SHELF LIFE OF FRESH-CUT SPINACH AS AFFECTED BY CHEMICAL TREATMENT AND TYPE OF PACKAGING FILM

Brazilian Journal

ISSN 0104-6632
of Chemical
Printed in Brazil
Engineering
Vol. 19, No. 04, pp. 383 - 389, October - December 2002
SHELF LIFE OF FRESH-CUT SPINACH AS
AFFECTED BY CHEMICAL TREATMENT
AND TYPE OF PACKAGING FILM
A.M.Piagentini* and D.R.Güemes
Instituto de Tecnología de Alimentos, Facultad de Ingeniería Química,
Universidad Nacional del Litoral, C.C. 266 - 3000 Santa Fe - Argentina
E-mail: ampiagen@fiqus.unl.edu.ar
(Received March 5, 2002 ; Accepted, June 13, 2002)
Abstract - Fresh-cut vegetables are an important and rapidly developing class of convenience foods. Their
storage life may be greatly reduced due to their high rates of respiration and transpiration and the possibility
of enzymatic and microbiological deterioration. Consequently, the objective of this work was to determine the
shelf life and the failure attribute that conditioned the shelf life of fresh-cut spinach treated with chemical
solutions and packaged in bags with different permeabilities. The shelf life of fresh-cut vegetables was
defined as the time of refrigerated storage at which any one of the sensory attributes scored below 7 or when
the microbiological counts exceeded 5.107 CFU/g. Fresh-cut spinach was treated with citric acid and ascorbic
acid solutions and packaged in mono-oriented polypropylene (OPP) bags or low-density polyethylene (LDPE)
bags. Sensory attributes and total microbial counts were evaluated throughout refrigerated storage. Response
surface methodology (RSM) was used to study the simultaneous effect of chemical treatment and refrigerated
storage time on sensory and microbiological quality of fresh-cut spinach. A quadratic polynomial regression
model was assumed for predicting off-odor, general appearance, wilting, browning, color, and mesophilic
aerobic population. Type of packaging film only influenced development of off-odor (p?0.001) and had no
effect on visual sensory attributes or microbiological counts (p>0.05). Development of off-odor was the
attribute that limited shelf life of fresh-cut spinach packaged in OPP bags. On the other hand, shelf life of
samples packaged in LDPE bags was dependent on a decrease in general appearance or an increase in
microbiological counts, depending on the chemical treatment used.
Keywords: shelf life, fresh-cut vegetables, packaging, spinach.
INTRODUCTION
(Huxsoll and Bolin, 1989). Therefore, spoilage of
fresh-cut vegetables may result from degradation of
Fresh-cut vegetables are an important and rapidly
sensory characteristics (color, texture, flavor, or
developing class of convenience foods. The
aroma) as well as from microbiological degradation.
suitability of these products is based on their
Of the methods available for controlling or
attributes of convenience and fresh-like quality. A
moderating these degradative processes, packaging
problem faced by these products is that their storage
technology is vitally important. In particular,
life may be greatly reduced as compared with the
utilization of permeable polymeric films moderates
unprocessed raw material from which they are made.
moisture loss, retards the onset of product
This problem may be due to their high rates of
senescence and reduces the rate of sensory quality
respiration and transpiration and the possibility of
degradation by affecting the concentration of gases
enzymatic and microbiological deterioration
and water vapor within the package (Schlimme and
*To whom correspondence should be addressed

