Redalyc. Acoustic impulse respon

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					Revista Ciencias Técnicas Agropecuarias
Universidad Agraria de La Habana
paneque@isch.edu.cu
ISSN (Versión impresa): 1010-2760
CUBA




                                                      2006
                           Antihus Hernández Gómez / Annia García Pereira / Jun Wang
                   ACOUSTIC IMPULSE RESPONSE POTENTIAL TO MEASURE MANDARIN FRUIT
                                         RIPENESS DURING STORAGE
                        Revista Ciencias Técnicas Agropecuarias, año/vol. 15, número 004
                                        Universidad Agraria de La Habana
                                                La Habana, Cuba
                                                    pp. 24-30




            Red de Revistas Científicas de América Latina y el Caribe, España y Portugal

                           Universidad Autónoma del Estado de México

                                      http://redalyc.uaemex.mx
     Revista Ciencias Técnicas Agropecuarias, Vol. 15, No. 4, 2006




                           AGRICULTURA DE PRECISIÓN
                           PRECISION FARM



     Acoustic impulse response potential
     to measure mandarin fruit ripeness during
     storage

     Respuesta al impulso acústico para medir
     la madurez de la mandarina durante
     el almacenamiento
                              Antihus Hernández Gómez1, Annia García Pereira1 and Jun Wang2


     ABSTRACT. The aims of this research work were directed to evaluate the capacity of acoustic signal response to monitoring the mandarin fruit
     firmness change during storage. The dominant frequency, firmness index and elasticity coefficient as a function of time could be expressed as a
     decreasing polinomial function. A good correlation was established between the acoustic parameters (firmness index and coefficient of
     elasticity), and fruit compression force, for these data, the correlation coefficients were 0,88 & 0,91 respectively. The results indicates that it
     might be possible to identify the ripeness state of an individual mandarin by using the present method, and that the nondestructive acoustic test
     could replace conventional compression test in order to determine mandarin fruit firmness and expected shelf- life.
     Key words: acoustic impulse response technique, non-destructive test, firmness; ripeness, mandarin, storage.
     RESUMEN. Los objetivos de este trabajo investigativo estuvieron encaminados a evaluar la capacidad de la señal acústica para supervisar el
     cambio de firmeza de la mandarina durante el almacenamiento. La frecuencia dominante, el índice de firmeza y coeficiente de elasticidad pueden
     expresarse como una función de tiempo polinomial decreciente. Una buena correlación se estableció entre los parámetros acústicos (índice de
     firmeza y coeficiente de elasticidad), y la fuerza de compresión de la fruta, para estos datos, los coeficientes de la correlación fueron de 0,88 &
     0,91 respectivamente. Los resultados indican que podría ser posible identificar el estado de madurez de una mandarina empleando el presente
     método, y que la prueba acústica no-destructiva podría reemplazar la prueba de compresión convencional para determinar firmeza de fruta
     durante la vida de estantería de la misma.
     Palabras clave: respuesta al impulso acústico, prueba no-destructiva, firmeza, madurez, mandarina, almacenamiento.


     INTRODUCTION                                                               From the spectrum of the response signal a firmness index
                                                                                is calculated.
          The acoustic impulse response method has been                              The development of low-cost, lightweight, and flexi-
     suggested by many researchers (ABBOTT et al., 1968,                        ble piezoelectric film sensors, reported by (SHMULEVICH
     GALILI et al., 1998) to measure firmness as related to the                 et al., 1996) added new possibilities for dynamic testing of
     elastic properties of fruits and vegetables. In this method,               agricultural products. They concluded that the sensor
     the fruit is excited by means of a hammer, and the response                could detect the decreasing resonance frequency or the
     signal is captured using a microphone or/and film sensors.                 Firmness Index, during time, of the tested apples and of



