IJERD – International Journal of Environmental and Rural Development (2021) 12-2

Research article
erd

Establishment of Optimized Manufacturing Conditions for
Cooked Rice -Part II- Moisture Transfer Kinetics of Cooked
Rice when Drying and Soaking in Water
YOSHIKI MURAMATSU*
Faculty of Regional Environment Science, 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, JapanSOKLY SORM
Graduate School of Agriculture, Tokyo University of Agriculture, Tokyo, JapanEIICHIRO SAKAGUCHI
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, JapanSHOTARO KAWAKAMI
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, Japan
Received 6 November 2020

Accepted 15 October 2021(*Corresponding Author)

Abstract The thin-layer drying characteristics of cooked milled rice during hot air drying
were measured at four temperatures (30, 40, 50, and 60 °C) and at a relative humidity of 40%.
The hot air-drying process of the sample was composed of the first falling rate, and theexponential model was applied to predict the changes in moisture content of the sample ateach temperature. The drying rate constant of the sample increased as temperature increased
and was expressed as an Arrhenius-type equation. The water absorption characteristics ofdried cooked milled rice when soaking in water were examined at four temperatures (70, 80,
90, and 98 °C). The ratios of the changes in moisture content from 8 to 150% (d.b.) wereconverted from the data of changes in moisture content. The first-order reaction rate equation
could be applied to explain the water absorption process of samples. The water absorption
rate constant had a tendency to increase with increasing soaking temperature and was
expressed as a function of soaking temperature by an Arrhenius-type equation.
Keywords cooked milled rice, hot air drying, dried milled rice, water absorption, exponential
model, first-order rate constant

INTRODUCTION
The development of processed food made from rice at production location is expected to lead toregional revitalization and increased income for farmers. A simply processed food made from rice isdried cooked rice or pregelatinized rice. A general manufacturing process of dried cooked rice is asfollows: 1) washing raw rice, 2) soaking the washed rice in water, 3) cooking or steaming the rice(the raw rice starch is changed to pregelatinized starch), 4) separating the rice kernel from the massof cooked rice to a single cooked rice, and 5) drying the cooked rice. The dried cooked rice is usuallyrehydrated with water or hot water and then eaten.
Recently, food that can be kept in case of emergency has been developed and manufactured inJapan to prevent food shortage in the case of natural disasters and infectious diseases. The interest in
dried cooked rice has increased as it can be used in preserved food, portable ration food, andⒸ ISERD

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emergency provisions. In addition, dried cooked rice has been used as a material in instant foodproducts, for example, an instant pot dried rice similar to an instant pot noodle; thus, the market ofdried cooked rice products has continued to increase. The data of the drying characteristics,
equilibrium moisture content, latent heat of vaporization for cooked rice, and water absorption(rehydration) characteristics of dried cooked rice are required for optimizing processing operations,designing equipment, and ensuring high quality.
We measured the equilibrium moisture content of the cooked rice, the hot air-drying
characteristics of the cooked rice, and the water absorption characteristics of the dried cooked rice.
This basic information is needed to establish the optimized manufacturing conditions for cooked rice.
The latent heat of vaporization of water for cooked rice was calculated based on the thermodynamic
theory by using the equilibrium moisture content data. The moisture transfer kinetics during thedrying and the water absorption was analyzed based on the drying theory of the first falling rate
period or the first-order reaction rate theory. We had reported the equilibrium moisture content andthe latent heat of vaporization of water in part I (Muramatsu et al., 2021). The equilibrium moisture
contents are needed to analyze the hot air-drying characteristics in addition to examining foodpreservation. Because the changes in moisture content occur during the drying and the water
absorption process, kinetic analysis to predict the changes in moisture content are required to
optimize the processing conditions. The moisture transfer kinetics was reported in this Part II. The
equilibrium moisture content data reported in Part I was used in the analysis of the moisture transfer
kinetics during drying.
Ramesh and Rao (1996) examined the drying of cooked rice (high amylose variety rice) with avibro-fluidized bed drier. Luangmalawat et al. (2008) reported on the effect of temperature on thedrying characteristics and quality of cooked rice (Jasmin rice: long grain rice, indica rice). Jiao et al.
(2014) used convective hot air, microwave, and combined microwave-hot air drying techniques forthe drying of cooked rice (hybrid indica rice) and evaluated the efficacy of these techniques. Jiao etal. (2014) also investigated the effect of microwave power and air temperature on the drying andrehydration kinetics of dried cooked rice and examined the changes in the color values of the productafter drying. Koide et al. (2016) measured the changes in the moisture content of cooked rice during
hot air drying and changes in the moisture content of dried cooked rice during water adsorption.
Almost all previous papers have reported the drying characteristics of indica rice. Additionally, thedrying characteristics of many kinds of food have been measured, and many mathematical models,
are used to describe the drying process of food. 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, knowing the optimum model is important for practical use.
In this study, we examined the thin-layer hot air-drying characteristics of cooked milled rice
(short grain rice, japonica) at four different temperatures. In addition, the water absorption(rehydration) characteristics of dried cooked milled rice were also measured at four different
temperatures.
OBJECTIVE
The objectives of this study were 1) to examine the hot air-drying and the water absorption
characteristics at several temperatures, 2) to derive a suitable mathematical drying and water
absorption models to describe changes in moisture content, and 3) to evaluate the relationshipbetween the drying rate constant or the water absorption rate constant and temperature.
METHODOLOGY
SampleNon-glutinous rough rice (cv. “Hoshinoyume”) was purchased from a Japanese agricultural
cooperative in Tohma, Hokkaido, Japan (JA Tohma) and was stored in a refrigerator at approximately
5 °C until the test. The “Hoshinoyume” was widely cultivated in Hokkaido when we conducted thisⒸ ISERD