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384 A.M.Piagentini and D.R.Güemes
Rooney, 1994). However, improperly modified
A quadratic polynomial regression model,
atmospheric conditions may adversely affect product
Eq. (1), was assumed for predicting individual
quality. Therefore, selection of suitable packaging
Yk response variables (Watada et al., 1987;
materials is critical to obtaining the environment
Montgomery, 1991):
desired for extending the shelf life of fresh-cut
vegetables.
Y
2
2
k=bko+bk1.X1+bk2.X2+bk3.X3+bk11.X1 +bk22.X2 +
Chemical treatments are also used on lightly
(1)
processed fresh produce mainly for controlling
+bk
2
33.X3 +bk12.X1.X2+bk13.X1.X3+bk23.X2.X3
decay, reducing browning and retaining firmness
(Brecht, 1995). Low-pH organic acid solutions are
where bko, bki, bkii, and bkij are regression
used as antimicrobials to control bacteria or as
coefficients and Xi the coded independent variables,
antioxidants to prevent browning; to reduce
linearly related to cC, cA, and t (Table 1).
discoloration of pigments; and to protect against loss
The factorial design described was used for
of flavor, changes in texture, and loss of nutritional
samples packaged in mono-oriented polypropylene
quality (Wiley, 1994). Chemical treatments with
bags (OPP) and in low density polyethylene bags
citric acid and/or ascorbic acid solutions prevent
(LDPE) (29 runs for each type of packaging film).
browning in fresh prepared potatoes (Langdon,
1987) and lettuce (Castañer et al., 1996).
Spinach Preparation
Unfortunately, products that are artificially acidified
to pH levels of 3.5-4 lose their fresh-like quality
Spinach (Spinacia oleracea L., var. Hybrid 424)
(Huxsoll and Bolin, 1989).
was obtained from a local farm near Santa Fe
Consequently, the objective of this work was to
(Argentina). Outer, damaged and yellowed leaves,
determine the shelf life and the failure attribute that
roots, and stems were removed. The remaining
conditioned the shelf life of fresh-cut spinach treated
leaves were cut into 30mm x 40mm pieces and
with chemical solutions and packaged in bags with
washed for 2 min in cold water (4-6°C) containing
different permeabilities.
120 mg/L available chlorine. Then they were rinsed
with running tap water and dipped in citric acid and
ascorbic acid solutions for 2 min as described in the
MATERIAL AND METHODS
experimental design. The spinach was centrifuged in
a basket-type centrifuge at 540 rpm for 3 min. Fresh
Experimental Design
cut spinach samples (70 g) were placed in mono-
oriented polypropylene bags 30 µm thick, or in low
Response surface methodology (RSM) was used
density polyethylene bags 40 µm thick, according to
to study the simultaneous effect of chemical
the experimental design. Gas transmission rates of
treatment and refrigerated storage time on fresh-cut
OPP for O2, CO2, and water vapor were 2,000
spinach. The experiments were based on a three-
cm3/m2.24h.atm, 6,000-7,000 cm3/m2.24h.atm (both
level, three-factor factorial design, with two
at 23 °C and 1 atm), and 3-5 g/ m2.24 h. (at 37 °C
replicates at the central point (Montgomery, 1991). It
and 90 % RH.), respectively. Gas transmission rates
was assumed that a mathematical function exists for
of LDPE for O2, CO2, and water vapor were
each response variable Yk (off-odor, general
3,500-4,000 cm3/m2.24h.atm, 12,000-15,000
appearance, wilting, browning, color, and mesophilic
cm3/m2.24h.atm (both at 23 °C and 1 atm), and 10 g/
aerobic population) in terms of three independent
m2.24 h. (at 37 °C and 90 % RH.), respectively.
variables: citric acid concentration (cC), ascorbic acid
Finally, the sealed bags were stored at 4.0 °C and
concentration (cA) and refrigerated storage time (t).
90% RH.
Table 1: Independent variables and their levels
Coded-variable levelsa
Independent variable
Symbol
-1
0
1
Concentration of citric acid (%, w/v)
X1
0.00
0.50
1.00
Concentration of ascorbic acid (%, w/v)
X2
0.00
0.50
1.00
Time (day)
X3
0
7
14
aCoding equations: X1= (cC - 0.50)/0.50; X2= (cA - 0.50)/0.50; X3=(t - 7)/14
Brazilian Journal of Chemical Engineering

