Kiwifruit and apricot ﬁrmness measurement.pdf

Postharvest Biology and Technology 19 (2000) 47–54

Kiwifruit and apricot firmness measurement by thenon-contact laser air-puff method

V. Andrew McGlone *, Robert B. JordanTechnology De6elopment Group, HortResearch, Ruakura Research Centre, Pri6ate Bag 3123, Hamilton, New Zealand

Received 9 June 1999; accepted 21 December 1999

Abstract

The laser air-puff method was investigated for non-destructive firmness measurements on kiwifruit (Actinidiadeliciosa (A. Chev.) C.F. Liang et A.R. Ferguson cv. Hayward) and apricot (Prunus armeniaca L. cv. CluthaGold).The method involves delivering a sharp puff of air onto a stationary fruit whilst recording the resulting surfacedeformation with a laser displacement sensor. At a 65-kPa maximum puff pressure the deformations on both kiwifruitand apricots ranged from 0.1 mm for hard fruit to about 1 mm for very soft. A fruit stiffness value (Epuff) wascalculated from the maximum deformation and compared with penetrometer firmness (Fpen) on cool-stored kiwifruitand mixed maturity apricots. Two measurements were made on opposite sides of each fruit using each technique andwere averaged before regression analysis. Reasonable regression results (R2�0.80, s=2.1 N) were obtained betweenFpen and Epuff on kiwifruit over the penetrometer firmness range 0–30 N. The relationship showed a trend ofincreasing scatter with increasing kiwifruit firmness. With apricots, over a greater penetrometer range (0–80 N), therelationship between Fpen and Epuff was best described by a simple power law regression (R2�0.80, s=8.2 N). Bothsets of fruit results suggest the laser air-puff method is only suitable for coarse screening of fruit into twopenetrometer firmness classes. Improved correlations may be possible by using higher puff pressures and/or moremeasurements per fruit. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: Firmness; Stiffness; Non-destructive; Penetrometer; Kiwifruit; Apricots

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1. Introduction

The laser air-puff method, invented by re-searchers at the University of Georgia (Prussia etal., 1993, 1994), is attractive in its potential as ahigh speed grading technique as no mechanical

contact is required between the fruit and thesensor. The method works by delivering a puff ofair to the fruit and simultaneously measuring thesurface deformation that occurs. The underlyingtheory is that the ratio of deformation to appliedforce can be linked to fruit stiffness and, further,that stiffness is related to the penetrometerfirmness.

Hung et al. (1999) recently reported on thedevelopment and use of a laser air-puff instru-

* Corresponding author. Tel.: +64-7-8562835; fax: +64-7-8584705.

E-mail address: [email protected] (V.A. McGlone)

0925-5214/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 5 - 5 2 1 4 ( 0 0 ) 0 0 0 6 8 - 5

V.A. McGlone, R.B. Jordan / Posthar6est Biology and Technology 19 (2000) 47–5448

ment for measuring the firmness of peaches. Theyestablished that the instrument could provide anon-destructive firmness measure that was signifi-cantly correlated (R2�0.74) with the penetrome-ter firmness of peaches over a large range (0–120N). We also have recently reported on the devel-opment of a similar laser air-puff instrument (Mc-Glone et al., 1999). That report concentrated oncharacterising the instrument and providing evi-dence that the method could indeed measure fruitstiffness. Preliminary kiwifruit measurements werereported that were encouraging in relating fruitstiffness to penetrometer firmness (R2�0.88) overa narrow penetrometer firmness range (0–12 N).

The objective of the current research was toinvestigate the laser air-puff method for fruit firm-ness measurement on kiwifruit and apricots. Forkiwifruit, the primary interest is in sorting fruitafter long-term cool storage when consignmentaverages go well below 30 N and the presence ofsoft fruit puts consignments in jeopardy in terms

of industry regulations and final consumer accep-tance in the market place. Consumers will gener-ally not accept fruit B6 N (Stec et al., 1989) andthis results in setting thresholds back at the coolstore at anywhere between 8 and 15 N dependingon the exact market requirements and post-stor-age handling conditions. With apricots, the inter-est is in harvest-time sorting of mixed maturityconsignments to remove immature or over-maturefruit. Penetrometer firmness has been suggested asa standard for determining maturity in stonefruit,and in particular, over-maturity (Crisosto, 1994).

