Proceedings
of The World Avocado Congress III, 1995 300 - 306
ULTRASONIC
DEVICE FOR AVOCADO SHELFLIFE PREDICTING AND MATURITY DETECTION.
Institute
of Agricultural Engineering
Agricultural Research Organization (ARO)
The Volcani Center, P.O.Box 6
50250
Bet Dagan, Israel
Abstract
An ultrasonic testing
technique was applied to a post-harvested avocado fruit to predict its shelf
life and to assess its quality. Measurements of ultrasonic wave amplitude and
transit time over the peel of the fruit were used for calculation of wave
attenuation and velocity in each individual fruit. Nondestructive ultrasonic
tests were performed daily on avocado fruits, until full ripening was detected.
Penetration measurements of firmness and physiological tests of dry weight and
oil content of the fruit tissue were conducted using accepted methods. Results
of the ultrasonic tests were compared with destructive penetration tests, and
with physiological tests. Average values of the ultrasonic wave attenuation
could be correlated well with storage time and with the ripening process of the
avocado. A linear model well represented the changes in attenuation of the
ultrasonic signals in the fruit and second-order polynomial expressions the
changes in the firmness of the fruit, during storage at room temperature. The
wave attenuation increased during storage while the firmness of the fruits
decreased. The dry weight of avocado and its oil content were correlated with
its ultrasonic parameters and mechanical properties. It is suggested that
changes in the ultrasonic parameters during storage may be used to evaluate oil
content and firmness to predict shelf life of avocado fruit.
Keywords: Ultrasonic, nondestructive testing, shelf
life, quality, fruit, avocado
1. Introduction
Quality-related factors in
avocado such as firmness, maturity, oil content and dry weight are important
for both growers and customers. Most of these factors are related to each
other. Studies have shown that avocado firmness correlates well with fruit
maturity and expected storage time (Lewis, 1978). The firmness decline rate is
moderate at the beginning, increases later and then stops at full maturity
(Zauberman and Fuchs, 1981). Firmness differences in avocado fruits are good
indicators of differences to be expected in their ripening stages, since a
softer fruit will fully ripen sooner than a harder one. Classifying fruits
according to firmness would allow proper distribution of avocado fruit to
distant markets and a longer shelf life. Nondestructive firmness control is
important, since even a small quantity of ripe fruits in a shipment may trigger
a chain reaction and cause premature ripening of the whole 'shipment, due to a
high level of ethylene production (Zauberman and Fuchs, 1973). Distinction
between hard (unripe), medium (firm ripe), and soft (overripe) fruit may be
made by hand. However, it is almost impossible for the human sorter to
distinguish between a high-quality freshly picked fruit and a fruit collected
from the ground, which may have low quality and short shelf life. This
classification is commonly done by a penetrometer, but this is a destructive
technique. A nondestructive method for sorting avocado fruit into two or more
firmness classes may be of considerable economic interest for all production
and consuming stages.
Mizrach et
al. (1989) evaluated the use of high-power, low-frequency ultrasonic
excitation for determination of fruit tissue properties. An experimental system
was designed for determination of basic acoustic properties of some fruits and
vegetables, namely, wave propagation velocity and attenuation. Further studies
by Mizrach et al, (1991, 1992) showed strong correlation between the
ultrasonic properties and some ripening parameters of the fruit tissue. Galili et
al. (1993) measured ultrasonic surface waves developed on the peel of a
fruit and examined their connection to its internal properties. They found that
the ultrasonic attenuation, monotonically increased with the duration of
storage time and that the wave velocity have a non-monotonic complex
relationship with the storage time.
Mizrach et al. (1994)
applied the ultrasonic testing technique to the nondestructive quality
evaluation of avocado fruits (cv. 'Ettinger') and suggested a technique to reduce
the scattering in results by normalizing the time scale of measurements to the
day of maximum ripeness. The authors used a non-linear regression procedure for
determining models for relating variation in the ultrasound parameters and
firmness to storage time, and concluded that models of this type may be used to
predict ripening and shelf life of avocado fruit.
