Kiwifruit and apricot firmness measurement by the
non-contact laser air-puff method
V. Andrew McGlone *, Robert B. Jordan
Technology 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 (Actinidia deliciosa(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 surface deformation with a laser displacement sensor. At a 65-kPa maximum puff pressure the deformations on both kiwifruit and apricots ranged from 0.1 mm for hard fruit to about 1 mm for very soft. A fruit stiffness value (Epuff) was calculated from the maximum deformation and compared with penetrometer firmness (Fpen) on cool-stored kiwifruit and mixed maturity apricots. Two measurements were made on opposite sides of each fruit using each technique and were averaged before regression analysis. Reasonable regression results (R20.80,s=2.1 N) were obtained between
Fpen and Epuff on kiwifruit over the penetrometer firmness range 0 – 30 N. The relationship showed a trend of increasing scatter with increasing kiwifruit firmness. With apricots, over a greater penetrometer range (0 – 80 N), the relationship betweenFpenandEpuffwas best described by a simple power law regression (R20.80,s=8.2 N). Both sets of fruit results suggest the laser air-puff method is only suitable for coarse screening of fruit into two penetrometer firmness classes. Improved correlations may be possible by using higher puff pressures and/or more measurements 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 et al., 1993, 1994), is attractive in its potential as a high speed grading technique as no mechanical
contact is required between the fruit and the sensor. The method works by delivering a puff of air to the fruit and simultaneously measuring the surface deformation that occurs. The underlying theory is that the ratio of deformation to applied force can be linked to fruit stiffness and, further, that stiffness is related to the penetrometer firmness.
Hung et al. (1999) recently reported on the development 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)
ment for measuring the firmness of peaches. They established that the instrument could provide a non-destructive firmness measure that was signifi-cantly correlated (R20.74) with the penetrome-ter firmness of peaches over a large range (0 – 120 N). We also have recently reported on the devel-opment of a similar laser air-puff instrument (Mc-Glone et al., 1999). That report concentrated on characterising the instrument and providing evi-dence that the method could indeed measure fruit stiffness. Preliminary kiwifruit measurements were reported that were encouraging in relating fruit stiffness to penetrometer firmness (R20.88) over a narrow penetrometer firmness range (0 – 12 N). The objective of the current research was to investigate the laser air-puff method for fruit firm-ness measurement on kiwifruit and apricots. For kiwifruit, the primary interest is in sorting fruit after long-term cool storage when consignment averages go well below 30 N and the presence of soft 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) and this results in setting thresholds back at the cool store at anywhere between 8 and 15 N depending on the exact market requirements and post-stor-age handling conditions. With apricots, the inter-est is in harvinter-est-time sorting of mixed maturity consignments to remove immature or over-mature fruit. Penetrometer firmness has been suggested as a 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-puff instrument were a tank of pressurised air, a so-lenoid switching valve, an outlet nozzle and a laser displacement sensor (Fig. 1). Fruit to be tested were positioned directly below the nozzle and supported, when necessary, in a cradle made from soft modelling clay. The cradle prevented sideways movements of the fruit under the puff and also minimised deformation on the bottom side of the fruit by spreading the load over a large contact area. The solenoid valve, nozzle and dis-placement sensor were all fixed in position on a shifting arm that was adjusted, prior to most measurements, to give a nozzle-fruit distance of 23.3 mm that brought the displacement sensor reading close to its zero setting. A short blast of air, the puff, was delivered to the fruit by com-puter control of the open duration (typically 80 ms) of the solenoid valve (VS3145, SMC, USA). The tank supplying the air had a capacity of 50 l and was regulated to a typical set air pressure of 100 kPa. The nozzle had an internal diameter of 8.2 mm and this delivered a steeply peaked pres-sure profile at the fruit surface of diameter, at half-width, of 10 mm. Fruit surface deformations during a puff were measured with the laser dis-placement sensor (Keyance LB-081, Osaka, Japan), which had a range of 915 mm around the zero setting and a resolution of 8 mm. The laser beam of the sensor passed through a glass
Fig. 2. A deformation curve (solid line) and a repeat (dotted line) measured at the same site on a soft apricot (Fpen5 N). The peak deformation was measured as shown, for the first curve, and was 0.54 mm for both curves. Puff duration was 80 ms and the tank pressure was 100 kPa. Timing of solenoid control 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 impacting on the apricot. The readings level off (the third stage) after a further 50 ms and reach a maximum after about 220 ms total elapsed time. The final stage occurs after 260 ms elapsed time, and some 70 ms after cessation of the solenoid signal, as the displacement readings decline exponentially back towards the baseline reading in response to diminution of the puff. The deformation curves are not necessarily exactly repeatable and, in par-ticular, the baseline reading is not recovered dur-ing the final stage (Fig. 2). Softer fruit, or higher peak puff pressure, generally resulted in a greater baseline discrepancy. Deformation curves for elas-tic objects (e.g. rubber balls) always showed com-plete recovery and so the baseline discrepancy observed with fruit is due to non-recoverable strains occurring at lower stress limits than those applying to elastic objects. These non-recoverable strains 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 that the fruit are not damaged by the puff although no rigorous damage or repeatability studies have yet been done to confirm this.
