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Twin-Screw Extrusion for Expanded Rice
Products Processing Parameters and
Formulation of Extrudate Properties
INTRODUCTION
Extrusion cooking of food materials containing mainly starch has been widely
used for corn and wheat products, while rice extrusion has been studied only in
recent years. Similarly, there are fewer extruded rice products on the market than
extruded corn or wheat products. Nevertheless, rice crackers have been gainingpopularity in western markets during recent years.
Since rice starch contains less amylose than cornstarch, the properties of the
extrudates may be different. It is the objective of this series of studies (Chiou,
1986; Pan et al., 1987; Wu et al., 1989; Chen et al., 1990 a, b) to understand the
physical as well as the sensory properties of rice extrudates. The effects of
processing parameters and ingredients formulations on extrudate properties were
compared, using corn grits extrudate as a reference.
DIFFERENCES IN SENSORY PROPERTIES OF RICE AND CORN
EXTRUDATES
When corn and rice went through the same extrusion cooking process with the
screw configuration shown in Table 1, the extrudates of corn showed the higher
expansion ratio and tasted more crispy. In addition the rice extrudates exhibited a
slightly chewy and sticky mouthfeel as shown in Table 2. Since rice was higher
in amylopectin content (Chang and Liu, 1988) and formed a more viscous melt in
the barrel than corn did, it needed more effective mixing and kneading to
produce extrudates comparable to corn products.
Table 1 Screw Configuration
Function Reversal Compression Semi Compression Conveying
Length, mm 50 100 50 100 200Pitch, mm -15 15 25 35 50
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EFFECTS OF RICE PROPERTIES
Variety
Waxy rice produced sticker, less expanded extrudates than the nonwaxy (i.e.,
Penglai) rice, while the extrudates of the latter were tougher than the former. The
nonwaxy rice extrudates were higher in water absorption index and lower in
water solubility index than the waxy rice products. It seemed that increases in
amylose content caused increases in water absorption index.
Particle Size
Rice grits produced less expanded extrudates than the whole grain rice (Table 3).
Howevwer, higher homogeneous cell structure and smaller cells, as well as a
thinner cell wall, were observed in extrudates of rice grits. Rice flour preground
to 60-100 mesh impoved the homogeneity of extrudates (Table 4), probably by
improving flow properties leading to reduced power consumption (fig. 1) andheat transfer of the rice materials in the barrel (Chen et al., 1990c). Rice grain
milled to 20-80 mesh resulted in crispy rice of homogeneous appearance (chiang
and lue, 1989).
Moisture Content
Moisture content is the most important factor affecting the crispness, bilk density
hardness and appearance of expanded rice products (Chiou, 1986; Pan et al.,
1987; Wu et al., 1989; Chen et al., 1990a,b). As the feed moisture increased, the
sectional expansion ratio of rice extrudates decreased linearly. The watersolubility index of waxy rice extrudates was higher than that of nonwaxy rice at
the same moisture content and operating conditions (fig. 2).
Table 2 Chemical Properties of Rice and Sensory Properties of the Extrudates
Material Water
solubility
index (%)
Rankingb
Variety Amylose
(% starch)a
Stickiness Expansion Crispness
Rice
Penglai,
nonwaxy
12.8 - 16.7 25 3 2 4
Long-grain,
waxy
< 0.8 25 40 2 3 2
Short-
grain, waxy
1.0 0.2 > 40 1 4 3
Corn grits 24 - 26 15 4
(Not sticky)
1 1
aFrom Rice Varieties in Taiwan, 1930-1987, pp. 205, 236, 285 and Chang and Liu, 1988.
b From 1 (highest) to 4 (lowest) by sensor evaluation.
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Table 3 Particle Size of Rice and Physicochemical Properties of Extruded Products
Particle size
(mm)
Product
moisture
(%)
WAI
(g gel/g dry
wt)
Shear force
(kg)
WSI
(g dry wt/g
dry wi)
Gelatinization
(%)
Bulk density
(g/cm3)
Penglai Rice
2.00-1.41 6.8 5.38 1.8 0.2 0.07 70.4 0.15 1.41-1.00 6.9 5.60 2.1 0.3 0.09 69.6 0.15 1.00-0.60 6.9 5.51 1.5 0.2 0.09 64.3 0.15
0.60-0.3 6.1 5.37 1.5 0.2 0.11 63.8 0.16 < 0.30 6.2 5.2 1.0 0.2 0.21 63.7 0.12
Long
Glutinous
Rice
2.00-1.41 8.2 2.3 1.2 0.2 0.35 54.2 0.19
1.41-1.00 7.4 2.2 1.4 0.3 0.28 53.8 0.19
1.00-0.60 7.6 2.3 1.2 0.3 0.38 46.6 1.17 0.60-0.30 8.1 2.08 1.2 0.3 0.65 48.4 0.14
< 0.30 5.9 1.55 0.9 0.2 0.60 46.8 0.14
Table 4Structure Changes Caused by Addition of -Cellulose to Rice Extrudates with 3 % Soybean Oil Added Using a
(barrel temperature, 160; screw speed. 180 rpm; and feed rate, 57kg/h).
