J Sc; Food Ayric 1989, 49, 369-317

Physical Properties of Keropok (Fried Crisps) in Relation to the Amylopectin Content of the Starch Flours*

Suhaila Mohamed, Norakiah Abdullah and Mangayar Karasi Muthu

Faculty of Food Science and Biotechnology, Universiti Pertanian Malaysia, 43400 Serdang, Selangor, Malaysia

(Received 23 February 1988; revised version received 6 February 1989; accepted 18 February 1989)

ABSTRACT

The physical characteristics of keropok made from various jlours (glutinous rice, rice, wheat, sago, tapioca and corn) were studied. The amount of’ water used to wet the flour in the initial stages ofmaking the keropok was very important, since it affected the linear expansion, oil absorption, elasticity and crunchiness of’ the resultant Pied crisps. Linear expansion, oil absorption and elasticity were positively correlated to the total amylopectin content in the whole flour (r2 =0.99, 0.97 and 0.97 respectively). The best fitted lines for prediction of linear expansion, oil absorption and elasticity oj the fried crisps were 21y 6 0 . 3 5 ~ - 162, y=0.15x- 74 and y=0~0008x-0~41 respectively, where x is the amylopectin content of the whole flour. Good keropok can be made from any starch-rich flour that contains a high proportion of amylopectin. These starches can be recognised physically by their clarity on gelatinisation, stability to retrogradation, low resistance to shear and high cold-paste viscosity. The addition of mung bean flour to tapioca flour helped increase the protein content and flavour of the keropok; it also reduced the oil absorption of the keropok during frying. Although linear expansion decreased with increasing mung bean flour content, the substitution of up to 600 g kg-‘ mung bean flour was acceptable since the linear expansion of mung beanltapioca crisps remained above 35 %. The experimental values for linear expansion of mung beanltapioca crisps correlates very well (r2 = 0.99) with the predicted linear expansion based on the amylopectin content of the

* Paper presented in part at the ASEAN Food Conference (poster session), Bangkok, 24-26 October 1988

369

J SCZ Food AgrLc 0022-5142/89/$03.50 0 1989 Society of Chemical Industry. Printed rn Great Britaln

370 S Mohutned, N 4hdul/uh, M K Muthu

combined f lours. Legume crisps, being high in protein uvid ,fibre, are u possible cheup ulternutiue to ,fish crisps for the production o f ' a nutritious snack food for growing children.

K e y words: Amylopectin, oil absorption, linear expansion, crunchiness, elasticity, fried crisps (keropok)

INTRODUCTION

Keropok is the name in Malaysia for crisps or crackers made from gelatinised starch pastes (usually tapioca or sago in origin) which are dried to a moisture content of between 80and 150 g kg- and then puffed by frying in hot oil (Siaw et a1 1985). The best known keropok in Malaysia is keropok ikan (fish crackers), which by regulation should contain 3 200 g kg- I protein, and keropok undang (prawn crackers), which should contain 3 60 g kg- ' protein.

The objective of the investigation was to study the properties of crackers made from different starch-rich flours (namely glutinous rice, rice, wheat, corn, tapioca and sago) and to relate the properties of these keropok to the chemical constituents of the various flours. The main objectives were to investigate why certain starch-rich flours make good keropok whereas others do not and to develop an understanding, so enabling crisps to be made from other starch/protein flours such as legume flours or a combination of legume/starch flours. The findings from these experiments should permit the identification of those kinds of starch flours that are most suitable for making keropok. Cereal flours were studied because they contain a relatively high protein content compared with starches originating from roots or stems; furthermore they have an amino acid profile complementary to that of legumes. Legume crisps may be the next best alternative to fish crackers as a nutritious snack food for growing children. Legumes are high in protein and fibre yet low in cholesterol, and hence are beneficial for those who have to reduce the intake of animal products for health or religious reasons. Moreover legumes are low cost materials. Different legumes can also provide a variety of flavours, in contrast to the bland taste of starch-based crisps, and this may improve product marketability. Examples of well known legume-based snacks are papadom (from black gram) and muruku (from chickpea) which are Indian in origin.

EXPERIMENTAL

Materials

The various starch flours were bought from a grocery near the University. Refined, blended and deodorised palm oil (Labour brand) was used for deep frying the keropok. Dehulled mung beans (Phaseolus aureus L) obtained from the market were cleaned and then ground using a double disc mill (Safe World Enterprise, Klang, Malaysia).

Physical properties of' keropok

Yield stress

37 I

Force (F)

Preparation of keropok (Siaw et a1 1985)

Flour, as purchased, and water were mixed in the relevant proportions and the mixture was stuffed into cellulose casings (40 mm diameter Visopan or Wienie Pak cellulose casing from Industria navarra de envolturas cellulosicas SA, obtained from Mark Aid Co, Ltd, Malaysia). After tying the ends securely, the stuffed casings were steamed for 2 h. They were then dipped in iced water and chilled overnight at 5°C before the casings were removed and the dough sliced to 3 mm thickness with a gravity slicer. The separated keropok slices were dried in an oven at 50°C for at least 5 h. All the keropok slices were fried for 10 s at 200"C, except wheat keropok slices which were fried at 170°C.

Measurement of oil absorption, linear expansion, moisture, elasticity and crunchiness

Moisture contents were determined by oven drying at 105°C overnight (AOAC 1980). Linear expansion was determined by measuring three parallel lines drawn across the diameter of the keropok before and after frying. Linear expansion= 100(L2--Ll)/L1, where L1 =length before frying, and L2=length after frying.