The key firmness parameter extracted from the deformation curve was the peak deformation, D, calculated as the difference between the maximum displacement reading and the average baseline reading occurring during the first stage (Fig. 2). Peak deformation is inversely related to fruit firm-ness with a larger deformation corresponding to a softer fruit. Peak deformation measurements are repeatable for a range of fruit and for fruit firm-ness at tank pressures up to 200 kPa (McGlone et al., 1999).
Fruit stiffness, defined here as Epuff (the mod-ulus of elasticity by the laser puff method), was calculated from the peak deformation measure-ment,D, using a formula based on the Boussinesq theory of die loading (Mohsenin, 1986)
Epuff=Pp(1−m2)a/2D (1)
window on top of the nozzle holder, down through the nozzle, and then onto the fruit (Fig. 1). The sensing element of the sensor was on the outside 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 the nozzle constitutes the main difference between our instrument and the University of Georgia instru-ments, which have either the laser beam (Fan et al., 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 of our particular solution, it will not matter to the general problem of fruit firmness measurement which arrangement is employed.
wherePis the peak puff pressure on the fruit,mis the poisson ratio andais an equivalent die radius of the puff. The peak puff pressure P was calcu-lated from the tank pressurePtankwith an empiri-cally derived formula (McGlone et al., 1999). A poisson ratio of m=0.4 was used in all calcula-tions and was chosen (not measured) on the basis of being a mid-range value for fruit between the apparent extremes of m=0.3 for apples and m= 0.5 for potatoes (Mohsenin, 1986). The equivalent die radius a has been estimated at 2 mm for the measurement conditions in this study. Justifica-tion for the use of the Boussinesq formula and the estimate of a=2 mm is given in McGlone et al. (1999).
2.2. Kiwifruit data set
Kiwifruit (‘Hayward’) from four orchard lines were obtained from a commercial cool store and place in a controlled laboratory cool store operat-ing at a set temperature of 0°C. The fruit were all between 0 and 4 weeks post-harvest, in a single weight band of 116 – 127g and \20 N penetrome-ter firmness at procurement. At inpenetrome-tervals over a period of 24 weeks the fruit were sampled from the laboratory cool store and warmed to room temperature (20°C) overnight before being measured for firmness, first with the laser air-puff instrument and then the penetrometer. At each sampling, between 15 and 20 fruit were randomly chosen from each of the four orchard lines. In total 379 kiwifruit were sampled over the 24-week period. Two peak deformation measurements, at 65 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 the same two sites on each kiwifruit. Data analysis centred on establishing the strength of relation-ship between the mean Epuff and Fpen measure-ments made on each fruit. In addition, the measurement pairs per fruit, for both Epuff and Fpen separately, were compared by regression analysis 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’ apricots was chosen for this study. The fruit were har-vested within a 2-h picking session and included a wide range of maturity. The fruit were immedi-ately sorted by eye into seven colour grades, consisting of 20 fruit each, and were sent overnight to the research laboratory where the laser puff and penetrometer firmness measure-ments were made. Central sites on each opposing cheek of the fruit were chosen to provide two firmness measurements per fruit. The firmness measurement procedures and analysis methods were then exactly those of the kiwifruit data set.
