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Measurement of avocado softening at various temperatures using ultrasound

Measurement of avocado softening at various temperatures using ultrasound
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  Postharvest Biology and Technology 20 (2000) 279–286 Measurement of avocado softening at various temperaturesusing ultrasound U. Flitsanov  a, *, A. Mizrach  b , A. Liberzon  a , M. Akerman  c , G. Zauberman  c a Faculty of Agricultural Engineering  ,  Technion  –  Israel Inst .  Tech .,  Haifa  32000  ,  Israel  b Institute of Agricultural Engineering  ,  Agricultural Research Organization  (  ARO  )  ,  the Volcani Center ,  PO Box  6  , Bet Dagan 50250  ,  Israel  c Institute for Technology and Storage of Agricultural Products ,  ARO ,  the Volcani Center ,  PO Box  6  ,  Bet Dagan  50250  ,  Israel  Received 12 October 1999; accepted 19 June 2000 Abstract The low-temperature storage of avocado affects its subsequent softening process and shelf life. One of the mainindices of ripeness in avocado fruit is firmness, which changes during the ripening and softening process. Thetemperature and duration of storage fundamentally influence the firmness of the stored fruit, and monitoring thesoftening of fruit enables us to regulate its shelf life. The objective of the present study was to use nondestructiveultrasonic tests to elucidate the influences of storage temperature and time on the softening process of avocado fruit.The attenuation of the ultrasonic waves transmitted through the fruit tissue changes as the fruit passes through thevarious softening stages during and after low-temperature storage. Four groups of avocados, each stored at adifferent low temperature, and a control group which was stored at room temperature (20°C) were examined duringand after their designated storage times, until they reached full ripeness at room temperature. Nondestructiveultrasonic tests and destructive penetration measurements were carried out in order to determine the attenuation andthe tissue firmness, respectively, of the avocados. Statistical analysis showed quite good correlation between thefirmness and the ultrasonic attenuation, and their dependence on previous storage time-temperature history. Thissuggests that the ultrasonic measurements could be used as a nondestructive method of monitoring avocado ripenessduring low-temperature storage. © 2000 Elsevier Science B.V. All rights reserved. Keywords :   Ultrasonic transducers; Firmness; Temperature measurements; Storage time; / locate / postharvbio 1. Introduction Horticultural maturity of avocado ( Perseaamericana  Mill. cv. ‘Ettinger’) fruit can be definedas the growth stage at which harvested fruit willundergo normal ripening. Mature avocados donot ripen on the tree, but soften several days afterbeing picked. When an immature avocado is har-vested, it will not ripen properly, if at all, whereasan over-ripe one will decay rapidly after harvest.One of the main indicators of ripening in avocadois their firmness. Previous studies have shown that * Corresponding author. Tel.:  + 972-4-8292802; fax:  + 972-4-8221529. E  - mail address : (U. Flitsanov).0925-5214 / 00 / $ - see front matter © 2000 Elsevier Science B.V. All rights reserved.PII: S0925-5214(00)00138-1 Reprinted with permission from ELSEVIER, Inc.Postharvest Biology and Technology Homepage:  U  .  Flitsano   et al  .  /   Posthar  est Biology and Technology  20 (2000) 279–286  280 avocado firmness correlates well with fruit ripe-ness and expected storage time (Lewis, 1978).Firmness declines initially at a moderate rate,which increases later until firmness falls to lessthan 5 N resistance of penetration at full ripening(Zauberman and Fuchs, 1981). The temperatureand time of storage fundamentally influence thefirmness and other mechanical parameters of thestored fruit. The response of avocados to low-temperature storage has been studied by Bleinrothet al. (1976), Zauberman et al. (1977), Zaubermanand Jobin-Decor (1995), Berger et al. (1982), andthe firmness change under these conditions hasbeen used as an indicator of ripening. Storagetemperatures generally range between 5 and 