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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.

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