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Postharvest Biology and Technology 43 (2007) 36–46

Postharvest physiology of ‘Aroma’ apples in relationto position on the tree

Torsten Nilsson ∗, Karl-Erik GustavssonDepartment of Crop Science, Swedish University of Agricultural Sciences, P.O. Box 44, SE-230 53 Alnarp, Sweden

Received 16 February 2006; accepted 30 July 2006

bstract

The effects of fruit position within the canopy on the onset of the respiratory climacteric and the rise in ethylene production as well ashanges in peel colour and chemical composition were studied in apples (Malus x domestica Borkh. cv. Aroma) during ripening in normal airt 20 ◦C for 6–8 weeks over two crop seasons. The commencement of the rise in both CO2 and ethylene production was equal independent ofruit position but the peak of ethylene was behind that of CO2 with a lag of several days. While the climacteric ethylene peak was considerablyigher in shaded inside apples, the internal ethylene concentration was at the same level independent of canopy position. During maturationn the tree outside fruit developed a red peel colour while inside fruit remained green. Outside fruit had a higher content of dry matter, solubleolids and soluble sugars but a somewhat lower amount of titratable acidity than inside fruit. High summer temperatures in the second yearesulted in a significantly higher content of soluble solids and organic acids independent of fruit position but diminished the soluble solidsifference between outside and inside fruit and increased the difference in malic and citric acid concentrations. High summer temperatureslso increased the difference in peel colour between outside and inside fruit. Independent of canopy position, the soluble solids concentrationsemained unchanged during ripening while the amounts of sucrose as well as malic acid and the titratable acidity decreased with a concomitant

ise in the cell sap pH. The higher content of soluble sugars and a somewhat lower amount of titratable acidity in outside red-coloured applesrobably contribute to improved fruit quality but the difference seems to be strongly dependent on the growing conditions, especially the sumf heat units.

2006 Elsevier B.V. All rights reserved.

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eywords: Malus x domestica Borkh; Fruit canopy position; Ripening; Col

. Introduction

At the time of commercial harvest the visual quality ofpples, such as size and colour as well as eating qualityn terms of sugar and acid concentration and fruit texturean vary considerably both between trees in an orchardnd within the tree (Johnson and Ridout, 2000). Changesn apple skin colour start when the fruit approaches matu-ation as a consequence of chlorophyll loss in conjunctionith biosynthesis of anthocyanins and carotenoids. How-

ver, some cultivars remain green, with a change to lightreen or yellow during ripening. Solar radiation in combi-ation with cool night temperatures is reported to promote

∗ Corresponding author. Tel.: +46 40415000; fax: +46 40415394.E-mail address: 046293826@telia.com (T. Nilsson).

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925-5214/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.postharvbio.2006.07.011

emical composition

e novo synthesis of anthocyanins (Saure, 1990; Honda etl., 2002), mainly cyanidin-3-galactoside (Sun and Francis,967). Many red-coloured cultivars therefore, only developed blush on the side of the fruit exposed to solar radiationn the top or outer branches while fruit on the inner, shadedart of the tree remain green or pale red (Jackson et al., 1971;ever et al., 1995).Krishnaprakash et al. (1983) found that top fruit had higher

ean scores for colour, appearance and texture but lowerean scores for juiciness, aroma, taste and soluble solidshile there were no significant differences in acidity. Thisosition effect was also highly significant with respect to

he faster disappearance of starch in green inside fruit. Inontrast, Seeley et al. (1980) reported that ‘Delicious’ applesith slight red colour development had reduced soluble solids

oncentrations and starch contents compared with highly

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oloured fruit, but no differences in pH, titratable acidityr firmness were present. Tustin et al. (1988) working withGranny Smith’ apples also found that fruit harvested fromhe inner canopy had lower soluble solids concentrations andgreener background colour compared with fruit on the outerranches. In a study of the effects of percent redness on qual-ty (sensory and analytical) of ‘McIntosh’ and ‘Jonagold’pples, Dever et al. (1995) reported that fruit with 85–95%ed colour were significantly heavier, sweeter and less sourlower titratable acidity) with higher pH values than thoseith 15–25% red colour development. The non-blush sideas, independent of cultivar, crisper, less sweet, had lowerH values and soluble solids concentrations than the blushide. Obviously, the composition of apples can vary bothithin the tree as well as within the fruit but the results pre-

ented so far on the effects of fruit position on the tree areontradictory.

