Effects of relative humidity on apple quality under

simulated shelf temperature storage

K. Tua,*, B. NicolaõÈb, J. De Baerdemaekera

aDepartment of Agro-Engineering and Economics, Katholieke Universiteit Leuven,

Kardinaal Mercierlaan 92, 3001 Heverlee, BelgiumbFlanders Centre for Post-Harvest Technology, W. de Croylaan 42, 3001 Heverlee, Belgium

Accepted 11 November 1999

Abstract

The effects of relative humidity (RH) on the quality of `Braeburn' and `Jonagold' apples were

studied at 208C under 30, 65 and 95% RH conditions. The aim of the research was to assess the

effects of RH on apple quality under retailers' shelf temperatures. Mealy texture may develop by

holding apples at high RH (95%), and 208C for some time depending on the cultivar. The 65% RH

treatment is a typical RH for shelf life and 30% is a low RH condition. Apple ®rmness, expressible

juice content, weight loss, pH, soluble solids content (SSC), and some other quality parameters

were determined instrumentally. Scanning electron microscopy (SEM) was applied to investigate

the structural changes at the level of the cell wall. The RH had signi®cant effects on weight loss,

®rmness, and SSC values. Acoustic non-destructive measurements showed that the ®rmness of

apples decreased more slowly at higher RH, and the weight loss was faster at low RH for both apple

cultivars. Mealiness was observed under high RH (95%) and was associated with low tensile

strength and an increase of cell separation following the simulated shelf storage. Higher RH could

maintain apple ®rmness and weight better than lower RH but it tended to promote the development

of mealy texture at 208C. # 2000 Elsevier Science B.V. All rights reserved.

Keywords: Relative humidity; Apple ®rmness; Mealiness; Scanning electron microscopy; Cell wall

Scientia Horticulturae 85 (2000) 217±229

* Corresponding author. Present address: Department of Biomechanical Systems, College of

Agriculture, Ehime University, Tarumi 3-5-7, Matsuyama 790-8566, Japan. Tel.: �81-89-9469823;

fax: �81-89-9469916.

E-mail address: kangtu@agr.ehime-u.ac.jp (K. Tu)

0304-4238/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 1 4 8 - X

1. Introduction

The behaviour of fruits on retailers' shelves is a very important factor affectingconsumer's choice and the market price, and it can be affected by relativehumidity (RH). Most of the information available is on the effect of RH on fruitmoisture or weight changes during storage. Little information is available on RHeffects on the quality of apple fruit at room temperature and their relation tostructural changes at the cellular level. Generally, the equilibrium moisturecontent of food will increase as the environmental RH increases at a giventemperature (Mohsenin, 1986). Verstreken and De Baerdemaeker (1994) studiedthe effects of storage time, temperature and RH on the ripening process ofnectarines and proposed a regression model to predict their weight loss duringstorage. Landrigan et al. (1996) reported the effects of RH on post-harvestbrowning in Rambutan at 208C with 95 and 65% RH. They inferred that enzymeswere involved in the browning of damaged tissue under high RH. At low RH,inhibitors were ineffective as desiccation was the dominant factor of browning.Hat®eld and Knee (1988) reported the effects of water loss on quality of apples instorage. They reported a method to determine internal air spaces (IAS) in applefruit and the higher IAS corresponded to mealy texture. Loss of water will resultin signi®cant wilting, softening, shrivelling and a poor, mealy taste.

In this study, texture development of two apple cultivars, `Braeburn' and`Jonagold', were monitored under three RH conditions at 208C. Scanningelectron microscopy observation was carried out to investigate cell wall structuralchanges in the ruptured surfaces of the samples after tensile test. The objectivesof the research were to investigate the effects of RH on apple quality at roomtemperature with destructive and non-destructive methods. Not only weight loss,but also other important texture parameters such as ®rmness, juiciness andmealiness were studied and related to cell structural changes.

2. Materials and methods

2.1. Materials

`Braeburn' and `Jonagold' apples were stored under ULO (ultra low oxygen,1.5% O2, 1.5% CO2 at 18C) conditions for 1 month before the experiment.Apples of each cultivar were randomly chosen and separated into three groupswith 100 in each group. The simulated shelf storage conditions were 30�0.2%RH, 65�0.2% RH and 95�0.2% RH at 20�0.18C and were maintained in threedifferent chambers. The quality of the apples stored at 65 and 95% RH, 208Cwas monitored for 30 days. Apples under 30% RH, 208C were monitored for18 days.