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Shelf Life of Fresh-Cut Spinach 385
Microbiological Analysis
terms of this factor (X1.X4, X2.X4, X3.X4) had no
effect (p > 0.05) on the visual sensory attributes and
Shredded spinach (10 g) was mixed with 90 mL
mesophilic aerobic population. Thus, the effect of cC,
0.1 % peptone water and homogenized with a
cA, and t on visual sensory attributes and
stomacher for 2 min. Further decimal dilutions, as
microbiological counts for both types of packaging
required, were prepared with the same diluent and
film was modeled as a whole (Montgomery, 1991).
plated, in duplicate, on PCA and incubated during 48
Di Pentima et al. (1996) studied sensory quality
hr at 30 °C. Viable counts were reported as log
of ready-to-use broccoli, whole spinach leaves and
CFU/g (FAM).
asparagus packaged in different plastic films, stored
at 4 °C. They also found no differences in general
Sensory Analysis
appearance, color, browning, and wilting in samples
packaged in different types of films. However,
A trained quantitative descriptive panel was
development of off-odor was significantly different
required to evaluate sensory quality attributes of
depending on the film used, with higher values for
spinach samples. The judges indicated their
the film with lower permeability.
perception of the intensity of each quality attribute
The ANOVA analyses of regression models
on a 100mm unstructured line, with anchored terms
indicated the models were highly significant (p ?
located 10 mm from either end. They scored the
0.001), exhibiting no significant lack of fit (p >
perceived intensity of each descriptor by placing a
0.05). The coefficients of determination (R2),
vertical line across the unstructured scale line.
explaining the variability of experimental data, were
Quantitation was accomplished by measuring the
satisfactory. Based on all these tests, the prediction
distance from the left end (0.00) to the vertical line,
models were accepted. The regression coefficients
reporting measurements in cm. The anchored terms
of the fitted models are presented in Table 2.
were indicated from left to right as severe to none for
Shelf life of the product must exceed the
off-odors, very poor to excellent for general
minimum distribution time required from the
appearance, very severe to none for wilting and
processor to the consumer to allow a reasonable
browning, and yellow to dark green for color. Larger
period for home storage and use. This shelf life is
numbers on these scales represented higher quality
defined as the time period that a product can be
for all attributes (Pirovani et al., 1997). The panelists
expected to maintain a predetermined level of quality
evaluated development of off-odor, general
under specified storage conditions (Shewfelt, 1986).
appearance, wilting, browning, and color of fresh-cut
Considering that fresh-cut vegetables must maintain
spinach on each testing day.
their fresh-like quality and the limit of mesophilic
aerobic population proposed by the French
Statistical Analysis
legislation at the end of the consumption stage, the
shelf life was defined as the time of refrigerated
All data were submitted to analysis of variance to
storage at which any one of the sensory attributes
determine the effect of cC (X1), cA (X2), t (X3), and
scored below 7 or when the microbiological counts
type of packaging film (X4). STATGRAPHICS Plus
exceeded 5.107 CFU/g (Anónimo, 1989; Pirovani et
was used to fit the experimental data to the second-
al., 1997).
order polynomial equations, to obtain the
The shelf life of a whole product at any given
coefficients of the equations, for regression analyses,
temperature is determined by the mechanism in any
and for analyses of variance. Response surfaces and
component which proceeds fastest and thus causes
contour plots were developed using the fitted
the shortest life. A major question for fresh-cut
quadratic polynomial equations obtained.
vegetables is whether sensory spoilage, as a result of
chemical or biochemical reactions, or growth of
spoilage organisms determine the shelf life. To
RESULTS AND DISCUSSION
answer this question, in this work, the shelf life of
fresh–cut spinach was determined by superimposing
The multifactor ANOVA indicated that storage
the contour plots of each sensory attribute over the
time affected (p ? 0.001) all sensory attributes
contour plot of mesophilic aerobic count for the
studied and total microbial counts. Type of
selected criteria. Contour plots represented lines of
packaging film (X4) only influenced (p ? 0.001)
equal sensory score or FAM as a function of time
development of off-odor of fresh-cut spinach. The
and ascorbic acid concentration for cC = 0.50%.
multifactor ANOVA also indicated that type of
The shaded areas in Figures 1, 2, and 3 show the
packaging film and the corresponding interaction
commercialization period for spinach samples, which
Brazilian Journal of Chemical Engineering, Vol. 19, No. 04, pp. 383 - 389 October - December 2002

