achieved by physical and chemical methods. Modified atmospheres with elevated CO
2
levels and low temperatures are effective in reducing the
incidence of decay Mass, 1981; Reyes and Smith, 1986; Brown, 1992. However, prolonged exposure
of berries to high CO
2
concentrations can cause off- flavor development Li and Kader, 1989; Ke et al.,
1994 and low temperature alone is not an effective means of control Brown, 1992. Although prophy-
lactic field sprays with systemic benzimidazoles are effective in controlling post-harvest fungal infec-
tions Dennis, 1975, there is an increased concern among consumers about the potentially harmful
health effects of chemical residues Klein and Lurie, 1991, and development of chemical tolerance in
post-harvest pathogens Spotts and Cervantes, 1986. Thus alternative approaches are necessary to
maintain the marketable quality of strawberries.
Chitosan, a high molecular weight b-1,4-glu- cosamine polymer, is an important structural com-
ponent of the cell wall of some plant-pathogenic fungi, especially Zygomycetes Bartnicki-Garcia,
1970. It is also produced from the chitin compo- nents of arthropod exoskeletons; by deacetylation.
Chitosan has been shown to have anti-fungal activity against a wide range of fungi El Ghaouth et
al., 1992a,b. Chitosan coating of harvested straw- berries protected them from infection by B. cinerea
and improved their quality El Ghaouth et al., 1991a. Chitosan coating also reduced the incidence
of decay of tomato, bell pepper and cucumber, delayed ripening of apple and tomato fruits and
reduced the desiccation of pepper and cucumber El Ghaouth et al., 1991c; Davis and Elson, 1994.
Chitosan-induced glucanohydrolase activity in strawberries was inhibitory against B. cinerea El
Ghaouth et al., 1991b. The present study reports the effects of prophylactic pre-harvest chitosan
sprays applied at intervals on the decay and keeping quality of harvested strawberries stored at 3 and
13°C.
2. Materials and methods
2
.
1
. Culti6ar Runner roots of the cultivar Seascape were
transplanted to 30 cm diameter plastic pots three roots per pot filled with potting mixture
containing four parts by volume of pasteurized organic soil, one part of peatmoss and one part
of perlite. The pots were randomized on green house benches, all within one large chamber.
Plants were maintained at 25°C 1212 h, light dark cycles and watered at regular intervals. At
the time of flowering plants were agitated to simulate bee transfer of the pollen for opti-
mum fruit setting. A total of 63 plants per treat- ment in three replications of 21 each were
maintained in a randomized complete block de- sign RCBD.
2
.
2
. Chitosan sprays Shrimp-shell chitosan was purchased from
Nova-Chem Ltd.
Dartmouth, Nova
Scotia, Canada and ground into a fine powder. The
purified chitosan was prepared by dissolving chi- tosan in 0.25 N HCl, and the undissolved parti-
cles were removed by centrifugation 15 min, 10 000 × g at 24°C. The solution was neutral-
ized with 2.5 N NaOH to a pH of 8.0 to pre- cipitate the chitosan. The precipitated chitosan
was recovered by filtration, washed extensively with deionized water to remove salts and was
subsequently lyophilized. Chitosan stock solution 10 g l
− 1
was prepared by dissolving chitosan in 0.05 N HCl and pH was adjusted to 5.6.
Different chitosan concentrations of 2, 4, and 6 g l
− 1
were prepared in water. Plants were sprayed with chitosan solution from a hand sprayer when
the fruit were just turning red. The spray was continued
until the
deposition of
chitosan droplets was uniform on the fruit surface. A set
of plants was also sprayed with sterile water as control. Sprays were repeated after an interval of
10 days.
2
.
3
. Har6esting and storage Fruit were harvested 5 and 10 days after each
spray designated as pick 1 and pick 2, respec- tively. Berries of uniform size, free of physical
damage and fungal infection were selected. For quality evaluation, 30 fruit were randomly dis-
tributed into replicates of 10 fruit for each treat- ment, temperature, spray, pick and storage inter-
val.
2
.
4
. Inoculation of chitosan sprayed fruits B. cinerea was isolated from infected strawber-
ries and maintained on potato dextrose agar PDA. Conidia of B. cinerea were recovered by
flooding 2-week-old cultures with sterile water containing 0.1 vv Tween 80 and filtering the
mycelial suspension through three layers of ster- ile
cheese cloth.
