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A
COMPARISON OF MODIFIED ATMOSPHERE PACKAGING
UNDER AMBIENT CONDITIONS AND LOW TEMPERATURES
STORAGE ON QUALITY OF TOMATO FRUIT
Mathooko FM
ABSTRACT
Low
temperature and modified atmosphere packaging (MAP) were evaluated
for their effectiveness in extending the postharvest storage life
of tomato (Lycopersicon esculentum Mill.) fruits. Fruits
were harvested at the mature-green stage of ripeness, washed, sorted
and treated with a solution of sodium hypochlorite (150 ppm chlorine).
The fruits were packaged in low density polyethylene bags (0.044
mm) and kept at ambient conditions of 24±2 0C
and relative humidity (RH) of 65±2% or held in environmental
chambers at 15 0C and RH of 65%
for 4 weeks. The fruits were then transferred to ambient conditions
of air, temperature and RH for a further one-week to simulate marketing
conditions. Samples were evaluated initially and thereafter every
week for weight loss, titratable acidity (TA), soluble solids content
(SSC), pH, chlorophyll content, ascorbic acid content and visual
appearance. MAP reduced weight loss better than low temperature
storage. MAP was also superior in delaying ripening in the fruits
for the entire four weeks as evidenced by delayed increase in SSC,
chlorophyll degradation and increase in ascorbic acid content which
are associated with the fruit ripening. After two weeks of storage
at 15 0C most of the fruits had
started shriveling, thereby leading to loss of brightness in color
and there were signs of mold infection. Under MAP the carbon dioxide
and ethylene concentrations in the bags increased to 9% and 31 ppm,
respectively. Upon transfer of fruits to ambient conditions, fruits
held under MAP ripened normally with no signs of carbon dioxide
injury. At the end of five weeks, 50% of fruits held at 15
0C were spoilt while only 26% of fruits under MAP were
spoilt. These results indicate that under tropical conditions the
quality and storage life of tomato fruits can be extended and ripening
delayed better by MAP than by low temperature storage owing to fruit
susceptibility to chilling injury.
Key
words: Ascorbic acid, Low temperature storage, Lycopersicon esculentum,
Modified atmosphere packaging, Tomato
French
COMPARAISON
DU CONDITIONNEMENT EN ATMOSPHERE MODIFIEE SOUS CONDITIONS ENVIRONNANTES
ET DE LA CONSERVATION A BASSE TEMPERATURE POUR LA QUALITE DU FRUIT
DE TOMATE
RESUME
La
conservation à basse température et le conditionnement
en atmosphère modifiée (modified atmosphere packaging
– MAP) ont été évalués pour leur
efficacité dans le but de prolonger la durée de conservation
après la récolte des fruits de tomate (Lycospersicon
esculentum Mill.). Les fruits ont été récoltés
à l’état de maturité vert avancé,
lavés, triés et traités avec une solution d’hypochlorite
de sodium (150 ppm de chlore). Les fruits ont été
emballés dans des sacs en polyethylene à basse densité
(0,044 mm) et conservés sous conditions environnantes à
24±2 °C avec une humidité relative (HR) de 65±2%
ou maintenus dans des pièces tempérées à
158C avec HR de 65% pendant 4 semaines. Les fruits ont ensuite été
transférés en conditions environnantes d’aération,
de température et de HR pendant encore une semaine pour simuler
les conditions de mise sur le marché. Les échantillons
ont été évalués au début de l’étude
et ensuite chaque semaine pour perte de poids, acidité titrable
(AT), teneur en matière solide soluble (TMS), pH, teneur
en chlorophylle, teneur en acide ascorbique et apparence visuelle.
Le MAP avait un meilleur effet réducteur de poids que la
conservation à basse température. Le MAP était
aussi supérieur pour retarder la maturation des fruits, pendant
toute la période de quatre semaines, ainsi que démontré
par une augmentation retardée du TMS, de la dégradation
du chlorophylle et de l’augmentation du contenu en acide ascorbique
qui sont associés à la maturation du fruit. Après
deux semaines de conservation à 158C, la plupart des fruits
commençaient à se ratatiner, entraînant une
perte du brilliant de la couleur, et il y avait des signes de moisissure.