In a study of ‘Delicious’ apples allowed to ripen on theree, Farhoomand et al. (1977) found that top and outsideruit appeared more mature than inside and low fruit, despitehe latter being frequently physiologically more advancedith a higher ethylene production compared to fruit from

he outer branches. However, Jackson et al. (1977) reportedhat unshaded, outside ’Cox’s Orange Pippin’ apples had aigher respiration rate and ethylene production than heavilyhaded fruit but the onset of the climacteric rise in respira-ion and ethylene production occurred at about the same timendependent of fruit position, indicating no marked shift inhe onset of ripening due to shading. Krishnaprakash et al.1983) on the other hand, found that apples at the bottom ofhe tree matured earlier than those on the middle and top butith a pronounced variation in the rate of maturation between

nterior as well as exterior fruit.Whether fruit appearance should be considered as an

cceptable criterion for maturity is uncertain since both greennd immature appearing fruit as well as red coloured fruit areeported to be at a more advanced stage of ripening due tohigher ethylene production. However, the reasons behind

hese inconsistencies are not well understood. Most apples arearvested commercially before they become ripe for eatingnd are then stored at low temperature in a controlled atmo-phere for several months. Investigations aimed at elucidatingostharvest changes in the onset and rate of respiration andthylene production as well as in chemical composition ofpples harvested from different canopy locations and allowedo ripe in common air at 20 ◦C, are therefore scarce.

The purpose of the present study was to examine if ando what degree postharvest ripening of sun-exposed out-ide apples differ from shaded inside apples with respecto the onset and rate of the respiratory climacteric and theurst of ethylene as well as to what extent outside andnside fruit deviate with regard to colour, dry matter con-

ent, soluble solids concentration, soluble carbohydrates, pH,rganic acids, and titratable acidity during ripening in air at0 ◦C. The study was conducted over two consecutive cropeasons.

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logy and Technology 43 (2007) 36–46 37

. Materials and methods

.1. Source of fruit

Apples (Malus x domestica Borkh. cv. Aroma), a Swedishultivar red-coloured on the sun exposed side were obtainedn two successive seasons (1998 and 1999) from the experi-

ental farm Kivik, located in the south-east region of Sweden∼55◦N). The trees growing on M.9 rootstocks were about0 years old and planted with 4 m between and 2 m within theows and pruned with circular canopies. Twelve trees similarn size and receiving sunlight uniformly were selected. Fromhe sun-exposed outer branches, top and outside red-colouredruit were harvested and from the shaded inside and bottomranches green-coloured fruit were harvested. Outside andnside fruit were held separately throughout the experimentaleriod. Harvest in both years was carried out on 14 Septembernd all fruit were immediately brought to Alnarp and storedvernight at 12 ◦C until the onset of the experiments.

Daily minimum and maximum temperatures and precipi-ation were measured at the farm and daily mean temperaturend heat units (base temperature 7 ◦C) (Pavel and Dejong,995) were calculated monthly each year from 1 May to theay of harvest.

.2. Storage conditions and sampling

On 15 September in both years (day 0) the apples (about00 of each type) were transferred to closed glass jars (5 l)laced in darkness at 20 ± 1 ◦C and ventilated continuouslyith compressed humidified air (500 ml min−1). Concur-

ently, five fruit of each colour were selected at random,umbered, weighed and enclosed singly in sealed glass jars870 ml) placed in darkness at 20 ± 1 ◦C and ventilated for–4 days with humidified air (120 ml min−1) before measure-ent of CO2 and ethylene production and colour. Each fruitas then cut vertically into 12 sectors (skin + cortex) with the

ore discarded. To reduce within-apple variation, sectors cutrom opposite sides were subdivided into four sub-samplesnd immediately stored separately in airtight plastic vials at80 ◦C until analysed. The sampling procedure was then

epeated 12 times until the beginning of November with newpples selected at random from various stock jars in order toecure an equal decreasing content in all jars. In both yearseasurements of CO2, ethylene and colour were performed

n days 2, 6, 9, 13, 16, 20, 24, 27, 30, 34, 37, and 43 andn 1999 also on day 55 while sampling for analyses werenly performed on days 2, 9, 16, 24, 30, 37, and 43 and in999 also on day 55. In order to elucidate differences in res-iratory climacteric and ethylene production between singlepples during ripening, five fruit of each colour were selectedt random, weighed and placed singly in similar sealed and

entilated (120 ml min−1) glass jars (870 ml) on day 0 andeld in darkness at 20 ± 1 ◦C for repeated measurements ofO2 and ethylene production at 2–4 days intervals during–8 weeks.

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.3. Respiration rate, ethylene production and coloureasurements

