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ABSTRACT
The preservation of quality of orange "Valencia late"
was evaluated during a 5 month storage at 6°C in air and
in air with ethylene removal by means of a Swingterm converter.
Every month, a sample of twenty fruit was checked, skin colour,
internal acetaldehyde and ethylene, %juice, ethanol, soluble
solids and titratable acidity in juice were measured. At the
end of storage, the percentage of fruit affected by decay
or physiological disorders and of those with green calyx was
recorded, and a taste test was carried out. Fruit stored in
ethylene-free room, showed a lower decay rate, a better skin
appearance, and a higher number of fruit with green calyx.
The same oranges had lower internal ethylene and acetaldehyde
contents, a higher acidity and a lower ethanol level in juice.
The taste test showed only minor difference between fruits
submitted to the two different storage treatments. The catalytic
ethylene scrubber, already successfully used in pear and kiwifruit
storage, seems to improve storability and quality also in
long-term storage of oranges "Valencia late", by
delaying senescence of fruit.
INTRODUCTION
The storage life of oranges is conditioned by such factors,
as fruit variety, temperature, physiological disorders and
the extent of moulds which develop. The optimal storage temperature
for "Valencia" oranges produced in Italy was found
to be around 6-7° C (Agabbio et al., 1986, 1987; Arras
and Schirra, 1988). Also well-known is the role which the
volatiles of citrus fruit play on senescence and in the development
of abnormal tastes in fruit (Norman, 1977). Researchers suggest
good ventilation and daily air change during storage (Wardowski
et al., 1986). Among these volatiles, ethylene seems to play
a primary role; in fact a number of researchers pointed out
the detrimental effects of high level of this gas on fruit
storability and quality (McGlasson and Eaks, 1972; Davies
et. al., 1974; Wild et. al., 1976,1977). Many methods were
proposed in the past, in order to purify storage room atmosphere
by absorbing or by destroying ethylene. The present work aimes
at verifying the advantage of ethylene removal on "Valencia
oranges" by means of a catalytic converter, already successfully
used in storage of ethylene sensitive species (Wojciechowski,
1988).
MATERIALS AND METHODS
"Valencia late" oranges were grown in Sicily and
picked at a proper maturity stage at the end of April 1991.
The fruits were, within 24 hours of harvest, treated with
TBZ in a commercial packinghouse and then sent to Milan. On
their arrival at I.V.T.P.A., they were inspected again removing
damages fruits and fruits affected by moulds. Fruits sorted
for uniformity of orange colour and size were randomized into
two groups each consisting of 20 plastic crates (approx. 80
fruits per crate). Then one group was placed in a storage
room without ethylene removal equipment, the other in a storage
with a catalytic converter. In both the storage rooms the
temperature was mainted at 6°C and the humidity ranged
from 85-90%. Ethylene in the control room was found varying
between 2 to 3 ppm before the daily air change, while in the
scrubber room it was kept between 0.02 and 0.03. Initially
and after 2, 3, 4 and 5 storage months the following quality
evaluations were carried out:
- The behaviour of peel colour by a Minolta Croma Meter CR
200 reflectance colorimeter on two samples of 40 fruit each;
- Ethylene and acetaldehyde contents inside the fruit atmosphere
by gas-chromatography;
- Soluble-solid content, titratable acidity, juice % by conventional
methods;
- Ethanol content in juice by an enzymatic method. For the
last 5 analyses individually 20 fruit were used for each analysis.
At the end of storage after 5 months, all fruits were inspected
and classified as healthy or unmarketable. Unmarketable fruits
were subdivided into two groups: the first group consisted
of fruit affected by moulds caused by Penicellium spp., whilst
the second group consisted of fruit affected by other moulds
or different physiological disorders (stem-end rind breakdown,
pitting, etc.). The healthy fruits were classified according
to the appearance of button (green calyx, black calyx and
detached). A sensory evaluation was performed by a triangle
assaying procedure, on the mixed juices of 10 fruits from
each treatment. The test was repeated over a period of 7 days,
twice daily, using randomized glasses of juice, indentified
by a three digit random number. Each taster was requested
to identify the odd sample, express his preference and state
the reasons. Furthermore on two samples of 5 kg of peeled
fruit, stored according to the two different procedures, the
GC/MS analysis of typical volatile compounds of the juice
was carried out. (SEE TABLE 1, TABLE 2).
RESULTS AND DISCUSSION
The behaviour of the colour of fruit skin during storage (Table
1) shows a substantially different trend in the two groups:
the ethylene-free fruit retains a higher brightness than the
test group, which is much more evident after 4 and 5 storage
months. As regards the red component (a*), no differences
were found between fruit from the two storage system. Yellowness
(b*) resulted higher in ethylene-free fruit, which showed
an increasing trend already after a 2-month storage time,
whilst in the control group a decreasing trend was observed.