     Recibido 15/02/06, trabajo 88/06, investigación.
     1
       Dr., Prof., Agricultural Mechanization Faculty, Havana Agricultural University, Cuba. E-:antihus@.isch.edu.cu
     2
      Dept. of Agricultural Engineering, Zhejiang University, Hangzhou 310029, China.
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                                                                     Revista Ciencias Técnicas Agropecuarias, Vol. 15, No. 4, 2006
several other fruits during storage; and that the measuring      greater than that of the penetrometer; this technique was
technique of fruit firmness with the piezoelectric film          integrated into an automated fruit firmness monitoring
sensors was found to be simple, fast, and repeatable.            system and tested successfully on ‘Jonagold’ apples in a
      According with LANDAHL et al.,2002, the water loss         commercial cool store. According with LANDAHL et al.,
has a considerable influence on the results of the acoustic      2002, the water loss has a considerable influence on the
impulse response technique.                                      results of the acoustic impulse response technique.
      The proper values of impacting mass and drop height        COOKE, 1972 and COOKE and RAND,1973, proposed a
depend on the physical design of the sensing unit and the        mathematical model for the interpretation of the vibrational
type of fruit being tested.                                      behavior of intact fruit. They showed that the Elastic
      Nondestructive firmness test would enable the              Modulus (or Young’s Modulus) could be estimated
firmness grading of every fruit and so result insignificant      satisfactorily as fallows:
improvement in the control and specification of the firmness
                                                                                            E = f 2m2/3 ρ1/3                  (2)
quality of fruit consignments.
      VAN WOENSELT et al., 1983, followed the ripening           where: E is the elasticity coefficient (Pa) and ρ the density
of apples, during their storage, using a wide band random        (kg/m3).
vibrator of the range 0 to 1 600 Hz to excite the fruit. The          Citrus fruit flesh generally is not as firm as that of other
excitation force was measured the vibrations on the              fruit, because of juice sacs. However, firmness is an important
opposite side. These two signals were used to detect the         characteristic in citrus because the firmness of fruit flesh
resonance frequency with the aid of FFT analyzer. The            influences the mouth feel of citrus, (MURAMATSU et al.,
results showed a clear change in the Firmness Index during       1996. Most instrumental techniques to measure the firmness
the entire season.                                               are destructive, involve a considerable amount of manual
      An experimental system for nondestructive firmness         work, and they are not practical for cultivars or storage
evaluation, based on the flexible piezoelectric sensors, a       stations.
microphone or and accelerometer was developed and tested              However, little detailed information is available on
on several fruit and other products, such as: apple (ZUDE et     firmness of mandarin fruit during storage using non-
al., 2004; YAMAMOTO et al., 1980; VAN WOENSELT et al.,           destructive impulse response (I-R). Therefore, the objective
1988; LILJIDAHL and ABBOTT 1994; CHEN, H., and DE                of this research work was directed to evaluate the capacity
BAERDEMAEKER 1993; CHEN, P. et al., 1992); tomatoes              of acoustic signal response to monitoring the mandarin
(DUPRAT et al., 1997), avocado (PELEG et al., 1990 and GALILI    fruit firmness change during storage.
et al., 1998); muskmelons (SUGIYAMA et al., 1994);
watermelon (DIEZMA et al., 2004); pear (WANG, 2004,              MATERIALS AND METHODS
WANG et al., 2004; HERNÁNDEZ et al., 2004); peach
(HERNÁNDEZ, et al., 2004); and many others.                           Mandarin «Zaojin Jiaogan» (C. reticulata) was selected
      To indicate firmness for spherical fruit, stiffness fac-   to the experiment. All samples (200 samples) were taken
tor (S) or firmness index (FI) (first introduced by ABBOTT       directly from a simple same orchard and randomly assigned
et al., 1968, modified by COOKE and RAND, 1973) can be           to be hand harvested at October 03, 2004, this date is
calculated as:                                                   consider as a mandarin commercial picking time. Upon
                                                                 arrival, each date the fruits were sorted and selected as
                         S = f 2 m2/3                     (1)
