Vol.4, No.9B, 12-22 (2013) Agricultural Sciences
http://dx.doi.org/10.4236/as.2013.49B003
Optimisation of extrusion cooking conditions and
characterization of rice (Oryza sativa)—Sweet potato
(Ipomoea batatas) and rice-yam (Dioscorea alata)
based RTE products
Elina Brahma Hazarika, Anjan Borah, Charu Lata Mahanta*
Department of Food Engineering and Technology, School of Engineering, Tezpur University, Assam, India;
*Corresponding Author:
Received June 2013
AB STRACT
Extrusion cooking has been extensively used to
produce variety of foods like ready to eat break-
fast cereals, baby foods, snack foods, etc. Tak-
ing rice flour as the base ingredient, two locally
available tuberous root vegetables, greater yam
(Dioscorea alata) and sweet potato (Ipomoea ba-
tatas) were used in this study for the develop-
ment of ready-to-eat breakfast products in a sin-
gle screw extruder. During extrusion cooking,
the screw speeds ranged from 132 to 468 rpm
and the barrel temperatures ranged from 103˚C
to 137˚C. T he extrudat es were th en analyzed for
various physical and physicochemical proper-
ties. Optimization was done following Response
Surface Methodology (RSM) using Central Com-
posite Design. Using screw speed, barrel tem-
perature and feed composition as the three in-
dependent variables, the three responses taken
were bulk de nsity, expansion in dex and breaking
strength. The optimized conditio n s were used for
developing 3 new products one of which also
contained tomato pulp powder. The products were
analyzed for their physical, proximate, sensory
and antioxidant properties. There was significant
colour change in all the three samples as indi-
cated by total colour change (∆E). Texture anal-
ysis of t he extrudat e samples showed hardnes s
values ranging from 28.68 N to 47.57 N. Amylose
content was found to be 15.3% in rice-sweet
potato extrudate, 14.7% in rice-yam extrudate
and 18.2% in r ice-sweet potato-tomato extrudate.
The antioxidant profile of the extrudates studied
through DPPH (2,2-diphenyl-picryhydrazyl) sc a-
venging activity and FRAP (ferric reducing an-
tioxidant property) showed that the antioxidant
capacity in all the 3 extrudates was very low.
Rice flour incorporated with sweet potato was
judged the best on sensory evaluation. The study
has shown that both sweet potato and greater
yam tubers can be commercially exploited for
the development of ready-to-eat (RTE) products .
Keywords: Extrusion; RSM; RTE; Physicochemical
Properties
1. INTRODUCTION
Consumers want snacks that taste good, smell good,
feel good, look good and in addition, are nutritionally su-
perior and healthy. Extrusion cooking is one of the most
important food processing technologies which have been
used since the mid 1930s for the production of breakfast
cereals, ready to eat snacks, and other textured foods. In
the past decade, extrusion cooking has been studied ex-
tensively to produce variety of specialty foods including
pasta products and ready to eat breakfast cereals, baby
foods, snack foods, texturised vegetable protein, pet foods,
dried soups and dry beverage mixes, as it not only im-
proves digestibility (Singh, Dartois, & Kaur, 2010) but
also improves the nutrients bioavailability (Gu, Hous
Rooney, & Prior, 2008) compared to conventional cook-
ing. The quality of the product depends on the process
conditions, such as the extruder type, the feed moisture,
the temperature profile in the barrel sections, the screw
speed and the feed rate (Thymi, Krokida, Pappa & Ma-
roulis, 2005).
Owing to the popularity and high nutritive value of
vegetables, their utilization has increased either in raw
form or processed form. Greater yam (Dioscorea alata)
and sweet potato (Ipomoea batatas) are two species of
large underground tuberous root vegetables seasonally
available and are gaining importance as processed food

E. B. Hazarika et al. / Agricultural Sciences 4 (2013) 12-22
13
sources. Work on extrusion cooking of sweet potato -
soybean blend (Iwe et al., 1998) and sweet potato-whole
wheat bran (Dansby et al., 2003) has been reported. Oke
et al. (2012) and Chiu et al. (2013) have worked on ex-
trusion cooking of yam (Dioscorea alata). However,
extrusion cooking of sweet potato and grater yam with
rice has not been reported. In this study sweet potato and
grater yam powders were blended with rice to obtain a
ready to eat breakfast product. The products were ana-
lyzed for their physical, physicochemical, nutritional and
sensory properties.
2. MATERIALS A ND METHODS
2.1. Materials
The tubers of Dioscorea alata (greater yam) and Ipo-
moea batatas (sweet potato) and tomatoes (Solanum ly-
copersicum) were obtained from the market place near
Tezpur University campus. The chemicals were obtained
from Merck, Hi-Media and Rankem, and the glasswares
used were from Borosil and Rankem. The extruder ma-
chine ( G.L. Ext rusio n System Pvt. Ltd , New Del hi) used
in the current study is a single-screw one, driven by 5
H.P. induction motor (DC).
