before reaching the markets. One of the hypothe- sized mechanisms for the rapid browning is that it
results from oxidation and polymerization of phe- nolic compounds, including the red anthocyanins,
by polyphenol oxidase PPO Akamine, 1960; Zauberman et al., 1989; Underhill et al., 1992. In
undamaged tissue, PPO is presumed to be local- ized in the plastids, while anthocyanins accumu-
late in the vacuole. The desiccation of the rind and formation of micro-cracks may be partly
responsible for cell degradation, resulting in asso- ciation of PPO with its potential substrates Un-
derhill and Critchley, 1992, 1993a. It should be noted however, that there is no conclusive evi-
dence for the role of PPO in anthocyanin degra- dation Underhill and Critchley, 1994.
A number of methods have been devised over the years to retain the appealing color of litchi
fruits Holcroft and Mitcham, 1996; Ray, 1998. One of the most widely used methods involves
fumigation of the fruits with SO
2
, several hours after harvest Swarts, 1983; Zauberman et al.,
1989. This causes the red color to be bleached to yellow, which is slowly and partially restored to
pink. SO
2
interacts with the membranes, making the rind pliable and leaky to solutes. Also SO
2
directly reacts with anthocyanins rendering them colorless and stabilizing them against degradation
Timberlake and Bridle, 1975, and it is thought to inhibit litchi PPO. The dipping of litchi fruits
in dilute hydrochloric acid was developed by Za- uberman et al. 1989 in order to restore the red
color of the fruit after bleaching: the lower pH of the rind restores the red color to the colorless
anthocyanins. This procedure was followed by dipping the fruits in prochloraz solution, but
Fuchs et al. 1993 found that prochloraz was better dissolved directly in the HCl solution, and
as such was more effective in preventing fungal development. After drying, the fruits are packed
and can be cold-stored for several weeks. This procedure enables the marketing of attractive
fruits, but SO
2
represents health risks for allergic people Taylor, 1993 and therefore, SO
2
treat- ments have been banned in the USA for every
purpose other then control of gray mold in table grapes Paull et al., 1995; Holcroft and Mitcham,
1996. Alternative procedures to SO
2
fumigation of litchi fruits have been proposed, e.g. steam treat-
ment Kaiser et al., 1995 or hot benomyl dipping Scott et al., 1982, but so far, no method has
been widely accepted or established commercially. Our objective was to develop a procedure that will
not compromise the quality of the fruits and that might be applied commercially upon demand
from the market or public health authorities. The principle of brushing fruits and vegetables under
hot water spray was reported recently for the postharvest treatment of bell peppers Fallik et
al., 1999 and mangos Prusky et al., 1999. The apparatus adopted for litchi fruits is different
from the systems used for other commodities and consists of a revolving drum internally covered
with a bristle-brushing surface fitted with hot water nozzles. During 1998, approximately 500 kg
of the cultivar ‘Mauritius’ from five different growers and in four different regions of the coun-
try were treated by hot water brushing HWB and evaluated for quality. Combined with acid
dipping, this procedure yielded high quality and safer litchi fruits.
2. Materials and methods
2
.
1
. Litchi fruits Litchi fruits Lychee chinensis Sonn. cv. Mauri-
tius were obtained from growers on the day of harvest, transported in an air-conditioned van
and treated at the Volcani Center, 3 – 9 h after harvest, with the newly-developed hot water
brushing HWB system or by fumigation with SO
2
as previously described Zauberman et al., 1991. Immediately after HWB or after an
overnight interval at 1.5°C following SO
2
treat- ment, the fruits were dipped in an acid solution
containing 4 food-grade HCl and 0.2 prochlo- raz Sportak, AgrEvo UK Ltd., Hauxton, Cam-
bridge, England. The fruits were dried under ambient conditions for 15 – 30 min and packed in
2 kg open cardboard boxes. Storage for 3 weeks 1.5°C, RH \ 95 was followed by shelf-life sim-
ulation for 3 or 5 days 18°C, RH 88 – 92 after which the fruits were evaluated for quality.
2
.