Immediately before and after the jars were sealed for 1 h,amples (10 ml) of the headspace air were withdrawn with aas tight syringe and the concentrations of ethylene and CO2ere determined. Actual atmospheric pressure was measured

nd the density of the apples was estimated to be 0.77 g ml−1.Ethylene and CO2 were assayed by a Varian 3700 gas

hromatograph equipped with both TCD and FID detec-ors and two Varian 4290 integrators. The air sample wasnjected manually into a split-valve dividing the sample intowo streams for CO2 and ethylene detection respectivelyefore entering a 200 �m loop connected with the columnet. For CO2 analysis we used a HayeSep Q 3.3 m × 3.2 mmolumn (100–120 mesh) and a TCD detector. The carrieras was helium at 30 ml min−1 and the filament, detector,ven and injector temperatures were 160, 110, 75 and 60 ◦Cespectively. For ethylene analysis we used a HayeSep Q.7 m × 3.2 mm column (80–100 mesh) and a FID detectorhydrogen 32 ml min−1, air 325 ml min−1). The carrier gasas nitrogen at 30 ml min−1 and the injector, column andetector temperatures were 60, 75 and 110 ◦C respectively.ompressed gas mixtures were used for calibration of CO2nd ethylene. The respiration rate and ethylene productionere calculated from the difference in the concentrationseasured before and after closure of the jar and expressed asg CO2 kg−1 h−1 and �l ethylene kg−1 h−1 respectively.In order to compare internal ethylene concentration with

thylene production, samples of 10 apples from both out-ide and inside fruit were collected at random in 1999 aftertorage at 20 ± 1 ◦C for 40 days. The apples were placedingly in sealed glass jars (870 ml) at 20 ± 1 ◦C for determi-ation of ethylene production as previously described. Afterentilation for 3 h the internal ethylene concentration wasssayed. The apple was immersed in water (20 ± 1 ◦C) androm the seed cavity a gas sample (0.2 ml) was withdrawnith a syringe and a hypodermic needle with the aperturen the side inserted from the calyx end of the fruit. Thexperiment was performed twice.

The surface colour of each fruit was assessed with ainolta Chroma Meter model CR 200. The CIELAB coor-

inates (L*, a*, b*) were measured at three random locationst the equatorial zone of each fruit with the average usedo calculate hue angle (H◦ = tan−1 b*/a*) where b* is the yel-ow/blue and a* is the red/green colour coordinates (McGuire,992). H◦ = 0◦ represents a totally red, H◦ = 90◦ a totallyellow and H◦ = 180◦ a totally green peel colour.

.4. Soluble solids concentration, pH and titratablecidity

Subsamples were allowed to thaw before homogenisingn a Waring blender (1 min) with ultra pure water (1:1; w/v).fter centrifugation (10,000 × g) for 10 min, percent solu-le solids in the supernatant were determined with a tabletop

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logy and Technology 43 (2007) 36–46

efractometer (RMF 80, Bellingham and Standley Ltd., UK).rom the supernatant, 5 ml was diluted with 20 ml ultra pureater and after pH measurement titrated with NaOH (0.05 M)

o pH 8.1 with a Radiometer TT80 Titrator (Radiometer, Den-ark). Titratable acidity was expressed as milliequivalents

cid 100 ml−1 juice.

.5. Organic acids

Samples (250 mg) of freeze-dried and ground sub-samplesf apple tissue were extracted with 50 mM H3PO4 (10 ml) inlosed test tubes. After vortexing, the tubes were placed in anltrasonic cleaner for 60 min whereupon samples of 1.2 mlere centrifuged (10,000 × g) for 10 min at 5 ◦C. Samples of00 �l were used for HPLC analysis.

For HPLC analysis, a Hewlett Packard model 1100 wassed (Hewlett Packard, Sweden) with a rheodyne injector,inary pumps, UV detector and an autosampler (Marathon,echlab, Germany). The separation was performed on aucleosil 100-5 C18 column (200 × 4.6 mm) (MacheryNagel, Germany). Mobile phase: 10 mM KH2PO4 (pH

.40): methanol (HPLC grade) (95:5) with a flow rate ofml min−1 at 30 ◦C and an injection volume of 20 �l. Dataere processed with the Prime Data Program (HPLC Tech-ology Ltd.) from external standards of malic and citriccid.

.6. Soluble sugars and dry matter

Samples (50 mg) of freeze-dried and ground apple tissueere extracted twice with 70% ethanol (1 ml). From the com-ined extract, 200 �l was diluted with double distilled watero 1000 �l and centrifuged (10,000 × g) for 7 min at 5 ◦C.

For HPLC analysis a Hewlett Packard model 1100 wassed (Hewlett Packard, Sweden) with a rheodyne injec-or, binary pumps, RI-detector (RI-4, Varian Instruments,nc.) and an autosampler (Marathon, Techlab, Germany).he separation was performed on a Sarasep CAR-H

300 mm × 7.8 mm) column (Sarasep Inc., Santa Clara, Cali-ornia, USA). Mobile phase: 8 mM H2SO4 with a flow rate of.5 ml min−1 at 25 ◦C and an injection volume of 5 �l. Dataere processed with the Prime Data Program (HPLC Tech-ology Ltd.) from external standards of fructose, glucose anducrose.

The dry matter content was determined in sub-samplesried in a ventilated oven at 70 ◦C for 24 h plus 2 h at 105 ◦C.