218 K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229

The physical properties of 10 apples from each storage condition weremeasured with non-destructive, destructive and analytical methods at 3-dayintervals during the simulated shelf temperature storage.

2.2. Non-destructive acoustic measurements

The ®rmness of apples was determined by an acoustic impulse responsetechnique. The equipment for acoustic impulse response measurement was thesame as used by Chen and De Baerdemaeker (1993). The peak in the resonancefrequency spectrum corresponding to the spherical mode shape of apple wasrecorded. The apple ®rmness was indicated by the stiffness factor which is calcu-lated as f 2M2/3 (f is the ®rst peak frequency in Hz; M is the apple mass in g) and thereexists a linear correlation between stiffness factor and Young's modulus of apple¯esh (Armstrong et al., 1990; Chen and De Baerdemaeker, 1993). In this experi-ment, f was represented by the mean value of three resonance frequencies measuredat three marked locations separated about 1208 around the apple equator.

2.3. Destructive measurements

A compression test was applied with a Universal Testing Machine System(UTS Test Systeme, GmbH, Germany) to determine the extractable juice contentof apple samples. Apple ¯esh was cut with a cylindrical cutter and a corticaltissue sample of 17 mm diameter and 5 mm thickness was subjected to thecompression test. The extractable juice content of the apple was measured asdescribed by Tu and De Baerdemaeker (1997). With this method, the weight of®lter paper (Whatman) was weighed before the compression. It was then placedon the top and bottom of a tissue sample (about 1.2 g), and after compression the®lter paper with expressed juice was weighed again. Expressible juiciness wasde®ned as weight gain (WafterÿWbefore) of the ®lter paper based on the initialsample weight (Wsample) and referred as % of expressible ¯uid in the fruit.

The tensile strength of ¯esh tissue was measured by the method of Verlindenand De Baerdemaeker (1994), and Tu et al. (1996) which was regarded as anindicator of the appearance of apple mealy texture. A higher tensile strengthre¯ects strong connections between apple cells and a lower tensile strengthindicates weak cell connections, which often occurs when apples becomeoverripe or mealy. The experimental arrangement can be seen in Fig. 1.

2.4. Analytical measurements

The soluble solids content (SSC) of apple juice was measured with a hand-heldrefractometer (0±85%; Zeiss, Germany) and the pH of the juice was determinedwith a pH meter (ORION, model 250A, USA) following a compression test. The

K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229 219

IAS of apple ¯esh was calculated following the formula suggested by Hat®eld andKnee (1988): IAS (%)�1ÿ(fruit density/speci®c gravity of juice)�100%. Fruitdensity can be calculated by apple weight/volume and the speci®c gravity of applejuice was the average of estimations using a pycnometer. Dry matter was determinedaccording to the AOAC1 (Association of Of®cial Analytical Chemists International)Of®cial Method of Analysis (1997), with 2 g of apple tissue dried at 708C (about24 h) until consecutive weighings made at 2 h intervals varied by less than 3 mg.

2.5. SEM observation

The ruptured samples after the tensile testing were kept in a solution (FAA)consisting of 10 ml of commercial formaldehyde (36%), 5 ml of acetic acid(100%), 85 ml of ethanol (94%). Five samples were taken of each cultivar in eachtest. One day later, the samples were subjected to critical point drying for furtherSEM. In this process, the samples were ®rst washed in 70% ethanol 2±3 times(5 min each). The ®xation was then carried out twice in formaldehyde dimethylacetal (C3H8O2) solution for 45 min each. After ®xation, the samples were putinto a critical point dryer (Balzers, CPD 030, Liechtenstein) and then gold coatedwith a S150A sputter coater (Edwards, UK) at 3 min with a 20 mA current. Aftercoating, the SEM was carried with a JEOL superprobe 733 (Japan) at 20 kV.

Tu et al. (1996) observed with light microscopy that a higher percentage oftissue cells separated instead of rupturing under a tensile force in `Granny Smith'apple when fruits became mealy. In this SEM observation, fresh, slightly mealyand mealy ¯esh tissue of apples can also be distinguished based on the quantity ofseparated cells observed after the tensile test.