2
. Fruit quality e6aluation Brown discoloration of the fruit was deter-
mined by examining the fruits usually 2 kg per package and assigning an index on a scale of
1 – 5 1, 100 of the fruits with brown discol- oration; 5, no brown discoloration. As 5 of
the fruits are equivalent to 0.2 index units within the range of 1 – 5, the index was calcu-
lated from the following formula: 5 − of brown fruits × 0.25. Fruits with an index be-
low 3.0 are considered commercially unaccept- able. Fruit firmness was evaluated manually
according to three categories: 1, very soft; 5, soft; 10, firm. Brown patches on the rind were
categorized as being either light or intense. Fruits with a brown patch only around the
pedicel were assigned to a separate category. Ten fruits from each replicate were peeled, and
their internal browning and taste were recorded.
2
.
3
. Hot water brushing Based on the technology described by Fallik
et al. 1999, a modified machine has been de- veloped for hot water brushing HWB of small
fruits. Instead of rolling brushes, a revolving drum, internally covered with a brushing sur-
face, and hot water nozzles, was built. Litchi fruits were rinsed in hot water at 55°C and the
angle of the drum was set to retain the fruits for 20 s inside it. The fruits were then dipped in
HCl solution, dried, packed and stored as de- scribed above.
2
.
4
. Prochloraz residues To determine prochloraz levels, the fruits were
treated by HWB or with SO
2
followed by HCl prochloraz 40.2 vv dipping, or were un-
treated. Twenty-five fruits in each of four replicates were submitted for chemical analysis
Plant Protection and Inspection Services, Pesti- cide Division, Bet Dagan, Israel. The samples
were prepared for analysis according to FDA- PAM, 1994, for non-lipid products.
2
.
5
. Leakage from peel discs After the litchi fruits had been treated as spe-
cified, the peel from four fruits per treatment was cut in half, peeled off from the flesh and lightly
dried on a paper towel to remove free juice. A 10-mm cork borer was used to cut out 15 peel
disks that were weighed and immediately placed in beakers containing 20 ml of 0.4 M mannitol
solution. The beakers were incubated on a rotat- ing shaker 100 rpm for 2.5 h at room tempera-
ture 23°C. An El-Hamma TH27 conductometer cell constant 0.99 cm was used to measure solu-
tion conductivity, and readings were taken in absolute mode on 300-mmho or 1-mmho scales.
Total conductivity was assayed after freezing the peel disks in solution, boiling and cooling to room
temperature.
2
.
6
. Polyphenol oxidase acti6ity and determination of pericarp pH
Fruits were treated by HWB 6, 7.5 and 9 h after harvest and were dipped in acid 5 – 20 min after
HWB. The fruits were stored at 20°C for 5 days to obtain an immediate assessment of their qual-
ity, or were stored for 3 weeks at 1.5°C and then for 5 days at 20°C. Fruits were frozen before and
after HWB, after acid treatment, and different time points during storage, as specified. Control
treatments included fruits treated by HWB with- out the acid treatment, or by hot water dipping
55°C for 20 s followed by acid dipping, or by SO
2
fumigation followed by acid dipping. Three fruits per treatment were thawed and peeled, and
2 g of pericarp tissue was homogenized in 0.1 M phosphate buffer, pH 6.6 and 0.5 g of insoluble
polyvinyl pyrollidone Merck for 30 s with a polytron
homogenizer Kinematica
GmbH, Kreins, Luzern, Switzerland; probe diameter, 20
mm set at speed 6. The homogenate was cen- trifuged for 10 min at 8000 × g in a Sorvall rotor
SS-34 at 4°C. The supernatant was collected and centrifuged again in 1.5 ml tubes at 20 000 × g for
10 min at 4°C. The supernatant was collected into a fresh tube and 0.75 ml was used for the PPO
assay in duplicate. The PPO assay was conducted by adding 0.12 ml 4-methyl catechol Sigma, St.
Louis, MO, USA freshly dissolved 0.25 g in 2 ml of ethanol and 10 ml of water final concentra-
tion, 23 mM. A Contron spectrophotometer was used to follow changes in absorbance at 410 nm
over 2 min, and the linear progress of the reac- tions was recorded between 30 and 90 s. Results