.7. Statistical analysis

The data were analysed by means of two-way factorialNOVA (Statgraphics Version 6, Manugistics, Inc., USA)ith fruit location on the tree and time of storage as fixed

actors. Both factors and their interaction were tested againsthe residual mean square. Due to significant interactions,ne-way ANOVA was also used in order to test differencesetween all means each year. Least significant differences

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LSDs) were calculated using Tukey HSD at P = 0.05. Cor-elation analyses were performed with the Origin computerrogram.

. Results

.1. Fruit weight

Mean fruit weight, as a measure of size for outsidepples was 138.6 ± 2.38 g in 1998 and 139.7 ± 3.38 g in999. Similar figures for inside fruit were 132.1 ± 2.90 and39.6 ± 2.55 g respectively. The variation of the variablesnalysed that could be attributed to a linear regression onruit weight (r2) was less than 30% (data not shown).

.2. Respiration rate

The respiration rate (Fig. 1A and B) followed the sameattern each year in both outside and inside fruit with

he respiratory climacteric peak reaching the same height27 mg CO2 kg−1 h−1) independent of fruit position. In 1998,he peak appeared around day 15 in outside fruit com-ared with day 18 for inside fruit. In 1999 (Fig. 1B) the

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ig. 1. Respiration, ethylene production and hue angle in apples cv. Aroma duymbols = outside fruit, filled symbols = inside fruit. Data represent means of five inll means.

logy and Technology 43 (2007) 36–46 39

eak appeared around day 4 independent of fruit loca-ion. When averaged over the same 12 sampling daysTable 1) the CO2 production was on the same level in998 (20.5 mg CO2 kg−1 h−1) independent of fruit loca-ion but in 1999 significantly (P < 0.001) higher for insideruit (21.0 mg CO2 kg−1 h−1) compared with outside fruit15.7 mg CO2 kg−1 h−1).

In apples subjected to repeated measurement of respirationFig. 2A and B) the CO2 production followed the same patternut the difference between inside and outside fruit in 1999as smaller (P > 0.05). The respiratory peak appeared arounday 19 (25.2 mg CO2 kg−1 h−1) in 1998 and around day 5 in999 (27.1 mg CO2 kg−1 h−1) independent of fruit locationdata not shown).

.3. Ethylene production

The ethylene production in both years reached a signifi-antly (P < 0.01) higher peak level in inside fruit comparedith outside fruit (Fig. 1C and D) with the most pronounced

ifference in 1999. When averaged over the same 12 samplingays (Table 1) the ethylene production for inside apples was3% higher in 1998 and 67% higher in 1999 compared withutside apples.

ring storage in darkness at 20 ◦C after harvest in 1998 and 1999. Opendividual fruit. Vertical bars indicate LSD (Tukey HSD) at P = 0.05 between

40 T. Nilsson, K.-E. Gustavsson / Postharvest Biology and Technology 43 (2007) 36–46

Table 1CO2 (mg kg−1 h−1) and ethylene (�l kg−1 h−1) production, hue angle and chemical composition of outside and inside apples cv. Aroma during ripening in airand darkness at 20 ◦C

CO2 Ethylene Hue DM (%) SS (%) Soluble sugars (g 100 g−1 Fwt) meq. 100 ml−1 juice mg 100 g−1 Fwt

Fructose Glucose Sucrose Total Titratableacidity

pH Malicacid

Citricacid

1998Outside fruit 20.04a 57.42a 88.45a 12.77a 10.94a 6.20a 1.16a 1.60a 8.96a 9.72a 3.40a 470.66a 1.83a

Inside fruit 21.02a 70.69b 107.94b 11.26b 9.79b 5.58b 1.04b 1.14b 7.76b 10.11a 3.44a 457.95a 1.74a

LSD 1.12 4.28 1.76 0.45 0.34 0.20 0.07 0.13 0.32 0.57 0.05 30.10 0.20

1999Outside fruit 15.71a 50.19a 70.04a 15.44a 13.39a 6.93a 1.32a 2.40a 10.65a 8.46a 3.55a 593.84a 4.02a

Inside fruit 21.00b 83.94b 100.99b 14.42b 12.69b 6.69a 1.38a 1.78b 9.85b 9.01b 3.60b 662.80b 5.21b

LSD 0.97 4.48 3.30 0.54 0.48 0.25 0.08 0.19 0.38 0.50 0.04 36.50 0.52

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SD values (Tukey HSD) at P = 0.05. Means followed by different superscrean values* of the same 12 (CO2, ethylene, hue) and seven (the other) sam

n = 60 (CO2, ethylene, hue) and n = 35 for the other.

The time when the climacteric rise in ethylene produc-ion started did not differ between inside and outside fruit.owever, in 1998 the ethylene production on day 2 was1 �l kg−1 h−1 in four of five apples independent of loca-

ion compared with a spread in ethylene production between.5 and 22.5 �l kg−1 h−1 on day 2 in 1999. This differenceetween the years was also present in apples subjected toepeated measurements (Fig. 2C and D).