Cellulose (%) Cell wall Thickness (m)a
Cell size (mm) Cell number (number/cm
0 83 14c 2.3 6.7 8.0 3.5g
1 66 9d
0.4 3.8 28.5 .70f
2 49 7e 0.1 - 3.5 51.9 7.3e
3 47 14e
0.1 3.2 79.9 7.3d
4 48 7e
0.1 3.0 118.3 11.8c
Rice flour 45 7e
0.1 3.0 84.8 7.5d
aValues followed by the same superscript c-g in the same column are not significantly different (p
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Feed rate
(kg/h)
t (s) at screw speeda
120
rpm
150
rpm
180
rpm
210
rpm
240
rpm
10 66 48 * * *
15 43 9 * * *
20 39 34 32 32 3525 30 31 29 28 28
30 27 27 24 26 2635 - 22 22 23 24
40 - 20 19 21 2045 - - - 19 19
50 - - - 17 19
Figure 3 Effect of screw speed and feed rate on the shear force of a glutinous rice product extruded with 3% soybean oil using a C
temperature of 160C (R2 - 0.85,p < 0.0001).
Figure 4 Effect of screw speed and feed rate on the water solubility index of a glutinous rice product extruded with 3 % soy
extruder at a barrel temperature of 160 C (R2 - 0.83, p < 0.0002).
Figure 5 Effect of screw speed and feed rate on the cross-sectional expansion ratio (mm/mm) of a glutinous rice productextrudedtwin-screw extruder at a barrel temperature of 160C (R2 = 0.94, p < 0.0001).
Table 6 Difference (t) between Mean Residence Time and Peak Residence Time with Changing Feed Rate and Screw Speed o
Rice with 3 % Oil Added.
Feed rate
(kg/h)
t (s) at screw speeda
120
rpm
150
rpm
180
rpm
210
rpm
240
rpm
10 19 11 * * *
15 6 3 * * *20 3
b----
0
----
2 2 7
25 -3 -1 1----
1 1
30 -5 -2 -1 1
----
1
35 - -5 -2 -1 1
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----
40 - -5 -5 -1 -245 - - - -2 -250 - - - -4 -2a-.beyond safety range of extruder (Clextral BC45) and, *, feed rate low for the screw speed.
b---- when t = 0.
Table 7 Contribution of R2 to Regression Cotxii-icicnis (bn.), and Significance (p) of Screw Speed (x1) and Fee
Extrudate (Y), using the, StatisticalModal: Y = b0 + b1x1 + b2x2 + b3x12 + b4X22 + b5x1x2 + b6x13 + b7x23+ b8x12X2 + b9
Coefficient Whiteness Moisture
content
Water solubility
index
Cross- sectional
Expansion ratio
Longitudinal
expansion
ratio
spec
Volum
B0
B1
B2
B3
B4
B5
B6
B7
B8
B9
R2
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p
parameters not at 0.05 significant level with the stepwise method of the SAS program are indicated by -.
Table 8: Regression Coefficients of Three Independent Variables (Oil Addition at 1-5 %, Barrel Temperature, a
Extrudates of Penglai Rice: 19 Degrees of Freedom
VariablesWater solubility
iridex, Y,
Color difference,
Y2
Shear force,
Y3
Oil addition, X1 -0.4872 -2.1834b 682.0875
Temperature, X2 -0.0051 -2.2086 -296.7894
Screw Speed, X3 -0.0170 -2.6347 a -117.95206
X12 -0.0170 0.7711a -10.1513
X22 -0.000 0.00430 0.7630
X32 -0.000 0.000 0.0709
XIX2 0.0016a 0.0325 -4.9000
XIX3 0.00190 -0.0575# 1.000
X2X3 0.0001 00065a 0.5625
R2 0.82 0.90 0.83
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a Sigrnificant = atp = 0 01.
b Sigrnificant = atp = 0 05.
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EFFECTS OF INGREDIENTFORMULATI ONS
Sugar
Addition of sugar induced cartelization of extrudates and decreased the
expansion ratio. Addition of sugar to rice flour increased the water holding
capacity and color intensity of crispy rice products (Tsiang, 1988).
Salt
Addition of salt improved the gelatinization of corn grits during extrusion.
Salt also increased the water holding capacity of the extruded product (Chiou,
1986; Chiang and Lue, 1989).
Polysaccharides
Addition of guar gum resulted in less expanded rice extrudates, while CMC
and pectin enhanced the expansion (Table 9) (Tsiang, 1988). Addition of a-
cellulose increased the longitudinal expansion of the extrudates, decreased
the power consumption (amperes) of the extruder, probably by acting as a
flow conditioner (Fig. l), and resulted in improved homogeneity in the cell
structure of rice extrudates (Table 4). In addition, stickiness and crispness of
extrudates were improved.
Monoglycerides
Addition of about 3% monoglyceride decreased the water solubility and
stickiness of the rice extrudates but was less effective in improving the
expansion ratio.
