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Long-term storage of "Sweetheart"
cherries using different plastic films
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A. Yommi1, S.
Horvitz1, C. Godoy2, A. López Camelo1
and S. San Martino2
1INTA, EEA Balcarce. CC 276. 7620. Balcarce, Argentina.
2Facultad de Ciencias Agrarias de Balcarce (UNMdP)
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2002
Abstract
The effects of LDPE, PVC and a commercial film (P)
on "Sweetheart" cherries quality were evaluated during 42 days at 0°C and
compared with unwrapped-fruit (control). Gas composition inside the bags and fruit quality
(weight loss, pedicel aspect, color, firmness, soluble solids content and decay) were
analyzed. The O2:CO2 equilibrium
levels were: 5.5%:12.8% (LDPE), 17.1%:5.2%
(PVC) and 1.3%:82.0% (P).
The modified atmosphere created inside the LDPE bags maintained
fruit quality and pedicel aspect during 6 weeks of storage, although an important decay
was detected. In this experiment, the control and the P and PVC-wrapped
cherries lost weight and storage was limited to 7 days mainly due to pedicel browning. CO2
accumulation in the P bags was higher than the recommended level and caused off-odors.
From day 21 of storage until the end of the experiment, Monilinia
sp. and Botrytis cinerea were detected, with lower incidence in P
and the control.
Introduction
Cherries postharvest life is very short. Loss of brightness, darkening of the peel
and pedicel decoloration are the main symptoms of deterioration that can be detected.
Also, senescent fruit might be softer and with less flavor. Quality loss rates vary with
different cultivars: cv. "Bing" has a lower respiratory rate and it’s less
susceptible to dehydration than other cultivars (Crisosto et al., 1993).
On the other side, "Burlat" cherries, an early cv., have a higher respiratory
activity than other intermediate and late cultivars (Jaime et al., 1999).
Cold storage (0°C) maintains fruit quality during at least two
weeks, (Hardenburg et al., 1986). Together with this technique,
modified atmospheres with high CO2 (>10%)
and low O2 levels (3-10%) extend even more
the commercial life of this fruit (Thompson, 1998). Low O2
concentrations (<2,5%) retained peel brightness, fruit acidity and
green stem color (Chen et al., 1981). Combinations of 15% CO2
and 5% O2 have been effective to maintain color and firmness
during storage (Chapon & Bony, 1990). However, inadequate levels of O2
may result in off-flavor due to ethanol and acetaldehydes accumulation (Flores
Cantillano et al., 1996).
In this experiment, quality of "Sweetheart" cherries packed with
different semipermeable films and stored during 42 days at 0°C was evaluated.
Materials and methods
"Sweetheart" cherries were harvested on December 20th, 1999, from
an orchard located in Lobería, province of Buenos Aires, Argentina (-37,45° Lat; 58,18°
Long). Fruit were hydrocooled (chlorinated water, 200 ppm active Cl), transported to the
Postharvest Lab., (INTA EEA Balcarce) within 4 hours of harvest and stored at 0°C for 24
hours.
Samples of 500 g of cherries, uniform in color and size were placed in plastic
trays and packed with bags (27.0 x 21.5 cm), using three different plastic films (LDPE,
PVC and a polymeric commercial film (P)). The control were
unwrapped fruit. All the treatments were stored at 0°C during 42 days, with no control of
the relative humidity.
O2 and CO2
levels inside the bags were measured on days 1, 3, 7, 10, 14, 21, 28, 35 and 42 with
a gas analyzer (Model ILLINOIS 3600), equipped with an
electrochemical sensor and an infrared detector for O2 and
CO2 measurement, respectively. Weight loss (%), pedicel aspect
(commercial: green color, turgids; non commercial: stained, dehydrated or darkened), color
changes related to the beginning (using the equation: y = Ö (L*m
– L*i)2 + (a*m
– a*i)2 + (b*m - b*i)2,
where L*m, a*m y b*m
are the values of Lab* (CIE) registered with a colorimeter (MINOLTA
CR 300) at each evaluation date and L*i, a*i
y b*i, the values at the beginning of the experiment (L*
= 33.0; a* = 30.6; b* = 11.9), firmness (DUROFEL,
CTIFL) with a relative scale from 0 to 100, soluble solids content (SSC,
ATAGO hand refractometer, 0-32° Brix), decay and their causal agent were
measured at the beginning of the experiment and on a weekly basis.
A completely randomized experimental design with four replications was carried
out. Data were statistically analyzed by GLM procedure and means were
compared using the Tukey-Welsh test (a<0,05).
Results and discussion
Gas composition changed with all types of films. The equilibrium O2:CO2
concentrations are shown in Table 1. The CO2 level stabilized
during the first three days in PVC and LDPE bags while
in P, it continued increasing reaching an equilibrium at the end of the experiment. O2
steady state was obtained during the first week for PVC and P
and after 14 days for
LDPE. Figure 1.
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Table 1: Equilibrium
O2:CO2 levels (%) using P, LDPE
and PVC films.
| |
P |
LDPE |
PVC |
| O2
(%) |
1.2 |
5.5 |
17.1 |
| CO2
(%) |
82.0 |
12.8
|
5.2 |
CO2 concentration in P was higher than 15%,
the maximum recommended level (Kader et al., 1989). The best O2:CO2
combination suggested to maintain quality of cherries (Thompson, 1998) was obtained with
the LDPE. Results of a sensory analysis indicate that the modified
atmosphere with this film retained the characteristic flavor of the fruit during all the
experiment, while the development of abnormal taste detected in P from
day 14 of storage (Trinchero et al., 2000) may be due to CO2
accumulation in these bags.