Dans les conditions de MAP, les concentrations de dioxyde de carbone
et versène dans les sacs ont augmenté jusqu’à
9% et 31 ppm, respectivement. Lors du transfert des fruits en conditions
environnantes, les fruits conservés par MAP ont mûri
normalement sans montrer de signes de dommage dû au dioxyde
de carbone. Après cinq semaines, 50% des fruits conservés
à 15 °C étaient abimés tandis que seulement
26 % des fruits l’étaient avec le système MAP.
Ces résultats indiquent que dans des conditions tropicales,
la qualité et la durée de conservation des fruits
de tomate peuvent être améliorées et la maturation
mieux retardée en utilisant le MAP que la conservation à
basse température, compte tenu de la susceptibilité
des fruits aux dommages causés par le refroidissement.
Mots
clés: acide ascorbique; conservation à basse termpérature;
Lycopersicon esculentum; conditionnement en atmosphère modifiée;
tomate.
INTRODUCTION
The
accumulation of carbon dioxide and depletion of oxygen to beneficial
levels by the application of modified atmosphere packaging (MAP)
is known to extend the postharvest life and quality of many horticultural
commodities [1, 2]. Indeed MAP has been used as a supplement or
even a substitute for refrigeration to prolong storage life of fresh
produce during transportation and retail handling [2, 3]. Kader
et al. [4] reported that controlled atmosphere (CA) storage
could retard quality deterioration in fresh tomato in a manner similar
to low temperature. Several other researchers have reported similar
results. However, the high degree of atmospheric regulation associated
with CA is capital intensive and expensive to operate especially
in developing countries, and is, therefore, amenable for long-term
storage of commodities such as apple [1, 5]. Alternatively, an inexpensive
way of delaying fruit ripening is the use of MAP, where fruits are
sealed in semipermeable plastic packages that enable the development
of a beneficial gas atmosphere created and maintained by the interaction
of fruit respiration and gas diffusion through the packaging film.
Few previous reports have evaluated the superiority of MAP to low
temperature storage. In Kenya, like in many developing countries,
low temperature storage is not commonly used due to lack of refreigeration
facilities, and hence the need to evaluate MAP as a suitable alternative.
Tomato
fruits are one of the major crops involved in rural-urban trade
in Kenya and are among the most perishable during retail marketing.
The deterioration of tomato fruits following harvest is influenced
by many factors including stage of ripeness, prepackaging treatment
and packaging material [6]. Most of the work done on postharvest
handling of tomato has been done in the developed countries where
emphasis has been laid on low temperature effects (probably due
to availability of cooling facilities) on the fruit quality. Although
a lot of work has been done on MAP, it has mainly been in combination
with low temperature. Information on real rather than simulated
tropical conditions seems to be lacking. Moreover, tomato fruit
being sensitive to chilling temperature can only be stored safely
at above 12 oC to ensure freedom from chilling injury,
good color and flavour profile [7]. Although optimum quality of
tomato is attained through vine ripening, ripe tomato fruits are
perishable and very labile to transport damage that consequently
leads to loss of quality and waste [8]. This is especially so in
developing countries due to poor postharvest handling systems and
transportation of fruits and vegetables over rough roads and uneven
surfaces [9]. For this reason fruits intended for distant markets
are usually harvested at mature-green or breaker stages so that
the fruits can endure the rigors of handling while maximizing shelf
life [10].
Since
development of a modified atmosphere within the polymeric film extends
the storage life of fruits and vegetables by reduction of ethylene
production and respiration [2, 11, 12]. MAP may provide a cheap
means for the storage of fruits and vegetables and serve as an alternative
to fungicides in extending their shelf life. Most recently, polyethylene
packaging of fruits (including tomato) during retail marketing has
become a common practice in Kenya [13] and this practice is also
extended to the household level. However, the effects of this mode
of packaging on deteriorative changes in tomato fruit have not been
reported. The viability of MAP as an alternative to low temperature
storage and especially under tropical conditions needs to be evaluated.
