IJERD – International Journal of Environmental and Rural Development (2022) 13-1

Research article
erd

Moisture Content Prediction of Dried Young Papaya during
Drying and Water Absorption
SOKLY SORM
Graduate School of Agriculture, Tokyo University of Agriculture, Tokyo, JapanYOSHIKI MURAMATSU*
Tokyo University of Agriculture, Tokyo, Japan
Email: y-murama@nodai.ac.jp

MASANORI HASHIGUCHI
Keisoku Engineering System Co., Ltd., Tokyo, JapanDAHAI MI
Keisoku Engineering System Co., Ltd., Tokyo, JapanEIICHIRO SAKAGUCHITokyo University of Agriculture, Tokyo, JapanSHOTARO KAWAKAMITokyo University of Agriculture, Tokyo, Japan
Received 14 January 2021

Accepted 6 December 2021(*Corresponding Author)

Abstract The effective use of agricultural products and the development of processed foods
with guaranteed safety and quality are needed in Kampong Cham Province, Cambodia.
Young papaya is an important agricultural product in this area. Dried young papaya is oneof the preserved foods. The main objective in this paper was to provide basic information
for the optimization of drying process. Two types of young papaya, raw or fresh papaya andblanched young papaya, were used in this study to investigate the influence of drying
pretreatment that is, blanching, on the changes in the moisture content during drying andwater absorption. The hot air-drying characteristics of each sample were measured at threetemperatures (30, 50, and 70°C), three air velocities (1, 2, and 3 m/s), and a relative
humidity of 40%. The examination of the effects of temperature, air velocity, and blanchingon the drying of samples resulted in the drying characteristics of the raw sample being very
similar to that of the blanched sample. A linear relationship existed in the moisture content
range between the initial moisture contents and 500% (d.b.). The exponential model wasapplied to predict the changes in the moisture contents of samples below500% (d.b.) at eachdrying condition. The drying rate constants in the period above or below500% (d.b.) wereincreased with increasing air temperature and air velocity and expressed as a linear functionof both temperature and air velocity, respectively. The water absorption characteristics ofthe dried young papaya and the dried blanched young papaya that were dried after
blanching were investigated at three temperatures (20, 30, and 40°C). Blanching before
drying of the sample had the effect of slowing the water absorption rate. The water
absorption rate constant tended to increase with increasing soaking temperature.
Keywords young papaya, drying characteristics, water absorption characteristics,
blanching, temperature and air velocity dependency,
prediction model of moisture contentⒸ ISERD

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IJERD – International Journal of Environmental and Rural Development (2022) 13-1