2. Materials and methods

2.1. The laser air-puff instrument

The major components of the laser air-puffinstrument were a tank of pressurised air, a so-lenoid switching valve, an outlet nozzle and alaser displacement sensor (Fig. 1). Fruit to betested were positioned directly below the nozzleand supported, when necessary, in a cradle madefrom soft modelling clay. The cradle preventedsideways movements of the fruit under the puffand also minimised deformation on the bottomside of the fruit by spreading the load over a largecontact area. The solenoid valve, nozzle and dis-placement sensor were all fixed in position on ashifting arm that was adjusted, prior to mostmeasurements, to give a nozzle-fruit distance of23.3 mm that brought the displacement sensorreading close to its zero setting. A short blast ofair, the puff, was delivered to the fruit by com-puter control of the open duration (typically 80ms) of the solenoid valve (VS3145, SMC, USA).The tank supplying the air had a capacity of 50 land was regulated to a typical set air pressure of100 kPa. The nozzle had an internal diameter of8.2 mm and this delivered a steeply peaked pres-sure profile at the fruit surface of diameter, athalf-width, of 10 mm. Fruit surface deformationsduring a puff were measured with the laser dis-placement sensor (Keyance LB-081, Osaka,Japan), which had a range of 915 mm aroundthe zero setting and a resolution of 8 mm. Thelaser beam of the sensor passed through a glass

Fig. 1. A schematic diagram (not to scale) of the laser air-puffinstrument. The nozzle is shown in cross-section with a shadedline following the vertical laser beam path to the fruit and thenthe imaging line (off-angle) back to the sensor.

V.A. McGlone, R.B. Jordan / Posthar6est Biology and Technology 19 (2000) 47–54 49

Fig. 2. A deformation curve (solid line) and a repeat (dottedline) measured at the same site on a soft apricot (Fpen�5 N).The peak deformation was measured as shown, for the firstcurve, and was 0.54 mm for both curves. Puff duration was 80ms and the tank pressure was 100 kPa. Timing of solenoidcontrol pulse shown at top.

give a baseline reading close to zero. At 150 ms,some 50 ms after initiation of the solenoid signal,the second stage occurs with displacement read-ings climbing quickly due to the puff impactingon the apricot. The readings level off (the thirdstage) after a further 50 ms and reach a maximumafter about 220 ms total elapsed time. The finalstage occurs after 260 ms elapsed time, and some70 ms after cessation of the solenoid signal, as thedisplacement readings decline exponentially backtowards the baseline reading in response todiminution of the puff. The deformation curvesare not necessarily exactly repeatable and, in par-ticular, the baseline reading is not recovered dur-ing the final stage (Fig. 2). Softer fruit, or higherpeak puff pressure, generally resulted in a greaterbaseline discrepancy. Deformation curves for elas-tic objects (e.g. rubber balls) always showed com-plete recovery and so the baseline discrepancyobserved with fruit is due to non-recoverablestrains occurring at lower stress limits than thoseapplying to elastic objects. These non-recoverablestrains never revealed themselves in visible dam-age on the fruit and did not affect the repeatabil-ity, at least for successive pairs of measurements,of the firmness parameter extracted from the de-formation curves (Fig. 2). Hence it appears thatthe fruit are not damaged by the puff although norigorous damage or repeatability studies have yetbeen done to confirm this.

The key firmness parameter extracted from thedeformation curve was the peak deformation, D,calculated as the difference between the maximumdisplacement reading and the average baselinereading occurring during the first stage (Fig. 2).Peak deformation is inversely related to fruit firm-ness with a larger deformation corresponding to asofter fruit. Peak deformation measurements arerepeatable for a range of fruit and for fruit firm-ness at tank pressures up to 200 kPa (McGlone etal., 1999).

Fruit stiffness, defined here as Epuff (the mod-ulus of elasticity by the laser puff method), wascalculated from the peak deformation measure-ment, D, using a formula based on the Boussinesqtheory of die loading (Mohsenin, 1986)

Epuff=Pp(1−m2)a/2D (1)

window on top of the nozzle holder, downthrough the nozzle, and then onto the fruit (Fig.1). The sensing element of the sensor was on theoutside of the nozzle holder and had an unob-structed view of the laser spot on the fruit. Bring-ing the laser beam of the sensor directly down thenozzle constitutes the main difference between ourinstrument and the University of Georgia instru-ments, which have either the laser beam (Fan etal., 1994) or the air-puff (Hung et al., 1999)striking the fruit surface at a non-normal angle.Whilst we certainly enjoy the technical elegance ofour particular solution, it will not matter to thegeneral problem of fruit firmness measurementwhich arrangement is employed.