2.2. Sample Preparation for Extrusion
The water yam and sweet potato tubers were washed,
peeled and cut into thin slices using Slicer (Alpha In-
struments, Delhi) while yam tubers were dipped in 0.5%
sodium metabisulphite solution to reduce browning. All
samples were dried in a tray drier at 55˚C. The dried
chips were subsequently milled into flour in a pulverizer
(Alpha Instruments, Delhi) to pass through a 1.00 mm
mes h sc ree n. T he flour samples were put in sealed pouch-
es and kept in covered plastic containers until used for
extrusion processes and analyses. Blends were prepared
from sweet potato flour and rice flour in definite ratios
for carrying out 18 different runs in the extruder. The
moisture contents of the blended samples for extruder
feed were adjusted at around 12% by adding calculated
amounts of water into each blend and then mixing them
properly. The blended mix in different ratios were kept
overnight in a tightly covered vessel for moisture equili-
bration. The extrusion processes were performed on a sin-
gle screw extruder. Raw materials were fed into the ex-
truder barrel a nd at the end of extrusion cooking was dis-
charged from the end of the barrel as the extrudates.
2.3. Sample Preparation for Analysis of
Extrudates
The samples were ground in a mixer grinder into powder
and ke pt in sealing pouches for t heir analysis.
2.4. Pasting Properties
Pasting properties of extrudate powders were deter-
mined using a Rapid Visco Analyser (RVA) model 2-D
(Newport Scientific Instrument) with Thermocline soft-
ware (3.0 version) by ICC Standard Method No. 162 (1995).
Briefly, sample suspension was prepared by placing 3.5 g
extrudate powder, in an aluminum canister containing 25
mL distilled water. A programmed heating and cooling
cycle was used. Each sample was stirred at 960 rpm for
10 s while heating at 50˚C, and then constant shear rate
(160 rpm) was maintained for the rest of the process.
Temperature was held at 50˚C for 1 min. Then the sam-
ples were heated from 50˚C to 95˚C within 3 min 42 s
and held at 95˚C for 2 min 30 s. Subsequently samples
were cooled down from 95˚C to 50˚C within 3 min 48 s
and the n held at 5 0˚C for 2 min. The RVA plot of viscos-
ity (cP) versus time (s) was used to determine peak vis-
cosity (PV) and final viscosity (FV).
2.5. Expansion Index (EI)
Expa nsion of e xtrudate s was meas ured usi ng a vernier
caliper (Mitutoyo, Japan) according to the method of
Ding et al. (2005). Measurements were taken on ten
randomly selected pieces of extrudates. The die diameter
of the extruder used in the stu dy is 2 mm.
2.6. Bulk Den sity (BD)
Bulk density (BD) of extrudates was determined fol-
lowing the method of Bhat nagar et al. (1995).
2.7. Color
The colour values of the rice extrudates incorporated
with sweet potato powder in terms of L, a, b values was
measured using colour measurement spectrophotometer
(Hunter Lab, Ultra-scan VIS).
2.8. Texture Profile Analysis (TPA)
Texture evaluation of the extrudates was performed
with texture analyzer (TA-HD-plus, Stable Micro Sys-
tems, UK) with a 5 kg load cell. Hardness (HRD), punc-
ture force (PF) and breaking strength (BS) of extrudates
were determined as the maximum force offered by ex-
trudates during compression and three point cutter test,
respectively.
Bre aki ng Str en gth ( N/m m2) = Peak breaking for ce/Cross
sectional area
2.9. Water Absorption Index (WAI) and W a ter
Solubility Index (WSI)
WAI and WSI were determined following the proce-
dure of Anderson et al. (1969).

E. B. Hazarika et al. / Agricultural Sciences 4 (2013) 12-22
14
2.10. Determination of Amylose
Amylose content was measured according to Sowba-
hagya & Bhattacharya (1979).
2.11. Sensory Properties
The extruded products were sensorily evaluated by ten
panelists. The samples were provided along with milk
(for soaking) and sugar (for taste). Based on a 9-point
Hedonic scale (0 - 9), the panel members were asked to
give their score s on 6 different attributes, namely, appear-
ance, colour, taste, texture, mouthfeel and overall accep-
tability.
2.12. Determination of Proximate Content
The moist ure, cr ude fat, crude protein, crude fiber and
ash were determined as per AOAC (1999). The total car-
bohydrate in the samples was calculated by the method
of difference on wet basis.
2.13. Determination of DPPH Activity
The radical scavenging activity of the extrudates was
measured by determining the inhibition rate of DPPH (2,
2-diphe nyl-1-picrylhydrazyl) radical (Brand-Williams,
1995).
2.14. Determination of Ferric Reducing
Ant i oxidant Property (F RAP)
FRAP activity of the samples were measured by the
method of Benzie and Strain (1999), after preparing ex-
tracts in the same way as for DP PH analysis.
2.15. Statistical Analysis
All the readings are average of minimum two readings.
Analyses of all parameters are reported with standard
deviation error bar.
3. RESULTS AND DISCUSSION
3.1. Experimental Design and Statistical
Analysis
Response Surface Met hodolo gy ( RSM), wh ic h explores
the relationship between several explanatory variables
and one or more response variables, was applied to the
experimental data using the package, Design expert ver-
sion 7.1.1, (STATE-EASE Inc, Minneapolis, USA. Trial
version). The same software was used for generation of
response surface plots, superimposition of counter plots
and optimizatio n of process variables (Altan et al., 2008;
Yagci & Gogus, 2008; Ding et al., 2005). The results are
reported as means o f three rep licate s. A four var iable (five
level of each variable) central composite design (CCD)
was employed (Montgomery, 2001; Yagci & Gogus, 2008).