The objective of the present
paper was to examine the influence of oil content and dry weight on the
acoustical measurements, and to assess the avocado properties non-
destructively, in terms of firmness, maturity and shelf life.
2. Experimental setup
An experimental setup was designed and
constructed for local ultrasonic inspection of whole fruit (Mizrach et al., 1989).
The basic setup included a Krautkramer USL 33 high- power low-frequency
ultrasonic pulser/receiver, a pair of 50-kHz narrow-band ultrasonic
transducers, an electronic gauge to monitor the contact force of the
transducers, and a microcomputer system for data acquisition and analysis (Fig.
1). Exponential Plexiglas energy concentrators were used to match each
transducer with a chisel-type contact at the fruit surface. A
through-transmission mode was selected for the ultrasonic setup. The pulser
caused the transmitter to oscillate and launch a narrow-band ultrasonic pulse
into the fruit peel this induced waves across the peel and fruit tissue and
activated the receiver. The output signal was amplified and displayed on the
CRT monitor, where the pulse amplitude and transit time could be visually
examined. In parallel, a built-in peek-detector and microprocessor-controlled
serial interface captured the signal amplitude and the transit time, and sent
the digitized data to the external microcomputer. The stored data used to
determine the velocity of wave propagation and the attenuation coefficient of
the fruit.
3. Materials and methods
3.1. Fruit selection and
procedure
Avocado fruits (Persea
americana Mill., cv. 'Fuerte'), were taken in a packinghouse during the harvest
season. The fruits were taken after the pre-cooling and sizing process and were
stored in a cooling room at 5'C for 24 h. The fruits were then placed in an
air-conditioned lab of 20'C and about 60% humidity, for acoustical, mechanical,
oil content and dry weight tests. Each fruit was marked on the peel in five
locations: one for ultrasonic non-destructive testing (NDT), and four for
penetration tests. The four locations for penetration tests were placed so that
the diagonals between them crossed at right angles, while the NDT location was
marked between two penetration spots. This arrangement minimized the influence
on the repeated NDT tests, of local bruising caused by the firmness penetration
test.
Destructive
tests were conducted after the ultrasonic tests, to determine some of the
quality-related parameters of the fruit. These included mechanical parameters
(measured by firmness penetration tests) and physiological properties (oil
content and dry weight percentage). The relevant test procedures are listed
below. All the avocado fruits were subjected to both, ultrasonic NDT and
destructive tests.
3.2. Ultrasonic tests
Each fruit was subjected to
ultrasonic NDT, daily at one point, and on every second day to a penetration
test at one of the other marked points. The acoustical parameters of the fruit,
namely the transit time of the acoustical wave and the pulse amplitude, were
measured daily until full ripening was detected. The propagation velocity, Cp,
of the acoustic wave in the material was obtained by measuring the time t
required for the pulse to traverse the gap, l, between the probes and
using the expression Cp=l/t. The attenuation of the ultrasonic signal
was calculated according to the exponential expression (Krautkramer and
Krautkramer, 1990): A = A0 e-al where A and A0,
respectively are the signal amplitude at the beginning and the end of a
distance l along the propagation path of the ultrasonic wave, and α
is the apparent attenuation coefficient of the signal.
3.3. Penetration tests
The penetration tests were
performed on unpeeled fruit, using a Chatillon durometer (John Chatillon &
Sons, New York) with a 6.35-mm diameter cone head and 60º cone angle. Each
fruit was subjected to a penetration test every second day at a different
marked point. The test was performed in the radial direction near the location
of the ultrasonic test (Mizrach et al., 1991). Maximum penetration force
(firmness) was recorded at a penetration rate of 3 mm/s. Maximum penetration
depth was about 7 mm.