It is against this background that we compared low temperature storage
and MAP as means of controlling ripening in tomato fruits. Since
the benefits of MAP are lost when the package is opened, we also
evaluated the residual effects of MAP on the ripening characteristics.
Fruits are a good source of vitamins. However, most of the work
done using MAP has been geared towards physiological effect and
little has been done on its effect on vitamins as has been done
for vegetables. We, therefore, further investigated the effect of
MAP on ascorbic acid content in mature-green tomato fruits where
maximum level has not been attained.
MATERIALS
AND METHODS
Plant material and treatment
Freshly harvested mature-green tomato (Lycopersicon esculentum
Mill. var. Cal J) fruits were obtained from a commercial supplier
in Thika Town, Kenya. The fruits were selected for uniformity in
size, color, absence of blemishes and fungal infection, and transported
to the laboratory within one hour. Before the experiment, the fruits
were washed with a solution of sodium hypochlorite (150 ppm chlorine).
The fruits were divided into two lots: one lot was stored in an
environmental chamber maintained at 15 oC and 65% RH.
This temperature was chosen since normal ripening does not occur
at below 12 oC [14]. The other lot was packaged in low
density polyethylene bags of 18 cm x 24 cm and 0.044 mm in thickness
and kept on a laboratory bench to simulate marketing conditions.
The temperature in the laboratory ranged from 23 oC to
27 oC while the RH ranged from 63% to 67% during the
experimental period. Each bag contained six fruits and were replicated
three times. The fruits were held at these conditions for four weeks
and then transferred to ambient conditions of 23 oC to
27 oC and 63% to 67% RH for a further one week to simulate
marketing conditions.
The
fruits were evaluated before treatment (time 0) and at weekly intervals
for weight loss, titratable acidity (TA), soluble solids content
(SSC), pH, chlorophyll content, ascorbic acid content and spoilage.
Carbon dioxide and ethylene concentrations in the polymeric films
were monitored on a weekly basis for six weeks.
Titratable
acidity (TA), pH and soluble solids (SSC) content
Three fruits from each treatment were analyzed for TA, SSC and pH.
The core was removed and the fruits were pureed in a blender. The
pureed samples were filtered to remove fibres. TA was determined
by titrating five ml sample with 0.1 N NaOH to pH 8.1. The acidity
is expressed as percent citric acid. pH was determined using a pH
meter (Toa, Kyoto, Japan). SSC was determined by refractometry using
an Atago N1 refractometer and expressed as oBrix (= %sucrose). This
represents not only the reducing sugar content but also organic
acids and minor constituents in the sap that contribute to the refractive
index [15].
Chlorophyll
content
Color change during the experiment was assessed both subjectively
and objectively. Since determination of either chlorophyll or lycopene
content can be used to indicate tomato color change [16], in the
objective assessment, chlorophyll content was used as an index of
ripening. Total chlorophyll was extracted according to the method
of Arnon [17]. Four grammes of pericarp tissue including the skin
was ground in a mortar and pestle with some cold 80% acetone and
in the presence of some acid-washed sand. The homogenate was filtered
and washed with cold 80% acetone until the filtrate was colorless.
The amount of acetone used in each case was noted. Total chlorophyll
content in the extract was determined spectrophotometrically by
measuring absorbance at 645 nm and 663 nm using a Shimadzu UV-visible
spectrophotometer (Model UV-1600, Shimadzu Corporation, Kyoto, Japan).
Total chlorophyll content in the extract was calculated according
to the Arnon formula [17].
In the subjective evaluation, fruit color change during storage
was assessed visually and the fruits classified into three groups:
Group I = mature-green fruits only; Group II = breaker and turning
fruits only and Group III = pink and red ripe fruits only.