                                                                 samples according with a uniform color and medium size
where: S is the stiffness coefficient (kg 2/3 s -2 ), f the      (672.5 mm). All fruits of each sample were individually
dominant frequency where response magnitude is the               numbered.
greatest (Hz) and m the fruit mass (g).
     This stiffness factor is significantly correlated with      Mandarin storage conditions
fruit firmness and sensory measurements (GALILI and
DE BAERDEMAEKER, 1996). To reduce errors caused by                   Two different storage treatments were performed;
the variance in shape inherent in horticultural products,        the mandarin fruit were matured under laboratory
it is advisable to take the average of three measurements        conditions (shelf live conditions). The mandarin fruits
equally spaced on the fruit equator (CHEN, H. 1993). Modal       were placed in 4 vacuum plastic bags and carton boxes
analysis on spherical products (apples) has shown that           respectively, with 20 mandarins each one (total 80) to
the best signal is produced when the response is recorded        each treatment, it were stored for 12 days at 20 ± 0,5 ºC,
0 or 180° from the place of impact (HUARNG et al., 1993).        relative humidity (RH) (60 %). Mandarin ((a bag/ a box)
RESEARCH of LANGENAKENS et al. ,1997, showed that                every time) were removed from storage at 3, 6, 9 and 12
the first resonance frequency for tomatoes corresponds           days and evaluated.
with the oblate–prolate spherical mode, which appears                Forty samples prior storage (at harvest time) and after
not to be influenced by the internal compartment                 each storage interval time (20 mandarins) were subjected
structure of the tomato. DE BELIE et al.,2000, in their          to compression test, immediately after mass, density and
research concluded that the acoustic technique is very           acoustic impulse response measurements.
reproducible, and its sensitivity to firmness changes was
                                                                                                                                     25
     Revista Ciencias Técnicas Agropecuarias, Vol. 15, No. 4, 2006
 Mass and density measurement                                         impedance sensor with electrical sensitivity 3.7 PC/N:
                                                                      measure range: force 2000 N, traction force 100 N and
     The mass of fruit was determined with the Electronic             inherent frequency 55 kHz.
 Precision Balance (Mettler Toledo, Max. 1000± 0.01 g). The                A multi channel combined type electrical amplifier YE5853
 mass loss was determined over all individual fruit during            was used with a frequency range from 1 Hz to
 each storage interval time. The percent of mass loss can             200 kHz, with three bit and decimal system, transducer degree
 be calculated as:                                                    regulate, all different sensitivity tone can reach normalization
                    ml= [(mi -mf)/ mi ] * 100 %                 (3)   process. Traduce sensitivity adjust range from 1- 10,99 PC/
                                                                      unit. Maximum electrical input quantity 105 PC, output gain 1,
 where:                                                               3, 10, 30, 100, 1 000 mv/unit (selected value 100 mv/unit).
 m1: mass loss, %                                                          The sign was transferred to a computer (PC9801VM2,
 mi : initial fruit mass, g                                           NEC) through an A/D converter (PCL- 1800). The sampling
 mf : final fruit mass, g.                                            frequency was 1 kHz photoelectric switch used as an auto-
      The fruit density was measured by Archimedes’                   trigger in order to obtain the same timing of data acquisition.
 principle using a purpose-built apparatus for fruit                  The data were analyzed by mean of Genie software
 volumetric measurement by full fruit immersion in 1 l of             (Interactive signal processing package) for Windows. This
 clean tap water. Fruit were pre-wet, prior to placement in           program gave a rapid visual depiction of the spectrum on
 the apparatus, to minimize air bubbles forming on the fruit          the computer screen in function of time. The selected sound
 surface during immersion. Appropriate procedures and                 waves were imported to Microcal Origin 6.0 software, the
 corrections were used to account for the volume of the               response from time to frequency by means of Fast Fourier