The peak in ethylene production appeared in 1998 betweenays 24 and 27 (mean, day 25) in outside fruit and betweenays 24 and 30 (mean, day 27) in inside fruit. In 1999 the

thylene peak appeared earlier similar to that of respirationnd between days 16 and 20 (mean, day 18) in outside fruitnd between days 16 and 24 (mean day 19) in inside fruit. Thepples subjected to repeated measurements (Fig. 2C and D)

3

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ig. 2. Repeated measurements of respiration and ethylene production in apples cv.pen symbols = outside fruit, filled symbols = inside fruit. Data represent means oetween all means.

rs (a and b) are significantly different.ates both years.

howed the same pattern with the ethylene peak between days7 and 29 (mean day 28) in 1998 and between days 13 and0 (mean day 19) in 1999 independent of fruit position (dataot shown). Consequently, the respiratory peak preceded thatf ethylene with 9–10 days in 1998 and with 14–15 days in999.

The internal ethylene concentration in apples stored for0 days at 20 ◦C in 1999 (Table 2) was similar, independentf fruit location on the tree, despite a 59% (P < 0.01) higherthylene production in inside apples.

.4. Hue angle

The peel colour (Fig. 1E and F) of inside fruit on daywas greener in 1998 as indicated by the difference in hue

Aroma during storage in darkness at 20 ◦C after harvest in 1998 and 1999.f five individual fruit. Vertical bars indicate LSD (Tukey HSD) at P = 0.05

T. Nilsson, K.-E. Gustavsson / Postharvest Bio

Table 2Comparison of internal ethylene concentration (�l l−1) and ethylene pro-duction (�l kg−1 h−1) of outside and inside apples cv. Aroma stored for 40days at 20 ◦C in 1999

Outside fruit Inside fruit LSD

Internal concentration 83.1 ± 6.85 85.0 ± 5.94 NSEthylene production 45.6 ± 5.08 72.4 ± 5.23 15.3L

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ngle (H◦ = 115.6 compared with H◦ = 109.8 in 1999). In bothears the hue angle of inside fruit decreased almost linearlyP < 0.001) with time due to a gradual change from green toellowish-green peel colour. The red colour of outside fruitFig. 1E and F) was stronger in 1999 with a mean hue angle of01.0 compared with 107.9 in 1998 (Table 1). The differencen hue angle between outside and inside fruit was significantP < 0.001) both years (Table 1) but larger in 1999; H◦ = 31.0ompared to H◦ = 19.5 in 1998. Outside fruit were thus moreeddish and inside fruit less greenish in 1999.

.5. Dry matter and soluble solids concentration

The ratio between dry matter content and soluble solidsoncentration was in the range 0.85–0.87 independent of yearnd fruit location on the tree and remained unchanged as fruitipening proceeded (data not shown), with a significant linearorrelation (r = 0.64**–0.79**) between dry matter contentnd soluble solids concentration (Table 3).

Outside fruit in both years had significantly (P < 0.01)igher dry matter contents and soluble solids concentrationsFig. 3A–D; Table 1) than inside fruit when averaged overhe same sampling days, but the differences were larger in998 (13%) than in 1999 (7%). The levels remained almostnchanged over the postharvest period in both years but

fter 37 days the difference between outside and inside fruiteemed to diminish. In 1999 the dry matter content was on anverage 21% higher in outside fruit and 28% higher in insideruit (P < 0.001) compared with the levels in 1998. Similar

able 3orrelation coefficients between variables analysed in outside and insidepples cv. Aroma over the same storage period each year

ariables compared Correlation coefficient (r)

1998 1999

Outside Inside Outside Inside

ry matter vs. soluble solids 0.793a 0.724a 0.644a 0.739a

otal sugar vs. dry matter 0.577a 0.221 0.804a 0.873a

otal sugar vs. soluble solids 0.667a 0.530a 0.693a 0.576a

ructose vs. glucose 0.875a 0.759a 0.857a 0.698a

onosaccharides vs. sucrose −0.136 −0.219 −0.727a −0.665a

itratable acidity vs. malic acid 0.970a 0.971a 0.968a 0.956a

itratable acidity vs. citric acid 0.875a 0.899a 0.776a 0.822a

alic acid vs. citric acid 0.870a 0.902a 0.776a 0.859a

itratable acidity vs. pH-value −0.819a −0.785a −0.800a −0.847a

a Denotes correlation coefficient significant at P = 0.01. n = 35 both years.

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logy and Technology 43 (2007) 36–46 41

gures for soluble solids concentrations were 22% and 30%espectively.

Since the main part of the soluble solids was composedf sugar (80%) independent of fruit position on the tree andear, significant linear correlations between total sugar andoluble solids were present (Table 3). However the propor-ion of the variance (r2) of the soluble solids that could bessociated with its linear regression on total sugar was some-hat higher in outside fruit (44–48%) compared with inside

ruit (28–33%).