Ascorbic
acid content
Ascorbic acid content in the fruit pericarp tissue was determined
by visual titration of extract with 2,6-dichlorophenolindophenol
solution according to AOAC methods [18].
Gaseous
monitoring
In the MAP treatment the concentrations of carbon dioxide and ethylene
in the polymeric films were monitored during six-week storage by
gas chromatography. Samples of internal gas were withdrawn through
a self-sealing septum affixed on the surface of the bag using a
1-ml gas-tight syringe. Carbon dioxide was analyzed by injecting
1 ml of the headspace gas into a Shimadzu GC 8A gas chromatograph
(Shimadzu Corporation, Kyoto, Japan) equipped with a thermal conductivity
detector and a Porapak Q column. Ethylene concentration was determined
by injecting 1 ml of headspace gas into a Shimadzu GC 9A gas chromatograph
(Shimadzu Corporation, Kyoto, Japan) equipped with a flame ionization
detector and an activated alumina column.
Visual
appearance and weight loss
The stage of ripening, appearance and decay of the tomato fruit
were evaluated by visual observation noting color development, shriveling
and presence of molds on the tomato surface. The results were expressed
as percent of the total sample. Weight loss was monitored every
week using a weighing balance and expressed as a percentage.
Results,
where applicable were analyzed using Duncan`s Multiple Range Test
at P = 0.05.
RESULTS
Weight
loss
Fruits held under MAP lost less weight compared to those held at
15 oC (Fig. 1). When the fruits were transferred to ambient
conditions to simulate market conditions, the rate of water loss
was higher in fruits previously held at low temperature than those
under MAP. This weight loss was associated mainly to loss of moisture.
In general, weight loss progressively increased with storage time
and was linear for fruits under MAP before transfer to normal air.
Titratable
acidity (TA), pH, soluble solids (SSC) content and SSC:TA ratio
The effects of low temperature storage and MAP on TA, pH, SSC and
SSC:TA ratio in the tomato fruits are shown in Table 1. SSC in fruits
under MAP remained fairly constant until transferred to normal air,
while in fruits held at 15 oC there was a general increase
in SSC. TA in fruits under MAP decreased slightly and remained fairly
constant, whereas in fruits held at 15 oC, TA increased
up to a maximum in the second week and then began to decrease and
was similar to that of fruits under MAP upon transfer to normal
air. There was a general increase in SSC:TA ratio under both treatments,
initially being higher in fruits under MAP than in fruits held at
15 oC. Fruits held at 15 oC had low pH values
which correlated to the TA values.
Color
change and gas composition
Color is the most visible ripening indicator, hence determination
of color development appears to be a logical factor in investigating
modified atmosphere storage of tomatoes [19]. MAP was more effective
in retarding chlorophyll degradation than low temperature storage
(Fig. 2).
Figure
1
Effects of low temperature storage and modified atmospere
packaging on weight loss of mature-green tomato fruit. The
fruits were transferred to ambient conditions of relative
humidity, air and temperature on week 4. Vertical bars are
SE of the mean of three replications. When absent, the SE
bars fall within the dimentions of the symbol. |
|
Figure
2
Effects of low temperature storage and modified atmosphere
packaging on chlorophyll degradation during tomato fruit ripening.
The fruits were transferred to ambient conditions of relative
humidity, air and temperature on week 4. Vertical bars are
SE of the mean of three replications. When absent, the SE
bars fall within the dimensions of the symbol. |
|
Although
low temperature delayed chlorophyll degradation, it did not prevent
it altogether and majority of the fruits had lost most of the green
color by the end of the second week. The effect of MAP was reversed
upon transfer of fruits to ambient conditions where color developed
normally. The carbon dioxide concentration in the polymeric films
increased to 9% while ethylene increased to 30 ppm (Fig. 3).