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386 A.M.Piagentini and D.R.Güemes
satisfies the following conditions simultaneously:
0.94 % and cC = 0.50% (Figure 3a) or cC = 1.00%, cA
sensory attribute scores ? 7 and FAM ? 7.70.
? 0.71% (Figure 3b), general appearance is the
It was found that wilting, browning, and color
attribute that conditioned shelf life of fresh-cut
scores were always higher than 7 when FAM = 7.70.
spinach packaged in LDPE bags up to 8 days.
Therefore, these visual attributes did not determine
Pirovani et al. (1997) also found that the failure
the shelf life of fresh-cut spinach packaged in OPP
attribute that conditioned the shelf life of minimally
or LDPE bags (Figure 1).
processed cabbage could be different depending on
The off-odor score of fresh-cut spinach packaged
the packaging film used.
in OPP bags decreased faster than the off-odor score
When deciding whether to buy a particular
of samples packaged in LDPE bags and decreased to
commodity, consumers evaluate fresh-cut vegetable
7 before the mesophilic aerobic population reached
quality by visual sensory attributes. Therefore,
the limit value of 7.70. (Figure 2). Therefore,
according to score ratings obtained for visual sensory
development of off-odor was the attribute that
attributes, some fresh-cut spinach samples could be
limited shelf life of fresh-cut spinach packaged in
considered suitable for commercial sale after 14 days
OPP bags up to 6 days (Figure 2a).
of storage. Thus, with no obvious signs of spoilage,
It was found that the general appearance score
mesophilic aerobic microorganisms might be present
could decrease to 7 before FAM = 7.70 for samples
at high concentrations, without anything to warn
packaged in LDPE. For a chemical treatment of cA ?
consumers of the danger.
Table 2: Regression coefficients of the second-order polynomial equations
Off- odor Off- odor
General
Coefficients
FAM
Wilting
Browning
Color
OPP
LDPE
Appearance
bo
7.752
6.941
8.266
8.025
8.191
7.968
8.136
b1
-0.523
-0.014
-0.144
-0.378
-0.176
-0.282
-0.089
b2
-0.316
-0.092
-0.391
-0.564
-0.180
-0.445
-0.157
b3
3.156
-2.211
-0.787
-1.380
-0.860
-0.986
-1.023
b11
0.201
-0.480
-0.051
0.0396
0.078
-0.0124
-0.033
b22
-0.0397
-0.314
0.109
-0.485
-0.187
-0.285
0.036
b33
0.946
0.278
-0.191
-0.363
-0.133
0.00087
-0.317
b12
0.122
0.307
-0.303
-0.301
0.0113
-0.054
-0.185
b13
-0.096
0.064
0.0025
-0.325
-0.1496
-0.097
-0.191
b23
-0.127
-0.042
-0.228
-0.488
-0.155
-0.292
-0.3198
Wilting
cC = 0.50%
Time (day)
14
7.5

8
FAM = 7.70
7

8.5
0
0
0.25 0.50
0.75 1
Ascorbic acid (%)
Brazilian Journal of Chemical Engineering

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Shelf Life of Fresh-Cut Spinach 387
Browning
Color
cC = 0.50%
cC = 0.50%
Time (day)
Time (day)
14
14
7.5
7.5


8
8
FAM = 7.70
FAM = 7.70
7
7
8.5
8.5


0
0
0
0.25 0.50
0.75 1
0
0.25 0.50
0.75 1
Ascorbic acid (%)
Ascorbic acid (%)


Figure 1: Contour plots of wilting, browning, and color superimposed over the
contour plot of mesophilic aerobic counts (FAM) at 7.70
(a) Off-odor OPP
(b) Off-odor LDPE
c
c
Time (day)
C = 0.50%
C = 0.50%
Time (day)
14
14