 suspension apparatus, and the fruit and water temperature.           Transform (FFT), as demonstrated in fig. 1 a, b.
      The density loss of each fruit was determined during
 each storage interval time. The density loss of fruit was
 determined using the next expression:
                                                                      Measurements of acoustic signals response
                     l= [(i -f)/ i] * 100 %                 (4)        During the test, the fruit (mandarin) was first placed on a
                                                                      soft foam support in order to create free support conditions
 where:
                                                                      and not to disturb the vibration pattern. The frequencies were
 1 : density loss, %
                                                                      obtained with a piezoelectric sensor placed at opposite side
 i : initial fruit density, g/cm 3
                                                                      of the impact point; the piezoelectric sensor was composed
 f : final fruit density, g/cm3.
                                                                      for a soft polyvinylidene fluoride (PVDF) film coated with
       For the calculations were assumed a water mass
                                                                      thin layers of conductors that were bonded to a soft
 density of 1 g/cm3.
                                                                      polyethylene-foam padding to allow free vibrations of the
                                                                      fruit. Fruits were then excited at the marked positions; the
 The fruit firmness                                                   frequency of all individual fruit was measured on the three
      The fruit firmness was quantified by maximum compression        positions along the equator approximately 120º between
 force (Fc). The maximum compression force required to                them; the pendulum consist of a wooden ball of diameter
 compress a fruit by 3 % of its diameter was recoded at a strain      24.5 mm and a 150 mm long plastic rod. The weight of the
 rates of 0.00016 m/s (10mm/min). The maximum compression             pendulum was 12 g, the impact angle was of 45. It was light
 force of all individual fruit was measured on the three positions    enough to avoid the damages. However, there may be slight
 along the equator approximately 120º between them, perpendi-         damage if the same point receives repeated impact. In order
 cular to the stem-bottom axis. The measurements were carried         to avoid this damage, it was verified that the location of the
 out a Universal Testing Machine (Model 5543 Single Column,           impact point did not affect the sound signals. Then, the
 Instron Corp., Canton MA. USA). The test was performed using         impact point was periodically changed to minimize damage
 parallel plates to compression test.                                 to the sample. The intensity of impacts at given point was
                                                                      limited to three repeated impact with a total, of 9 per fruit. In
                                                                      the resulting frequency spectrum, the first resonance
 EQUIPMENT
                                                                      frequency (f) was selected (DE BAERDEMAEKER, 1988;
 Experimental equipment                                               CHEN et al., 1992). Arbitrarily, only frequencies of which
                                                                      the peak amplitude was larger than 50 % of the overall peak
     The acoustic signal was sensed by a piezoelectric                amplitude were considered in this selection. Fig. 1b shows a
 transducer type sensor acceleration (CA-YD-139) with the             typical frequency spectrum for a mandarin and the selected
 following levels: sensitivity 0.05 PC/ms -2, maximum                 peak. Each signal was processed and normalized by maximum
 transverse ratio d 5 %, maximum velocity increment                   sound intensity. The Firmness Index was calculated by
 105 ms-2, mass 5 g with a flat frequency response between            equation 1, and the Elastic Modulus (or Young’s Modulus)
 4 Hz-4 kHz, used sensor of force CL-YD-331, force                    estimation was done using equation 2 (elasticity coefficient).
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                                                                     Revista Ciencias Técnicas Agropecuarias, Vol. 15, No. 4, 2006