INTRODUCTION
The effective use of agricultural products and the development of processed foods with guaranteedsafety and quality are needed in Kampong Cham Province, Cambodia. Young papaya is an
important agricultural product in Kampong Cham. Recently, in Japan, young papaya has beenrecognized as a regional biological resource for the diversification of primary producers into
processing and distribution, and it is cultivated in the Kanto region.
Although young papayas can be eaten fresh, they can also be used after drying. Drying is oneof the oldest preservation methods and does not require special machines or devices in almost allcases. Drying not only increases the shelf life of the product but also increases its food value. Dried
young papaya has been sold as a processed food. Usually, dried young papayas are rehydrated tosome degree before eating. Pretreatments, for example, blanching, are often added to enhance thequality of dried vegetables. Blanching, a mild thermal treatment similar in temperature-time
intensity to pasteurization, is applied to fruits and vegetables primarily to inactivate enzymes thatcatalyse degradation reactions (Park et al., 2014). Lydia et al. (2017) investigated the effects ofpretreatments, including soaking in certain solutions and drying methods, on some qualities ofdried mango fruit.
Some researchers (EI-Aouar et al., 2003; Lagunez-Rivera et al., 2007; Fernando et al., 2008;
Lemus-Mondaca et al., 2009; Udomkun et al., 2015) have measured and analysed the drying
kinetics of papaya. Almost all previous papers have reported the drying characteristics of mature
papaya. However, very few studies have measured the hot air-drying characteristics of young
papaya with different air velocities, as performed in this study. In previous references (EI-Aouar etal., 2003; Lagunez-Rivera et al., 2007; Fernando et al., 2008; Lemus-Mondaca et al., 2009;
Muramatsu et al., 2012; Udomkun et al., 2015), some mathematical models (drying models) wereused to describe the relationship between the moisture content of the sample and drying time.
Optimum drying and water absorption models, including the values of parameters, are particularly
useful for easily predicting the changes in the moisture content of materials. Therefore,
determination of the optimum model is important for practical use.
In this study, we examined the hot air-drying characteristics of two types of young papaya:
raw and blanched young papaya at three different temperatures and air velocities. In addition, thewater absorption (rehydration) characteristics of dried and dried blanched young papaya were alsomeasured at three different temperatures. The results obtained in this study provide important basic
information and are needed to optimize drying processes and to design dryers.
OBJECTIVE
The objectives of this study were 1) to examine the effects of air temperature, air velocity, andblanching treatment on the drying characteristics of the sample, 2) to investigate the influence of
blanching treatment and soaking temperature on the water absorption characteristics of the sample,
3) to derive suitable mathematical drying and water absorption models to describe changes inmoisture content with time, and 4) to obtain empirical equations to express the relationshipbetween the drying or water absorption rate constants and the measurement conditions.
METHODOLOGY
Sample Preparation
Young papaya (Carica papaya) imported from Thailand was purchased at a supermarket in Tokyo,
Japan. Before the test, the young papayas were stored in a refrigerator at approximately 4°C. Afterthe peel was removed from the young papaya, the young papaya was cut into a rectangularparallelepiped (50 mm in depth, 20 mm in width, and 2.5 mm in height) by using a commercial
vegetable cutter. We call this sample the “raw sample” in this manuscript. For the blanchingtreatment, the raw young papayas were immersed in boiling water at 100°C for 30 s and cooled inⒸ ISERD

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IJERD – International Journal of Environmental and Rural Development (2022) 13-1

cold water at 0°C for 2 min. We describe this sample as the “blanched sample” in this manuscript.
Hot Air-drying Test
The changes in the moisture content of raw and blanched samples using the hot-air drying method
were measured at three temperatures (30, 50, and 70°C), three air velocities (1, 2, and 3 m/s), and a
relative humidity of 40%. A constant temperature and humidity chamber (KCL-2000, EYELA Co.,
Ltd.) was used as a drying chamber in this study to maintain air temperature and humidity during
the drying test. The air velocity was adjusted with a fan (CUDC 12B4, Japan Servo Co., Ltd.)
connected to a DC power source (AD-8735D, A&D Co., Ltd.). A stainless-steel basket (50 mm indepth, 20 mm in width, and 2.5 mm in height with a 3 mm aperture) was utilized as a sample tray.
Approximately 100 g of sample was placed on the sample tray and dried in the drying chamber.
The sample tray was removed out from the drying chamber at a preset time to measure the masschange of the sample. The mass change of the sample was measured with a digital balance (GX4000, A&D Co., Ltd.) and recorded at time intervals every 20 min for the initial 2 h and every 30
min from 2 h until the sample's moisture content became 15% (d.b.). After weighing the sample
mass, the sample tray was quickly returned to the drying chamber. When the moisture content of
the sample reached approximately 15% (d.b.), the mass of dry matter in the sample was determinedusing a forced hot air oven (VTEC-166, Isuzu Seisakusho Co., Ltd.) at 105°C for 24 h. Utilizing
the measured value of the mass of dry matter in the sample, the changes in mass during drying
were converted to changes in the moisture content of the sample.
Measurement of Water Absorption Characteristic
To measure the water absorption characteristics, the raw and blanched samples were dried by the
hot-air drying method at a temperature of 30°C, an air velocity of 3 m/s, and a relative humidity of
40% with a drying apparatus as previously mentioned. We describe the dried sample that wasadded to the blanching treatment before drying as the “dried blanched sample” in this manuscript.
Each dried sample was stored in a polyethylene bag and kept for 5-10 days at room temperature
before the water absorption test to ensure the uniform distribution of moisture in the bag andequilibrate the moisture content of the sample. The moisture content changes of the samples soaked
in water were measured at three different temperatures (20, 30, and 40°C). Approximately 1 g ofsample was immersed in a glass beaker containing 200 ml distilled water in a water bath at a preset
temperature. At time intervals every 1 min for the initial 6 min, 2 min from 6 to 10 min, 5 min from10 to 30 min, and then 30 min from 30-90 min, the sample was removed from the water, and thesurface of the sample was wiped with Kimwipes® (S-200, Nippon Paper Crecia Co., Ltd.) toremove residual liquid. These measurements were conducted until the moisture content of eachsample reached the same moisture content as fresh young papaya (approximately 94% (w.b.)). The
moisture content of the sample after soaking was determined by the oven method (105°C for 24 h).
RESULTS AND DISCUSSION
Hot Air-drying Characteristics
Fig. 1 (a) shows the changes in the moisture content of each sample at an air velocity of 3 m/s and atemperature of 30°C. The moisture contents of both samples decreased linearly with elapsed timefrom the initial moisture content to approximately 500% (d.b.) and decreased, exhibiting a gentle
downward curve below 500% (d.b.). As shown in Fig. 1 (b), the slope of the relationship between
the drying rate and moisture content at each sample was changed at approximately 500% (d.b.). Fig.
1 indicates that the drying characteristics, i.e., changes in the moisture content and drying rate of
the raw sample had almost the same trend as the blanched sample and the hot air-drying process ofboth samples had two periods, i.e., the first and second periods. Similar tendencies were obtainedfor other measurement conditions not shown in Fig. 1.
Ⓒ ISERD

