Journal of Scientific & Industrial Research

Vol. 65, February 2006, pp. 165-168

Physico-chemical characteristics of extruded snacks prepared from rice

(Oryza sativa L), corn (Zea mays L) and taro [Colocasia esculenta (L) Schott] by

twin-screw extrusion

P Bhattacharyya, U Ghosh, H Gangopadhyay and U Raychaudhuri*

Department of Food-Technology & Biochemical Engineering, Jadavpur University, Kolkata 700 032

Received 11 August 2005; accepted 22 November 2005

Rice, corn and taro starch blends were used to prepare snacks by a twin-screw extruder at 141, 150 and 159oC

respectively. Sodium dodecyl sulphate (SDS) and phosphate buffer (pH 6.9) were found to extract more protein than plain

buffer solution from extrudates. Loss of carbohydrate (maltose) was documented in extruded snacks. A universal texture

analyzer (model-4301 Instron, London, UK) was used in the compression mode to record the required force to break the

extruded products. The results showed a trend toward increasing expansion, decreasing density, and decreasing breaking

force with barrel temperatures of 141-159°C. The surface morphology of the extrudates was examined through scanning

electron microscopy (SEM).

Keywords: Corn, Extruded products, Protein solubility, Rice, SDS, SEM, Taro, Texture analyzer

IPC Code: A23J1/12

Introduction Snacks contribute an important part of many

consumers’ daily nutrient and calorie intake1.

Consumers can enjoy snack foods with good taste,

flavour, pleasing textural mouth feel and get “fun

eating” with nutrient supplements also2. Starchy

materials like rice, corn, potato, taro etc have been

popular feed materials for extrusion because of their

ready availability and good functional properties. The

introduction of twin screw extruders has widened the

scope of food extrusion technology for the

manufacture of many cereal-based products including

ready-to-eat breakfast cereals, infant food

formulations, snack foods and modified starches3.

Starch is used in a wide array of products from paper

to prepared foods4-5

, a wide variety of food and

nonfood products6 and a variety of fat substitutes

7-9.

The major component of the corn, taro root, rice etc.

is starch, which is responsible for the mechanical

properties of the dough10

and consequently plays an

important role as a determinant of the food product

quality. Actually, the crystalline order of starch

granules is lost during gelatinization11

.

The objectives of the present study are: a) To

examine the changes of textural properties of

extrudates; b) To compare the protein solubility in

different extracting solution and maltose content of

unextrudate and extrudate; and c) To examine the

surface morphology of the extrudates through

scanning electron microscopy (SEM).

Materials and Methods Materials

Corn seeds (Zea mays L), Rice (Oryza sativa L)

and Taro [Colocasia esculenta (L) Schott] root were

procured from local market. 3,5 Dinitrosalicylate

(Loba Chemie, Mumbai), sodium phosphate dibasic

anhydrous purified (Loba Chemie, Mumbai), and

sodium dihydrogen orthophosphate (Qualigens Fine

Chemicals, Mumbai) were purchased. Sodium

dodecyl sulfate (S.D. Fine Chem. Pvt Ltd, Boisar-

401501) and urea (Sisco Research Laboratories Pvt.

Ltd, Mumbai) were also procured. The rice, corn and

taro flours (100:10:9) were blended. Extruder and Extrusion Cooking

A co-rotating fully intermeshing twin-screw

extruder (Model No P1, Basic Technology Pvt Ltd,

Kolkata) was used (Screw Profile: L/D, 12:1; barrel

length, 336 mm; barrel diam, 98 mm; screw diam,

28 mm; conveying angle, 30o; intermeshing screws,

24 mm apart) using a 3 mm diam die. The moisture

content (15%) of the dough was adjusted. The

__________________

*Author for correspondence

Tel: 91-33-2414-6663; Fax: 91-33-2414-6822

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166

temperature of the extruder barrel was maintained at

141, 150 and 159°C respectively throughout the

barrel. The screw speed was set at 475 rpm, while the

feed rate was maintained constant at 28 g/min.

Protein Solubility

Protein solubility was performed on both

unextrudate material and extruded products. The

following solutions were prepared for measuring the

protein solubility: 1) 0.1 M Phosphate buffer solution

of pH 6.9 (45 ml of 0.1 M monobasic sodium

phosphate +55 ml of 0.1 M dibasic sodium phosphate,

diluted to a total of 200 ml); 2) 1% Sodium dodecyl

sulfate (SDS), an agent known to disrupt non-covalent

interaction and hydrophobic interactions in buffer

solution; and 3) 8M Urea, an agent known to interrupt

with the hydrogen bond in buffer solution12

.

About 50 g samples were finely chopped with a

blender (Bajaj Mixer Grinder, GM-550) for 1 min.

The moisture content of each sample was measured

before extraction. Sample (500 mg) from each

treatment was extracted with 10 ml of solvent for

2.5 h in BOD incubator (Sambros India Ltd, SM-

0067) at 40°C with shaking (100 rpm). The

suspension was taken and centrifuged at 10,000 g for

20 min. The protein content of the supernatant was

determined using Folin-phenol reagent13

and was

expressed as bovine serum albumin (BSA) equivalent.

Moisture Content

Moisture content of control and extruded products

were measured using standard air oven (Model No-

06104, S C Dutta & Co, Kolkata) according to

AOAC14

NO14.002.

Texture Analysis

A universal texture analyzer (model-4301 Instron,

London, UK) was used in compression mode to

record the required force to break extruded products.