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research because this rice had a good eating quality or tasty. This non-glutinous rough rice was firsthulled with a roll-type rice huller (THU35B, Satake Co., Ltd.). Brown rice (800 g) was put into anabrasive-type (stir-type) rice-milling machine (SKM-5B (1), Satake Co., Ltd.) and milled for 5.0 min
to produce milled rice. The degree of milling (or milling yield) was adjusted to be approximately
92%.
The milled rice was cooked with commercial rice cookers (NP-FB10, Zojirushi Co., Ltd.) in
accordance with the operation manual of the rice cooker. To uniformly gelatinize the rice, the rice
was soaked in tap water at room temperature for 1 h before cooking. The cooking time was 45-60
min and the maximum temperature in the rice cooker was around 104 °C. To obtain a single grain of
cooked rice from the mass of cooked rice, the cooked rice was rinsed with tap water for 3 s, and theexcess water was drained. After the rinse, the surface of the cooked rice was wiped with Kimwipes ®
(S-200, Nippon Paper Crecia Co., Ltd.) to remove excess water.
Cooked milled rice was used for measuring the drying characteristics. Dried cooked milled rice
was used as a sample for the water absorption test. For the preparation of this dried milled rice,cooked milled rice was dried at a temperature of 40 °C and a drying time of 6 h with a commercial
hot air dryer (LH-103D, Satake Co., Ltd.).
Hot Air Drying Test

Fig. 1 Schematic of the hot air-drying apparatus
The changes in the moisture content of the cooked rice using the hot air-drying method were
measured at four temperatures (30, 40, 50, and 60 °C) and at a relative humidity of 40 %. Fig. 1
shows a schematic of the hot air-drying apparatus used in this study. The apparatus consisted of threeunits: a drying chamber at constant temperature and humidity, a ventilation unit, and a recording unit.
A constant temperature and humidity chamber (IG420, Yamato Scientific Co., Ltd.) was used as thedrying chamber. The drying chamber dimensions were a width of 500 mm, a length of 390 mm, anda height of 600 mm. The temperature and relative humidity in the drying chamber were measured
with a T-type thermocouple and a relative humidity sensor, and recorded with a data logger (GL200,
GRAPHTEC Co., Ltd.). The airflow rate in the chamber was adjusted to 2.0 m/s by a fan (CUDC
12B4, Japan Servo Co., Ltd.) using a DC power source (model 526, Metronix Co., Ltd.). The DC
power source was controlled with a timer (ELT-3, AS ONE Co., Ltd.), and the airflow was stopped
when the sample mass was measured. A grid screen was fitted below the fan to straighten the air
velocity pattern. Changes in mass, i.e., the changes in moisture content, when drying were measured
using a digital balance (EK-300i, A&D Co., Ltd.), and the changes in sample mass wereautomatically recorded on a personal computer. A stainless steel basket with a diameter of 100 mm,
a height of 15 mm, and a 3.0 mm aperture, was utilized as a sample tray.