The sharp decrease observed in the yellow component at storage
end is very pronounced. Ethylene and acetaldehyde measured
in the air inside the fruit show increasing values with increasing
storage time (Table 2). Low ethylene storage drastically reduced
both these compounds. In fact the average value was of 1/4
for internal ethylene and 1/6 for acetaldehyde. The juice
and soluble solids contents which, for the sake of brevity,
are not reported, showed sligth differences among treatments.
The behaviour of acidity during storage was sensibly higher
in fruit kept in low ethylene (Table 3). Also the ethanol
content in juice confirms the trend of other parameters, showing
an increase proportional to the storage time and values which
are costantly lower in fruit stored in low ethylene.
(SEE TABLE 3)
After 5 month's storage the amount of ethanol in control
fruit results to be double that found in fruit stored in a
scrubbed atmosphere. The results of fruit storability can
be clearly seen in Table 4, 5 and 6. The percentage of healthy
fruit stored with catalytic system was higher than in fruit
stored under traditional conditions. The incidence of decay,
predominantly green and blue mould (Penicillium digitatum
and italicum) was significantly less in fruits kept in the
storage room with ethylene removal equipment. These results
confirmed those of McGlasson and Eaks (1972) who reported
less mould incidence in stored "Valencia" oranges
in a low ethylene environment. A similar response with lemons
was obtained by Wild et. al. (1976). El-Kazzaz et.al. (1983)
working on the effect of various concentration of ethylene
on the in vitro and in vivo growth of postharvest fruit-infecting
fungi found that ethylene significantly stimulated spore germination
of P.digitatum and italicum. Observing the appearance of the
healthy fruit, 81% of the fruit stored in low ethylene retained
(SEE TABLE 4, TABLE 5, TABLE 6) green calyx. In fruit kept
in the normal storage room, only 5%retain green calyx, and
in most of them a black calyx was observed (67%). The sensory
evaluation did not show such evident results as the objective
analyses, because only half the tests carried out resulted
statistically different (SEE TABLE 7), and overall, only 62%
of tasters succeded in finding out the right answer in the
triangli test. Among the tasters who gave the right answer,
approximately 60% gave their preference to the juice from
ethylene-free fruit. As to the reasons of preferences, most
used adjectives were: more acid, sweeter, less aromatic, presence
of off-flavours, but not always were such adjectives attributed
to the same samples. So there is some doubt the sensorial
difference existing between treatments might be smaller than
the variability existing between individual oranges. In the
chromatograms were pointed out, which are regarded as typical
for volatile compounds. Among the main ones that can be mentioned
here are: limonene, ß-pinene, linalool, trans- and ciscarveol
and valenvenve, which is regarded as the impact component
for this cultivar. The intensity of peaks was higher in fruit
stored under traditional conditions, whilst the catalytic
purification reduced the intensity. In particular, in the
juice from ethylene-free oranges, small amounts of linalool
and the disappearance of cis-carveol were observed.
RESULTS
Our purpose was to verify the long-term storability of ranges
"Valencia late" and their quality characteristics,
and the storage results showed interesting differences. The
percentage of healthy fruits was 90% vs. a value of 35% observed
in the control, and, above all, the appearence and the freshness
of fruit stored under ethylene-free conditions was considerably
better. In fact, 81% of fruit showed a green calyx vs. 5%
of those stored by the traditional method. Besides other specific
defects of skin, the lack of calyx, or its darkening are regarded
by the market as evident sympotoms on fruit senescence. Physical-chemical
analysis demonstrated considerable difference in internal
ethylene and acetaldehyde, and also in the juice there was
a difference in ethanol content and acidity. The colour of
the skin was also brighter in fruit stored using the catalytic
scrubber. All these analytical indexes confirm the higher
freshness and vitality of fruit purified by the new method.
It should be furthermore stressed that the catalytic scrubber
was capable of destroying not only ethylene, but also the
other volatile substances of air. The evaluations carried
out by a taste panel did not find such substantial differences
as the analytical assays did; perhaps such differences, which
anyway emerged, even if to a minimal extent, may have been
attenuated by the variability existing between individual
fruit. The facility of the working of the catalytic scrubber,
which makes unnecessary air exchanges, suggests that it could
be very advantageous in long-term storage of oranges. Not
only for tradespeople, who gain their highest prices during
the months of September or October, but also for the customers,
who get healthy looking and better tasting oranges, without
the typical off-flavours of fruit stored for long time.
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