3.4. Oil content and dry
weight determination
The oil content
of the fruit was determined by means of the refractive index technique
(Shannon, 1949). The procedure is described in detail by Lee (1981). Avocado
tissue is ground with chloronaphthalene (Halowax oil) and the refractive index
of the mixture is measured to determine the oil content.
The dry weight
of avocado was determined according to Lee et al. (1983). A sample of
about 10 g of avocado tissue was taken from the location in which NDT had been
performed earlier, and was weighed and dried in a 105ºC forced-air oven for 3 h
before being re-weighted for dry weight percentage calculation.
4. Results
The means, standard
deviation, and the minimum and maximum values obtained in of the ultrasonic NDT
and the destructive tests, which were primarily measured in the avocado batch
received from the packing house, are shown in Table 1. Data obtained from of
ultrasonic NDT and destructive tests for avocado fruits between harvest and
full ripening at room temperature, ranged between 2-5 dB/mm, for attenuation,
105-450 m/s for velocity and 5-100 N for firmness. The oil content of avocado
fruits (cv. 'Fuerte') ranges from 10% up to 20%.
Originally, the
fruits were tested seven times during 7 days. The variation among fruits in oil
content on the first day indicated that the fruits were probably harvested at
different stages of maturity. By means of a technique suggested by Mizrach et
al. (1994), the results were analyzed after normalizing at the same scale
of maturity and time base was expended to 216 h in 24 h intervals. The results
of measurements of the acoustical, mechanical and physiological properties of
the avocado fruits from harvest to full ripeness after normalization procedure
are summarized in Table 2.
5. Discussion
When comparing the results
measured on the first day with those obtained in the previous studies, the data
recorded (Table 1) show wide variability in attenuation and oil content and a
relatively narrow distribution of firmness values. The attenuation measured
corresponds to the oil content of the fruit. Figure 2 relates the attenuation
and oil content measurements on the first day: the measured attenuation was
found to be greater for higher then for lower oil content. An exponential
expression was selected for the direct relation between the parameters
(Chi2=0.0515). It might be suggested that nondestructive ultrasonic attenuation
measurements of avocado on the first day after harvest provide an indication of
the oil content.
Dry weight (DW) is an acceptable and convenient
indicator for evaluation of oil content in avocado when performing the
physiological tests. In several previous studies (see, e.g., Lee et al., 1983),
the authors indicated a close correlation between oil content and dry weight
during maturation. Figure 3 matches the attenuation measurements in avocado
fruits against their % DW on the 7th day of this study. It was found that the
dry material content of the avocado, which was measured destructively on the
7th day, correlated quite closely with the ultrasonic attenuation of the fruit
on the same day (R=0.8127). However, the trend in Figure 3 is opposite to that
for oil content mentioned in Figure 2. It might be suggested that the
attenuation is influenced by the % DW much more than by oil content.
The calculated velocities and
attenuation of the NDT ultrasonic wave in avocado fruits and the firmness, as
measured by the destructive tests, during 9 days are illustrated in Figure 4.
The firmness diminished with passing time, but not at a constant rate. Small
changes could be observed in the mechanical parameters of the fruits during the
first days of storage. Then, within about 24 hours, the firmness reduced
sharply and the fruits became much softer.
A non-linear regression
procedure was used for determining curve fitting to relate variations in
firmness and ultrasound parameters to storage time. The nature of the curves
for the present study was suggested in previous work by Mizrach et al. (1994).
A parabolic function was
fitted to the data mean values to describe the model relating the variation of
firmness with time and it provided relatively good correlation (R=0.9946). It
suggests that the firmness of avocado could be predicted satisfactorily from
nondestructive measurements of the ultrasonic attenuation in the fruit during
the softening on-shelf processes. A monotonic increase in the attenuation of
the ultrasonic signal with time suggested selection of a linear model
(R=0.9941). A cubic equation was chosen for the velocity versus time curve
(R=0.8204). Models of this type may be used for ripening prediction and
shelf-life evaluation of avocado fruit.
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