The parameters and their levels were chosen based on
literature available on rice based extrudates (Yagci and
Gogus, 2008; Ding et al., 2005; Upadhyaya, 2008). The
ingredients used for the extrudates were: rice flour, sweet
potato po wder, yam powder and tomato pulp powder. The
independent variables included screw speed (SS), barrel
temperature (BT ) and feed composition (FCM). Re sponse
variables were expansion index (EI), bulk density (BD)
and breaking strength (BS). T he five levels of the process
variables were coded as −1.682, −1, 0, 1, 1.682 (Mont-
gomery, 2001) and the design in coded (x) form is given
in Table 1. A quad ratic p olyno mial equatio n was fitted to
the data to obtain a regression equation. The statistical
significance of the terms in the regression equation was
examined by analysis of variance (ANOVA) for each re-
sponse. The extrusion processing parameters were opti-
mized by using a conventional graphical method of RSM
in order to obtain extrudates with acceptable properties.
All the processing variables were kept within range while
the responses were either maximized (for expansion in-
dex) or kept less than a sp ecific value ( bulk den sity, break-
ing str ength) . The ANO VA re sults a re shown i n Table 2.
The Optimized parameters thus obtained from the above
model are s hown in Table 3.
Finally, the optimized solution that was obtained with
the values of predicted responses are shown in Ta b l e 4 .
The desirability level was 0.84. The Coefficients of va-
riables in the predictive model for response variables
(coded units) are:
2
123 1
22
23 1213
23
EI 4.790.16x0.44x0.032 x0.048 x
0.25x0.090x0.19xx0.11 xx
0.021 xx
=++ +−+
+++ ⋅−⋅
−⋅
(1)
33
12 3
2 232
12 3
3 33
1213 23
BD
0.177.70510x0.026 x9.18110x
0.010 x0.015 x4.40510x3.150
10 xx3.15010 xx1.05010 xx
−−
−
− −−
=+−×−+ ×
−−+× +
×⋅−×⋅+× ⋅
(2)
3
12 3
32 22
1 2312
13 23
BS
0.290.025x0.098 x8.44610x7.975
10x0.021x0.021 x0.011 xx
0.017 xx0.013 xx
−
−
=+−−+ ×−
×− −−⋅
+ ⋅−⋅
(3)
For EI, R2 = 0.761
For BD , R2 = 0.742
For BS, R2 = 0.706
3.2. Properties of the Extrudates of Rice and
Sweet Potato Powder
Various parameters of the 20 extrudates of rice and
sweet potato were studied. Their colour profile is given

E. B. Hazarika et al. / Agricultural Sciences 4 (2013) 12-22
15
Table 1. Experimental design fo r ext rusion experiment with coded and actu al variable levels.
Sample
cod es Coded Un coded
x1 x2 x3 X1 X2 X3
RS(92:8)A −1 1 −1 200 130 80
RS(80:20)A −1 1 1 200 130 200
RS(86:14)A 1.682 0 0 468 120 140
RS(80:20)B 1 1 1 400 130 200
RS(86:14)B 0 0 0 300 120 140
RS(86:14)C 0 0 0 300 120 140
RS(80:20)C 1 −1 1 400 11 0 200
RS(86:14)D 0 0 0 300 120 140
RS(86:14)E 0 0 0 300 120 140
RS(92:8)B 1 1 −1 400 130 80
RS(86:14)F 0 0 0 300 120 140
RS(76:24) 0 0 1.682 300 120 240
RS(86:14)G 0 0 0 300 120 140
RS(86:14)H 0 1.682 0 300 137 140
RS(92:8)C −1 −1 −1 200 110 80
RS(86:14)I −1.682 0 0 132 120 140
RS(92:8)D 1 −1 −1 400 110 80
RS(80:20)D −1 −1 1 200 110 200
RS(96:4) 0 0 −1.682 300 120 40
RS(86:14)J 0 −1.682 0 300 103 140
x1 and X1, screw Speed (rpm); x2 and X2, extrusion temperature (˚C); x3 and X3, concen t ration of swee t pot at o powde r (g ∙kg−1); R denotes rice flour; S denotes
sweet pota to .
Table 2. Analysis of variance results for fitted models of product properties.
Response Source df Sum of square Mean squares F-value P-value
EI
Regres sion 9 4.30 0.48 3.54 0.0307∗
Lack of Fit 5 0.40 0.080 0.42 0.8159
Pure error 5 0.95 0.19
Residual 10 1.35 0.13
Total 19
BD
Regres sion 9 0.017 1.842 × 10
−3
3.19 0.0426∗
Lack of Fit 5 2.519 × 10−3 5.039 × 10−4 0.77 0.6 078
Pure error 5 3.259 × 10−3 6.519 × 10−4
Residual 10 5.779 × 10−3 5.779 × 10−4
Total 19
BS
Regres sion 9 0.16 0.017 2.67 0.0 708
Lack of Fit 5 0.028 5.564 × 10−3 0.75 0.6203
Pure error 5 0.037 7.424 × 10−3
Residual 10 0.065 6.494 × 10
−3
Total 19
∗Sig nifica n t a t P < 0.05; df: degree of freedom.