The raw data from the instrument was calledthe deformation curve and consisted of a timedsequence of displacement sensor readings,recorded at a sampling rate of 1 kHz, during theapplication of a puff to the fruit. A typical defor-mation curve (Fig. 2) shows four distinct stages.The first is up to 150 ms of baseline displacementreadings of the undeformed fruit surface. Thesolenoid signal was delayed by 100 ms to ensure areasonable baseline period. The shifting arm ofthe instrument had been previously adjusted to


where P is the peak puff pressure on the fruit, m isthe poisson ratio and a is an equivalent die radiusof the puff. The peak puff pressure P was calcu-lated from the tank pressure Ptank with an empiri-cally derived formula (McGlone et al., 1999). Apoisson ratio of m=0.4 was used in all calcula-tions and was chosen (not measured) on the basisof being a mid-range value for fruit between theapparent extremes of m=0.3 for apples and m=0.5 for potatoes (Mohsenin, 1986). The equivalentdie radius a has been estimated at 2 mm for themeasurement conditions in this study. Justifica-tion for the use of the Boussinesq formula and theestimate of a=2 mm is given in McGlone et al.(1999).

2.2. Kiwifruit data set

Kiwifruit (‘Hayward’) from four orchard lineswere obtained from a commercial cool store andplace in a controlled laboratory cool store operat-ing at a set temperature of 0°C. The fruit were allbetween 0 and 4 weeks post-harvest, in a singleweight band of 116–127g and \20 N penetrome-ter firmness at procurement. At intervals over aperiod of 24 weeks the fruit were sampled fromthe laboratory cool store and warmed to roomtemperature (�20°C) overnight before beingmeasured for firmness, first with the laser air-puffinstrument and then the penetrometer. At eachsampling, between 15 and 20 fruit were randomlychosen from each of the four orchard lines. Intotal 379 kiwifruit were sampled over the 24-weekperiod. Two peak deformation measurements, at65 kPa peak puff pressure and 80 ms puff dura-tion, were made on opposite sides of each fruit.The deformation measurements were each trans-formed into Epuff measurements before being av-eraged to provide a mean value for the fruit.Penetrometer measurements were made at thesame two sites on each kiwifruit. Data analysiscentred on establishing the strength of relation-ship between the mean Epuff and Fpen measure-ments made on each fruit. In addition, themeasurement pairs per fruit, for both Epuff andFpen separately, were compared by regressionanalysis to assess the degree of within-fruit vari-ability associated with each measurement method.

2.3. Apricot data set

One orchard line of 140 ‘CluthaGold’ apricotswas chosen for this study. The fruit were har-vested within a 2-h picking session and included awide range of maturity. The fruit were immedi-ately sorted by eye into seven colour grades,consisting of 20 fruit each, and were sentovernight to the research laboratory where thelaser puff and penetrometer firmness measure-ments were made. Central sites on each opposingcheek of the fruit were chosen to provide twofirmness measurements per fruit. The firmnessmeasurement procedures and analysis methodswere then exactly those of the kiwifruit data set.

2.4. Penetrometer firmness

Penetrometer firmness (Fpen) measurements onkiwifruit and apricots were made in accordancewith standard industry practice. Hand-held pen-etrometers (Effegi) were used with a 7.9-mm di-ameter plunger. Two measurements were made onopposite sides of the fruit (same sites as Epuff

measurements) and then averaged to give a meanvalue for the fruit. A 1-mm thick slice of skin wasremoved from each measurement site prior to themeasurement. The maximum depth of penetrationwas 8 mm and, as is industry practice, the rate ofpenetration was subjectively controlled at about 5mm/s (2 s to reach maximum depth). Penetrome-ter measurements are reported in Newtons (N)with 10 N being approximately 1 kgf (standardindustry units of kilogram-force).

3. Results

3.1. Kiwifruit firmness

The Fpen distribution for the kiwifruit data setranged from 2 to 29 N with a mean value of 9.7N and standard deviation of 4.7 N. As fruitsoftened during storage the peak deformationsmeasured with the laser air-puff method rangedfrom as low as 0.1 mm for hard fruit (\20 N) toas high as 1 mm on very soft fruit (B5 N). Therelationship between Fpen and Epuff was strongly


Fig. 3. Kiwifruit penetrometer firmness Fpen against the laserair-puff stiffness Epuff. The line is a linear regression fit to thedata (R2=0.80).

Fig. 5. Apricot penetrometer firmness Fpen against laser air-puff stiffness Epuff. The line is a power law regression fit to thedata (R2=0.80).

linear although the data look to be increasinglymore scattered above 10 N penetrometer firmness(Fig. 3). Regression statistics of R2=0.80 and s(mean residual error)=2.1 N were obtained forFpen against Epuff. The mean residual error is 22%of the mean fruit firmness (9.7 N). The Epuff

measurements made on opposites sides of thefruit, reflecting the within-fruit variability, areonly moderately correlated to each other (Fig.4(b)) with regression statistics of R2=0.66 ands=0.37 MPa. In contrast the Fpen measurements

from opposites sides are highly correlated (Fig.4(a)) with regression statistics of R2=0.93 ands=1.3 N.