The extruded sample (5 cm long) was placed on the

platform transversally over a metal sheet support

(1 cm thick) and operated in the compression mode

with a sharp testing blade (3 mm thick, 6.93 mm

wide). The texturometer head moved the probe down

at a rate of 15 mm/min until it broke the extrudates.

SEM

Scanning electron microscope (JEOL, JSM5200,

TOKYO, JAPAN) were taken at an accelerating

voltage of 20 km to view extrudates in three

dimensions and to determine the shape and surface

features of extruded products. Extrudates from all the

treatments and the control (unextrudate) sample were

mounted stubs with adhesive tape and sputters coated

gold approx 190 A° thick for 2.5 min at 10 mA before

observation with SEM. One micrograph was taken for

each starch sample at 50 X magnification for extruded

products and at 1000 X magnification for unextruded

sample. All the images for each sample showed

representative results.

Determination of Starch Digestion

Effects of extrusion on starch damage were

estimated by determining reducing sugar (maltose) of

each sample. Reducing sugar was determined by

AOAC14

(925.10,1990) method.

Statistical Analysis

Data were analyzed using single factor ANOVA in

EXCEL (Microsoft Office 2000). Significance level at

P≤ 0.05 was applied to the results to test the

significant difference.

Results and Discussion Protein Solubility

Protein solubility test was performed to investigate

the forces that were responsible for stabilizing the

extrudate during the extrusion. Moisture content of

each sample was determined before protein

extraction. The extractable protein decreased in all

solvents after extrusion (Table 1) probably due to

formation of some protein with new chemical

linkages, such as non-disulfide covalent bonds that

were not disrupted by the solvents used or the

formation of some polymers with very high molecular

weight15

. The amount of protein extracted by

phosphate buffer in both raw material and extrudates

was significantly less than amounts extracted by other

solvents may be because the phosphate buffer alone

only dissolved protein molecules that were in their

native states16

. When a second solvent was combined

Table 1—Protein solubility in different extracting solutions of

unextrudate and extrudate

Samples Protein solubility, %

PB PB+8 (M) U PB+SDS

Unextrudate 0.096±0.004 0 .118±0.005 0.127±0.002

A 0.046±0.002 0.064±0.008 0.089±0.006

B 0.039±0.002 0.065±0.007 0.094±0.007

C 0.038±0.002 0.060±0.009 0.082±0.005

A, extrusion at 141oC; B, extrusion at 150oC; C, extrusion at

159oC

PB: Phosphate Buffer (pH 6.9); SDS: Sodium dodecyl

sulphate; 8(M) U: 8 molar urea.

BHATTACHARYYA et al: CHARACTERISTICS OF EXTRUDED SNACKS FROM RICE, CORN AND TARO

167

with the phosphate buffer, protein solubilities

increased, which suggest that the protein aggregated

with more than one type of chemical bond.

Breaking Force

The results showed a trend toward increasing

expansion, decreasing density, and decreasing

breaking force with barrel temperatures from

141-159°C. Variations (Fig. 1) observed in the

breaking force of extruded products indicated that

barrel temperature is important for this characteristic

in starch mix flour extrusion. Breaking force is

influenced by the expansion index and starch layer

alignment for formation of final structure17

.

Starch Digestion

The expansion of starch (or cereals) depends mainly

on its degree of gelatinization, which in turn is

determined by temperature18

. Extrusion cooking is one

processing method that has been used to modify the

digestible characteristics of starchy materials. The

amount of starch digestion was determined by

comparing the absorbance of each sample to a standard

maltose calibration curve, and reporting the results as

percent (%) maltose content19

. The main difference in

the digestion profiles was due to temperature. The flours

extruded at 141°C digested significantly slower than

those extruded at 150 and 159°C (Fig. 2). The maltose

content was highest for the products extruded at 159°C.

Fig. 1—Texture analysis of extrudates: Sample 1, extrusion at

141°C; Sample 2, extrusion at 150°C; Sample 3, extrusion at

159°C

Fig. 2—Determination of starch damage (5 maltose content):

Sample 1, unextrudate; Sample 2, extrusion at 141°C; Sample 3,

extrusion at 150°C; Sample 4, extrusion at 159°C

Fig. 3—Scanning electron micrographs (SEM): (A), unextrudate; (B), extrusion at 141°C; (C), extrusion at 150°C; (D), extrusion

at 159°C

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168

Microscopic Examinations

Extruded samples had porous, open-celled

structures. Fig. 3A shows intact starch granules in raw

mixed (rice, corn and taro) starch flour. Important

granule damage occurred during extrusion. Extrudates

showed large numbers of flattened and sheared

granules. Damage was highest in case of extrusion at

159°C (Fig. 3D) followed by 150oC (Fig. 3C) and

141°C(Fig. 3B).

Conclusions

The extrusion process parameter (barrel

temperature) markedly affect the texture, starch

digestible characteristics and surface morphology of

taro, rice and corn starch blends extrudates. The

lowest value (P<0.05) for the breaking force of

extrudates (13.03N±0.88086) was found for the

extrusion at 159°C.More protein was soluble in SDS

containing buffer than in plain phosphate buffer

(pH 6.9). Microscopic observations showed large

numbers of flattened and sheared granules. The

organized structure of starches was modified during

the extrusion process.

Acknowledgements

Authors thank Departmental Special Assistance

Programme Phase III (UGC, Govt. of India, New

Delhi) and Department of Biotechnology, Govt. of

India, New Delhi for their financial support.

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