KKU ENGINEERING JOURNAL 2016;43(S3):459-461 Research Article

KKU Engineering Journal

https://www.tci-thaijo.org/index.php/kkuenj/index

Effect of drying methods on property of Thai rice cracker

P. Thanompongchart*1), P. Pintana1), K. Phimphilai2) and N. Tippayawong2)

1)Faculty of Industrial Technology, Uttaradit Rajabhat University, Uttaradit, Thailand.
2)Department of Mechanical Engineering, Chiang Mai University, Chiang Mai, Thailand.
Received April 2016
Accepted June 2016

Abstract

Fried glutinous rice cracker is a popular snack for local Thais. One of the production steps usually requires drying. Method of
drying is an important process affecting products, shelf life and quality. In this research study, quality of rice cracker obtained
from various drying methods (direct solar radiation, solar dome, and hot air drying oven) was investigated and compared.
Properties of the rice cracker being analyzed were shrinkage ratio, expansion ratio, color, hardness and crispness. The rice
cracker samples were in rectangular slap. Experimental results showed that the final moisture content of the rice cracker was
in the range of about 8-10% wet basis. Direct solar drying gave minimum shrinkage ratio of 1.34 and maximum expansion
ratio of 4.99. The color of dried rice cracker was given as L* a* b* color space. The highest hardness of the rice cracker (2,200
g) can be reached by hot air drying, and the best crispness (18,000 g.s) was obtained by direct solar drying.

Keywords: Dehydration, Food processing, Food quality, Snack

1. Introduction The glutinous rice was obtained locally. Before

experimentation, it was soaked in water overnight, after
Fried glutinous rice cracker or “Khaotan” is a traditional which it was washed and steamed. Then the cooked rice may
snack of local Thai communities. Khaotan is popular in north be mixed with other ingredients, such as fruit syrup. The rice
of Thailand. From food preservation in small groups of was then molded into shape with volume of about 12 cm3.
housewives, khao tan is now an economically important The moisture content was about 60-70 % dry basis.
product with export to the World. It helps generate
significant extra income to local people. There are many 2.2 Drying methods
shapes of rice cracker such as round, oval, rectangular or
irregular shaped. In the production of rice cracker, drying is Three methods of drying were used for removing
an essential step to preserve the rice cracker and prevent moisture of the rice cracker. The configuration of the three
fungal spoilage. Subsequent step is frying. Fried rice cracker dryers is shown in Figures 1, 2 and 3. For solar drying, it was
had moisture content about 8-10 % dry basis [1-2]. Drying operated between 9.00 am. – 17.00 pm. A hot air drying oven
process can be challenging due mainly to uncertainty and is a square room. It was operated at temperature of about 40-
fluctuation in final moisture content of product, which in 45oC. Air distribution throughout the room was about 3-3.5
turns, depending on method of drying. Normally, drying by m3/s. Hot air entered at the bottom of the room. Moist air
direct solar radiation is adopted to reduce moisture content exited at the top of the drying room. Moisture content was
of the steamed rice before frying. The energy is provided for analyzed using a Mettler Toledo model MJ33. The final
free but the method has limitation regarding weather moisture content of rice cracker was about 8-10 % dry basis
conditions. It can take long period of up to 2-3 days and risk before storage. Temperature, humidity and intensity of solar
contamination from insects, dust and others foreign materials irradiation were also monitored.
that may damage the product [3-5]. Indirect solar drying
using greenhouse dome dryer or drying with hot air from 2.3 Property analysis
burning of fuel gas can offer alternative solution. However,
these drying techniques can result in varying quality and The following physical properties were considered;
property of the rice cracker. shrinkage ratio, expansion ratio, color, hardness and
Hence, in this work, we aimed to study property of dried rice crispness. Shrinkage and expansion ratios were calculated
cracker for each drying method and compare between them. from change in volume [6]. Color analysis was based on CIE
system; lightness (L*), redness (a*) and yellowness (b*). It
2. Materials and methods was determined using Hunter Lab Color Flex [7]. Hardness
and crispness were based on texture analysis using TAXT
2.1 Rice crack samples Plus. Three point bend, pre-test, test and post-test speeds of

*Corresponding author. Tel +6686 671 7937

Email address: patipat.than@gmail.com
doi: 10.14456/kkuenj.2016.155
460 KKU ENGINEERING JOURNAL 2016;43(S3)

