Directory UMM :Data Elmu:jurnal:S:Scientia Horticulturae:Vol85.Issue3.Agust2000:
Scientia Horticulturae 85 (2000) 217±229
Effects of relative humidity on apple quality under
simulated shelf temperature storage
K. Tua,*, B. NicolaõÈb, J. De Baerdemaekera
a
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.
PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 1 4 8 - X
218
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
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.
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
219
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 calculated as f 2M2/3 (f is the ®rst peak frequency in Hz; M is 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 experiment, f was represented by the mean value of three resonance frequencies measured
at 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 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
The soluble solids content (SSC) of apple juice was measured with a hand-held
refractometer (0±85%; Zeiss, Germany) and the pH of the juice was determined
with a pH meter (ORION, model 250A, USA) following a compression test. The
220
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
Fig. 1. Device for apple tensile strength measurement. Cross-section S1S2(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.
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
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
221
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).
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.
222
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
Fig. 3. Weight loss of `Braeburn' and `Jonagold' apples at 208C and different RH (The bars
indicate standard errors).
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
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
223
Fig. 4. Tensile failure force changes 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 at P1%.
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
224
Cultivar
RH (%)
Fresh
After storage
Dry matter
(%)
Expressible IAS (%)
juice
content (%)
pH
SSC (%)
Dry matter
(%)
Expressible IAS (%)
juice
content (%)
pH
SSC (%)
Braeburn
30
65
95
12.90.2
13.10.2
13.00.2
31.40.3
31.50.4
31.50.3
13.80.2
13.80.2
13.60.2
3.470.02 13.00.2
3.470.02 13.10.2
3.470.01 13.10.2
15.30.2
14.30.2
13.90.2
24.30.2
25.30.5
25.80.4
16.40.2
15.10.2
15.30.2
4.020.02 13.50.2
4.070.02 13.20.2
3.940.02 13.70.2
Jonagold
30
65
95
13.10.2
13.60.2
13.50.2
32.30.3
32.70.5
31.90.3
18.70.2
17.70.2
17.40.2
3.380.02 11.30.2
3.380.02 11.60.2
3.380.02 11.70.1
15.50.2
14.70.2
14.10.2
22.70.4
24.60.3
24.90.3
23.10.2
20.50.2
20.10.2
4.060.02 12.30.2
4.060.02 12.40.2
3.960.02 12.90.2
a
Mean value (SE, n 10), IAS: internal air spaces and SSC: soluble solid content.
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
Table 1
Some characteristics of `Braeburn' and `Jonagold' apples before and after storage for 4 weeks at 208C and at three different RH levelsa
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
225
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.
Upon mastication in the mouth, this type (overripe) of breakdown
probably leads to an accumulation of clumps of intact cells which in turn
are responsible for a rough and grainy sensory impression. Surface structure of
the breakdown particles is important for mealiness perception. Although
mealiness is usually observed only in ripe, soft apples whose tissue strength is
already low, it is the type (separation between cells) rather than the extent
of failure (rupture) which determines whether an apple is perceived as mealy
or not (Harker and Hallett, 1992). They found similar results of cell separation
of overripe fruits based on observation of late picked `Braeburn' apples in
cold storage.
226
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
Fig. 5. SEM pictures of `Braeburn' apples (60, 20 kV, (a) fresh apple; (b) slightly mealy; (c) mealy apple) at 208C and 95% RH.
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
Fig. 6. SEM pictures of `Jonagold' apples (60, 20 kV, (a) fresh apple; (b) slightly mealy; (c) mealy apple) at 208C and 95% RH.
227
228
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
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.
References
AOAC1, 1997. Of®cial Method of Analysis of Association of Of®cial Analytical Chemists
(AOAC) International, 16th Edition, Dr. Patricia A. Cunniff (Ed.).
Armstrong, P., Zapp, H.R., Brown, G.K., 1990. Impulsive excitation of acoustic vibrations in apples
for ®rmness determination. Trans. ASAE 33 (4), 1353±1359.
Chen, H., De Baerdemaeker, J., 1993. Effect of apple shape on acoustic measurements of ®rmness.
J. Agric. Eng. Res. 56, 253±266.
Harker, F.R., Hallett, I.C., 1992. Physiological changes associated with development of mealiness
of apple fruit during cool storage. HortScience 27, 1291±1294.
Harker, F.R., Stec, M.G.H., Hallett, I.C., Bennett, C.L., 1997. Texture of parenchymatous plant
tissue: a comparison between tensile and other instrumental and sensory measurements of tissue
strength and juiciness. Postharvest Biol. Technol. 11 (2), 63±72.
Hat®eld, S.G.S., Knee, M., 1988. Effects of water loss on apples in storage. Int. J. Food Sci. Tech.
23, 575±583.
Landrigan, M., Morris, S.C., Gibb, K.S., 1996. Relative humidity in¯uences postharvest browning
in Rambutan (Nephelium lappaceum L.). HortScience 31 (3), 417±418.
Mohsenin, N.N., 1986. Physical Properties of Plant and Animal Materials, 2nd Edition. Gordon and
Breach, New York.