.6. Soluble sugar contents

Fructose in both years (Fig. 3E and F) was the major sol-ble sugar and made up 65–72% of the total soluble sugarontent in both fruit types compared with 12–14% for glucosend 15–23% for sucrose (Table 1). Outside fruit in 1998 con-ained 11% more fructose, 12% more glucose and 40% moreucrose than inside fruit (P < 0.01) but in 1999 only sucroseas significant higher in outside fruit (>35%; Table 1). The

orrelation between fructose and glucose in both years wastronger in outside fruit (r = 0.86**–0.88**) than in inside fruitr = 0.70**–0.76**) (Table 3).

The pattern of changes over the postharvest period in theontent of soluble sugars was equal in both fruit types buteviated between the years (Fig. 3E and F). In 1998 theoncentration of all three sugars increased between day 2nd day 9 but was most pronounced for fructose and glu-ose (P < 0.001), whereupon the monosaccharides remainednchanged with a concomitant fall in the sucrose con-ent. In 1999 the monosaccharides increased almost linearlyP < 0.01) over the entire postharvest period while the sucroseontents decreased. A concomitant rise in the amounts ofructose and glucose due to the fall in sucrose was thusnly evident in 1999 as indicated by the significant nega-ive linear correlation between monosaccharides and sucrosen both outside (r = −0.73**) and inside (r = −0.67**) fruit inhat year (Table 3). For all soluble carbohydrates quantified,he differences between outside and inside apples tended toisappear at the end of the postharvest period each year.

The total amount of soluble sugars (fructose, glucose anducrose) (Fig. 3E and F; Table 1) when averaged over theame postharvest period was significantly (P < 0.001) highern outside fruit in both years, but the difference was larger in998 (15%) compared with the 1999 fruit (8%).

.7. Titratable acidity, pH and organic acidoncentration

The titratable acidity (Table 1) when averaged over theame sampling days was at the same levels in both fruit typesn 1998 but significantly (P < 0.05) higher in inside fruit in

999. From day 2 to day 43 the concentrations (Fig. 4And B) declined almost linearly with time at the same rate0.2 meq. 100 ml−1 juice day−1) in both years independentf fruit position on the tree. Concurrently, the pH (Fig. 4A

42 T. Nilsson, K.-E. Gustavsson / Postharvest Biology and Technology 43 (2007) 36–46

Fig. 3. Dry matter, soluble solids and soluble sugars in apples cv. Aroma during storage in darkness at 20 ◦C after harvest in 1998 and 1999. ( ) Dry mattero uble suf uit. Ver

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utside fruit, (×) dry matter inside fruit. (� �) soluble solids, (♦ �) total solruit, filled symbols = inside fruit. Data represent means of five individual fr

nd B) increased from 3.2 to 3.7 in 1998 and from 3.4 to 3.8n 1999 when averaged over both fruit types.

Contrary to titratable acidity the malic acid contentTable 1) was lower in both fruit types in 1998 but the dif-erence between outside and inside fruit was similar to thatf titratable acidity, only significant in 1999 with 12% morealic acid in inside fruit (P < 0.001). Independent of fruit

ype, malic acid decreased approximately linearly with timeFig. 4C and D) in 1998, but after day 37 in 1999 the lin-ar decrease levelled off. Despite the lower concentration atarvest in 1998 the rate of malic acid decrease was similar10 mg 100 g−1 Fwt day−1) independent of fruit position onhe tree and year.

The concentration of citric acid (Table 1) was much lowern 1998 than in 1999 and similar to malic acid, the content wasnly significantly (P < 0.001) higher in inside fruit in 1999.hen averaged over both fruit types, citric acid amounted in

998 to 0.38% and in 1999 to 0.75% of the total acid contentnalysed (malic + citric). In both years, the amount of citriccid was opposite to that of malic acid, being unchanged upo day 16 then decreasing to day 30, whereupon it remained

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gar, (��) fructose, (� �) sucrose, (© �) glucose. Open symbols = outsidetical bars indicate LSD (Tukey HSD) at P = 0.05 between all means.

nchanged for the rest of the postharvest period (Fig. 4E and). This pattern was similar in both types of fruit, although theagnitude was more pronounced in 1999 due to the higher

oncentration.As could be expected there were strong linear correlations

etween malic acid and citric acid as well as between titrat-ble acidity and malic acid and citric acid and pH respectivelyn both inside and outside fruit each year (Table 3). However,he higher levels of malic acid and citric acid in 1999 werendependent of fruit type linked to lower levels of titratablecidity and higher pH values compared with the oppositeattern in 1998 (Table 1).

