Effects of relative humidity on apple quality under
simulated shelf temperature storage
K. Tu
a,*, B. NicolaõÈ
b, J. De Baerdemaeker
aa
Department of Agro-Engineering and Economics, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, 3001 Heverlee, Belgium
b
Flanders 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
*
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.
1. Introduction
The behaviour of fruits on retailers' shelves is a very important factor affecting consumer's choice and the market price, and it can be affected by relative humidity (RH). Most of the information available is on the effect of RH on fruit moisture or weight changes during storage. Little information is available on RH effects on the quality of apple fruit at room temperature and their relation to structural changes at the cellular level. Generally, the equilibrium moisture content of food will increase as the environmental RH increases at a given temperature (Mohsenin, 1986). Verstreken and De Baerdemaeker (1994) studied the effects of storage time, temperature and RH on the ripening process of nectarines and proposed a regression model to predict their weight loss during storage. Landrigan et al. (1996) reported the effects of RH on post-harvest browning in Rambutan at 208C with 95 and 65% RH. They inferred that enzymes were 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 in storage. They reported a method to determine internal air spaces (IAS) in apple fruit and the higher IAS corresponded to mealy texture. Loss of water will result in 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. Scanning electron microscopy observation was carried out to investigate cell wall structural changes in the ruptured surfaces of the samples after tensile test. The objectives of the research were to investigate the effects of RH on apple quality at room temperature with destructive and non-destructive methods. Not only weight loss, but also other important texture parameters such as ®rmness, juiciness and mealiness 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 groups with 100 in each group. The simulated shelf storage conditions were 300.2% RH, 650.2% RH and 950.2% RH at 200.18C and were maintained in three different chambers. The quality of the apples stored at 65 and 95% RH, 208C was monitored for 30 days. Apples under 30% RH, 208C were monitored for 18 days.
The physical properties of 10 apples from each storage condition were measured with non-destructive, destructive and analytical methods at 3-day intervals during the simulated shelf temperature storage.
2.2. Non-destructive acoustic measurements
The ®rmness of apples was determined by an acoustic impulse response technique. The equipment for acoustic impulse response measurement was the same as used by Chen and De Baerdemaeker (1993). The peak in the resonance frequency spectrum corresponding to the spherical mode shape of apple was recorded. The apple ®rmness was indicated by the stiffness factor which is calcu-lated asf2M2/3(fis the ®rst peak frequency in Hz;Mis the apple mass in g) and there exists 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,fwas represented by the mean value of three resonance frequencies measured at three marked locations separated about 1208around 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 content of apple samples. Apple ¯esh was cut with a cylindrical cutter and a cortical tissue sample of 17 mm diameter and 5 mm thickness was subjected to the compression test. The extractable juice content of the apple was measured as described by Tu and De Baerdemaeker (1997). With this method, the weight of ®lter paper (Whatman) was weighed before the compression. It was then placed on 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 was de®ned as weight gain (WafterÿWbefore) of the ®lter paper based on the initial
sample weight (Wsample) and referred as % of expressible ¯uid in the fruit.
The tensile strength of ¯esh tissue was measured by the method of Verlinden and De Baerdemaeker (1994), and Tu et al. (1996) which was regarded as an indicator of the appearance of apple mealy texture. A higher tensile strength re¯ects strong connections between apple cells and a lower tensile strength indicates weak cell connections, which often occurs when apples become overripe or mealy. The experimental arrangement can be seen in Fig. 1.
2.4. Analytical measurements
IAS of apple ¯esh was calculated following the formula suggested by Hat®eld and Knee (1988): IAS (%)1ÿ(fruit density/speci®c gravity of juice)100%. Fruit density can be calculated by apple weight/volume and the speci®c gravity of apple juice was the average of estimations using a pycnometer. Dry matter was determined according 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 (about 24 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 each test. One day later, the samples were subjected to critical point drying for further SEM. 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 dimethyl acetal (C3H8O2) solution for 45 min each. After ®xation, the samples were put
into a critical point dryer (Balzers, CPD 030, Liechtenstein) and then gold coated with a S150A sputter coater (Edwards, UK) at 3 min with a 20 mA current. After coating, 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 of tissue cells separated instead of rupturing under a tensile force in `Granny Smith' apple when fruits became mealy. In this SEM observation, fresh, slightly mealy and mealy ¯esh tissue of apples can also be distinguished based on the quantity of separated 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-sectionS1S2(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.
apples under 95% RH lost their ®rmness more slowly than the others. The stiffness factor decreased the fastest under 30% RH conditions. From the results of acoustic measurements it was found that ®rmness was maintained better at higher RH in both cultivars. The softening of apple tissue was considered to be associated with moisture loss and the loss of turgor pressure (Van den Berg, 1981). However, the measurement was not continued after 18 days for the apples stored under 30% RH conditions since they were rotting and were no longer acceptable 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 cause signi®cant wilting, low ®rmness, shrivelling and poor taste. Higher RH levels reduce apple weight loss, as can be seen in Fig. 3. The respiration rate can be 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 a change 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
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 decreased from 95 to 65% than from 65 to 30%. The greater vapour pressure de®cit may be the main reason of the faster loss of weight under 208C, 30% RH conditions.
