Postharvest Biology and Technology 18 2000 235 – 244
Hot water brushing: an alternative method to SO
2
fumigation for color retention of litchi fruits
Amnon Lichter
a,
, Orit Dvir
a
, Ilana Rot
a
, Miryam Akerman
a
, Rafi Regev
b
, Aharon Wiesblum
b
, Elazar Fallik
a
, Giora Zauberman
a
, Yoram Fuchs
a
a
Department of Posthar6est Science of Fresh Produce, Institute for Technology and Storage of Agricultural Products, ARO, The Volcani Center, PO Box
6
, Bet Dagan
50250
, Israel
b
Department of Controlled Agriculture and En6ironmental Engineering, Institute of Agricultural Engineering, ARO, The Volcani Center, PO Box
6
, Bet Dagan
50250
, Israel Received 26 July 1999; accepted 18 November 1999
Abstract
Distribution of high-quality litchi Lychee chinensis Sonn. fruits to global markets depends exclusively on postharvest treatments to suppress peel browning. The current standard treatment of litchi fruits in Israel includes
fumigation with sulfur dioxide SO
2
followed by dipping the fruits in hydrochloric acid containing the fungicide prochloraz. As part of the effort to reduce the use of potentially hazardous chemicals in agriculture, we developed a
new procedure that may enable SO
2
to be avoided. Instead of fumigation, litchi fruits are sprayed with hot water while being brushed in a revolving drum, after which the fruits are subjected to hydrochloric acid treatment. Fruits
that are processed in this way maintain a uniform red color for at least 35 days, without apparent deterioration in external or internal quality, or taste. Physiological studies demonstrate that polyphenol oxidase PPO activity is
reduced by the hot water brushing HWB procedure as compared with controls but not to the same extent as inhibition by SO
2
treatments. In addition to its effect on PPO activity, HWB may lead to reduced pH of the pericarp, or more uniform distribution of the acid in it. This result suggests that HWB may act by bruising the external layer
of the pericarp allowing the peel to be uniformly exposed to the acid which may inhibit PPO activity and maintain the anthocyanins in their red-pigmented form. © 2000 Elsevier Science B.V. All rights reserved.
Keywords
:
Postharvest treatment; Polyphenol oxidase; Hot water www.elsevier.comlocatepostharvbio
1. Introduction
Litchi Lychee chinensis Sonn. fruits are in high demand as exotic commodities, because of their
appealing natural color and rich taste and aroma. However, if the fruit is not treated after harvest,
the red pericarp becomes brown and desiccated
Corresponding author. Tel.: + 972-3-9683915; fax: + 972- 3-9683622.
E-mail address
:
ntlichnetvision.net.il A. Lichter 0925-521400 - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 5 - 5 2 1 4 9 9 0 0 0 7 7 - 0
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
