were
three replicates. Data were tested by the analysis of variance using SPSS version
7.5. Least Significance Differences (LSDs) were calculated to compare significant
effects at the 5% level.
Results and Discussion
Effects of ATP on Peel Browning and Disease Development
Litchi fruit rapidly deteriorated after harvest due to the browning and
decay (Huang and Scott, 1985; Johnson and Sangchote, 1994; Ray, 1998). Skin
browning index increased markedly with storage time, while disease developed
rapidly (Fig. 1 and 2). Dipping litchi fruit
into 1 mM ATP solution reduced browning index and disease development during
storage. Similar results were reported by Duan et al. (2004) who found
that reduced skin browning of litchi fruit after pure oxygen treatment was associated
with higher level of energy status of peel tissues. In this study, as ATP concentrations
used increased, there was not enhanced effect of the browning inhibition (data
not shown). Thus, the best concentration of ATP used requires further investigation.
Effects of ATP on Membrane Permeability
Browning of plant tissues was partly caused by membrane deterioration (Toivonen,
1992). Membrane permeability of litchi fruit, which was correlated negatively
with membrane integrity (Marangoni, 1996), gradually increased during storage.
The fruit dipped into 1 mM ATP solution had a lower leakage rate in association
with reduced browning index, compared with the control fruit (Fig.
1 and 3). This investigation showed that application of
exogenous ATP relatively maintained membrane integrity, which was consistent
with Eilam (1965) who suggested that energy was essential for maintenance of
membrane integrity. Saquet et al. (2001) reported that the flesh browning
of Conference pears was associated with lower ATP concentrations in the fruit
tissues, which might be due to loss in membrane integrity. Therefore, it could
be suggested that the reduction of litchi fruit skin browning by ATP treatment
could be accounted for maintenance of membrane integrity.
| Fig. 1: | Effect of ATP on browning index of litchi fruit. Fruit were
treated with 0 ( |
| Fig. 2: | Effect of ATP on disease incidence of litchi fruit. Fruit
were treated with 0 ( |
| Fig. 3: | Effect of ATP on electrolyte leakage rate of litchi fruit.
Fruit were treated with 0 ( |
Effects of ATP on Total Soluble Solids, Titratable Acidity and Ascorbic
Acid
Total soluble solids, titratable acidity and ascorbic acid are important
factors in assessing flavour and nutritive quality of litchi fruit (Jiang and
Fu, 1998). As shown in Table 1, the concentrations of total
soluble solids, titratable acidity and ascorbic acid of litchi flesh decreased
after 4 and 5 days of storage, possibly due to an enhanced respiration and oxidation
activities (Peng, 1998). Compared with the control, the fruit treated with 1
mM ATP had higher concentrations of total soluble solids, titratable acidity
and ascorbic acid after 5 days of storage, particularly in the concentration
of ascorbic acid (Table 1).
| Table 1: | Effects of ATP treatment on contents of total soluble solids,
titratable acidity and ascorbic acid of litchi fruit after 4 and 5 days
of storage |
| Means within cloums followed by some latter(s) are not significantly different at 5% level | |
Saquet et al. (2001) and Veltman et al. (2003) suggested that brown core initiation in Conference pears might be a result of a limited availability of energy and antioxidants, with low ascorbic acid level and ATP production. The effectiveness of exogenous ATP treatment on the browning of litchi fruit may be due to the enhancement of antioxidants, but this needs further investigation.
In conclusion, treatment with 1 mM ATP effectively reduced skin browning, disease development and membrane permeability and maintained eating quality of litchi fruit, particularly in the ascorbic acid level. However, further investigation into the effect of the ATP treatment on the fruit could be needed during storage at low temperature because the low temperature storage is used widely in commercial situations.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 30425040) and the International Foundation for Science (Grant No. E2265/3F).