       FIGURE 1. Typical acoustic signal of Satsuma mandarin (mean 9 impacts): (a) time domain, (b) frequency domain.

RESULTS AND DISCUSSIONS                                          carton box; it was minor due to the lowest weight loss in
                                                                 this period.
Mandarin compression force behavior during                            The results of regression analysis between Fc and
storage                                                          storage time for mandarin stored in bag and box are given
                                                                 in table 2,5. In both cases the compression force response
    The compression force in mandarin stored in bag and          to a polynomial equation, specified deg=2, moreover the
box are shown in Fig. 2. As can seen, in this figure             relationship between the Fc and storage time for mandarin
depending upon the storage two-type treatments at the            stored in plastic bag and carton box is high, which means
same temperature (20 C) the longer storage period gave          that 97 and 98 % of the variation in Fc is explained by
the smaller Fc. The Fc loss of the fruits stored in bag was      storage time.
higher than the Fc loss of fruits stored in box.                      In citrus fruits, the relation between the degradation
                                                                 of the cellular wall and the loss of firm ness that
                                                                 accompanies fruit maturation has been observed as well
                                                                 (TING and ROUSEFF, 1986; MURAMATSU et al., 1996a).

                                                                 Acoustic signal response to mandarin during
                                                                 different maturity states in storage
                                                                     The figures 3a, b & c, show the change of the
                                                                 frequency, firmness index and elasticity coefficient against
                                                                 time, during Satsuma mandarin ripeness in storage at 20 ºC
                                                                 and 552 % RH for each storage type or treatment. The
                                                                 higher f, FI and E corresponds to the firmer mandarins prior
                                                                 storage.
                                                                     The frequency in mandarin stored in bag and box are
                                                                 shown in Fig. 3a. As can seen, in this figure depending
FIGURE 2 Compression force changes in mandarin according         upon the storage two-type treatments at the same
         to storage time at 20 ºC and 55 % R.H.                  temperature (20 C) the longer storage period gave the smaller
                                                                 resonant frequency. However starting from the seven-
    At the end of twelve days storage period the highest
                                                                 observation day for the mandarins stored in carton box a
Fc loss was obtained (22,89 N) from the fruits in carton box
                                                                 light increased in frequency can be observed. This
and the lowest Fc loss was obtained (17,68 N) from the
                                                                 phenomenon can be associated to the high fruit water lost
fruits stored in plastic bag. The average of compression
                                                                 value due to the fruit was exposed to the surrounding air
force loss every three days it was 4.42 and 5,68 N for
                                                                 bring to as a consequence an excessive drying of the
mandarin stored in bag and box respectively. The
                                                                 mandarin; due to high dehydration the skin loss its natural
compression force values of mandarin rapidly decrease
                                                                 turgor, this phenomena is accompanied with the peel creasing
during their first six storage days (day 3 & day 6) after this
                                                                 and an increment of the skin resistance to penetration. Due
time the fruit compression force slowly decreased during
                                                                 to this phenomenon the resonant frequency, increase its
the rest of intervals. It is also shown that the fruit Fc loss
                                                                 values at the last observation, not meaning this that the
value descends from 4,62 and 8,1 N in the first three day
                                                                 internal mandarin firmness increased with storage time. In
down to a 0.61 and 0.23 N in its last three storage days in
                                                                 the reality, the mandarin firmness measured through of
bag and box respectively. Moreover, the compression for-
                                                                 compression force deceased all the time.
ce loss during the last three days of the mandarin kept in
                                                                                                                                     27
     Revista Ciencias Técnicas Agropecuarias, Vol. 15, No. 4, 2006




               FIGURES 3a, b & c. Changes in mandarin mean frequency value during its ripening in storage (20 mandarins).

          The resonant frequency values of mandarin stored in bag, decrease during whole storage time from 375 to 287 Hz. This
     result means a frequency decrease of 23,32 % during all storage time. During the three first days can be see a quickly change
     in frequency with a fall of 43 Hz this value represent 11,27 % of the total frequency lost. After this period a moderate decreased
     in frequency, change is observed. The average of frequency loss every three days it was 22 Hz (5,83 %) for mandarin stored in
     bag, this result infers that the decrease in resonant frequency for each fruit is 1,94 % per day.
          The results of regression analysis between puncture force and storage time for mandarin stored in bag and box
     are given in Table 1. In both cases the resonant frequency response to a polynomial equation, specified deg=3,
     moreover the relationship between the frequency and storage time for mandarin stored in plastic bag is good, with
     a determination coefficient of 0,98, which means that 98 % of the variation in frequency is explained by storage time.

       TABLE 1. Regression analysis results between each mandarin acoustic parameters and storage time (20 mandarins)

                     Equation                                                                R2          P          SD
                     Storage in bag
                     f= 373,93357- 16,04821d + 1,47325d2- 0,06194d3                          0,98013     0,17887    9,59171
                                                          2               3
                     FI=3,14266- 0,18806d+ 0,00703d - 1,57685e-5d                            0,98992     0,12759    0,13217
                                                                  2            3
                     E= 0,30522- 0,01783d+ 5,95861E-4d + 5,5328e-6d                          0,98922     0.13194    0,13769
                     Storage in box
                     f= 375,1652- 33,89789d+ 2,93869d2- 0,07256d3                            0,99997     0,00729    0,01017
                                                              2       3
                     FI= 3,17258- 0,55545d + 0,05909d - 0,0021d                              0,99866     0,04652    0,06749
                                                          2               3
                     E= 0,30733- 0,05387d + 0,00558d - 1,94444e-4d                           0,99967     0,02327    0,00275