Lecithin
The expansion ratio of rice extmdates was not improved by the addition of
lecithin at 2%, but the appearance of the puffed extrudates was smoother
and whiter. Unfortunately the high extrusion temperature caused lecithin to
decompose and produced an ammoniac odor (Chen et al., 1990c). However,
addition of 0.2% did not yield the undesirable odor. The volatile
components resulting from the addition of 0.2% lecithin are shown in
Table 10 (Hau et al., 1990).
Gum Cross-
sectional
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expansio
n ratio
Control 2.81
Agar 2.71
Guar gum 2.65
CMC 3.07Pectin 3.16 Addcd at 2% of rice on dry weight basts (Tsiang, 1998).
Table 10 Volatile Compounds Formed in Extrusion of Starch and 0.2% Lecithin
Octanoic acid
1,2-Benzene dicarboxylic acid Tridecanoic acid
Heptadecanoic acid
Hexadienoic acid
9,12-Octadecadienoic acid 1-Pentanol
1.3-Di (isobutoxyearboxv1)- 2-4,Trirnethy1 pentane2-Propenamide
Dioxylphthalate
Source: Hau et al. (1990).
Soybean Oil
The addition of soybean oil at 3-4% to whole-grain white rice increased the
sectional expansion ratio of die extrudate by two- to four-fold (Fig. 6) and
produced greater crispness (Fig. 7). However, the water solubility index and
stickiness of mouthfeel also increased (Fig. 8). When the amount of soybean
oil added was adjusted to between 0 and 205v and the barrel temperature
between 120 and 180C at a feed moisture of 13%, the extrudate was the
highest expansion ratio and water solubility index, as well as the bes(
crispness, was obtained at a barrel temperature of 158C and 3% added oil.
Scanning electron microscopy showed that the rice extrudates consisted mainly
of closed cells in lamina
Figure 6 Expansion ratio (%) of rice extrudates as affected by the addition of
soybean oil and by barrel temperature using a Clextral BC45 twin-screw
extruder (screw speed, 120 rpm; and feed rate, 28 kg/h): (A) glutinous rice and
(B) Penglai rice.
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Figure 7 Shear force (g) of rice extrudates as affected by the addition of soybean oil and
by barrel temperature using a Clextral BC45 twin-screw extruder (screw spced, 120
rpm: and feed rate, 28 q/h): (A) glutinous rice and (B) Penglai rice.
Figure 8 Water solubility index (g dry wt/g dry wt) of rice extrudates as affected by
the addition of soybean oil and by barrel temperature using a Clextral BC45 twin-screw
extruder (screw speed, 20 rpm: and feed rate, 28 kg/h): (A) glutinous rice and (B)
Penglai rice.
Figure 9 Scanning electron micrographs of glutinous rice extrudates exLmI-rci
layers and appeared flaky and porous. Those without added oil showed open cell
network structure (Fig. 9). The browning intensity of the extrudates was
reduced by the addition of 3% soybean oil, probably as a result of improved
mixing and reduced friction of the feed material in the barrel. Water solubility
ndex, bulk density color difference, whiteness, and shear force of the extrudates
varied with increasing le-,el of oil addition (R2 > 0. 80, p < 0.01).
Figure 10 Effect of ratio of fish paste (2.5% NaCI) to com grits on expansion ratio (%) of
extrudates at feed moisture 40%, barrel de temperature 150C, and screw speed 120
rpm using a Clextral BC45 twin-csrew extruder
Proteinaceous Ingrediente
The addition of fish paste (
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increase in breaking force at 14-35 % (Chiang, 1988). The addition of banana
flour affected significantly (p < 0.01) the expansion ratio, hardness, adhesion,
brittleness, and viscosity of the extrudates (Hsu. 1989).
Bran
The addition of wheat bran to rice resulted in a saving in energy consumption
(Fig. 1) and in a homogeneous structure of the extrudates (Fig. 11). Wheat bran
not only improved the equality of extrudates but also increased the fiber content.
Figure 11 Cross section of whole-grain rice extruded with 3% soybean oil added as
affected by the addition of - cellulose and wheat bran using a Clextral BC45 twin-
screw extruder (barrel temperature, 160C; screw speed, 180 rpm; and feed rate, 57
kg/h).
CONCLUSION
The variety of rice plays and important role in determining the types and
properties of extrusion products developed. The amylose contents of the
major types of rice, which may become the dominant factor in product
properties, varied from 0.1% to about 26%. Stickiness in mouthfeel seems
to be a drawback of the expanded rice products. The high viscosity of the
rice melt conveyed through the screw, as shown by the amperes consumed,
indicated that methods or ingredients capable of lowering rice melt viscosity
will probably enhance crispness and reduce product stickiness, in addition
to saving energy. Ingredient having laminar structures may serve as flow
conditioners to the rice melt. The mechanism and means of eliminating
stickiness are flavored of interest in rice extrusion.
At the present time, rice extrusion products are flavored mainly by
postextrusion treatment. Application of flavor precursor to develop flavor
during the extrusion process seems to be another area of future development for
rice extrusion.
ACKNOWLEDGMENT
The financial support of the Council of Agriculture of the Republic of China
for this series of rice extrusion studies and funding for the purchase of
Clextral BC45 twin-screw extrude,- is greatly appreciated.