Table 3. Optimized parameters i n the response optimizer.
Response Goa l Lower Limit Upper Limit Lower Weight Upper Weight Importance
EI Maximum 4.388 6.22 1 1 3
BD Minimum 0.101 0.22 1 1 3
BS Minimum 0.085 0.46 1 1 3
Table 4. Optimized solution obtained using the response optimizer.
Opt imal solution Predicted r esponses Desirability
X1 (rpm) X2 (˚C) X3 (g/kg) EI BS, N/mm2 BD, g/cm3
400 130 150 5.84 0.13 0.12 0.84

E. B. Hazarika et al. / Agricultural Sciences 4 (2013) 12-22
16
in Tabl e 5 and physical properties are given in Ta b le 6.
The 3-D graphical representations of the 3 responses (ex-
pansion index, bulk density and breaking strength) against
different screw speeds (SS), barrel temperatures (BT) and
feed composition (FCM) are given in Fig u re 1. T he water
absorption index (WAI) and water solubility index (WSI)
of the 20 samples are give n in Table 7.
3.3. Selection of Best-5 Extrudates Based
on Their A ppearances
Based on the appearence of the 20 samples, a score
rangi ng fr om 1 t o 9 was give n to eac h sa mple as gi ven i n
Ta b le 7 . The 5 samples that got the highest scores of 9
were selected for sensory evaluation and RVA studies.
For convenience of terms, the product codes of the 5
samples with the highest score of 9 are changed as given
in Table 8.
3.4. Pasting Profile of the Ex trudates
The RVA study of the extrudates showing their peak,
hold and fi na l visc o si tie s a long wit h t hei r b r ea kd o wn a nd
setback values are given in Table 8. The Table shows
Table 5. Color profile of the 20 extrudate samples developed from rice flour and sweet potato.
Sample
Pre extrusion
Pos t extrusion
∆E
L
0
a
0
b
0
L
a
b
RS(92:8)A
76.64
0.05
8.15
65.18
2.09
10.75
11.93
RS(80:20)A 78.75 0.50 9.42 69.05 2.04 11.56 10.05
RS(86:14)A 76.58 0.26 8.93 65.65 2.91 12.17 11.70
RS(80:20)B 77.45 0.54 9.14 65.85 2.10 11.10 11.87
RS(86:14)B 75.97 0.40 8.76 67.34 0.94 9.33 8.67
RS(86:14)C
75.61
0.55
9.07
66.58
1.61
10.19
9.16
RS(80:20)C 78.23 0.75 9.42 66.35 2.32 11.41 12.15
RS(86:14)D 76.33 1.09 10.24 64. 45 1.95 10. 45 11.91
RS(86:14)E
75.70
0.41
8.36
65.34
1.89
10.72
10.73
RS(92:8)B 76.03 0.51 9.00 67.81 2.93 12.67 9.32
RS(86:14)F
75.27
0.95
9.12
68.40
2.56
11.54
7.46
RS(76:24) 75.68 0.62 9.51 66.35 2.92 12.60 10.09
RS(86:14)G 75.97 0.40 8.76 67.34 0.94 9.33 8.67
RS(86:14)H 73.91 0.41 7.78 68.03 3.01 11.95 7.66
RS(92:8)C 75.76 0.39 8.91 67.41 1.07 9.59 8.41
RS(86:14)I
75.61
0.55
9.07
66.58
1.61
10.19
9.16
RS(92:8)D 75.89 0.55 8.05 67. 76 2.13 10.94 8.77
RS(80:20)D 76.67 0.59 9.16 67.39 1.55 10.46 9.42
RS(96:4) 75.90 0.50 7.53 64. 43 1.07 8.93 11.57
RS(86:14)J 77.45 0.55 8.59 65.58 1.14 8.5 11.88
Table 6. Physical propert ies of the 20 extrudate sampl es made from rice flour and sweet potato.