1, 2 and 10 mm/s, distance of 10 mm and trigger force of 5g 3. Results and discussion

were used.
For direct solar drying in ambient air, experiments on
clear days showed average temperature and humidity of
outdoor ambience at 41.5oC and 45% RH. Average solar
power was 756 W/m2 with maximum of 1,100 W/m2. Drying
time was 8 h or about one day of work. Final moisture
content was about 8-9% dry basis. For indirect solar drying
with greenhouse dome dryer, average temperature, air
ventilation and dome solar power were found to be about 45-
50oC, 1.0 m3/s and 560 W/m2, respectively. Average drying
time was 13 h or about one and a half day of work. For hot
air drying in the LPG fired oven, temperature was set at 40-
Figure 1 Direct solar drying on a tray 45oC with air circulation of about 3-3.5 m3/s. Average drying
time was about 12 h, or about one night of work.
After frying of the dried rice crackers, their shrinkage
and expansion ratios, color, hardness and crispness were
analyzed and shown in Tables 1, 2 and 3, respectively.
Physical appearance of the products after drying and frying
was shown in Figures 4 and 5.
From the experimental results, it was clear that minimum
drying time was obtained from direct solar drying (about 8
h). Maximum and minimum shrinkage of dried rice crackers
were 35% from hot air drying oven and 26% from direct solar
drying. Maximum expansion after frying was 400% from
Figure 2 Indirect solar drying with greenhouse dome direct solar drying. Shrinkage of dried rice crackers and
expansion after flying were found to depend on temperature
and time in drying process. Dried rice crackers from direct
solar drying was lighter, lower redness and yellowness than
those obtained from other methods. Change in product colors
was also dependent on drying time and temperature. Color of
the fried rice cracker did not change significantly. For dried
rice crackers, maximum hardness and crispness were
4,676.87 ± 582.43 g of force from hot air drying oven and
14,193 ± 1,180.32 g.s from direct solar drying. For solar
dome dryer, hardness and crispness were quite low due to
long drying time where evaporation of water occurred slowly
and regularly. For fried crackers, maximum hardness and
crispness were obtained from hot air drying oven and direct
solar heating dryer, respectively.
Figure 3 Hot air drying in an oven with LPG combustion

Table 1 Shrinkage after drying and expansion after frying process

Shrinkage Percent Expansion Percent

Drying methods S.D. S.D.
ratio shrinkage ratio expansion
Direct solar drying 1.34 25.61 ±0.91 4.99 399.85 ±6.00

Solar dome dryer 1.41 28.95 ±1.10 4.71 371.58 ±4.27

Hot air drying oven 1.54 34.93 ±0.50 4.68 368.37 ±2.34

Direct solar drying Solar dome dryer Hot air drying oven

Figure 4 Color of rice crackers after drying

KKU ENGINEERING JOURNAL 2016;43(S3) 461

Table 2 Color of rice crackers

Rice cracker after drying Color Rice cracker after frying Color
process L* a* b* process L* a* b*
Direct solar drying 60.35 0.95 17.22 Direct solar drying 69.41 9.315 29.73
S.D. ± 0.220 0.064 0.330 S.D. ± 0.350 0.420 0.250
Solar dome dryer 62.39 3.74 22.45 Solar dome dryer 68.62 10.79 30.41
S.D. ± 0.610 0.098 0.200 S.D. ± 1.330 1.170 1.730
Hot air drying oven 58.91 5.95 22.35 Hot air drying oven 66.92 10.2 29.96
S.D. ± 0.310 0.077 0.130 S.D. ± 0.007 0.410 0.470

Direct solar drying Solar dome dryer Hot air drying oven

Figure 5 Color of rice crackers after frying

Table 3 Hardness and crispness of rice crackers

Rice cracker after drying Hardness Crispness Rice cracker after frying Hardness Crispness
process (g) (g.s) process (g) (g.s)
Direct solar drying 4,105.06 14,193.87 Direct solar drying 1,883.42 17,976.87
S.D. ± 517.32 1,180.32 S.D. ± 591.65 1,747.83
Solar dome dryer 2,840.39 10,787.31 Solar dome dryer 1,275.85 13,481.66
S.D. ± 423.32 958.56 S.D. ± 23.89 1,471.17
Hot air drying oven 4,676.87 13,780.66 Hot air drying oven 2,188.67 10,223.54
S.D. ± 582.43 1,125.43 S.D. ± 306.65 1,081.48

4. Conclusions [3] Limpaiboon K, Wiriyaumpaiwong S. Drying kinetics

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This work was supported by the National Research Journal of Food Properties 2004;7(1):37-44.
Council of Thailand, Uttaradit Rajabhat University and [7] Larrain RE, Schaefer DM, Reed JD, Use of digital
Chiang Mai University. images to estimate CIE color coordinate of beef. Food
Research International 2008;41(4):380-385.
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