3.2. Apricot firmness

The Fpen measurements of the apricot data setranged from 2 to 93 N with a mean of 22.8 N anda standard deviation of 18.5 N. Peak deforma-tions ranged from 0.1 to 1 mm in going from veryhard to very soft fruit, respectively. The plotting

Fig. 4. Correlation scatter plots between penetrometer firmness (a) and laser air-puff stiffness (b) measurements made on oppositessites on each kiwifruit (R2=0.93 and 0.66, respectively). Straight lines are the 1:1 relationship.


of Fpen against Epuff (Fig. 5) shows slight curva-ture, and a power law relationship Fpen=2.6×(Epuff)1.7 gave the best regression fit with anR2=0.8 and s=8.3 N. The mean residual error isequivalent to 36% at the mean fruit firmness of 23N. The Epuff measurements made on oppositessides of the fruit, reflecting the within-fruit vari-ability, are only moderately correlated to eachother (Fig. 6(b)) with regression statistics of R2=0.64 and s=0.37. The corresponding Fpen mea-surements from opposites sides are bettercorrelated (Fig. 4(b); R2=0.8, s=8.3 N).

4. Discussion

The reasonably high correlations (R2�0.8) be-tween Fpen and Epuff with the kiwifruit and apricotdata sets means that the laser air-puff method canassess the same general firmness differences as thepenetrometer method does with cool-stored ki-wifruit and ripening apricots. For the kiwifruitdata set the correlation is linear over the 2–30-Nrange of Fpen measurements (Fig. 3). For theapricots the correlation was less linear and bettermodelled as a power law relationship over theFpen range 0–90 N (Fig. 5). The correlations areslightly better than those achieved by Hung et al.(1999) in using the laser air-puff method onpeaches (R2B0.8; value depending on cultivar).

However a fair comparison is not possible be-cause Hung et al. (1999) have used the peakdeformation (D) directly, rather than a derivedmeasure such as stiffness (Epuff), and re-expressionas stiffness might well have improved the linearityand significance of their correlations. Stiffness is,in broad terms, the ratio of stress to strain in aloaded material and as such is a completely differ-ent physical property from the maximum stress orrupture force property that the penetrometer mea-sures. Nevertheless, and as observed in this study,the empirical evidence (Abbott et al., 1997) is thatstiffness and rupture force properties are oftenlinearly correlated.

In terms of penetrometer firmness predictionthe results suggest only moderate accuracy andwould probably only allow a coarse screening offruit into high or low firmness grades. The abso-lute values of the mean residual errors, in regress-ing Fpen against Epuff, are quite different at 2.1 and8.3 N for the kiwifruit and apricot data sets,respectively. However the data set ranges are alsoquite different (�30 and 80 N, respectively) andin both cases the residual errors represent thesame 45% fraction of the respective distributionalstandard deviations.

Grading scenarios can be created, on the basisof the regression models for Fpen against Epuff, toget estimates of likely grading error rates. Toreject, with at least 95% confidence, all kiwifruit

Fig. 6. Correlation scatter plots between penetrometer firmness Fpen (a) and laser air-puff stiffness Epuff (b) measurements made onopposites sites of each apricot (R2=0. 80 and 0.64, respectively). Straight lines are the 1:1 relationship.


with an actual Fpen measurement below 10 Nwould require a sorting threshold on the laserair-puff predictions at about 14.2 N (twice themean residual error above the 10-N maximum).The errors around this threshold can be estimatedand result in an error rate of 23% of the fruitactually \10 N being rejected. With the kiwifruitdata set there is also increasing scatter observableat higher firmness values (Fig. 3) which will makethe grading error rates even greater for firmerfruit. The increasing scatter is most likely due tothe inverse proportionality between Epuff and thedeformation D (Eq. (1)) that effectively amplifieserrors in D, at lower deformations, relative tothose at greater deformations. In addition, thefractional error in D becomes necessarily larger atlower deformation because of the finite resolutionof the laser displacement sensor (�0.008 mm).For apricots a penetrometer firmness threshold at20 N might be necessary to reject fruit that aretoo soft to survive days of delay in reachingdistant retail markets. To be at least 95% confi-dent of removing all such soft fruit, a sortingthreshold at about 37 N is necessary on the laserair-puff predictions. This would mean that 26% ofthe fruit \20 N were being rejected in error.