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IJERD – International Journal of Environmental and Rural Development (2022) 13-1

In the first period (above 500% (d.b.)), the moisture contents decreased linearly with anincrease in elapsed time. Therefore, the measured data at each sample and each measurement
condition were fitted to Eq. (1).

M = −k1t + M 0(1)

where M: the moisture content (% (d.b.)), M0: the initial moisture content (% (d.b.)), k1: the drying
rate constant in the first period (1/min), and t: the drying time (min). The measured value of M0 andthe values of k1 determined by using the least squares method and the root mean square error(RMSE) are shown in Table 1. In the second period (below 500% (d.b.)), because the drying rate
decreased linearly with decreasing moisture content, the following exponential model (Muramatsu
et al., 2012) was used to analyse the changes in moisture content.

M − Me= exp − k2 ( t − tc ) 
500 − M e(2)

where Me: the equilibrium moisture content (% (d.b.)), k2: the drying rate constant in the second
period (1/min), and tc: the drying time at a moisture content of 500% (d.b.) (min). The value of tc in
Eq. (2) was calculated from Eq. (1). The measured drying data in the second period were fitted toEq. (2) using the nonlinear least squares method, and the values of k1 and Me were determined ateach sample and each measurement condition. The values of the parameters and RMSE of Eq. (2)are given in Table 2. The comparisons of the observed moisture contents with the results calculated
from Eqs. (1) and (2) at an air velocity of 3 m/s for the raw sample are shown in Fig. 2. The solidlines in Fig. 2 show the results calculated from Eq. (1) or (2). As shown in Fig. 2, the measured
results matched well with the calculated results. Under all measurement conditions, the changes in
the moisture content of the sample caused by hot air-drying could be estimated by Eqs. (1) and (2).

Fig. 1 Drying curve of each sample at an air velocity of 3 m/s and a temperature of 30°C(a) Changes in moisture content and (b) drying characteristics curves

Fig. 2 Comparisons of observed moisture content changes with the results calculated from(a) Eq. (1) and (b) Eq. (2) at air velocity of 3 m/s for the raw sample during dryingⒸ ISERD

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IJERD – International Journal of Environmental and Rural Development (2022) 13-1

Fig. 3 Relationships between the drying rate constants in the first period (above a moisture
content of 500 % (d.b.)) of the raw sample and (a): temperature and (b): air velocity
Table 1 Initial moisture content (M0), drying rate constant in the first period (above 500 %(d.b.)) (k1), and root mean square error (RMSE) of Eq. (1) under each set ofmeasurement conditions
SampleAir velocity(m/s)

Temperature(°C)

M0(% (d.b.))k1(1/min)RMSE(% (d.b.))