Sample Hardness (N) BS (N/mm2) EI (mm) BD (g/cm3)
RS(92:8)A 12.15 ± 0 .11 0.21 ± 0.03 4.90 ± 0.48 0.14 ± 0.02
RS(80:20)A
20.34 ± 0.22
0.34 ± 0.01
4.94 ± 0.22
0.12 ± 0.01
RS(86:14)A 20.42 ± 0.09 0.34 ± 0.04 5.00 ± 0.24 0.12 ± 0.0
RS(80:20)B 14.17 ± 0.01 0.26 ± 0.02 4.60 ± 0.43 0. 11 ± 0.01
RS(86:14)B 24.35 ± 0.16 0.37 ± 0.02 5.55 ± 0.33 0.14 ± 0.01
RS(86:14)C 22.36 ± 0.09 0.37 ± 0.05 5.00 ± 0.35 0.13 ± 0.01
RS(80:20)C
12.95 ± 0.16
0.22 ± 0.01
4.89 ± 0.44
0.12 ± 0.0
RS(86:14)D 24.35 ± 0.16 0.37 ± 0.02 5.55 ± 0.33 0.14 ± 0.01
RS(86:14)E 22.36 ± 0.09 0.37 ± 0.05 5.00 ± 0.35 0.13 ± 0.01
RS(92:8)B 15.66 ± 0.25 0.26 ± 0.03 5.08 ± 0.04 0.11 ± 0.0
RS(86:14)F 14. 11 ± 0.03 0.22 ± 0.01 5.39 ± 0.36 0.13 ± 0.0
RS(76:24)
14.42 ± 0.21
0.24 ± 0.01
5.04 ± 0.37
0.13 ± 0.0
RS(86:14)G 31.21 ± 0.18 0.37 ± 0.02 4.51 ± 0.35 0.16 ± 0.01
RS(86:14)H 19.33 ± 0.14 0.31 ± 0.05 5.22 ± 0.28 0.12 ± 0.0
RS(92:8)C 11.97 ± 0.11 0.21 ± 0.03 4.77 ± 0.38 0.12 ± 0.01
RS(86:14)I 23.46 ± 0.52 0.37 ± 0.01 5.12 ± 0.18 0.17 ± 0.01
RS(92:8)D
13.32 ± 0.27
0.21 ± 0.01
5.17 ± 0.55
0.13 ± 0.01
RS(80:20)D 14.54 ± 0.16 0.23 ± 0.02 5.34 ± 0.45 0.17 ± 0.01
RS(96:4) 19.24 ± 0.14 0.33 ± 0.04 4.82 ± 0.27 0.20 ± 0.0 1
RS(86:14)J 19.06 ± 0.28 0.32 ± 0.04 5.04 ± 0.42 0.19 ± 0.0

E. B. Hazarika et al. / Agricultural Sciences 4 (2013) 12-22
17
Figure 1. The nine 3-D graphs showing changes in EI, BD and BS with respect to changes in screw speeds
(SS), barrel temperatures (BT) and feed compo s ition (FCM).
4.5919
4.91182
5.23174
5.55166
5.87157
EI
200.00
250.00
300.00
350.00
400.00
110.00
115.00
120.00
125.00
130.00
A: SS
B: BT
4.66461
4.80401
4.94341
5.08282
5.22222
EI
200.00
250.00
300.00
350.00
400.00
80.00
110.00
140.00
170.00
200.00
A: SS
C: FCM
4.59567
4.85048
5.10529
5.3601
5.61491
EI
110.00
115.00
120.00
125.00
130.00
80.00
110.00
140.00
170.00
200.00
B: BT
C: FCM
0.117484
0.135081
0.152679
0.170276
0.187874
BD
200.00
250.00
300.00
350.00
400.00
110.00
115.00
120.00
125.00
130.00
A: SS
B: BT
0.153433
0.162587
0.171741
0.180895
0.190049
BD
200.00
250.00
300.00
350.00
400.00
80.00
110.00
140.00
170.00
200.00
A: SS
C: FCM
0.126657
0.144459
0.162262
0.180065
0.197867
BD
110.00
115.00
120.00
125.00
130.00
80.00
110.00
140.00
170.00
200.00
B: BT
C: FCM
0.130983
0.192157
0.253331
0.314505
0.375679
BS
200.00
250.00
300.00
350.00
400.00
110.00
115.00
120.00
125.00
130.00
A: SS
B: BT
0.213751
0.238004
0.262257
0.28651
0.310762
BS
200.00
250.00
300.00
350.00
400.00
80.00
110.00
140.00
170.00
200.00
A: SS
C: FCM
0.148583
0.205369
0.262155
0.318942
0.375728
BS
110.00
115.00
120.00
125.00
130.00
80.00
110.00
140.00
170.00
200.00
B: BT
C: FCM

E. B. Hazarika et al. / Agricultural Sciences 4 (2013) 12-22
18
Table 7. Water absorption and solubility indices of the 20 extrudate samples made from rice flour and sweet potato.
Sample WA I(g gel/ g) WSI (%) Score on App earance
RS(92:8)A 7.67 ± 0.6 9 11.93 ± 0.71 7
RS(80:20)A 7. 83 ± 0. 71 9.23 ± 0.81 8
RS(86:14)A 7. 84 ± 0. 31 12.68 ± 0.66 9
RS(80:20)B 7. 52 ± 0.51 10. 8 6 ± 0.43 9
RS(86:14)B 6. 62 ± 0.43 10. 0 0 ± 0.36 6
RS(86:14)C 8. 16 ± 0.37 8.00 ± 0. 3 2 6
RS(80:20)C 8. 27 ± 0.93 11.28 ± 0.19 6
RS(86:14)D 8. 17 ± 0. 23 8. 09 ± 0. 61 8
RS(86:14)E 6.62 ± 0.44 10. 1 5 ± 0.31 7
RS(92:8)B 6.84 ± 0.7 5 13.2 0 ± 0. 3 2 9
RS(86:14)F 7. 06 ± 0. 73 13.36 ± 0.31 8
RS(76:24) 7.02 ± 0. 4 9 11.53 ± 0.11 8
RS(86:14)G 7. 04 ± 0. 40 5. 29 ± 0. 15 7
RS(86:14)H 7. 25 ± 0. 15 10.54 ± 0.73 9
RS(92:8)C 7.15 ± 0.8 3 9 . 86 ± 0.41 7
RS(86:14)I 7. 02 ± 0. 21 9. 42 ± 0. 19 6
RS(92:8)D 7.48 ± 0.3 5 12.0 4 ± 0.10 9
RS(80:20)D 7. 37 ± 0. 92 9. 10 ± 0. 47 7
RS(96:4) 7.29 ± 0. 5 7 6.39 ± 0. 2 7 8
RS(86:14)J 6.89 ± 0. 5 2 6 . 74 ± 0.38 7
Table 8. Change of sample codes and RVA profile of the rice-sweet potato extru dates.