All
treatments lost weight significantly, from the first week, except LDPE.
Daily weight loss was 0.62 %, 0.23% and 0.18% for the control, P and PVC,
respectively and negligible for LDPE. These high rates, associated to
dehydration can be related to the relative humidity during storage and probably to the
higher permeability of PVC and P to water vapor.
Pedicel aspect is an indicator of cherries quality (Gatti et al., 1984) and
is affected by dehydration even more than fruit (Patterson & Kupferman, 1983). 50% of PVC
and P-packed fruit presented non-commercial pedicels from day 7 of
storage Figure 2. The control shown important decoloration and dehydration symptoms, while
LDPE retained green and turgid pedicels during the 42 days of storage.
Using the same temperature and relative humidity conditions, Chen et al.(1981)
found that O2 levels of 0.5-2.5% maintained pedicel greenness,
while no effects were found with high CO2 concentration. If
relative humidity is not controlled, dehydration may be the main cause of this problem and
the effects of the modified atmospheres inside the bags become less evident. Figure
2.
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Significant differences in color changes between films were observed from day 28.
Daily color change rate was higher in the control than PVC and P and, negligible in
LDPE, Figure 3.
A decrease in L*, a* and b* values during storage was
the main cause for color changes. Bernalte et al. (1999) found that both L* (lightness) and a*
(green to red color) decrease in cherries as they ripen. According to
Chen et al. (1981), color maintenance in LDPE can be associated
with the modified atmosphere generated by this film.
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No
changes were found in "Lapins" cherries firmness packed with LDPE
during 8 weeks of cold storage (Meheriuk et al., 1995). Moreover, under the same
storage conditions, Meheriuk et al. (1997) observed an increase in
"Sweetheart" cherries firmness, stored during 6 weeks. Our results indicate that
no significant changes in firmness were registered with LDPE and PVC,
while softening of the fruit was observed in the other treatments, mainly at the end of
the experiment, Figure 4a.
Significant
differences between films were found from day 35, when the control and P
were softer than LDPE and PVC.
SSC only increased significantly in the
control from day 14, Figure 4b. This can be explained by a sugar concentration due to the
important dehydration observed in this treatment (Bernalte et al., 1999).
Differences between films were found from the second week. The control presented the
highest values and LDPE the lowest, while PVC and P
were intermediate. Figure 4.
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In
coincidence with Meheriuk et al. (1997), our results indicate the possibility of
storing "Sweetheart" cherries in LDPE bags during 6 weeks
obtaining an acceptable quality in terms of color and brightness, firmness and pedicel
aspect. When this period was extended, they found taste loss and decay development as the
limiting factors.
Rot development during storage was important and started from day 21. As fruit was
affected before harvest and no fungicide applications were made, we presume that part of
the fruit used in the experiment was infected. Decay incidence increased during the
storage period in PVC and LDPE, reaching 15% and 20%,
respectively after 42 days. In the control this value was less than 5%, probably because
of the low relative humidity. The same happened with P probably due to the high CO2
concentration, which reduces rot incidence (De Vries-Paterson et al., 1991, cited
by Looney et al., 1996). On days 21 and 28, Botrytis cinerea and
Monilinia spp. were found, regardless of the film used. From day 35, the main
pathogen was Botrytis spp. but also Penicillium
spp. was observed (Troglia, pers.comm.) Inhibition of Monilinia spp.
by the end of the experiment could be attributed to the low temperature and high CO2
levels.
Conclusions
LDPE film created the most adequate
modified atmosphere to maintain fruit quality along the evaluation period, reducing water
and taste loss. With film P, an important pedicel darkening and weight
loss together with off-odors and off-flavors were detected since the first week. In the PVC
bags, about 50% of the fruit were affected by pedicel deterioration after only 7 days.
Dehydration of the fruit could be reduced with the use of humidifiers inside the cold
storage.
Considering pedicel aspect as the quality limiting criteria and with a better
sanitary program in the preharvest period, storage can be extended up to 6 weeks, using LDPE
bags. Control and, PVC and P films maintained quality
only during 7 days. In this experiment, pedicel deterioration was more related to the
relative humidity than to the O2 and CO2
levels inside the bags.
Resume
L’effet de l’utilisation de LDPE, PVC
et d’un film commerciel (P), sur la qualité des cerises
"Sweetheart" en stockage pendant 42 jours à 0°C a été évalué et comparé
avec un témoin sans emballage. On a mesuré la composition gaseuse à l’intérieur
des emballages, ainsi que la qualité du fruit (perte de poids, aspet des pédoncules,
contenu de solides solubles et de pourritures). En l’equilibre, les concentrations de
O2:CO2 ont été 5.5%:12.8% (LDPE),
17.1%:5.2% (PVC) et 1.3%:82.0% (P). L’atmosphère
modifiée des emballages LDPE a maintenu la qualité du fruit et
l’aspet des pédoncules pendant 6 semaines de stockage, mais on a constaté un niveau
de pourritures assez important. Le témoin et les emballages P et PVC
ont perdu du poids et le stockage a été limité à 7 jours seulement, surtout, par la
couleur brunâtre des pedoncules.
L’accumulation
de CO2 pardessus des niveaux recommandés dans les emballages P
a produit l’apparition de saveurs anormales. Dès le 21 jour jusqu’à la fin de
l’expérience on a constaté la présence de Monilinia spp et
Botrytis cinerea, avec moins d’importance en P
et le témoin.
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