8°C,but some research performed as low as 2°C foundthat storage at 2–8°C could extend shelf life andfacilitate the distribution of avocados to distantmarkets.In order to control the ripening process and toensure longer shelf life, it is advisable to monitorthe firmness of the avocado fruit during low-tem-perature storage. Frequently removing samples of fruit and performing penetrometer tests is ameans of doing this, but it is a destructive tech-nique that does not allow continuous monitoring,and there is a great need for a nondestructivemethod. Various researchers have suggested di-verse nondestructive methods for maturity andfirmness evaluation of avocado fruit: nuclear mag-netic resonance (NMR) methods (Chen et al.,1993), applying vibrations to one side of the fruit,while measuring the transmitted vibration energyon its other side (Peleg et al., 1990), acoustictesting based on mechanical impulse excitationand flexible piezoelectric sensors (commerciallyavailable as Firmalon) (Galili et al., 1998); nearinfrared spectroscopy (Schmilovitch et al., 1997);ultrasonic excitation (Mizrach et al., 1996), etc.None of these studies included measurements atvarious storage temperatures. The present authorspropose an ultrasonic technique for measurementsof the avocado softening process under low-tem-perature storage conditions. An ultrasonic tech-nique has been suggested in the past, as adedicated method for nondestructive firmnessevaluation of fruit and vegetables (Mizrach et al.,1989); it used a high-power, low-frequency ultra-sonic system for excitation of fruit tissue, andexploited the basic acoustic properties of somefruit and vegetables, i.e. wave propagation veloc-ity and attenuation. In addition, Mizrach et al.(1994) found a strong interdependence betweenultrasonic properties and post-harvest ripeningparameters of the fruit tissue, and they patented adevice for nondestructive determination of fruitquality parameters. Models of ultrasonic parame-ters to assess avocado properties and shelf lifewere suggested as well (Mizrach et al., 1996). Allthe above-mentioned studies of the ultrasonictechnique were performed on avocados placed inroom-temperature storage immediately after har-vest. Recently, Mizrach et al. (2000) suggested amethod for monitoring avocado softening in low-temperature storage using nondestructive ultra-sonic measurements. The objective of the presentstudy was to use this technique to analyze ultra-sonic signals in avocados stored at several lowtemperatures and to examine the effects of thetemperature and storage time on the softeningprocess in the low-temperature storage and whileremoving the fruit to room-temperature storage. 2. Materials and methods The basic experimental arrangement included ahigh-power, low-frequency ultrasonic pulser-re-ceiver (Krautkramer, model USL33), a pair of 50-kHz ultrasonic transducers and a microcom-puter system for data acquisition and analysis.Exponential-type Plexiglas beam-focusing ele-ments were used to reduce the 55-mm diameter of the beam from each transducer to that of thedesired area of contact with the fruit. The trans-ducers were mounted with an angle of    120°between their axes, enabling an ultrasonic signalto be transmitted and received over a short dis-tance across the peel of the fruit (Mizrach et al.,1994). The head arrangement allowed relative mo-tion of the ultrasonic probes along a transverseaxis, while a controlled contact force on the fruitpeel was maintained.The through-transmission mode was selected,with one transducer acting as a transmitter andthe other as a receiver. The pulser caused the  U  .  Flitsano   et al  .  /   Posthar  est Biology and Technology  20 (2000) 279–286   281 transmitter to oscillate and emit a narrow-bandultrasonic pulse obliquely into the peel andthrough the fruit tissue. Previous study had shownthat the thin peel of avocado fruit (cv. ‘Ettinger’)did not interfere with the ultrasonic waves(Mizrach et al., 1994). The ultrasonic energy inputinduced waves through the fruit tissue, whichactivated the receiver. The output pulse was dis-played on a cathode ray tube (CRT) monitor, onwhich the pulse amplitude and transit time couldbe visually determined. In parallel, a built-in peakdetector and microprocessor-controlled serial in-terface captured the signal amplitude and thetransit time, and sent a digitized read-out to anexternal microcomputer. The collected data wereused to determine the attenuation of the wavesand hence to calculate the attenuation coefficientof the fruit, by means of common formulationsdescribed by Mizrach et al. (1989).Mature avocado (cv. ‘Ettinger’) were harvestedand graded by weight, and those of OECD stan-dard (OECD, 1995) size group ‘16’ (weight be-tween 236 and 265 g) were selected. Fruit weretransported to the laboratory within 48 h andtransferred to well ventilated rooms at 2, 4, 6 and8°C. Fifteen avocados were stored continuously ina ventilated laboratory at 20°C (  85% humid-ity); they served as controls and were assesseddaily. Each week for 4 weeks, 15 fruit were re-moved from each temperature regime for ultra-sonic nondestructive tests and destructivepenetration tests at five successive ripening times.The peel of each fruit was marked at six locationson the circumference (‘equator’) of the largestcross–section perpendicular to the blossom end-stem end axis; one for ultrasonic nondestructivetesting and the remaining five for penetration testsat successive ripening times. The six locationswere spaced 60° apart around the circumference,to minimize the influence on the repeated NDTtests and of local bruising caused by the firmnesspenetration test. Destructive tests were conductedimmediately after the ultrasonic tests, to deter-mine the firmness of the avocado fruit.Each fruit was subjected daily to a nondestruc-tive ultrasonic test. The pulse amplitude of thetransmitted ultrasonic signal was measured at themarked point, for five different spacings (5, 12,14, 16 and 18 mm) between the two probes. Theattenuation of the ultrasonic signal was calculatedaccording to the exponential expression(Krautkramer and Krautkramer, 1990):  A = A 0  e −  l  , where  l   is the spacing between the inputand collection probes,  A  and  A 0 , respectively, arethe ultrasonic signal amplitudes at the beginningand end of the propagation path,  l   of the ultra-sonic wave, and    is the apparent attenuationcoefficient of the signal.The penetration tests were performed on un-peeled fruit (a recognized method for firmnessdetermination in all varieties of avocado fruit),with a durometer (John Chatillon & Sons, NewYork) with a 6.35-mm diameter conical head and60° cone angle (Mizrach et al., 1996). The peakforce, measured in newtons (N), was recorded at apenetration rate of 3 mm / s, to a maximum pene-tration depth of    7 mm. Each fruit was sub- jected to the penetration test at the remainingmarked locations, in sequence, every second dayuntil they reached eating ripeness (i.e. firmnessequal to 10 N). 3. Results and discussion The softening process of the control avocados(20°C) started immediately after harvest, and theirfirmness dropped rapidly from 99.5 N on the firstday to a low value of 12.1 N after 7 days (Table1). The ultrasonic attenuation of this group in-creased rapidly over the same period from 2.09dB / mm initially to 4.03 dB / mm on the last day.The correlation coefficient between the attenua-tion and the firmness for the control avocadoswas determined once, immediately after storage,and found to be relatively high. A correlationanalysis was performed for the controls (15 fruit)and for the totals of the weekly values over 4weeks (60 fruit per storage temperature).As shown in Table 1, the avocados at 6 and8°C softened more rapidly than those stored at 2and 4°C. During four weeks of storage at thelower temperatures (2 and 4°C), the firmness de-creased, although insignificantly, to 89.2 and 79.2N, respectively. Fruit from higher temperaturestorage (i.e. 6 and 8°C) changed their firmness  U  .  Flitsano   et al  .  /   Posthar  est Biology and Technology  20 (2000) 279–286  282 more significantly to 12.5 and 10.9 N, respec-tively. The attenuation measured on avocadossampled from low-temperature storage initiallydecreased during the first storage week, and av-ocados from lower storage temperatures showedlower attenuation. The signal amplitude attenua-tion changed from an initial value of 2.09 dB / mmto lower values: 1.56, 1.89, 1.94 and 2.03 dB / mm,for fruit stored at 2, 4, 6 and 8°C, respectively.This phenomenon has been reported in previousstudies (Mizrach et al., 1989; Mizrach and Flit-sanov, 1995; Mizrach et al., 1996). Then, from theend of the first week of low-temperature storageuntil 28 days, the attenuation increased with timefor all groups: to 1.87, 2.36, 3.02 and 3.19 dB / mmfor those stored at 2, 4, 6 and 8°C, respectively.The firmness values and the amplitude attenua-tion for 4 weeks of low-temperature storage arepresented in Fig. 1 as dots connected with dottedlines. The fruit stored at 4 and 6°C gave bettercorrelation coefficients between destructive andnondestructive tests ( r = 0.978, and 0.979, respec-tively) than those stored at 2 and 8°C ( r = 0.926and 0.9, respectively).The second stage of the experiment was theanalysis of the ripening process at room tempera-ture (20°C), after low-temperature storage. Aftereach week, during 4 weeks of cold storage, 15fruit were removed from each group and kept atroom temperature (20°C) until their firmnessreached the ripe level (i.e. 10 N). The fruit firm-ness changes were monitored with nondestructive Table 1Variation of wave amplitude attenuation and firmness of avocado, during 4 weeks of low-temperature storage (at 2, 4, 6 and 8°C),and control group stored at room temperature (20°C)Time (days) Firmness Correlation coefficient between firmnessStorage temperature (°C) Attenuationand attenuation r S.D. a NS.D. a dB / mm0.1420 99.50 5.1 0.9642.09 0.15 75.11 4.42.375.564.40.162.552 3 2.90 0.26 50.1 4.04.330.20.233.104 4.823.30.283.415 3.89 0.26 15.1 4.067 4.03 0.33 12.1 5.97 2.03 0.258 51.5 3.0 0.9003.831.70.172.4114 21 2.48 0.16 19.7 2.628 3.19 2.810.90.186 0.137 54.7 4.0 0.9791.940.12 3.414 2.27 34.721 2.47 0.26 28.2 3.51.028 3.02 0.20 12.52.489.30.111.8974 0.978 0.14 85.8 2.92.13140.132.1721 3.083.1 2.36 0.1728 79.2 4.195.4 3.52 0.9267 1.56 0.120.12 92.9 2.614 1.571.78 0.13 91.3 3.52128 1.87 0.14 89.2 3.0 a Standard deviation (S.D.).  U  .  Flitsano   et al  .  /   Posthar  est Biology and Technology  20 (2000) 279–286   283Fig. 1. Variation of the mean values of firmness (  ) and wave attenuation (  ) with time at room temperature for avocado removedweekly from 2, 4, 6 and 8°C storage temperatures. Dotted lines represent the variation with time of the firmness and attenuationduring low-temperature storage. ultrasonic attenuation measurements and mea-sured with the destructive penetration test. Thechanges with time of the mean values of theresulting firmness and ultrasonic wave attenuationdata are presented in Fig. 1. The correspondingdata for the control group are also presented inFig. 1, labeled ‘week 0’. Firmness and attenuationdata for each sample of fruit were recorded fromtheir removal to room temperature until theyreached a firmness of 10 N. The graphs show thatthe softening process of avocado fruit which hadbeen stored at 2 and 4°C was slower than that of those stored at 6 and 8°C (Fig. 1). Thus, after thefirst week of low-temperature storage, the ripen-ing of the fruit stored at 2 and 4°C took 7 and 8days, respectively, while those stored at 6 and 8°Cripened faster, taking 5 days. The time data (indays) for room temperature ripening (i.e. shelf life) are presented in Table 2; the ripening timedecreased with increasing storage time. Thus, af-ter 3–4 weeks of storage at the higher tempera-tures, the fruit softening took 2 days or 1 day forfruit stored at 6 or 8°C, respectively. Avocadosstored at 2°C for 4 weeks ripened within 3 days,but those stored at 4°C took a mean of 4 days toripen fully. Some of the fruit stored at 2°C for 4weeks sustained chilling injury defects.In parallel to destructive firmness measure-ments, nondestructive ultrasonic measurementswere performed. Fig. 1 shows that attenuationincreased during the ripening at room-tempera-ture, after removal of the avocados from coldstorage. A good correlation was found betweenthe firmness and the ultrasonic wave attenuationdata presented in Fig. 1, and summarized in Table3. The correlation coefficients for the groups,
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