3. Results and discussion

The RH level in storage had a signi®cant effect on changes in stiffness factor(Fig. 2). The stiffness factor decreased for both apple cultivars at 208C but those

Fig. 1. Device for apple tensile strength measurement. Cross-section S1�S2�(Dÿd)�thickness of

the sample. In the experiment, D (sample outer diameter)�17 mm, d (inner diameter)�10 mm and

the thickness of the sample is 5 mm.

220 K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229

apples under 95% RH lost their ®rmness more slowly than the others. Thestiffness factor decreased the fastest under 30% RH conditions. From the resultsof acoustic measurements it was found that ®rmness was maintained better athigher RH in both cultivars. The softening of apple tissue was considered to beassociated with moisture loss and the loss of turgor pressure (Van den Berg,1981). However, the measurement was not continued after 18 days for the applesstored under 30% RH conditions since they were rotting and were no longeracceptable in the market.

Both apple cultivars lost weight most rapidly at 208C, 30% RH conditions.Peleg (1985) reported that weight losses of more than 5±10% usually causesigni®cant wilting, low ®rmness, shrivelling and poor taste. Higher RH levelsreduce apple weight loss, as can be seen in Fig. 3. The respiration rate canbe stimulated by water stress, which is induced by lower than optimum RH(90±98%) in the air surrounding the fruit. It also has been reported that achange in RH would have a much larger effect at high than at low humidity.

Fig. 2. Changes in the stiffness factor of cortex tissue of `Braeburn' and `Jonagold' apples kept at

208C and different RH (The bars indicate standard errors).

K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229 221

For example, a change from 98 to 93% increases evaporation by 250%,while a change from 85 to 80% only increases evaporation by 33% (Weichmann,1987). It can be seen in Fig. 3 that apples lost more weight as the RH decreasedfrom 95 to 65% than from 65 to 30%. The greater vapour pressure de®citmay be the main reason of the faster loss of weight under 208C, 30% RHconditions.

It was also found (Fig. 3) that `Jonagold' apples lost weight faster than'Braeburn' apples under the 30 and 65% RH conditions at 208C.

3.1. Destructive measurement results

Tensile rupture force decreased for both apple cultivars under the three RHconditions (Fig. 4). However, the differences in tensile rupture force between thethree RH conditions were not signi®cant. The tensile rupture force of the`Braeburn' apples decreased linearly, but that of the `Jonagold' apples decreasedexponentially. After 2±3 weeks at 208C, 95% RH, apples possessed less tensile

Fig. 3. Weight loss of `Braeburn' and `Jonagold' apples at 208C and different RH (The bars

indicate standard errors).

222 K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229

strength which was considered a symptom of mealy texture. Harker et al. (1997)reported that low tensile strength was associated with less juiciness or mealy tasteof fruits including apple.

It was found that for both apple cultivars, the non-destructive and destructivemeasured ®rmness results (data not shown) were correlated but the correlationwas not very high (correlation coef®cient 0.55±0.8). Increase in apple weight losswas negatively correlated with the decrease of ®rmness.

Duncan's multiple range test (SAS Version 6.0, 1989) showed that RH had asigni®cant effect on some measured parameters. For both `Braeburn' and`Jonagold' apples, the weight loss, compression force, and stiffness factor underthe three different RH conditions were signi®cantly different at P�1%.

3.2. Analytical results

From Table 1 it can be seen that the fruit dry matter content increased duringstorage, and the apples kept at 30% RH showed the highest dry matter content

Fig. 4. Tensile failure force changes of `Braeburn' and `Jonagold' apples at 208C and different RH

(The bars indicate standard errors).

K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229 223

Table 1

Some characteristics of `Braeburn' and `Jonagold' apples before and after storage for 4 weeks at 208C and at three different RH levelsa

Cultivar RH (%) Fresh After storage

Dry matter

(%)

Expressible

juice

content (%)

IAS (%) pH SSC (%) Dry matter

(%)

Expressible

juice

content (%)

IAS (%) pH SSC (%)

Braeburn 30 12.9�0.2 31.4�0.3 13.8�0.2 3.47�0.02 13.0�0.2 15.3�0.2 24.3�0.2 16.4�0.2 4.02�0.02 13.5�0.2

65 13.1�0.2 31.5�0.4 13.8�0.2 3.47�0.02 13.1�0.2 14.3�0.2 25.3�0.5 15.1�0.2 4.07�0.02 13.2�0.2

95 13.0�0.2 31.5�0.3 13.6�0.2 3.47�0.01 13.1�0.2 13.9�0.2 25.8�0.4 15.3�0.2 3.94�0.02 13.7�0.2