In all of this the laser puff method wouldappear at least comparable in accuracy to othernon-destructive methods that have been recentlyreported. Kiwifruit studies with the impactmethod (McGlone and Schaare, 1998), the dy-namic force-deformation method (Abbott andMassie, 1996) and sonic resonance method (Ab-bott and Massie, 1998) all reveal similar measure-ment errors of around 2 N for a 10-Npenetrometer firmness. There appears to be noformally published work on the non-destructivefirmness testing of apricots. For another stone-fruit, peaches, a recent article (Stone et al., 1998)reports a linear correlation between the penetrom-eter and acoustic impulse methods of R2=0.76where the measurements are averages made onopposite cheeks. That correlation is only slightlylower than that achieved here with the laser air-puff method.

The prediction errors might be reduced by us-ing higher puff pressures to improve the correla-tion between Fpen and Epuff. Correlations between

the two properties depend on the measurementconditions, and in particular, how damaging ornon-recoverable the strains are during the stiffnessmeasurement. In general the larger the strains andthe closer the stresses are to the maximum sus-tainable by the material then the higher the expec-tation of better correlations (Bourne, 1982). Theneed for the measurement to be non-destructivemeans the stress applied by the puff must bebelow the maximum sustainable by the fruit. Themaximum pressure on the fruit during a puff (65kPa) can be achieved with the 7.9-mm diameterpenetrometer tip applied to the fruit with a forceof 3.2 N. This force is significant considering the5 N or less penetrometer firmness of the softerfruit. Therefore it is probably not realistic toincrease the puff pressure to improve the correla-tions, without significantly damaging softer fruit.

Another possibility for improving the correla-tions would be to take averages over more mea-surements per fruit, especially for the stiffnessmeasurements that have much higher within-fruitvariability than the penetrometer measurements(Figs. 4 and 6). Such an approach has been usedwith other non-destructive methods, involving av-erages over four or more measurement sites perfruit, to improve firmness predictions for peaches(Stone et al., 1998) and melons (Ozer et al., 1998).We speculate that the higher variability recordedhere for Epuff, compared to Fpen, is most likely dueto surface morphology differences between mea-surement sites that affects the laser air-puffmethod but not the penetrometer method forwhich the surface layer has been removed. How-ever, it could also be that the fruit stiffness prop-erty is less homogenous around a fruit than therupture force property that the penetrometer mea-sures. Accurate stiffness measurements on excisedtissue from around the fruit would be necessary toresolve this issue.

The laser air-puff method should be suited toany sorting situations where fruit firmness isjudged on the basis of stiffness alone. Stiffnessmeasures of firmness are used in industry but areoften quite subjective, such as in hand-sorting ofproduce, or slow as in the standard compressionmethod (ASAE Standards, 1984). We are notaware of any sorting situations that enforce a


stiffness type firmness measure, and readily agreewith Abbott et al. (1997) that most fruit andvegetable industries appear to have accepted theconventional rupture force type methods, such asthe penetrometer, as the standard firmness ormaturity measure. A shift away from these stan-dard measures, towards a non-destructive stiffnesstype measure, would require a large research ef-fort to provide the industries with the necessaryquality evidence (e.g. sensory trials) and marketconfidence in a new measure. To date, researchinitiatives along this path have only served toconfirm that rupture force type measures providethe best standard methods (Pitts et al., 1997)despite their many known limitations (Abbott etal., 1997). In the case of the laser air-puff methodfurther work might also be necessary to resolvethe cause, and/or reduce by taking averages overmore measurements per fruit, the high with-infruit variability of the Epuff measurements.

5. Conclusion

The laser air-puff method provides fruit stiff-ness estimates that correlate reasonably highlywith penetrometer firmness measurements on ki-wifruit and apricots. However, the correlationsagainst penetrometer firmness are not yet highenough to provide accurate predictions for morethan a very coarse screening of fruit into twopenetrometer firmness classes. So whilst themethod continues to appeal as a high speed grad-ing technique, because it is fast and non-contact,it is unlikely to find application in practical fruitfirmness grading situations where accuracy isjudged solely on the basis of penetrometer firm-ness. Options for improving the correlations in-clude using a higher puff pressure and/or makingmore than two measurements per fruit.

Acknowledgements

We thank Barry Stevenson for mechanical de-sign and construction of the laser air-puff instru-ment and Jill McLaren for organising the apricotdata set for us. This research was supported by

the New Zealand Foundation for Research, Sci-ence and Technology.

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Kiwifruit and apricot ﬁrmness measurement.pdf