Effect of drying and frying time on textural and sensory characteristics of popped rice

As. J. Food Ag-Ind. 2009, 2(02), 166-170

Asian Journal of
Food and Agro-Industry
ISSN 1906-3040
Available online at www.ajofai.info

Research Paper

Effect of drying and frying time on textural and sensory
characteristics of popped rice

Samatcha Phanitcharoen, Anon Maliket and Prasong Siriwongwilaichat*

Department of Food Technology, Faculty of Engineering and Industrial Technology,
Silpakorn University, Nakhon Pathom 73000 Thailand.

*Author to whom correspondence should be addressed, email: siriwong@su.ac.th

This paper was originally presented at Food Innovation Asia 2009, Bangkok, Thailand, June 2009.


Abstract

The objective of this study was to investigate the effect of drying time and frying time on
the textural and sensory characteristics of popped rice to be used as an ingredient base for
snack bars. The broken sticky rice was steam-cooked before drying at 65oC for 2, 3 and 4
hours. The popped rice was then prepared by deep fat frying the dried rice at 210oC for 5,
10 and 15 seconds. The product quality was evaluated for moisture content, water activity,
colour, texture and sensory perceptions. It was found that the drying time significantly
affected the product colour (p<0.05). However, the effect of frying time on final product
quality was not significant at any given drying time (p>0.05). The colour of the fried
product obtained from cooked broken sticky rice dried for 2 hours was significantly lighter
than that dried for 3 and 4 hours. The hardness of product measured as peak compression
force increased with increasing drying time. Sensory evaluation indicated that the product
obtained from drying cooked sticky rice for 3 hours, followed by deep fat frying for 10
seconds was the most acceptable.

Keywords: snack food, popped rice, drying, frying, textural characteristics, Thailand

Introduction

Sticky or glutinous rice is produced and widely consumed in Thailand. Its lack of amylase
provides good properties for producing various porous textured products, such as popped,
fried or baked snacks [1]. Nevertheless, the broken sticky rice has not been fully
developed for value added products. Popped rice is one of the most well known puffed
snacks in Thailand. It could be also used as an ingredient base for various snack bars, thus
considered for this study. The main process of popped rice preparation includes steaming,
drying and frying [2]. Initial moisture content of rice before frying is a major factor

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influencing final product quality. With lower initial moisture content, the lower oil content
is absorbed inside the fried product [3]. Thus, pre-fry drying is a significant process
determining the final product quality [4]. Frying time and temperature also influence the
product characteristics, especially texture and colour [5, 6, 7]. It has been reported that
increasing frying time and temperature causes deep fried rice crackers to become harder
[1].


The objective of this study was to investigate the effect of drying time and frying time on
the textural and sensory characteristics of popped rice to be used as an ingredient base for
snack bars.

Materials and Methods

Materials
Broken sticky rice, Palm oil, Sucrose, Salt, Water

Equipment
Thermometer, Deep fat fryer, Tray dryer, Texture analyzer (TA-XT2i, New York, USA),
Colour-view Spectrophotometer, Water activity meter (thermoconstanter), hot air oven.

Methods
For each treatment, sticky rice of 200 g was washed and soaked in water for 1 h, then
drained before steaming at 100oC for 30 min. The steamed rice was dried in a tray dryer at
65oC followed by frying at 210oC. Two variables, drying time and frying time, were
included in this study. Drying time was varied at 2, 3, and 4 h. Frying time was varied at
5, 10 and 15 s, respectively using 32 factorial design. Two replications were performed for
each experiment. The fried popped rice was evaluated for moisture content, water activity,
colour, texture (probe p/100, compression plate, loading speed of 1.5 mm/s, with a 25 kg
load cell) and sensory perceptions. For sensory evaluation, popped rice bar was prepared
using binding syrup comprising 2 g of salt, 175 g of sucrose and 175 g of glucose syrup
with 650 g of popped rice. Out of six treatments, the top three samples were selected using
a ranking test with 40 untrained panelists as tasters. Hedonic scale (from 1-extremely
dislike to 9-extremely like) was also applied with 40 untrained panelists as tasters for
product acceptance evaluation. The product colour, biting hardness, crispiness and overall
liking were evaluated.

Results and Discussion

Rice Steaming and Drying Preparation
Steamed rice was prepared and dried before frying to obtain popped rice. During drying,
samples were collected at 2, 3 and 4 hours for moisture content determination. The results
are presented in Table 1. Rice moisture content decreased with increased drying time. The
samples with varied levels of moisture content were then used for frying experiments.

