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Harvesting by Peel Color to ReduceBruising of ‘Golden Delicious’ ApplesKay Mitsuhashi-Gonzalez a , Eric A. Curry a , John K. Fellman a ,Marvin J. Pitts a & Carter D. Clary aa Department of Horticulture and Landscape Architecture,Washington State University, Pullman, Washington, USA

Available online: 08 Jun 2010

To cite this article: Kay Mitsuhashi-Gonzalez, Eric A. Curry, John K. Fellman, Marvin J. Pitts &Carter D. Clary (2010): Harvesting by Peel Color to Reduce Bruising of ‘Golden Delicious’ Apples,International Journal of Fruit Science, 10:2, 166-176

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International Journal of Fruit Science, 10:166–176, 2010Copyright © Taylor & Francis Group, LLC ISSN: 1553-8362 print/1553-8621 onlineDOI: 10.1080/15538362.2010.492334

WSFR1553-83621553-8621International Journal of Fruit Science, Vol. 10, No. 2, May 2010: pp. 0–0International Journal of Fruit Science

Harvesting by Peel Color to Reduce Bruising of ‘Golden Delicious’ Apples

Harvesting by Peel ColorK. Mitsuhashi-Gonzalez et al.

KAY MITSUHASHI-GONZALEZ, ERIC A. CURRY, JOHN K. FELLMAN, MARVIN J. PITTS, and CARTER D. CLARY

Department of Horticulture and Landscape Architecture, Washington State University, Pullman, Washington, USA

‘Golden Delicious’ apples were harvested at three peel color stagesand equilibrated at 22°C for 2 days before inducing bruising to aconstant depth using an artificial silicon finger attached to anInstron universal material testing instrument. After 48 hr, bruisedtissue was sliced sequentially from the fruit surface in a planeperpendicular to the direction of the applied force until discolorationwas no longer evident. Thickness and discolored area of eachtissue slice was measured using a digital caliper and total volumeof bruised tissue was estimated. Analysis showed that there was asignificant difference in bruise volume between green and yellowpeel stages. Susceptibility to bruising of fruit at the white (inter-mediary) stage appeared to vary with environmental conditions(year to year). Compared with the previous year, the 2008 growingseason was cooler and shorter and bruise volume at all stages wasgreater. Analysis of fruit maturity suggested bruise volume wasinfluenced by ripening-induced changes in cell structure, size,and integrity. Where fruit peel color changed but flesh firmnessdid not, bruise volume increased. We conclude that peel color is abetter indicator of bruise susceptibility than fruit flesh firmness.

KEYWORDS bruising, bruise volume, skin color, firmness, Malus

Address correspondence to Carter D. Clary, Department of Horticulture and LandscapeArchitecture, Washington State University, P.O. Box 646120, Pullman, WA 99164-6414, USA.E-mail: cclary@wsu.edu

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Harvesting by Peel Color 167

INTRODUCTION

‘Golden Delicious’ apples are often harvested in a single pick depending onpeel ground color, compared to other cultivars, such as ‘Gala’, that areharvested more than once based on both red pigmentation and changingground color. ‘Golden Delicious’ apples are harvested approximately 130days after full bloom (DAFB) (Jackson, 2003) at which time, depending onthe season, peel color may vary from green (less mature) to yellow (moremature).

Roth et al. (2005) reported that bruising occurs more in firmer apples.Hertog et al. (2003) found that flesh firmness measured by a Magness-Taylorfruit penetrometer (Stable Micro Systems Ltd., Godalming, Surrey, UK) isaffected by the mechanical strength of the tissue. Others have reported thatapple fruit flesh firmness is determined by the physical anatomy of thetissue, including such components as cell size, cell shape, and tissuearrangement (Toivonen and Brummell, 2008). Moreover, cell wall thicknessand strength, cell to cell adhesion, and turgor may influence bruise volume(Harker et al., 1997). Development of internal fruit structure is influenced byseasonal temperatures, which may alter the number of cells in the cortex(Lakso et al., 1995). Small cells tend to have greater cell to cell contact, alower amount of intercellular air space, and less cell sap (cytoplasm andvacuole), making the tissue firmer and less juicy (Harker et al., 1997).

During ripening, fruit flesh undergoes degradation resulting in reducedcell wall thickness and lessened intercellular adhesion (Toivonen andBrummell, 2008). Fruit parenchyma cells often have relatively thin andweak primary cell walls compared to those from more vegetative (struc-tural) tissues, which have a higher proportion of cells with thickened andlignified cell walls, thereby conferring greater strength (Kays, 1997;Toivonen and Brummell, 2008). Roth et al. (2005) also reported that softerfruits are less sensitive to damage by impact than firmer fruit and, therefore,are less susceptible to impact damage. They showed that reduction in firmnessof ripened fruit was attributable, in part, to increased enzymatic activityleading to cell wall breakdown. Specifically, polygalacturonase activityincreased exponentially during the first 30 days after harvest.