Jonagold 30 13.1�0.2 32.3�0.3 18.7�0.2 3.38�0.02 11.3�0.2 15.5�0.2 22.7�0.4 23.1�0.2 4.06�0.02 12.3�0.2

65 13.6�0.2 32.7�0.5 17.7�0.2 3.38�0.02 11.6�0.2 14.7�0.2 24.6�0.3 20.5�0.2 4.06�0.02 12.4�0.2

95 13.5�0.2 31.9�0.3 17.4�0.2 3.38�0.02 11.7�0.1 14.1�0.2 24.9�0.3 20.1�0.2 3.96�0.02 12.9�0.2

aMean value (�SE, n � 10), IAS: internal air spaces and SSC: soluble solid content.

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corresponding to the highest water loss. The expressible juice content decreasedafter storage, but the apples kept at 95% RH had the highest expressible juicecontent. The SSC, pH, and IAS of the fruit increased during storage. However, theSSC did not change much, and the decrease in acidity (increased pH) may resultin an increased sensory perception of sweetness.

Under 208C, 30% RH conditions, the IAS of both apple cultivars increasedmore than that under the other two storage conditions. This indicates that thecells of the apple tissue were connected less tightly under these conditions. Thecells are more likely to separate when under an external force (i.e. tensile) andless cell content will be released so that apples will taste less juicy and moremealy.

3.3. Results of SEM observation

As seen in the SEM picture (Fig. 5), cells of fresh `Braeburn' apple breakduring the tensile test (Fig. 5a). As apples became ripe and overripe (turnedmealy), fracture occurred between cells due to a breakdown of the inter-lamellarregion rather than breakage of the cells themselves as more unbroken cells can beseen in Fig. 5b and c than in Fig. 5a. In Fig. 5c, the IAS was around 16%. Thetensile strength of tissue was calculated (tensile rupture force/rupture area) to beas low as 0.06±0.08 MPa when apple cells showed obvious separation undertensile testing.

Similar behaviour was observed in the `Jonagold' apples (Fig. 6). Most cellswere ruptured resulting in many broken surfaces being visible after tensile test ona fresh apple (Fig. 6a). As the apple became overripe after 4 weeks at 208C, morecell separation was observed (Fig. 6c at 95% RH) after tensile testing. Thisindicated that fewer cells ruptured under the same tensile strength andaccordingly, less cell content was released which may lead to less extractablejuice and a granular, ¯oury sensory feeling. These results suggested that high RH(95%) at 208C may maintain fruit ®rmness and weight better than low RH but itmay introduce a mealy texture.

Upon mastication in the mouth, this type (overripe) of breakdownprobably leads to an accumulation of clumps of intact cells which in turnare responsible for a rough and grainy sensory impression. Surface structure ofthe breakdown particles is important for mealiness perception. Althoughmealiness is usually observed only in ripe, soft apples whose tissue strength isalready low, it is the type (separation between cells) rather than the extentof failure (rupture) which determines whether an apple is perceived as mealyor not (Harker and Hallett, 1992). They found similar results of cell separationof overripe fruits based on observation of late picked `Braeburn' apples incold storage.

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Fig. 5. SEM pictures of `Braeburn' apples (60�, 20 kV, (a) fresh apple; (b) slightly mealy; (c) mealy apple) at 208C and 95% RH.

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Fig. 6. SEM pictures of `Jonagold' apples (60�, 20 kV, (a) fresh apple; (b) slightly mealy; (c) mealy apple) at 208C and 95% RH.

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4. Conclusions

This study provides information on the effects of RH during simulated shelftemperature storage of apples. The 95% RH treatment maintained apple ®rmnessand weight better but it tended to promote the development of a mealy texturethat was con®rmed by SEM pictures. Low RH (e.g. 30%) caused more weight(water) and stiffness loss. The normal storage condition in the market is about65% RH. Combining these data with other experimental data on storagetemperature and time, it is possible to further develop models of apple textureduring post-harvest storage and shelf life.

Acknowledgements

This study was performed within the framework of FAIR (Food Agro-IndustrialResearch) project: FAIR1-CT95-0302. The authors wish to thank the EuropeanCommunity, the Research Council of Katholieke Universiteit Leuven and theFlemish minister of Science and Technology for the ®nancial support. AuthorBart NicolaõÈ is postdoctoral research fellow with the Flemish Institute forScienti®c Research.

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