2.4. Penetrometer firmness
Penetrometer firmness (Fpen) measurements on kiwifruit and apricots were made in accordance with standard industry practice. Hand-held pen-etrometers (Effegi) were used with a 7.9-mm di-ameter plunger. Two measurements were made on opposite sides of the fruit (same sites as Epuff measurements) and then averaged to give a mean value for the fruit. A 1-mm thick slice of skin was removed from each measurement site prior to the measurement. The maximum depth of penetration was 8 mm and, as is industry practice, the rate of penetration was subjectively controlled at about 5 mm/s (2 s to reach maximum depth). Penetrome-ter measurements are reported in Newtons (N) with 10 N being approximately 1 kgf (standard industry units of kilogram-force).
3. Results
3.1. Kiwifruit firmness
Fig. 3. Kiwifruit penetrometer firmnessFpenagainst the laser air-puff stiffnessEpuff. The line is a linear regression fit to the data (R2=0.80).
Fig. 5. Apricot penetrometer firmness Fpenagainst laser air-puff stiffnessEpuff. The line is a power law regression fit to the data (R2=0.80).
linear although the data look to be increasingly more scattered above 10 N penetrometer firmness (Fig. 3). Regression statistics of R2
=0.80 and s (mean residual error)=2.1 N were obtained for Fpenagainst Epuff. The mean residual error is 22% of the mean fruit firmness (9.7 N). The Epuff measurements made on opposites sides of the fruit, reflecting the within-fruit variability, are only moderately correlated to each other (Fig. 4(b)) with regression statistics of R2
=0.66 and s=0.37 MPa. In contrast the Fpen measurements
from opposites sides are highly correlated (Fig. 4(a)) with regression statistics of R2
=0.93 and s=1.3 N.
3.2. Apricot firmness
The Fpen measurements of the apricot data set ranged from 2 to 93 N with a mean of 22.8 N and a standard deviation of 18.5 N. Peak deforma-tions ranged from 0.1 to 1 mm in going from very hard to very soft fruit, respectively. The plotting
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 an
R2=0.8 ands=8.3 N. The mean residual error is equivalent to 36% at the mean fruit firmness of 23 N. The Epuff measurements made on opposites sides of the fruit, reflecting the within-fruit vari-ability, are only moderately correlated to each other (Fig. 6(b)) with regression statistics ofR2= 0.64 and s=0.37. The corresponding Fpen mea-surements from opposites sides are better correlated (Fig. 4(b); R2
=0.8, s=8.3 N).
4. Discussion
The reasonably high correlations (R20.8) be-tweenFpenandEpuffwith the kiwifruit and apricot data sets means that the laser air-puff method can assess the same general firmness differences as the penetrometer method does with cool-stored ki-wifruit and ripening apricots. For the kiki-wifruit data set the correlation is linear over the 2 – 30-N range of Fpen measurements (Fig. 3). For the apricots the correlation was less linear and better modelled as a power law relationship over the Fpen range 0 – 90 N (Fig. 5). The correlations are slightly better than those achieved by Hung et al. (1999) in using the laser air-puff method on peaches (R2B0.8; value depending on cultivar).
However a fair comparison is not possible be-cause Hung et al. (1999) have used the peak deformation (D) directly, rather than a derived measure such as stiffness (Epuff), and re-expression as stiffness might well have improved the linearity and significance of their correlations. Stiffness is, in broad terms, the ratio of stress to strain in a loaded material and as such is a completely differ-ent physical property from the maximum stress or rupture force property that the penetrometer mea-sures. Nevertheless, and as observed in this study, the empirical evidence (Abbott et al., 1997) is that stiffness and rupture force properties are often linearly correlated.
In terms of penetrometer firmness prediction the results suggest only moderate accuracy and would probably only allow a coarse screening of fruit into high or low firmness grades. The abso-lute values of the mean residual errors, in regress-ingFpenagainstEpuff, are quite different at 2.1 and 8.3 N for the kiwifruit and apricot data sets, respectively. However the data set ranges are also quite different (30 and 80 N, respectively) and in both cases the residual errors represent the same 45% fraction of the respective distributional standard deviations.