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Table 1. Moisture content of dried rice.
Moisture content (% dry basis)
Drying time at 65oC (h)
(mean±s.d.)
2
9.85±0.02
3
5.45±0.01
4
3.47±0.01

Rice Frying
Rice frying experiments were conducted to examine the effect of initial moisture content
of rice before frying and frying time on popped-rice characteristics. After frying, the
product’s water activity varied between 0.3-0.4. It was found that the colour of popped-
rice as indicated by L*, a*, and b* (Table 2) became a darker yellow when initial moisture
content of rice was lower (longer drying time). However, only popped rice dried for 2
hours with the highest initial moisture content gave significantly different colour from that
of rice dried for 3 and 4 hours (p<0.05).

Table 2. Effect of drying and frying time on popped rice colour.
Colour1
Drying time (h)
Frying time

(mean±s.d.)

(S)
L*
a*
b*
5 74.12 ± 4.21 a 2.49 ± 1.61 b 20.33 ± 2.10 c
2
10 72.96 ± 2.00 a 2.96 ± 1.63 b
22.86 ± 2.58 c
15 72.73 ± 3.52 a 3.94 ± 1.84 b 22.57 ± 2.02 bc
5 65.16 ± 0.26 b 8.51 ± 0.91 a 26.97 ± 0.22 a
3
10 64.94 ± 1.59 b 7.29 ± 0.58 a 24.57 ± 0.61 ab
15 64.44 ± 0.45 b 8.05 ± 0.87 a 26.79 ± 1.01 a
5 62.69 ± 0.56 b 8.47 ± 0.85 a 26.92 ± 0.98 a
4
10 62.38 ± 0.04 b 7.96 ± 0.44 a 27.33 ± 0.69 a
15 61.234 ± 0.58b 8.04 ± 0.68 a 27.025 ± 1.02 a
Note: 1Letters a, b, and c indicate significantly different mean values in the same column.

Texture of popped rice was measured as peak compression force as shown in Figure 1.
The results indicated that at a given drying time, the peak compression force of fried
product tended reach the maximum at 10 minutes of frying. However, the peak
compression force increased with increasing drying time or when the initial rice moisture
content was lower. This was in agreement with the results reported by Sukumar et al [8],
that the hardness of fried chickpea flour-based snack increased at the lower moisture
content of the pre-fried product.

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169
a
10,000
a
g
)
a
r
c
e (
8,000
n
Fo
Drying 2 hours
ressio
6,000
mp
Drying 3 hours
b
Co
b
a
Drying 4 hours
e
ak
4,000
b
a
e
P
a
v
erag
2,000
A
0
0
5
10
15
20
Frying Time (seconds)

Figure 1. Effect of rice drying and frying time on popped rice hardness.
Note: Letters a and b indicate significant difference in mean values of the same line.

Three product treatments including drying 2 hours, frying 10 seconds, drying 2 hours,
frying 15 seconds and drying 3 hours, frying 10 seconds were chosen by ranking
preference test. To evaluate the consumer’s preference of the popped rice, plain rice snack
bar was prepared. Hedonic score was employed.



9

8


7

b
6
b
a
e
a
a

ab
b
c
or
a
5

a
a
a
n
g
s
i
ki

b
L
4

3


2

1


0

Color Biking hardness Crispiness Overall liking


Drying 2 h, frying 10 s
Drying 2 h, frying 15 s
Drying 3 h, frying 10 s


Figure 3. Preference score of popped rice snack bar.

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The results shown in Figure 2 indicate that the product prepared from 3 hours of drying,
followed by 10 seconds of frying, received the highest overall liking score (p<0.05). It was
noticed that the most acceptable product also provided the highest colour and texture
liking scores. Nevertheless, the crispiness liking scores of all samples were not
significantly different (p>0.05).

Conclusions

The experimental results showed that popped rice characteristics were associated with the
initial moisture content prior to frying and frying time. According to this study, drying
cooked rice for 3 hours (moisture content of about 3.47%d.b.) followed by 10 seconds of
deep-fat frying was the best condition for popped rice preparation.

Acknowledgement

This research was sponsored by the Research and Development Institute, Silpakorn
University, Thailand.

References

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Maneerote, J., Noomhorm, A. and Takhar, P.S. (2009). Optimization of processing
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