Cortex tissue from ripe apples has more intercellular spaces and is notas dense as younger, less mature tissue (Glenn and Poovaiah, 1987; Roth etal., 2005). According to Alvarez et al. (2000a, 2000b), the denser the cells,the smaller the intercellular airspaces are, and the less damage occurs whena force is applied. Indeed, Knee (1993) has suggested an increase in thenumber and size of individual air spaces during ripening may partiallyaccount for the decline in flesh firmness.

Previous research on bruising of ‘Golden Delicious’ focused oncomparing peel color stages green vs. yellow for bruising susceptibility.Hyde and Ingle (1968), Kvaale et al. (1968), Diener et al. (1979), and Saltveit

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168 K. Mitsuhashi-Gonzalez et al.

(1984) all concluded, to reduce bruising of ‘Golden Delicious’ apples at harvest,it is advantageous to pick the fruit while the peel is green.

Preliminary observations of bruising at harvest within several commercial‘Golden Delicious’ orchards in central Washington (Mitsuhashi, unpublished)indicated a transitory ‘white’ peel color, which occurs between green andyellow, and may also be related to bruise susceptibility. The objective ofthis research was to determine if bruising of ‘Golden Delicious’ apples atharvest could be reduced by evaluating differences in bruise susceptibility(volume) of fruit harvested at the white peel ground color compared withthat of fruit harvested with green or yellow peel color.

MATERIALS AND METHODS

Experimental Design and Treatments

This study was completed over two harvest seasons (2007–2008) in acommercial apple (Malus domestica Borkh cv. ‘Golden Delicious’) orchardlocated in Orondo, WA. The experiment was laid out as a completelyrandomized design with three treatments (peel color) each consisting ofthree fruits harvested from each of 20 trees. Sixty green, white, or yellowapples were harvested at 143, 150, and 158 days after full bloom (DAFB),respectively, in 2007, and 132, 139, and 146 DAFB, respectively, in 2008, aspreviously described by Mitcham et al. (2006). The orchard was managedorganically and all trees were treated similarly.

Bruise Evaluation and Fruit Quality Analysis

Apples were harvested individually into tri-layered European-style (Euro)apple boxes and nested in cylindrical holes cut in low-density foam. Anadditional foam sheet lined the bottom of the box to protect the applesfrom damage due to handling after harvest (not the focus of this work).Fruit were transported to the laboratory, weighed individually, and kept at22°C for 48 hr to establish temperature equilibrium.

To simulate bruising by a picker, a silicon finger was attached to anInstron Model 1350 (Instron Industrial Products, Grove City, PA, USA) anddisplaced 10 mm in 23 sec into the side of the apple at approximately thewidest equatorial diameter (shoulder). Rate of displacement of the siliconfinger was the same for all apples. Fruit were then placed back into theEuro foam boxes, bruised side up, and kept at 22°C and 68% relativehumidity for 48 hr prior to evaluation.

Bruise volume was measured by removing serial 1 mm tangentialsections through each bruised area, perpendicular to the direction of theapplied force, using a stainless steel blade, until discoloration (brown) wasno longer evident. The bruised (brown) region of each slice was measured

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Harvesting by Peel Color 169

at the widest diameter and again at 90° to the widest diameter using a digitalcaliper (Model CD67-6, Mitutoyo Corporation, Kawasaki, Japan) and averagedto provide the mean diameter of a circular (assumed) area. Bruise volumeper tissue slice was estimated to be the area × thickness (depth). Totalbruise depth was calculated as the sum of the depths of affected slices.Total bruise volume was calculated in cubic millimeters using the sum ofbruise volumes of individual slices.

Apple Quality Evaluation

After fruit were bruised, but before slicing for bruise volume determinations,flesh firmness was measured using a Texture Analyzer (FTA, Güss Manufactur-ing (Pty) Ltd., Strand, SA) on opposite sides of each apple after removing about1 mm of peel tissue. Average values per fruit were used in statistical analysis.

After fruit were processed and bruise volume determined, solublesolids content was measured from the juice of the apple using an ABBERefractometer (American Optical, Model 10450 Mark II, Digital Refractometer,Southbridge, MA) and recorded as °Brix. To measure the starch clearingindex, each fruit was cut transversely through the equatorial line and the cutsurfaces dipped in 2% iodine/potassium iodide solution (1:4) for 10 sec andleft to equilibrate for about 1 min. The stained area of each fruit was com-pared to a standard scale of 1 to 6 (1 = maximum starch content, 6 = nostarch content) and the values recorded.