         The Fig. 3b shows the firmness index behavior of                     mandarin as storage time function. The coefficient of elasticity
     mandarin as storage time function. The FI of mandarin has                of mandarin has a decline during whole observation time. Its
     a decline during whole observation time. The firmness index              values fall from 0,31 to 0,19 MPa and from 0,31 to 0,13 in
     values fall from 3,16 to 1.93 *104 kg2/3s-2 and from 3,16 to             mandarin stores in bag and box respectively. This result means
     1,38 *10 4 kg 2/3s -2 in mandarins stores in bag and box                 a diminution of 56,32 % and 58,31 % in the elasticity coefficient
     respectively. This result mean a diminution of 39,92 % and               value respective to its initial values (at harvest time) during
     56,32 % in the firmness index value respective to its initial            whole storage time.
     values (at harvest time) during whole storage time. The                       The E average value for each interval of storage time
     firmness index average value for each interval of storage                (3 days) was 0,03 and 0,04 MPa in mandarins stored in bag
     time (3 days) was 0.31 and 0,45 *104 kg2/3s-2 in mandarins               and box respectively during all storage time.
     stored in bag and box respectively.                                           The firmness index of mandarin decrease during their
         The decrease in the firmness index during the last three             perma nence in storage, a ccording to third-degree
     observation days in mandarin stored in carton box only                   polynomial curve, see table 1 for each storage treatments.
     archived a 3,37 % of the total, consequently with the                    The relationship between E and storage time is excellent,
     increase of the resonant frequency in this period.                       with a determination coefficient of 0,98 and 0,99 in mandarin
         The firmness index of mandarin decrease during their                 stored in bag and box respectively.
     perma nence in storage, a ccording to third-degree                            On the contrary to frequency, the firmness index an
     polynomial curve, see table 1 for each storage treatments.               elasticity coefficient decrease its values during all mandarin
     The relationship between FI and storage time is excellent,               storage time in bag and box, give it the true behavior of the
     with a determination coefficient of 0,98 and 0,99 in mandarin            fruit firmness during storage. The change in fruit firmness
     stored in bag and box respectively.                                      more significant was given by elasticity coefficient; the
         The Fig. 3c shows the elasticity coefficient behavior of             elasticity coefficient values overcoming in 0.16 and 1,98 %
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                                                                    Revista Ciencias Técnicas Agropecuarias, Vol. 15, No. 4, 2006
the values of firmness given by the firmness index in           elasticity coefficient. The higher f, FI and E corresponds
mandarins stored in box and bag respectively during the         to the firmer fruit prior storage. The change in fruit firmness
12 days of mandarin storage time. For what it, can be           more significant was given by elasticity coefficient. For
considered the elasticity coefficient as the best indicator     what can be considered the elasticity coefficient as the
of the fruit firmness when the acoustic signal response         best indicator of the fruit firmness when the acoustic signal
technique is used to measure the firmness of the fruit during   response technique was used to measure the firmness of
storage periods.                                                the fruits during the storage periods. For this reason it is
     In mandarin stored in box high fruit water lost values     recommended to use the firmness index and elasticity
can be observed, due to the fruit was exposed to the            coefficient to study the firmness behavior of the fruit during
surrounding air, bring to as a consequence a excessive          their storage live.
drying of the mandarin skin. This desiccation phenomenon             Regression was carried out with the means of 20
gave an increase of the fruit internal resonant frequency,      determinations of each mandarin parameter except in the
but this doesn’t mean that the fruit firmness given by the      beginning (start day/ day 0) where were took 40 samples.
firmness index and elasticity coefficient diminishes
consequently with the storage time. It can be concluded         CONCLUSIONS
that the acoustic response technique is also efficient to
measure the mandarin firmness when the fruit is stored in       • The firmness index and elasticity coefficient were clearly
carton box.                                                       identified and steadily decreased with storage time, for
     The mandarin ripening in bag is characterized for a          what can make sure that the Acoustic impulse Response
progressive decrease in the frequency, firmness index and         gives a reliable indication of the change or/and ripeness
elasticity coefficient. The higher f, FI and E corresponds        status of mandarin fruit during the storage conditions.
to the firmer mandarin prior storage. In the case of the        • A negative strong correlation was found between each
mandarin stored in box a different behavior was observed          mandarin acoustic analyzed parameters and storage
to the last tree days of observation with an increase of the      time. In all cases, the process was characterized by
frequency in the case of firmness index and elasticity            polynomial model.
coefficient can be express the true firmness behavior. Same     • T h e n on dest ruct i ve a coust i c t est m a y r epla ce
result was found by DUPRAD et al., (1997), in Golden              conventional destructive test mandarin in order to de-
Delicious apples. They showed that the acoustic response          termine fruit firmness and expected shelf- life.
of the fruit fall with storage time.                            • No sing of bruising was observed in the fruits tested,
     The fruit ripening in storage is characterized for a         during and after the experiments, confirming the non-
progressive decrease in the frequency, firmness index and         destructiveness of this technique.

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            La Universidad Autónoma Chapingo y su Departamento
              de Ingeniería Mecánica Agrícola felicitan a la RCTA,
         por sus 20 años al servicio de la información científica y técnica




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