Former sample code New s amp le co de Peak viscosity (cP) Hold Viscosity (cP) Final Viscosity (cP) Breakdown (cP) Set b ack (cP)
RS(86:14)A RS-1 1693 153 251 1540 98
RS(80:20)B RS-2 1749 187 300 1562 113
RS(92:8)B RS-3 1534 158 247 1376 89
RS(86:14)H RS-4 3437 321 528 3116 207
RS(92:8)D RS-5 1738 177 286 1561 109
that sample RS-4 has the highest peak viscosity of 3437
cP. It reaches its final viscosity of 528 cP, which is also
the hi ghest a mong all the othe r sample s. T he sample RS-
3 has the lowest peak viscosity of 1534 cP and also the
lowest final viscosity of 247 cP. It was observed that
sample RS -4 that was extruded at a barrel temperature of
137˚C had the highest peak viscosity of 3437 cP while
the other extrudates that were processed at barrel tem-
perature less than 137˚C had lower peak viscosities. This
may be attributed to the higher degree of gelatinised
starch in the extudate that allowed it to absorb large
quantities of wate r. Accord ing to Liu et al. (2006), starch
conte nt in the fl our, othe r comp one nts i n the st ar ch -water
system and processing of flours are very critical to past-
ing properties. The interaction of other components and
the degree of starch damage during extrusion could affect
the peak viscosity of extruded flours. Screw speed and
feed composition did not have any effect as the other
samples showed similar RVA profiles.
3.5. Sensory Profile
The extrudates were subjected to sensory analysis for
the attributes of appearance, color, taste, mouthfeel, tex-
ture and overall acceptability by a panel of 10 semi trained
panellists. The graphical representation of the same is
given in Figure 2. The graph of sensory evaluation re-
vealed that R S-3 ha d the hi ghe st ra nki ng scores in all the
6 attributes-appearance, colour, taste, texture, mouthfeel
and overall acceptability. The sample with the lowest
score in all the attributes was RS-5. It is clearly seen that
mouthfeel was the attribute with the most significant dif-
ference follo wed b y textur e and over all a ccep tab ilit y. There
was least difference among the samples for appearance.
The attributes of colour and taste also showed evident
differences, but with their ranges much smaller than that
of mouthfeel.
3.6. Development of Rice Extrudates under
Optimized Conditions
The optimized conditions obtained from RSM were:
Screw speed: 400 rpm
Barrel temperature: 130˚C
Concentratio n o f sweet potato: 150 g/kg
The optimized conditions for making rice-sweet potato
extrudate were followed to make rice-greater yam extru-
date, rice-sweet potato extrudate and rice-sweet potato-

E. B. Hazarika et al. / Agricultural Sciences 4 (2013) 12-22
19
tomato-chilli extrudate. The feed compostion of the blends
used for processing into extrudates is given in Ta bl e 9.
The photographs of the extrudates are given in Figures
3(a)-(c).
3.7. Properties of Rice Extrudates Prepared
under Optimized Conditions
Physical Properties
The changes in L, a and b values and the total color
change in RS, RY and RST are shown in Table 10. The
color profile showed that there was a significant decrease
Fi g ure 2. Sensory profile of the extrudates.
in the L value i.e. lightness of the sa mples post-extrusion.
Also, as indicated by the results, extrusion resulted in an
increase in a value i.e. redness of the samples. a was
stro nger i n R ST ha vi n g to mat o . The yel lowness ( b value)
increased in RST, but in RY and RS, there was a slight
decrease probably due to destruction of pigments. The
total difference in color values (∆E) of RST showed a
much significant colour change compared to RY and RS.
Shih et al. (20 09) fo und that air-dried sweet pota to slices
had a ∆E values of 7.80. High temperature of extrusion
cooking (130˚C) mi ght ha ve a hi ghe r effect o n the co lour
of the extrudates.
The texture profile of RS, RY, and RST are shown in
Table 10. Product hardness is the average force required
for a probe to penetrate the sample. T he results of texture
analysis showed that RST has the highest ha rdness with a
value of 47.57 N, while RS had the lo west with 28.68 N.
Springiness was also found to be the hi ghest in RST, b ut
RY and RS had equal springiness value of 0.51 mm. The
higher degree of tomato incorporated extrudate may be
because of the effect of tomato pulp. It has got a good
amount of fibre in it that could have added to the in-
creased hardness of RST.
Physico-chemical properties Pasting Properties
The pasting properties of the 3 extrudates-R S , RY a n d
RST are shown in Table 11. All the samples had an equal
pasting temperature between 50.1˚C and 50.2˚C. From
Table 9. Feed composition for extrusion under optimized conditions.