Peleg, K., 1985. Produce, Handling, Packaging and Distribution. The AVI publishing company Inc.,
Westport, Connecticut.
SAS Institute Inc., SAS Version 6.0, 1989. SAS User's guide. SAS Campus Drive, Cary, NC,
USA.
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Tu, K., De Baerdemaeker, J., 1997. A study on prestorage heat treatment effect on apple texture:
destructive and non-destructive measurements. J. Food Processing Preser. 21 (6), 495±506.
Tu, K., De Baerdemaeker, J., Deltour, R., De Barssy, T., 1996. Monitoring post-harvest quality of
Granny Smith apple under simulated shelf life conditions: destructive, non-destructive and
analytical measurements. Int. J. Food Sci. Tech. 31, 267±276.
Van den Berg, L., 1981. The role of humidity, temperature and atmospheric composition in
maintaining vegetable quality during storage. ACS Symp. Ser. 170, 95.
Verlinden, B.E., De Baerdemaeker, J., 1994. Development and testing of a tensile method for
measuring the mechanical properties of carrot tissue during cooking. In: World Congress and
AgEng `94 Conference on Agricultural Engineering, 29 August±1 September, Milan, Italy,
1994, pp. 874±875.
Verstreken, E., De Baerdemaeker, J., 1994. In¯uence of different storage conditions on the
evolution of the maturity of nectarines. Acta Horticulturae 368, 43±50.
Weichmann, J. (Ed.), 1987. Postharvest Physiology of Vegetables. Marcel Dekker, New York and
Basel, ISBN 0-8247-7601-1.
Effects of relative humidity on apple quality under
simulated shelf temperature storage
K. Tua,*, B. NicolaõÈb, J. De Baerdemaekera
a
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.
PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 1 4 8 - X
218
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
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.
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
219
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 calculated as f 2M2/3 (f is the ®rst peak frequency in Hz; M is 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 experiment, f was represented by the mean value of three resonance frequencies measured
at 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 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
The soluble solids content (SSC) of apple juice was measured with a hand-held
refractometer (0±85%; Zeiss, Germany) and the pH of the juice was determined
with a pH meter (ORION, model 250A, USA) following a compression test. The
220
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
Fig. 1. Device for apple tensile strength measurement. Cross-section S1S2(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.
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
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
221
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).
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.
222
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
Fig. 3. Weight loss of `Braeburn' and `Jonagold' apples at 208C and different RH (The bars
indicate standard errors).
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
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
223
Fig. 4. Tensile failure force changes 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 at P1%.
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
224
Cultivar
RH (%)
Fresh
After storage
Dry matter
(%)
Expressible IAS (%)
juice
content (%)
pH
SSC (%)
Dry matter
(%)
Expressible IAS (%)
juice
content (%)
pH
SSC (%)
Braeburn
30
65
95
12.90.2
13.10.2
13.00.2
31.40.3
31.50.4
31.50.3
13.80.2
13.80.2
13.60.2
3.470.02 13.00.2
3.470.02 13.10.2
3.470.01 13.10.2
15.30.2
14.30.2
13.90.2
24.30.2
25.30.5
25.80.4
16.40.2
15.10.2
15.30.2
4.020.02 13.50.2
4.070.02 13.20.2
3.940.02 13.70.2
Jonagold
30
65
95
13.10.2
13.60.2
13.50.2
32.30.3
32.70.5
31.90.3
18.70.2
17.70.2
17.40.2
3.380.02 11.30.2
3.380.02 11.60.2
3.380.02 11.70.1
15.50.2
14.70.2
14.10.2
22.70.4
24.60.3
24.90.3
23.10.2
20.50.2
20.10.2
4.060.02 12.30.2
4.060.02 12.40.2
3.960.02 12.90.2
a
Mean value (SE, n 10), IAS: internal air spaces and SSC: soluble solid content.
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
Table 1
Some characteristics of `Braeburn' and `Jonagold' apples before and after storage for 4 weeks at 208C and at three different RH levelsa
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
225
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.
Upon mastication in the mouth, this type (overripe) of breakdown
probably leads to an accumulation of clumps of intact cells which in turn
are responsible for a rough and grainy sensory impression. Surface structure of
the breakdown particles is important for mealiness perception. Although
mealiness is usually observed only in ripe, soft apples whose tissue strength is
already low, it is the type (separation between cells) rather than the extent
of failure (rupture) which determines whether an apple is perceived as mealy
or not (Harker and Hallett, 1992). They found similar results of cell separation
of overripe fruits based on observation of late picked `Braeburn' apples in
cold storage.
226
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
Fig. 5. SEM pictures of `Braeburn' apples (60, 20 kV, (a) fresh apple; (b) slightly mealy; (c) mealy apple) at 208C and 95% RH.
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
Fig. 6. SEM pictures of `Jonagold' apples (60, 20 kV, (a) fresh apple; (b) slightly mealy; (c) mealy apple) at 208C and 95% RH.
227
228
K. Tu et al. / Scientia Horticulturae 85 (2000) 217±229
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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