Ascorbic
acid content
Ascorbic acid is fairly labile and its retention is often followed
when evaluating postharvest storage effects on nutritional quality
in fruits and vegetables. In fruits held at 15 oC, ascorbic
acid content increased to a maximum within two weeks and then started
to decline (Fig. 4) while MAP delayed the increase in ascorbic acid
content that is associated with ripening. Upon transfer of fruits
from MAP to normal air, ascorbic acid content increased to the maximum
observed for fruits at 15 oC after two weeks of storage.
This increase was in parallel with the increase in other parameters
associated with ripening. Therefore, MAP could be an inexpensive
way of preserving the nutritional quality of tomato fruits and other
fruits and vegetables.
Figure
3
Changes in carbon dioxide concentration and ethylene concentration
within the tomato packages stored at ambient temperature.
Vertical bars are SE of the mean of three replications. |
|
| |
Figure
4
Effects of low temperature storage and modified atmosphere
packaging on ascorbic acid content during tomato fruit ripening.
The fruits were transferred to ambient conditions of relative
humidity, air and temperature at week 4. Vertical bars are
SE of the mean of three replications. |
|
Ripening
and spoilage
The effects of low temperature storage and MAP on ripening and spoilage
of the fruits is depicted in Table 2. Although storage at 15 oC
delayed ripening process, it did not stop it altogether. By the
end of three weeks, most of the fruits were either at the pink or
red stages of ripeness. Most of the fruits were shriveled due to
excessive water loss and we observed some traces of molds. On the
other hand, MAP significantly reduced the rate of ripening. By the
end of the fourth week 55% of the fruits were still at the mature-green
stage of ripeness. This correlated well with the results of chlorophyll
content. However, we observed traces of molds on the fruits and
we attributed this to condensation of water on the fruit surface.
Upon transfer of fruits to air for one week, 50% of fruits previously
held at 15 oC were spoilt, mainly due to excessive water
loss and mold infection whereas only 26% of fruits previously under
MAP were spoilt, mainly due to mold infection.
DISCUSSION
The
loss of moisture results in a reduction in the fresh weight of harvested
product, which when sold on a weight basis is translated into loss
in value. Generally, the loss of only 5-10% moisture renders a wide
range of products including tomato fruits unsellable [20]. Previous
studies have shown that MAP reduces weight loss in various commodities
including tomato [8] although none of these reports had assessed
its superiority to low temperature storage. Meir et al.
[21] observed that MAP reduced water loss in bell pepper by 40-50%.
Water loss of fresh tomato fruits is primarily due to transpiration
and respiration. Transpiration, the greatest contributor to reduction
in weight is the mechanism by which water is lost due to differences
in vapor pressure of water in the atmosphere and the tomato surface
while respiration causes weight reduction because each time a carbon
atom is lost a molecule of water is produced [22]. The reduced weight
loss by fruits held under MAP is most likely due to the high RH
maintained inside the polyethylene bags. Bhowmik and Pan [22] observed
that high RH under CA considerably reduced weight loss in tomato
fruits. The considerable water loss from fruits held at 15 oC
was associated with shrinkage of the skin leading to impairment
of appearance. The fruits were unattractive due to formation of
wrinkles on the surface. According to Artes and Escriche [7] and
Bhowmik and Pan [22], this leads to loss of brightness in color,
results which are consistent with our observation.
It
has been proposed that the lower TA in fruits held at high RH as
would be encountered under MAP is primarily due to their higher
retention of water and, therefore, the concentration effect caused
by water loss may be reflected in TA values [22]. Interestingly,
we did not observe any change in TA upon transfer of fruits previously
under MAP to air. A similar observation has previously been made
[8]. The increase in SSC:TA ratio was probably due to the changes
observed in SSC and TA under both treatments during the storage
period. High quality tomato fruits are characterized by containing
more than 3% SSC, 0.32% TA and having a SSC:TA ratio greater than
10 [4]. Our results, therefore, indicate that with respect to these
parameters, fruits from both treatments were of good quality. The
observation that MAP had little effect on SSC is in good agreement
with that of Nakhasi et al. [8] and supports the conclusion
reached by Goodenough et al. [23] that MAP may not equally
control all ripening processes. Yang and Chinnan [24] reported that
an appropriate MAP delays changes in SSC among other parameters.