7.5

6

8
FAM = 7.70
FAM = 7.70
7
7
7
8.5


8
0
0
0 0.25
0.50
0.75 1
0 0.25 0.50
0.75 1
Ascorbic acid (%)
Ascorbic acid (%)

Figure 2: Contour plots of development of off-odor superimposed over
the contour plot of mesophilic aerobic counts (FAM) at 7.70
Brazilian Journal of Chemical Engineering, Vol. 19, No. 04, pp. 383 - 389 October - December 2002

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388 A.M.Piagentini and D.R.Güemes
(a) General appearance
Time (day)
cC = 0.50%
14
7

8
FAM =7.70
7

0
0 0.25
0.50
0.75 1
Ascorbic acid (%)

(b) General appearance
Time (day)
cC = 1.00 %
14

7


FAM = 7.70
7
8

0
0 0.25 0.50
0.75 1
Ascorbic acid (%)

Figure 3: Contour plots of general appearance superimposed over
the contour plot of mesophilic aerobic counts (FAM) at 7.70
Brazilian Journal of Chemical Engineering

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Shelf Life of Fresh-Cut Spinach 389
CONCLUSIONS
Castañer M., Gil M. I., Artes F., and Tomas-
Barberan F. A. (1996). Inhibition of browning of
Type of packaging film only influenced
harvested head lettuce. J. Food Sci. 61(2):314-316.
development of off-odor (p?0.001) and had no effect
Di Pentima, J.H., Güemes, D.R., Pirovani, M.E., and
on visual sensory attributes or microbiological
Piagentini, A.M. (1996). Estudio del
counts (p>0.05). Development of off-odor was the
comportamiento de vegetales listos para usar,
attribute that limited shelf life of fresh-cut spinach
envasados con diferentes láminas plásticas. La
packaged in OPP bags. On the other hand, the shelf
Alimentación Latinoamericana, 213, 46-50.
life of samples packaged in LDPE bags was
Huxsoll, C.C. and Bolin, H.R. (1989). Processing
dependent on a decrease in general appearance or an
and distribution alternatives for minimally
increase in microbiological counts, depending on the
processed fruits and vegetables. Food Technol.,
chemical treatment used.
43 (2), 124-128.
For fresh-cut spinach stored at 4 ºC and 90% HR
Langdon, T.T. (1987). Preventing of browning in
a level of quality between excellent and good can be
fresh prepared potatoes without the use of
maintained for eight days using LDPE bags or for six
sulfiting agents. Food Technol., 41(5), 64-67.
days using OPP bags.
Montgomery, D.C. (1991). Diseño y análisis de
experimentos. 3rd Ed México: Grupo Editorial
Iberoamérica.
ACKNOWLEDGMENTS
Pirovani, M.E., Güemes, D.R., Piagentini, A.M., and
Di Pentima, J.H. (1997). Storage quality of
We thank Universidad Nacional del Litoral
minimally processed cabbage packaged in plastic
(Santa Fe - Argentina) for financial support under
films. J. Food Quality, 20(5), 381-389.
Project CAI+D. We gratefully acknowledge the
Schlimme, D.V. and Rooney, M.L. (1994).
technical assistance of M. S. Salsi and M.A.
Packaging of minimally processed fruits and
Moguilevsky (microbiological analysis). We also
vegetables. In: Wiley, RC, ed. Minimally
extend our gratitude to the staff of the Instituto de
Processed Refrigerated Fruits and Vegetables.
Tecnología de Alimentos, who participated in the
New York: Chapman & Hall, Inc., pp.135-182.
sensory evaluation.
Shewfelt, R.L. (1986). Postharvest treatment for
extending the shelf life of fruits and vegetables.
Food Technol., 46 (5), 70-80/89.
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Brazilian Journal of Chemical Engineering, Vol. 19, No. 04, pp. 383 - 389 October - December 2002

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