Grading scenarios can be created, on the basis of the regression models for Fpen against Epuff, to get estimates of likely grading error rates. To reject, with at least 95% confidence, all kiwifruit
with an actual Fpen measurement below 10 N would require a sorting threshold on the laser air-puff predictions at about 14.2 N (twice the mean residual error above the 10-N maximum). The errors around this threshold can be estimated and result in an error rate of 23% of the fruit actually \10 N being rejected. With the kiwifruit data set there is also increasing scatter observable at higher firmness values (Fig. 3) which will make the grading error rates even greater for firmer fruit. The increasing scatter is most likely due to the inverse proportionality between Epuff and the deformation D(Eq. (1)) that effectively amplifies errors in D, at lower deformations, relative to those at greater deformations. In addition, the fractional error inDbecomes necessarily larger at lower deformation because of the finite resolution of the laser displacement sensor (0.008 mm). For apricots a penetrometer firmness threshold at 20 N might be necessary to reject fruit that are too soft to survive days of delay in reaching distant retail markets. To be at least 95% confi-dent of removing all such soft fruit, a sorting threshold at about 37 N is necessary on the laser air-puff predictions. This would mean that 26% of the fruit \20 N were being rejected in error.
In all of this the laser puff method would appear at least comparable in accuracy to other non-destructive methods that have been recently reported. Kiwifruit studies with the impact method (McGlone and Schaare, 1998), the dy-namic force-deformation method (Abbott and Massie, 1996) and sonic resonance method (Ab-bott and Massie, 1998) all reveal similar measure-ment errors of around 2 N for a 10-N penetrometer firmness. There appears to be no formally published work on the non-destructive firmness 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
=0.76
where the measurements are averages made on opposite cheeks. That correlation is only slightly lower 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 betweenFpenand Epuff. Correlations between
the two properties depend on the measurement conditions, and in particular, how damaging or non-recoverable the strains are during the stiffness measurement. In general the larger the strains and the closer the stresses are to the maximum sus-tainable by the material then the higher the expec-tation of better correlations (Bourne, 1982). The need for the measurement to be non-destructive means the stress applied by the puff must be below the maximum sustainable by the fruit. The maximum pressure on the fruit during a puff (65 kPa) can be achieved with the 7.9-mm diameter penetrometer tip applied to the fruit with a force of 3.2 N. This force is significant considering the 5 N or less penetrometer firmness of the softer fruit. Therefore it is probably not realistic to increase 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 stiffness measurements that have much higher within-fruit variability than the penetrometer measurements (Figs. 4 and 6). Such an approach has been used with other non-destructive methods, involving av-erages over four or more measurement sites per fruit, to improve firmness predictions for peaches (Stone et al., 1998) and melons (Ozer et al., 1998). We speculate that the higher variability recorded here forEpuff, compared toFpen, is most likely due to surface morphology differences between mea-surement sites that affects the laser air-puff method but not the penetrometer method for which 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 the rupture force property that the penetrometer mea-sures. Accurate stiffness measurements on excised tissue from around the fruit would be necessary to resolve this issue.
stiffness type firmness measure, and readily agree with Abbott et al. (1997) that most fruit and vegetable industries appear to have accepted the conventional rupture force type methods, such as the penetrometer, as the standard firmness or maturity measure. A shift away from these stan-dard measures, towards a non-destructive stiffness type measure, would require a large research ef-fort to provide the industries with the necessary quality evidence (e.g. sensory trials) and market confidence in a new measure. To date, research initiatives along this path have only served to confirm that rupture force type measures provide the best standard methods (Pitts et al., 1997) despite their many known limitations (Abbott et al., 1997). In the case of the laser air-puff method further work might also be necessary to resolve the cause, and/or reduce by taking averages over more measurements per fruit, the high with-in fruit variability of theEpuff measurements.
5. Conclusion
The laser air-puff method provides fruit stiff-ness estimates that correlate reasonably highly with penetrometer firmness measurements on ki-wifruit and apricots. However, the correlations against penetrometer firmness are not yet high enough to provide accurate predictions for more than a very coarse screening of fruit into two penetrometer firmness classes. So whilst the method 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 fruit firmness grading situations where accuracy is judged solely on the basis of penetrometer firm-ness. Options for improving the correlations in-clude using a higher puff pressure and/or making more 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 apricot data set for us. This research was supported by
the New Zealand Foundation for Research, Sci-ence and Technology.
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