In tomato fruits held under CA, SSC remained low during storage
probably due to suppression of respiration and delay in ripening
and then increased upon return to air [22]. The low pH values for
fruits held at 15 oC could be attributed to the low RH
in the storage chambers. These results are similar to those reported
earlier by other researchers [22]. However, Wright and Kader [25]
observed that persimmons stored in air or atmospheres containing
12% carbon dioxide had similar pH.
MAP
maintained chlorophyll content in the fruits. This implies that
MAP decreased the metabolic processes responsible for both chlorophyll
degradation and lycopene synthesis or any process that may facilitate
unmasking of preexisting lycopene. Kidd and West [26] observed that
the color change in tomato fruits from green to red, indicative
of ripening could be retarded by high carbon dioxide, low oxygen
or a suitable combination of these gases. The increase in the concentrations
of carbon dioxide and ethylene is in agreement with the results
reported earlier [8]. The increased carbon dioxide concentration
may have prevented accumulation of higher levels of ethylene, and
also counteracted the biological activity of ethylene in enhancing
chlorophyll degradation. Color development (i.e. loss of chlorophyll)
is delayed by storing tomato fruits under CA [22]. MAP has also
been shown to delay color change in tomato fruits although these
treatments were in combination with low temperature storage [8,
24]. Gong and Corey [10] showed that tomatoes under MAP kept for
three weeks at 20o C without reaching the pink stage that is in agreement
with our observation. Reduced respiration rates combined with lowered
ethylene production and reduced sensitivity to ethylene results
in decreased chlorophyll degradation [2]. MAP has also been shown
to delay chlorophyll degradation in green vegetables [27].
In
tomato fruits ascorbic acid content increases with maturity and
stage of ripening [28]. It has been indicated that once fruits reach
full ripe stage, ascorbic acid content starts to decline [18]. It
seems that MAP suppresses the synthesis of ascorbic acid but does
not impair the fruit capability to synthesize the vitamin. MAP may
inhibit the activity of L-galactono-ƒ-lactone dehydrogenase,
which is required for the synthesis of ascorbic acid, a possibility
we are currently investigating. Factors such as high RH and temperature
have also been shown to affect ascorbic acid content [28]. Bhowmik
and Pan [22] observed that tomato fruits held at 12 oC
started ripening within a short time and over 80% were lost within
four weeks of storage. Although the carbon dioxide concentration
in the package increased to 9%, we did not observe any carbon dioxide
injury, either when fruits were in the package for upto four weeks
or when subsequently ripened in air at 25 oC. This is
in agreement with the observation of Gong and Corey [10]. Consistent
with the observation of Shirazi and Cameron [3], the limiting factor
in our MAP experiment seemed to be appearance of mold that we associated
with the high in-package RH.
In
conclusion, water loss seemed to be the overriding factor influencing
visual appearance and quality of tomato. Therefore, eliminating
water loss through packaging holds greater potential for extending
postharvest life of tomato fruits. Further study will, however,
be required to optimize packaging conditions especially relative
to disease control. To this end we are investigating the effects
of modifying in-package RH on tomato fruit ripening. We conclude
that MAP of tomato fruits is more superior to low temperature storage
with respect to delaying ripening and associated quality changes.
This is mainly due to maintenance of high RH and elevated carbon
dioxide and/or low oxygen concentrations in the package. Therefore,
implementation of MAP for tomato fruits in Kenya and other developing
countries where refrigeration facilities are rare will retard ripening
processes and extend fruit shelf life, thereby facilitating postharvest
handling and reducing waste.
ACKNOWLEDGEMENT
The
author is grateful to Japan International Cooperation Agency (JICA)
for financial support and to Ms. Mercy W. Mwaniki for technical
assistance.
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