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The sample (approximately 10 g) was placed on the tray. The drying test was terminated whenthe moisture content of the sample was approximately 15 %, on a dry basis (d.b.). The final moisture
content of the sample was determined using a forced hot air oven (DX-600, Yamato Scientific Co.,
Ltd.) at 135 °C for 24 h. Through the use of measured values of final moisture content, the changes
in mass when drying were converted to the moisture content of the sample.
Measurement of Water Absorption Characteristics
Moisture content changes of the dried cooked rice when soaking in water were measured at fourwater temperatures (70, 80, 90, and 98 °C). Approximately 5 g of sample was put into a sample net
after weighing the sample mass with a digital balance (FX-300, A&D Co., Ltd.), and soaked in awater bath (SH-12, TAITEC Co., Ltd.). The sample net was removed from the water bath at eachpreset soaking time (1.0, 2.0, and 5 min intervals), and the surface of the sample was wiped with
Kimwipes® (S-200, Nippon Paper Crecia Co., Ltd.) to remove residual liquid. The sample was thenreweighed. The increase in sample mass when soaking in water was considered to be an increase in
the moisture content of the sample. The moisture content of the sample after soaking was determinedfrom the values of the initial moisture content of the dried cooked rice and the mass changes datawhen soaking in water.
RESULTS AND DISCUSSION
Hot Air-Drying Characteristics
The moisture content of cooked rice decreased over elapsed time and exhibited a gentle downward
curve from the beginning of the drying process at each temperature. The changes in the moisture
content of the sample at four different temperatures (30-60°C) and at a relative humidity of 40% areshown in Fig. 2. The drying rate (% (d.b.)/h) of the sample was obtained by numerical differentiation(Gregory-Newton forward method) using the moisture content measurements. The drying rate of thesample increased with an increase in temperature. The linear relationship between the drying rate
and the free moisture content (|M-Me|) was found at each temperature. The above result indicates thatthe hot air-drying process of the sample comprises the first falling rate drying period.
Therefore, the measured drying data were fitted to the following exponential model (Eq. (1))(Koide et al., 2016) using the least squares method, and the values of the drying rate constant (k1)were determined at each temperature.

M − Me= exp ( −k1t )M0 − Me(1)

Where M: the moisture content at each preset time (% (d.b.)), M0: the initial moisture content(% (d.b.)), Me: the equilibrium moisture content (% (d.b.)), k1: the drying rate constant (h-1), and t:
the time (h). The values of Me in Eq. (1) was also calculated from following the equation (Muramatsu
et al., 2021). at each measurement condition.



rh = exp −50.61T −0.3675 exp ( −2.577  10−7 T 2.326 M e )(2)

The values of k1 and the root mean squared error (RMSE) of Eq. (1) are given in Table 1. The
solid line in Fig. 2 shows the results calculated from Eq. (1). 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 all samples caused by hot air drying could be estimated by Eq. (1).
The temperature dependency of k1 for grain (Murata, 1982) and beans (Muramatsu et al., 2007)
was expressed by the following Arrhenius-type equation:Ⓒ ISERD

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IJERD – International Journal of Environmental and Rural Development (2021) 12-2 b 
k1 = a1  exp  − 1 
 T

(3)

where a1 and b1 are parameters. Eq. (3) shows that the relationship between k1 and the reciprocal ofabsolute temperature (1/T) is approximately linear in a semilogarithmic plot. The values of k1 shownin Table 1 were fitted by the least squares method to Eq. (3). The values of the parameters and the
RMSE of Eq. (3) were: a1 = 772.5, b1 = 1838, and RMSE = 9.081×10-2. Eq. (3) was applicable for
examining the relationship between the drying rate constant and the temperature of the sample.

Fig. 2 Changes in the moisture content of the sample with drying
Table 1 Parameters in Eq. (1)Temperature (°C)
30
40
50
60

M0 (% (d.b.))
184.4
198.5
183.5
184.6

k1 (h-1)
1.746
2.311
2.506
3.115

Me (% (d.b.))
12.5
11.6
10.7
9.89

RMSE (% (d.b.))1.338
0.89951.052
0.6286

Water Absorption Characteristics
The initial moisture contents of the dried cooked rice were approximately 8 % (d.b.). The water
absorption curves of the sample are shown in Fig. 3. The moisture content of the sample increased
over time, exhibiting a gentle upward curve from the beginning of soaking, and did not approach
equilibrium under these measurement conditions. The water absorption rate of the sample increased
with an increase in soaking water temperature and on the other hand decreased with an increase insoaking time at each soaking temperature.
The moisture content of cooked rice (nonglutinous japonica rice) is usually approximately 65 %(w.b.), i.e., 150 % (d.b.). In this study, to evaluate the water absorption characteristics of the sample,
the ratios of the change in moisture content (Kubota, 1990) from 8 to 150 % (d.b.) (x) were calculated
from Eq. (4).