The concentration
of the
conidial suspension was adjusted to 2 × 10
5
conidia per ml. Berries to be inoculated were transferred to plastic containers in three repli-
cates of ten each with three layers of moistened blotters at the bottom. Fruit were inoculated in-
dividually with 20 ml of conidial suspension. The boxes were closed with perforated lids and subse-
quently
transferred from
ambient temperat-
ure to 3 or 13°C for storage. Strawberries were evaluated weekly for disease symptoms, and
spoiled fruit were discarded to avoid secondary infection.
2
.
5
. Assessment of quality The effect of pre-harvest chitosan sprays on
post-harvest quality of strawberries was assessed each week. A sample of seven to nine berries was
randomly removed from each replicate and ana- lyzed for firmness, titratable acidity and an-
thocyanin content. For firmness, berries were sliced into halves and each half was punch tested
on a texture analyzer using a 4-mm flat plunger Texture Technologies Corp., Scarsdale, NY.
Acidity was determined using a 10 g aliquot of puree in 40 ml of deionized water and titrating
with 0.1 N NaOH to an end point of pH 8.1. Titratable acidity was expressed as g of citric
acid per l. Anthocyanins were extracted with aci- dified ethanol from a 2 g aliquot of homogenate
according to the method of Fuleki and Francis 1968. Anthocyanin content was expressed as
mg anthocyanin per g fresh weight of strawberry homogenate.
2
.
6
. Experimental design and analysis The experiment was a completely randomized
4 × 2 × 2 × 4 factorial design. The factors were chitosan concentration, number of sprays, num-
ber of picks and storage interval. The experiment was conducted twice, and within each repetition
the treatment order was random. Analysis was carried out with triplicate data from each repeti-
tion. Data were pooled across number of sprays and picks 2 sprays × 2 picks and S.E.M. were
determined.
2
.
7
. Kinetics of fruit quality deteriorationchange Shelf life based on a specific criterion of qual-
ity acceptability at a specified environmental con- dition can be estimated from kinetic models
Labuza, 1982:
− dq
dt =
k[q]
n
1 where [q] is any quality characteristic, t the time,
k the rate constant, and n is the order of the quality deteriorationchange. The application of
chitosan is expected to modify the kinetics of the deteriorationchange of strawberry quality during
storage. Therefore, the rate of quality loss can be modeled as:
− dq
dt =
k −
k
i
c[q]
n
2 where c is chitosan concentration, k
the rate constant at 0 chitosan concentration, and k
i
is the inhibition constant at different chitosan con-
centrations. The value of n can range from 0 zero order kinetics up to 2 second order for
various reactions. Integration of Eq. 2 yields Eqs. 3 – 5 for zero, first and second order ki-
netics, respectively.
[q] = [q ] − k
− k
i
ct, n = 0
3 ln[q] = ln[q
] − k −
k
i
ct, n = 1
4 1
[q] =
1 [q
] +
k −
k
i
ct, n = 2
5 with
k
c
= k
− k
i
c 6
where k
c
is the rate constant at different chitosan concentrations.
At zero chitosan concentration, Eq. 2 becomes,
− dq
dt =
k [q]
n
7 On integration, Eq. 7 yields Eqs. 8 – 10 for
zero, first and second order kinetics, respectively: [q] = [q
] − k t,
n = 0 8
ln[q] = ln[q ] − k
t, n = 1
9 1
[q] =
1 [q
] +
k t,
n = 2 10
Where [q ] is the value of the quality attribute
at time 0. From the experimental data k , k
c
and k
i
c can be determined. The sensitivity of a food material or a reaction
to temperature changes can be expressed by Q
10
, the degree by which the process is accelerated by
a rise of 10°C. Expressing a modified Q
10
factor for various chitosan concentrations with respect
to 0 chitosan concentration, Q
10
= k
c
at T + 10°C k
at T°C 11
Mean inhibition constant k
i
is given by, k
i
= k
i1
c
1
+ k
i2
c
2
+ k
i3
c
3
+ …
c
1
+ c
2
+ c
3
+ …
12
2
.
8
. Determination of kinetic parameters The quality characteristics considered include
decay, texture, anthocyanin and titratable acidity. An optimization routine uses the values of the
independent variable the t values to predict the value of a dependent variable the [q] value. The
routine uses the Marquardt – Levenberg algorithm Marquardt, 1963 to find the parameters of the
independent variable t that give the best fit between the model and the data.
The model used obeys the following differential equation:
g d[q]
dt =
k[q]
n
13 with the initial condition at, t = 0; [q] = [q
]; with g =
1, if [q] increases with time and g = − 1, if [q] decreases with time.
The agreement between experimental and pre- dicted values was judged acceptable when the
mean deviation was less than the mean experi- mental error.
3. Results