Acid content expressed in percent malic acid equivalents was measuredby a titration method using the Model TIM 850 titration workstation (Radi-ometer Analytical SAS, Lyon, France).

Peel ground color was measured using a Konica Minolta Croma MeterCR 300 (Konica Minolta Sensing, Inc., Tokyo, Japan). Values were expressedusing the Commission Internationale de l’Eclairage color system with theL*a*b axis representing lightness, green-red and blue-yellow, respectively,and transformed to degrees (°) hue according to Mclellan et al., 1995.

Internal ethylene concentration (IEC) in each fruit was evaluated byfirst inserting an 18-gauge needle equipped with a rubber septum throughthe fruit calyx into the central cavity and withdrawing 1 mL of core gas.Ethylene in the withdrawn gas sample was measured by injecting 0.5 mL ofthe gas into an HP 5880A gas chromatograph with flame ionization detector(GC-FID, Hewlett Packard, Avondale, PA, USA) equipped with a 1 m × 3.2 mmi.d. glass column packed with 80–100 mesh Porapak Q (Waters Corporation,Milford, MA, USA) in an oven at 40°C according to standard methods.

Statistical Analysis

Data collected from the two-year trial were combined and analyzed forstatistical significance using the general linear model (GLM) procedure of

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SAS v.9.1 software (SAS Institute, Cary, NC, USA). Differences among treat-ments were identified using the Duncan’s New Multiple Range Test at P ≤ 0.05.Data within variables were distributed normally.

RESULTS

Fruit Quality 2007 vs. 2008

In 2008, anthesis was delayed by about 2 weeks due to cooler than normalambient temperatures (data not shown). Because fruit were harvested atabout the same calendar date as the previous year, the 2008 growing seasonwas, however, about 10–12 days shorter. Although peel color at the time ofsampling was similar in the two years, comparisons of flesh firmness, starchindex, soluble solids, and titratable acidity among years illustrates effect ofseasonal temperature differences on fruit quality (Table 1).

In 2007, fruit flesh firmness in apples at green and yellow peel colorstages was less than that of fruit sampled at the white stage, whereas in2008, firmness of green apples was statistically different than that of whiteand yellow apples (Table 1). In 2007 and 2008, firmness was higher inwhite apples compared to green and yellow apples.

Starch content differed significantly among the three color stages anddecreased with successive harvests in both years (Table 1). IEC alsoincreased with each subsequent sampling, however, only the yellow peelstage showed IEC greater than 1.0 μl·l−1. In 2007, although green apples hadthe lowest titratable acidity, there was no significant difference betweenwhite and yellow apples, whereas, in 2008, green and yellow applesshowed no significant difference, yet both differed significantly from whiteapples. Measurement of soluble solids content in both 2007 and 2008showed all stages were significantly different, although in 2008 there was anunexplained increase during the white peel stage (Table 1). Also in 2008,titratable acidity in white apples was lower than that of green or yellowapples. Though uncommon, this pattern occurs occasionally (Curry, unpub-lished), but is inconsistent with typical progressive ripening. Although peelcolor was correlated with starch content, we did not find significant correlationsamong starch, titratable acidity, and soluble solids content in either year(data not shown).

Bruise Volume

The relationship between peel color and bruise volume was similar in bothyears showing a general trend toward greater bruise volume with increasingfruit maturity (Fig. 1). In contrast, the degree to which bruise volumeoccurred at both green and white peel stages was significantly differentamong years. Bruise volume of apples harvested at the white color stage

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171

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was similar to that of fruit at green stage in 2007 and of the yellow stage in2008. In other words, apples at the white peel stage had a tendency tobehave as green apples one year and yellow apples the next. Thus, whitepeel color stage may not be useful for predicting bruise volume unless specificenvironmental and fruit quality parameters are evaluated as well.

DISCUSSION

Using starch index and IEC as criteria, yellow apples in 2007 and 2008 wereat similar stages of physiological maturity and showed similar values forbruise volume, even though fruit firmness differed by more than 2 lbs(Table 1, Fig. 1). This suggests that fruit firmness is not correlated withbruising. Indeed, Pearson correlation coefficients for bruise volume vs. fleshfirmness for 2007 and 2008 are quite low (Fig. 2). Further analysis indi-cates that at none of the peel color stages was fruit firmness, as measuredusing the FTA, correlated with imposed bruise volume (Table 2). In thisstudy, we chose to evaluate apple flesh characteristics (firmness) using theFTA rather than a handheld Magness-Taylor fruit firmness tester to: (1)reduce operator and instrument variability and thereby improving accuracy ofthe measured values, (2) permit these experiments to be reproduced moreprecisely elsewhere, and (3) correlate with a previous study (Mitsuhashiet al., 2010).