x=

M − M0M −8=
M T − M 0 150 − 8(4)Where x: the ratios of the change in moisture content from 8 to 150 % (d.b.), and MT: the target
or final moisture content (% (d.b.)). The values of x change from 0 to 1, and when “x = 1”, themoisture content of the sample has reached 150 % (d.b.).
The tendency of the changes in the values of x had the same with changes in the water absorption
rate. The values of x for the sample increased exponentially with an increase in soaking time.
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Therefore, the following first-order reaction rate equation (Kubota, 1990) was applied for analyzing
the water absorption characteristics of the sample.

dx= k2 (1 − x )dt(5)Where k2 is the water absorption rate constant (min-1). The unit of time (t) in Eq. (5) is “min”.
The values of k2 determined by the least squares method and the RMSE of Eq. (5) for eachtemperature are shown in Table 2. From the RMSE values, it was confirmed that the measured values
agreed well with those calculated from Eq. (5); thus, the ratios of changes in the moisture content of
the sample between 8 and 150 % (d.b.) could be estimated by Eq. (5).
The values of k2 shown in Table 2 increased with an increase in soaking temperature. The valuesof k2 were expressed as a function of temperature by using the following Arrhenius-type equation(RMSE = 0.04288).

 2606 
k2 = 681.0  exp  −

T 


(6)

Fig. 3 Changes in the moisture content of the sample during water absorption
Table 2 Parameters in Eq. (4)Temperature (°C)
70
80
90
98

k2 (min-1)
0.33400.46500.46630.6374

RMSE (-)
0.058930.054160.06033
0.05380

CONCLUSION
The thin-layer drying characteristics during hot air drying of cooked rice were measured at several
temperatures and relative humidity levels. The hot air-drying process of the sample was composedof the first falling rate, and the exponential model was applicable to predict the changes in moisture
content of the sample at each temperature. The drying rate constant of the sample increased as
temperature increased and was related to temperature by an Arrhenius-type equation.
The water absorption characteristics of dried cooked rice when soaking in water were measured
at four temperatures. The first-order reaction rate equation could be applied to explain the water
absorption process of samples. The water absorption rate constant was expressed as a function ofsoaking temperature by an Arrhenius-type equation.

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The changes in moisture content of the cooked rice during drying and the dried cooked rice
during water absorption can be easily predicted by using the drying model and the water absorption
model, respectively. These models will be useful in the determination of the endpoints or treatment
time for drying or water absorption. This information saves the input energy and the treatment time
and would lead to the establishment of optimized manufacturing conditions for cooked rice.
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Jiao, A., Xu, X. and Jin, Z. 2014. Modelling of dehydration-rehydration of instant rice incombined microwavehot air drying. Food and Bioproducts Processing, 92 (3), 259-265.
Koide, S., Orikasa, T., Koide, F., Muramatsu, Y. and Tagawa, A. 2016. Moisture content prediction of driedcooked rice during drying, preservation, and water absorption. J. Japan Assoc. Food Preservation Scientists,
42 (2), 59-64.
Kubota, K. 1990. Kinetics of quality conversion. In Reaction Engineering for Food, Kubota, K., Uno, J. andKobayashi, T. (Eds.), Tokyo, 1-45.
Luangmalawat, P., Prachayawarakorn, S., Nathakaranakule, A. and Soponronnarit, S. 2008. Effect oftemperature on drying characteristics and quality of cooked rice. LWT-Food Sci. Tech., 41 (4), 716-723.
Muramatsu, Y., Hashiguchi, M., Mi, D., Sorm, S., Sakaguchi, E. and Kawakami, S. 2021. Establishment of
optimizing manufacturing condition for cooked rice -Part I-, Equilibrium moisture content and latent heatof vaporization of cooked rice. Int. J. Environmental and Rural Development, 12 (1), 130-135.
Muramatsu, Y., Tagawa, A., Sakaguchi, E. and Kasai, T. 2007. Prediction of moisture content changes ofkidney beans during drying. J. Japan Assoc. Food Preservation Scientists, 33 (6), 295-302.
Murata, S. 1982. Characteristics of grain and estimation of drying rate. J. Japanese Soc. Agric. Machinery, 44(3), 517-523.
Ramesh, M.N. and Rao, S.P.N. 1996. Drying studies of cooked rice in a vibrofluidised bed drier. J. Food Eng.,
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