. Discussion

The pronounced difference between the 2 years in the rate

f CO2 production at the start of postharvest storage at 20 ◦Cndicate that on 15 September the 1998 apples were harvestedust before or at the onset of the respiratory rise and there-ore less mature than the apples in 1999, which were close

T. Nilsson, K.-E. Gustavsson / Postharvest Biology and Technology 43 (2007) 36–46 43

Fig. 4. Titratable acidity, pH-value, malic acid and citric acid in apples cv. Aroma during storage in darkness at 20 ◦C after harvest in 1998 and 1999. (♦ �)T n symboi n all m

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itratable acidity, (� �) pH-value, (��) malic acid, (© �) citric acid. Opendividual fruit. Vertical bars indicate LSD (Tukey HSD) at P = 0.05 betwee

o the respiratory peak at harvest. The greener peel colour ofnside fruit at harvest in 1998 also indicates a delayed matu-ity. The earlier ripening in 1999 was probably caused byhe higher daily mean temperatures from June to harvest inhat year (Table 4). Such temperature effects on the develop-ental rate of apples have been reported earlier (Luton andamer, 1983; Chu, 1984), but a within-tree variation was alsoresent. However, when outside and inside apples are com-ared, no pronounced differences in the pattern of respirationere present in any of the years besides the significant lower

O2 production in outside fruit in 1999.

The peak in ethylene production in both years was behindhat of CO2 production, with a lag time of 9 days in 1998nd 15 days in 1999, independent of fruit position. Reid

sand

able 4emperatures, accumulated heat units (base temperature = 7 ◦C) and rainfall at the E

onth Mean temperature (◦C) H

1998 1999 1

ay 10.7 9.4 1une 13.9 14.9 2uly 14.8 17.5 2ugust 14.5 16.3 2eptembera 14.1 16.1 1

ean or totalb 13.6 14.8 8a September 1–14.b Means of monthly temperatures or sums of heat units and rainfall.

ls = outside fruit, filled symbols = inside fruit. Data represent means of fiveeans.

t al. (1973) reported a similar delayed ethylene productionn ‘Cox’s Orange Pippin’ apples but the lag was only 12 h. Inontrast, Lu et al. (1992) found that in ‘White Winter Pear-ain’ apples stored at room temperature, ethylene peakeddays before the maximum respiratory rate was achieved.his inconsistency indicates that the ripening behaviour ofpples is influenced by the cultivar. The typical climactericise in respiration has by tradition been regarded as a con-equence of the increase in endogenous ethylene rather thancentral event in fruit ripening (Knee, 1993). It is however

afe to assume that the onset of the respiratory climacteric inttached fruit of the ‘Aroma’ cultivar in 1999 was not coordi-ated with the increased ethylene production and therefore aevelopmentally regulated event. The prolonged lag this year

xperimental farm Kivik, during the growing seasons 1998 and 1999

eat units (7 ◦C) Rainfall (mm)

998 1999 1998 1999

15 88 27 8708 237 93 10740 326 106 3332 289 13 13300 129 1 1

95 1069 240 361

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4 T. Nilsson, K.-E. Gustavsson / Postharv

ay also indicate that the rise in ethylene but not in CO2 pro-uction was inhibited due to an unknown tree factor (Lau etl., 1986; Blanpied, 1993), when harvest was delayed com-ared with the apples harvested before or close to the rise inO2 production in 1998. However, in detached fruit a slightly

ncreased endogenous ethylene concentration not detectables a rise in the rate of production may be sufficient to initiatehe respiratory rise (McGlasson et al., 1978).

In both years the peak in ethylene production was signif-cantly higher in inside, shaded fruit compared with outsideruit with red blush. This is in agreement with Farhoomandt al. (1977) but contrary to the data of Jackson et al. (1977)ho reported that shading reduced ethylene production butnly in October-picked fruit. The onset of the climactericise in ethylene production, and in that connection ripening,as simultaneous in both outside and inside fruit, which is

n agreement with Jackson et al. (1977) who also found noarked difference in the onset of ripening in ‘Cox’s Orangeippin’ fruit due to shading.

The higher ethylene production in inside ‘Aroma’ applesas not accompanied by an enhanced internal ethylene con-

entration. It can be assumed that this discrepancy was aesult of a higher ethylene production and a lower resis-ance to ethylene diffusion due to differences in intercellularpace, volume and cell size between inside and outside fruit.he rate of transformation of stomata into lenticels as wells thickness and composition of the cuticle wax layer dur-ng fruit growth and maturation (Blanke and Lenz, 1989)

ay also differ due to fruit canopy position. However, theeasons why inside green apples have a higher ethylene pro-uction during ripening than outside red coloured apples andf this pattern is independent of cultivar or not calls for furthernvestigation.