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 RH conditions (Fig. 4). However, the differences in tensile rupture force between the three RH conditions were not signi®cant. The tensile rupture force of the `Braeburn' apples decreased linearly, but that of the `Jonagold' apples decreased exponentially. 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).
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 taste of fruits including apple.
It was found that for both apple cultivars, the non-destructive and destructive measured ®rmness results (data not shown) were correlated but the correlation was not very high (correlation coef®cient 0.55±0.8). Increase in apple weight loss was negatively correlated with the decrease of ®rmness.
Duncan's multiple range test (SAS Version 6.0, 1989) showed that RH had a signi®cant effect on some measured parameters. For both `Braeburn' and `Jonagold' apples, the weight loss, compression force, and stiffness factor under the three different RH conditions were signi®cantly different atP1%.
3.2. Analytical results
From Table 1 it can be seen that the fruit dry matter content increased during storage, 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
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
Braeburn 30 12.90.2 31.40.3 13.80.2 3.470.02 13.00.2 15.30.2 24.30.2 16.40.2 4.020.02 13.50.2
65 13.10.2 31.50.4 13.80.2 3.470.02 13.10.2 14.30.2 25.30.5 15.10.2 4.070.02 13.20.2
95 13.00.2 31.50.3 13.60.2 3.470.01 13.10.2 13.90.2 25.80.4 15.30.2 3.940.02 13.70.2
Jonagold 30 13.10.2 32.30.3 18.70.2 3.380.02 11.30.2 15.50.2 22.70.4 23.10.2 4.060.02 12.30.2
65 13.60.2 32.70.5 17.70.2 3.380.02 11.60.2 14.70.2 24.60.3 20.50.2 4.060.02 12.40.2
95 13.50.2 31.90.3 17.40.2 3.380.02 11.70.1 14.10.2 24.90.3 20.10.2 3.960.02 12.90.2
aMean value (SE,
n10), IAS: internal air spaces and SSC: soluble solid content.
corresponding to the highest water loss. The expressible juice content decreased after storage, but the apples kept at 95% RH had the highest expressible juice content. The SSC, pH, and IAS of the fruit increased during storage. However, the SSC did not change much, and the decrease in acidity (increased pH) may result in an increased sensory perception of sweetness.
Under 208C, 30% RH conditions, the IAS of both apple cultivars increased more than that under the other two storage conditions. This indicates that the cells of the apple tissue were connected less tightly under these conditions. The cells are more likely to separate when under an external force (i.e. tensile) and less cell content will be released so that apples will taste less juicy and more mealy.
3.3. Results of SEM observation
As seen in the SEM picture (Fig. 5), cells of fresh `Braeburn' apple break during the tensile test (Fig. 5a). As apples became ripe and overripe (turned mealy), fracture occurred between cells due to a breakdown of the inter-lamellar region rather than breakage of the cells themselves as more unbroken cells can be seen in Fig. 5b and c than in Fig. 5a. In Fig. 5c, the IAS was around 16%. The tensile strength of tissue was calculated (tensile rupture force/rupture area) to be as low as 0.06±0.08 MPa when apple cells showed obvious separation under tensile testing.
Similar behaviour was observed in the `Jonagold' apples (Fig. 6). Most cells were ruptured resulting in many broken surfaces being visible after tensile test on a fresh apple (Fig. 6a). As the apple became overripe after 4 weeks at 208C, more cell separation was observed (Fig. 6c at 95% RH) after tensile testing. This indicated that fewer cells ruptured under the same tensile strength and accordingly, less cell content was released which may lead to less extractable juice 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 it may introduce a mealy texture.
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 shelf temperature storage of apples. The 95% RH treatment maintained apple ®rmness and weight better but it tended to promote the development of a mealy texture that 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 about 65% RH. Combining these data with other experimental data on storage temperature and time, it is possible to further develop models of apple texture during post-harvest storage and shelf life.
Acknowledgements
This study was performed within the framework of FAIR (Food Agro-Industrial Research) project: FAIR1-CT95-0302. The authors wish to thank the European Community, the Research Council of Katholieke Universiteit Leuven and the Flemish minister of Science and Technology for the ®nancial support. Author Bart NicolaõÈ is postdoctoral research fellow with the Flemish Institute for Scienti®c Research.
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