30
1525
7.572
19.2
50
1625
12.23
20.9
70
1615
16.03
2.6
2
30
1359
8.909
25.8
50
1253
12.87
14.7
70
1331
17.83
3.0
3
30
1643
13.76
47.0
50
1797
22.63
54.3
70
1800
29.92
20.0
1
30
1926
9.189
27.8
Blanched sample
50
1963
14.35
31.6
70
2008
19.77
10.2
2
30
1807
10.69
63.2
50
1576
15.12
12.9
70
1661
21.51
2.2
3
30
1794
15.31
39.3
50
1896
23.82
47.3
70
2302
37.15
18.1
The values of M0 were measured data. The values of k1 were determined by the least squares method based on themeasured results in the first period (above 500 %(d.b.)). The values of RMSE were calculated from the measured
results and the results calculated from Eq. (1).

Raw sample

1

As shown in Tables 1 and 2, the drying rate constants varied with temperature and air velocity.
Figure 3 represents the temperature and air velocity dependencies of k1 for the raw sample.
Although the values of k1 for both samples were not much difference between 1 m/s and 2 m/s, thevalues of k1 at 2 m/s were higher than that at 1 m/s for each temperature. The first period (above
500% (d.b.)) was considered as the apparent falling drying rate period including the sample
shrinkage, i.e., the constant drying rate period from the measurement results of changes in thesample surface temperature. In the constant drying rate period, the drying or water vaporization ratedepends on the heat transfer coefficient, the thicknesses of boundary layers for temperature and air
velocity, and so on. Since there was not much difference in the values of heat transfer coefficientand the thicknesses of boundary layers between 1 m/s and 2 m/s, the effect of air velocity on thedrying rate in the first period might be weak below 2 m/s. The k1 and k2 of each sample increased
almost linearly with the increase in air velocity at each temperature and the increase in thetemperature at each air velocity. Therefore, we derived the following empirical equation to express
the temperature and air velocity dependencies.

k1 or 2 = aTV + bT + cV + d(3)Ⓒ ISERD

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IJERD – International Journal of Environmental and Rural Development (2022) 13-1

where T: temperature (°C), V: air velocity (m/s), a, b, c, d: constant. The regression results for Eq.
(3) are shown in Table 3. The solid lines in Fig. 3 are the results calculated from Eq. (3). The k1 andk2 values of each sample were represented as a function of both temperature and air velocity by Eq.
(3).
Table 2 Drying time at a moisture content of 500 % (d.b.) (tc), drying rate constant in thesecond period (below 500 %(d.b.)) (k2), equilibrium moisture content (Me), and root
mean square error (RMSE) of Eq. (2) under each set of measurement conditions
Air velocity(m/s)

SampleTemperature
(°C)

tc
(min)

k2
(1/min)Me(% (d.b.))

RMSE(% (d.b.))30
135
1.224×10-2
16.09
11.2
50
92
2.058×10-2
9.542
14.2
70
70
2.940×10-2
5.132
14.0
2
30
96
1.708×10-2
14.37
12.8
50
59
2.549×10-2
8.183
12.0
70
47
3.649×10-2
5.153
16.2
3
30
83
2.052×10-2
13.36
16.0
50
57
3.438×10-2
8.604
11.8
70
43
5.157×10-2
5.226
16.1
1
30
155
1.136×10-2
13.65
23.0
Blanched sample
501021.845×10-2
5.345
21.9
70
76
2.785×10-2
0.02047
24.5
2
30
122
1.759×10-2
14.63
9.9
50
71
2.302×10-2
5.895
18.8
70
54
3.315×10-2
0.488123.6
33085
1.931×10-2
11.37
18.4
50
59
3.643×10-2
9.005
10.0
70
49
4.249×10-2
5.628
26.7
The values of tc were calculated from Eq. (1) by using the values of M0, k1, and M=500. The values of k2 and Me were
determined by the nonlinear least squares method based on the measured results in the second period (below
500 %(d.b.)). The values of RMSE were calculated from the measured results and the results calculated from Eq. (2).
1