The significantly higher contents of dry matter and solu-le solids as well as total soluble sugars in outside applesoth years indicate a close relationship between the lightevel within the tree canopy and the concentration of storedarbohydrates at harvest. This is in agreement with ear-ier investigations (Jackson et al., 1977; Seeley et al., 1980;obinson et al., 1983; Tustin et al., 1988). However, the dif-

erence in soluble solids concentration between outside andnside fruit was on an average twice as high in 1998 as in999 mainly due to the higher contents of fructose, glucosend sucrose in outside fruit in 1998 while in 1999 the dif-erence was limited to sucrose. Concurrently, the differencen peel colour between outside and inside fruit was, whenxpressed as hue angle, 60% higher in 1999 compared with998, indicating that fruit appearance is not an acceptableriteria for fruit quality. Despite the area of leaves associatedith the fruit (Ferguson et al., 1995), spur age (Robinson et

l., 1983) and the number of fruit per cluster (Beruter, 1985,989), which contribute beside light to the influence of fruit

anopy position on the amount of soluble solids, it cannot bexcluded that the for Swedish conditions, an unusually warmummer in 1999 contributed to the diminished effect of fruitosition on the tree.

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logy and Technology 43 (2007) 36–46

The starch content was not analysed in the present investi-ation but the rapid increasing amounts of all soluble sugarsnalysed in both fruit types during the first ten days the applesere kept at 20 ◦C in 1998 must originate from starch degra-ation. The minor increase during the same period in 1999hen the fruit had reached the respiratory climacteric at har-est supports this assumption. Obviously, accumulation ofoluble sugars due to degradation of starch followed the sameattern independent of fruit position and was almost finishedy the time the respiratory climacteric peak was reached. Laut al. (1986), Brookfield et al. (1997) and Duque et al. (1999)ave also reported a similar pattern of starch degradation.

In both years the content of sucrose decreased significantlyfter day 9 independent of fruit position on the tree with annchanged fructose/glucose ratio, but a concomitant increasef the hexoses was only present in 1999. Sucrose degradationn different apple cultivars including ‘Aroma’ during storaget 3 ◦C has been reported earlier, but the magnitude variedetween the cultivars (Suni et al., 2000). The major respira-ory substrates in apples are sugars and malic acid (Hulme andhodes, 1971) but the postharvest changes in the amounts of

hese substrates were apparently unrelated to the respiratoryise and fall during the climacteric, which is in agreementith Duque et al. (1999).The amounts of malic and citric acid were considerably

igher in 1999 especially in inside, shaded fruit. A similarifference was not present in 1998. The synthesis and accu-ulation of organic acids in apples is most pronounced at

he beginning of the cell expansion period (Hansen, 1979)fter which the concentration decreases for the remainder ofhe growth period (Hulme and Rhodes, 1971; Knee, 1993).ince full bloom at the experimental farm normally occursuring the second half of May, the higher mean air temper-tures during June in 1999 (Table 4) may therefore explainhe elevated organic acid content at harvest this year but theeasons behind the lower level in outside fruit this year areo far unknown.

When the titratable acidity is compared with the amount ofrganic acids it is obvious that the higher organic acid contentn 1999 did not increase the titratable acidity. Differences inhe proportion between malic acid and malate may explainhis discrepancy. The malic acid concentration is thereforeot an acceptable measure of apple sourness (Gorin and Klop,982). Fruit canopy position had only a minor influence onhe titratable acidity and pH value, which is in agreementith earlier findings (Seeley et al., 1980; Krishnaprakash et

l., 1983; Daugaard and Grauslund, 1999).The fall in malic acid content during maturation and ripen-

ng of apples with a concomitant decrease in titratable aciditynd raised pH value is a well known event (Ackermann etl., 1992; Knee, 1993; Brackmann et al., 1994; Suni et al.,000), but despite the higher initial concentration in 1999 the

ecreasing rate was similar both years and uninfluenced byruit position in the canopy. The fall (50–60%) after 43 days at0 ◦C is in agreement with earlier investigations (Ackermannt al., 1992; Knee, 1993; Brackmann et al., 1994).

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T. Nilsson, K.-E. Gustavsson / Postharv

. Summary

In this study we have shown that for the Swedish appleultivar ’Aroma’ fruit position on the tree has no influ-nce on the onset and course of ripening. The climactericeak in ethylene production was however, behind that ofespiration with a lag of more than 9 days, which has noteen earlier reported for other apple cultivars. The higherthylene production in inside apples was not accompaniedy an increased internal ethylene concentration indicatingn effect of fruit position on both ethylene production andate of diffusion outwards. The compositional differencesetween outside and inside fruit with a higher content ofoluble solids in outside fruit were more pronounced for sol-ble solids than for titratable acidity and pH value, but highummer temperatures diminished this difference. Indepen-ent of fruit position on the tree, high summer temperaturesncreased soluble solids and organic acid contents but hado influence on titratable acidity. The red peel colour ofutside fruit was also temperature dependent while insideruit remained green. A warm summer therefore resulted innside green apples with higher soluble solids content thann outside fruit with red blush harvested after a cooler sum-

er. Peel colour is thus not an acceptable criterion for fruituality.

cknowledgements

This work was supported by the Swedish Council fororestry and Agricultural Research. We are grateful to Miss. Persson for skilful technical assistance.

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