Sl.No. Composition Product Code
1 Rice flour(85%) + sweet potato powder(15%) + 3% wat er RS
2 Rice flour(85%) + yam powder(15%) + 3% water RY
3 Rice flour(80%) + sweet potato powder (15%) + Tomato pulp powde r ( 5% ) + 3% Chilli powder RST
Numbers in the brackets indicate percentage level of incorporation.
(a) (b) (c)
Figure 3. (a) RS extrudates; (b) RY extrudates; (c) RST extrudates.
Table 10. Changes in colour values and texture profile of RS, RY and RST.
Sample Pre extrusion Post extrusion ∆E Texture
L0 a0 b0 L a b Hardness (N) Springiness (mm)
RS 76.28 3.90 12.31 7 3.11 4.16 11.53 3.33 28.68 0.51
RY 62.58 3.09 9.30 60. 10 3.80 8.17 2.82 34.53 0.51
RST 71.10 6.39 11.98 68.22 11 .8 1 8.63 9.09 47.57 0.95
6
6 .5
7
7 .5
8
Appearance
Colour
Taste
Mouthfeel
Texture
Overall
acceptabilit
y
RS-1
RS-2
RS-3
RS-4
RS-5

E. B. Hazarika et al. / Agricultural Sciences 4 (2013) 12-22
20
Table 11. RVA profile, Water Absorption Index (WAI) and Water Solubility Index (WSI) of RS, RY and RST.
Sample Pasting properties WA I (g gel/ g) WSI(%)
PT(˚C) PV(cP) HV(cP) FV(cP) BD(cP) TSB(cP)
RS 50.2 704 220.5 337.5 483.5 117. 0 8.16 ± 0. 3 5 3.22 ± 0. 0 4
RY 50.1 661 187.0 295.0 474.0 108.0 8.59 ± 0.75 3. 41 ± 0. 14
RST 50.1 644 159.0 254.5 481.0 95.5 8.16 ± 0.73 4.25 ± 0.25
the viscosity profile, it can be seen that extrudate having
sweet potato had the highest peak viscosity, hold viscos-
ity and final viscosity. Setback and breakdo wn were also
therefore the hi ghest in rice-sweet potato extrudates. RST
sample tha t had to mato p ulp powder had diluted the starch
and therefore its viscosity was lo wer than RS. The higher
viscosity of rice-sweet potato extrudate than rice-yam ex-
trudate indicates the susceptibility of the sweet potato
starch to extensive gelatinisation and the higher water
absorption of the gelatinised starch. According to El-
Dash et al. (1984), the intensity and extent of the break-
down of a starch granule will depend on the type of starch,
mec ha nic al shear, and temperature present during the ge-
latinization of the starch.
The water absorption index (WAI) and water solubility
index ( WSI) o f RS, RY and RST are s hown in Table 1 1.
Water absorption index (WAI) was found to be the high-
est in RY (8.59). However RST and RS had an equal
WAI value of 8.16. However, Water solubility index was
highest in RST (4.25%) and lowest in RS (3.22%). The
WAI measures the amount of water absorbed by starch
and is related to the degree of starch gelatinization. T hus,
higher value of WAI can be attributed to higher degree of
gelatinized starch in extrudates. According to Anderson
et al. (1969), WSI is related to the amount of low mole-
cular weight products of starch degradation, which are
easily soluble because of reduced entanglement. L ikima-
ni et al. (1991) accounted the higher value of WSI a re-
sult of starch damage during extrusion due to high tem-
perature.
Amylose content
The amylose content in RS, RY, RST, yam powder,
sweet potato powder and rice powder are shown in Table
12. Amylose content was found to be highest in the rice
flour (24.8%). The raw yam powder had slightly higher
amylo se content than the sweet potato powder. On blend-
ing, the amylose composition of rice-sweet potato and
rice-yam extrudate were reduced. The amylose content in
the sample containing yam powder (RY) reduced slightly
more (14.7%) than that of the sample containing sweet
potato powder, RS (15.2%). Among the extrudates, sam-
ple RST had the highest amylose content (18.2%). Apri a-
nita et al. (2009) reported that the amylose content of
yam flour to be 14.60%, while that of sweet potato flour
to be around 18.12%. Chinnaswamy et al. (1988) found
that the expansion ratio of extruded corn starches increased
from 8 to 16.4 as amylose content increased from 0% to
Table 12. Amylose content for various samples.
Sample Amylose (%)
Rice Powder (RP) 24.8 3 ± 0.77
Sweet Potato Powder (SPP) 20.0 6 ± 0.78
Yam Powder (YP) 21.94 ± 0.94
RS 15.29 ± 0.15
RY 14.7 4 ± 0. 16
RST 18.2 3 ± 0. 24
50%. The bulk density of the extrudates decreased with
increased amylose content. In contrast, shear strength of
starch extrudates increased with increasing amylose con-
tent. Similar results were reported by Guha et al. (2006)
on the effect of amylose content on product. The low-
amylose extrudates gave the higher expansion ratio, fol-
lowed by intermediate-amylose and high-amylose con-
tent extrudates. Extrudate products with a higher expan-
sion ratio are preferred.