Raw sample

Table 3 Values of the parameters and root mean square error (RMSE) of Eq. (3) for eachdrying rate constant and sample
Drying rate
constant(1/min)k1
k2

Samplea(s/min･°C･m)b(1/min･°C)c(s/min･m)d(1/min)RMSE(1/min)Raw
Blanched
Raw
Blanched

9.626×10-2
1.407×10-1
1.736×10-4
7.535×10-5

8.708×10-2
7.896×10-2
2.163×10-4
3.151×10-4

2.687×10-1-1.541-1.305×10-3
2.234×10-3

1.238
3.613
1.962×10-3-1.735×10-3

2.052
2.469
1.438×10-3
2.251×10-3

The values of a, b, c, and d were determined by the least squares method based on the values of k 1 and k2 shown in Tables
1 and 2. The values of RMSE were calculated from the values of drying rate constant (k 1 or k2) shown in Table 1 or 2 andthe results calculated from Eq. (3).

Water Absorption Characteristics
The moisture content of dried and dried blanched samples increased with the elapsed time,
exhibiting a gentle upward curve from the beginning of soaking at all three temperatures, as shownin Fig. 4. The water absorption rate of dried blanched samples was slower than that of driedsamples at each temperature. The dried blanched sample was harder and had a smaller size than thedried sample. The difference in organizational structure for the samples after drying could becaused by the difference in water absorption characteristics. The exponential model Eq. (4)(Tagawa, et al. 1997) was used to analyse the changes in moisture content from the initial moisture
content of 15% (d.b.).

M − Ms= exp ( − k3tWA )15 − M s(4)Ⓒ ISERD

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IJERD – International Journal of Environmental and Rural Development (2022) 13-1

where Ms: saturated moisture content (% (d.b.)), k3: water absorption rate constant (1/min), and tWA:soaking time (min). The solid lines and broken lines in Fig. 4 represent the results calculated from
Eq. (4). The values of k3, Ms, and RMSE of Eq. (4) are shown in Table 4.
The k3 of each sample increased with increasing soaking temperature and was related tosoaking temperature by the following Arrhenius equation.

k3 = d  exp − E R (T + 273.15)

(5)

where R: ideal gas constant 8.314 (J/mol･K), d, E: constant. The values of d and E were 6.047×102
and 2.477×105 for the dried sample and 6.444×103 and 3.278×105 for the dried blanched sample,
respectively.
Table 4 Water absorption rate constant (k3), saturated moisture content (Ms),and root mean square error (RMSE) of Eq. (4) for each sample
SampleRaw

Blanched

Temperature
(°C)
20
30
40
20
30
40

k3
(1/min)2.417×10-2
2.994×10-2
4.640×10-2
9.129×10-3
1.499×10-2
2.153×10-2

Ms(% (d.b.))
2572
2748
2525
3549
2999
2696

RMSE
(% (d.b.))
43.8
42.1
37.6
35.0
38.0
59.3

The values of k3 and Ms were determined by the nonlinear least squares method based on the measured
results. The values of RMSE were calculated from the measured results and the results calculated from Eq.
(4).

Fig. 4 Changes in moisture contents during water absorption for each sample
CONCLUSION
The hot-air drying characteristics of two types of young papaya, raw or fresh papaya and blanchedyoung papaya, were measured at three temperatures (30-70°C), three air velocities (1-3 m/s), and a
relative humidity of 40%. The drying characteristics of both samples had almost the same tendency.
The linear function and the exponential model were used to express the changes in the moisture
content of the sample with drying times above or below 500% (d.b.), respectively. The drying rate
constant of each sample and each drying period (above or below 500% (d.b)) increased with anincrease in air temperature and air velocity and was expressed as a linear function of both airtemperature and air velocity, respectively.
The water absorption characteristics of the dried young papaya and the dried blanched young
papaya were measured at three temperatures (20, 30, and 40°C). The water absorption rate of dried
young papaya that did not receive the blanching treatment before drying was faster than that of theⒸ ISERD

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IJERD – International Journal of Environmental and Rural Development (2022) 13-1

sample that was blanched before drying (dried blanched sample). The exponential model could beapplied to explain the water absorption process of both samples. The water absorption rate constantincreased with increasing soaking temperature.
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