Although, these data may simply indicate an FTA is not the instrumentof choice to predict cell to cell attachment and bruising susceptibility, otherinvestigators (Lidster and Tung, 1980; Klein, 1987; Ericson and Tahir, 1996)also failed to find significant correlations between firmness, as measured

FIGURE 1 Bruise volume (imposed) at harvest as a function of year and peel color: green(G), white (W), and yellow (Y). Bars indicate ± standard error of the mean.

0

2007 2008

G W WY YG

Peel visual color

Bru

ise

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3 )

3000

1000

4000

2000

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Harvesting by Peel Color 173

with a Magness Taylor penetrometer and cortex tissue density, peel color,or bruising susceptibility. Ericson and Tahir (1996) bruised apples by drop-ping them from a specified height, and determined that smaller fruit wereless prone to bruising from high impact energy (dropping) than were largerfruit. Moreover, they reported no differences in bruising susceptibility withharvest dates. Viljoen et al. (1996) and Ericson and Tahir (1996) harvestedfruit based on DAFB and found no significant relationship between bruisevolume and peel color. In the present study, apples were approximately thesame weight, but still exhibited different levels of bruising.

In fruit such as apple, softening involves degradation of large pectinmolecules in the middle lamella that bind cells walls together (Kays, 1997).During ripening, enzymatic degradation of cell walls results in a texturalalteration (Kays, 1997). Fruit cell firmness decreases during storage whilesoluble pectin increases and detachment of cells occurs (Glenn and Poovaiah,1987). We found the number of damaged cells also increases with ripening,as does the size of intercellular spaces (Fig. 3).

FIGURE 2 Linear regression and Pearson correlation coefficients (r) for fruit flesh firmnessvs. imposed bruise volume for ‘Golden Delicious’ in 2007 (—) and 2008 (- - - - -).

12 14 16 18 20 22FIRM

0

20082007

Bru

ise

volu

me

(mm

3 )

Fruit flesh firmness (lb)

r = - 0.27 r = - 0.18

2000

8000

10000

6000

4000

TABLE 2 Pearson Correlation Coefficients for Fruit Flesh Firmness vs. Imposed BruiseVolume at Three Peel Color Maturity Stages for ‘Golden Delicious’ in 2007 and 2008

Year Harvest (DAFB) Peel color (visual) Correlation coefficient (r)

2007 143 Green −0.141150 White 0.012158 Yellow −0.445

2008 132 Green −0.064139 White −0.192146 Yellow −0.214

DAFB = Days after full bloom.

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FIGURE 3 Grayscale light micrographs of ‘Golden Delicious’ apple peel cross section atprogressive harvests in 2007: green (A), white (B), and yellow (C) peel color. Note thicken-ing parenchyma and differences in size of intercellular airspace (s) as apples ripen. Individ-ual image view field = 1.0 mm. Bars represent 500 μm.

A

B

C

s

s s

s

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s

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Harvesting by Peel Color 175

In this study, there was a significant difference in bruise volume withpeel color. Among years, variation of bruise volume with peel color significant,especially at the green and white peel stages. This may have been due toenvironmentally-induced differences in tissue characteristics, since cell divisionand cell number influence fruit tissue characteristics. Lakso et al. (1995)reported ‘Golden Delicious’ apples exhibited a positive curvilinear growthpattern of growth from 32 to 74 days. Moreover, cultivar and environmentcontributed to the number of cells in the cortex. Smith (1940, 1950) foundthat cultivar differences in apple size were due to differences in cell numberand cell size. Such differences could explain, in part, why some applecultivars are more bruise resistant than others. In many traditional apple andpear growing areas, fruit tend to be smaller during cooler seasons or whengrown under cooler climates by reason of latitude or altitude. Perhaps, bydeveloping models of fruit growth using multiple environmental variables,we might be able to predict tissue properties and bruising sensitivity ofapples.

CONCLUSIONS

Our data suggest severity of bruising in ‘Golden Delicious’ might bereduced by harvesting apples while they are green. On the other hand,because green apples are normally more firmly attached to the tree thanriper fruit, a greater force is usually required to remove them, which could,potentially, increase bruising incidence. Thus, teaching proper technique inharvesting apples remains key to minimizing picker-induced bruising.

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