Proximate p rofile
The proximate composition of RS, RY and RST are
shown in Table 13. The proximate profile of the extru-
date samples showed mixed results. The moisture content
of all samples was almost uniform around 9%. However,
differences were observed in ash, protein and crude fiber
contents. Ash, fat, protein and crude fiber percentages
were 2.4%, 0.29%, 6.3% and 1.35%, respectively in sam-
ple RST. RY had the lowest ash, fat and crude fibre con-
tent among all; 0.83%, 0.19% and 0.5%, respectively.
Protein content was found to be equal in both RY and RS
being 5.6%. By difference method, total carbohydrate
content was found to be highest in RY with 83.04% and
lowest in that of RST with 79.98%. Iwe et al. (2001)
found that in extruded soy flour and sweet potato blends,
fat content was 0.5%, ash 3% and carbohydrate around
79%. Aprianita et al. (2009) found that the total carbo-
hydr a te c o n te nt of ya m f lo ur wa s hi g her t ha n t ha t o f sweet
potato flour.
Sensory profile
The extrudates were subjected to sensory analysis for
the attributes of appearance, color, taste, mouthfeel, tex-
ture and overall acceptability by a panel of 10 semi
trained panelis ts. T he se nsor y pro file of RS, RY and RST
are graphic ally gi ven in F igu re 4. Among t he 3 sa mples,
the graph suggests that the sample with the best sensory
score in all the attributes i s RS, while the sample with the
least favourable a ttr ibute is RY, for its colour.
The attribute having the most prominent difference

E. B. Hazarika et al. / Agricultural Sciences 4 (2013) 12-22
21
Table 13. Proximate composition of RS, RY and RST.
Composition (w. b.)
Sample
RS
RY
RST
Moisture %
9.82 ± 0.02
9.84 ± 0.02
9.66 ± 0.07
Ash %
1.81 ± 0.34
0.83 ± 0.19
2.45 ± 0.18
Fat % 0.22 ± 0.01 0.19 ± 0.08 0.26 ± 0.01
Protein %
5.60 ± 0.0
5.60 ± 0.0
6.30 ± 0.0
Crude fib er %
0.99 ± 0.27
0.5 ± 0.14
1.35 ± 0.06
Total carbohydrate % (by difference)
81.56 ± 0.58
83.04 ± 0.22
79.98 ± 0.81
En erg y (kcal/100 g)
350.62
356.27
347.46
Figure 4. Graphical representation of the sensory profile of the
extrud ates RS, RY and RST.
among the samples is colour, followed by appearance.
However, as it is evident, there are no significant differ-
ences in the scores of the 3 samples in the other 4 attributes
namely-taste, mouthfeel, texture and overall acceptability.
RS had the b est ta ste a mong a ll. RST was the o ne getti ng
the least score in taste, but had the highest score in ap-
pearance.
Antioxidant Profile
The results of antioxidant properties of RS, RY and
RST are given in Table 14. By carrying out the radical
scavenging acti vity test, DP PH and the FRAP test for the
presence of antioxidants, the antioxidant content in the
extrudates were found to be very low. Both the tests showed
a similar result for all the 3 extrudates where it was ob-
served that the radical scavenging activity was highest in
RST than the other two being 3.6%. RY had a higher
radical scavenging activity at 3.22% than RS at 1.36%.
Similarly the Ferric reducing antioxidant power was
found to be higher in RST being 218 μM of Fe/100 g
than the other two. RY and RS had a closer FRAP values
being 148 and 118 μM of Fe/100 g. The higher antioxi-
dant activity in RST is due to the presence of tomato
pulp powder in its co mposition that naturally has a good
amount of antioxida nts in it. The low antioxidant conte nt
in the extrudates is due to the fact that the molecules
acting as antioxidants that were present in the raw sam-
ples are generally destroyed during the high temperature
used in extrusion cooking. According t o Sh ih et al. (2009),
temperatures greater than 6 0˚C is regar ded as unfa voura-
ble, due to the possibility of inducing oxidative conden-
sation or decomposition of thermolabile compounds, such
as catechin. A similar decrease in reducing power has been
Table 14. Antioxidant profile for RS, RY and RST.
Sample DPPH
(% radica l scavenging activity) FRAP (µM of Fe/100g sample)
RS 1.36 ± 0.08 118 ± 0.12
RY 3. 22 ± 0. 16 14 8 ± 0.11
RST 3.60 ± 0. 1 5 21 8 ± 0.14
reported by other authors like Xu et al. (2008) upon ther-
mal processing in different cereals.
4. CONCLUSION
The studies have shown that both sweet potato (Ipo-
moea batatas) and greater yam tubers (Dioscorea alata)
can be utilized for development of ready to eat (RTE)
products. Both the extruded products of rice incorporated
with sweet potato powder and yam powder have shown
to possess good functional properties and desirable nutri-
tive value. RTE breakfast cereal products available in the
market are generally expensive, so if such products are
developed that are incorporated with locally available
vegetables, not only those products can be made afforda-
ble enough, but it will also help the local farmers and
traders in improving their economy and vegetation. Le-
vels of fat, carbohydrate, protein and kilocalories of ener-
gy that these products were carrying, were found to be
comparable with that of a popular RTE breakfast product
available in the market. So it can be concluded fr om this
study that nutritionally enhanced breakfast cereals can be
made from rice-sweet potato extrudates and rice-yam ex-
trudates with addition of mine rals and vitamins.
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