For oil absorption, 100 g of keropok slices were weighed before and after frying in oil and then ground and oven dried at 105°C overnight. Oil absorption= lOO(W2- Wl)/Wl where W2= weight of dried fried keropok and W1= weight of dried unfried keropok.

Elasticity was determined using a Magnus Taylor probe attached to the Instron Universal testing machine by compressing the samples at a crosshead speed of 5 cm min- ' . The elastic modulus ( E ) was calculated, where E = stress/strain (N m-') . The crunchiness is taken as the yield stress (ie maximum stress before fracture (N m-')). A typical Instron graph of stress/strain is shown in Fig 1.

The texture measurements on fried keropok were done after it had been carefully stored overnight under dry conditions in hermetically sealed polypropylene packaging. All measurements were done on at least six samples.

Determinations of starch content in flour

Excess protein, fat and sugars were removed from 0.2 g flour, and the starch was gelatinised and hydrolysed with 0.2 ml amyloglucosidase (in 0.1 M acetate buffer,

312 S Mohamed, N Abdulluh, M K Muthu

pH 45). The amount of glucose released was determined by the Nelson-Somogyi method and multiplied by a factor of 0.9 to give the total starch content in the sample (Southgate 1976).

Determining the amylopectin content of the flour

Amylose was determined by the reaction with KI/12 (Morrison and Laignelet 1983). The amylopectin content of the starch was calculated by difference, and the amylopectin content of the whole flour was calculated.

RESULTS AND DISCUSSION

The optimum amount of water for keropok manufacture

The amount of water used to make the wet keropok dough is very important because it influences linear expansion, oil absorption and crunchiness of keropok made from different starch flours (Figs 2a-f). Insufficient water causes incomplete gelatinisation of the starch during steaming and results in keropok that does not expand well. Water in excess of the optimum level for maximum expansion limits expansion, probably due to the reduced solids content of the moist keropoks which have been sliced to constant thickness; such keropoks will be thinner after drying. Too much water also results in doughs that are too soft and difficult to slice.

Keropok made from glutinous rice, tapioca, wheat, rice and sago showed this increase and subsequent decrease in expansion with increasing water content of the moist keropok. Keropok made from cornflour does not show a decrease above the optimum water content (1200 g kg- ’ flour basis) probably because the two opposing factors balance each other.

The optimum water additions to the flour as purchased were 600,800,1000, I 100, 1200 and 1200 g kg-’ flour, for tapioca, wheat, sago, rice, glutinous rice and corn flour respectively.

For rice, glutinous rice, corn and sago flours the oil absorption appears to increase with increasing initial amounts of water used (Figs 2a-f). For tapioca and wheat flour, as the level of water used increased, there was an initial increase in oil absorption followed by a decrease. The increase appears to follow the same pattern as linear expansion. This could be because more oil is trapped in the surface layer of the bigger air ‘cells’ when expansion occurs. The optimum water level is the range that provides a compromise between maximum expansion and excessive oil absorption for each flour. Figures 2a-f also show the effect on crunchiness or hardness of using different amounts of water for gelatinisation of the starches. There appears to be an inverse relationship between linear expansion and crunchiness. Larger linear expansion reduced the thickness of the layer of starch molecules surrounding the air cells, so less force was needed to break the crisps. Crunchiness values below 2500 N m-’ can be considered acceptable for this type of keropok. Keropok made from wheat flour was the hardest to break, followed by that made from tapioca. The crunchiness of keropok made from rice, corn and sago did not differ significantly. Keropok made from glutinous rice was the least ‘crunchy’, ie the most brittle.

Physicul properties of' keropok 313

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Fig 2. Characteristics of keropok made with different amounts of water added to dry flour. Linear expansion (y ; ) , 0 oil absorption (961, + crunchiness (IOONm-'), il elasticityx 100 ( N m - I ) . (a) Glutinous rice crisps; (b) rice crisps; (c) tapioca crisps; (d) maize crisps; (e) sago crisps; (f) wheat crisps,

On the other hand the elasticity of keropok appears to be positively related to its linear expansion. Except for tapioca crisps, increasing linear expansion results in an increase in elasticity. The larger air cells may have resulted in the layers of starch molecules surrounding the air cells being more fully stretched so that greater force was needed to deform them, ie they had a greater elastic modulus.

Characteristics of various flours in relation to their ability to form good keropok

Table 1 is a summary of the characteristics of the various flours and keropok made from them. Gelatinised starch from glutinous rice flour was the only one that

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Physicul properties o/ keropok 315

showed some degree of freeze/thaw stability. This can be explained by the low amylose content since the amylose fractions are the ones responsible for retrogadation of starches and hence for freeze/thaw instability.

For flours that have similar protein contents, increasing amylopectin levels appear to result in increased expansion, oil absorption and elasticity in the resultant keropok. For flours that have similar amylose/amylopectin ratios, increasing protein content appears to cause decreases in linear expansion and oil absorption.

Table 1 also shows that starch-based flours which make good keropok, ie with good linear expansion, are those that are clear or translucent on gelatinisation (ie flours from glutinous rice, tapioca and sago). This clarity may be related to the high amylopectin content or low protein content of the starches. The opacity of the gelatinised starches is due to the tendency of the amylose portion to associate and crystallise out because of hydrogen bonding between hydroxyl groups of adjacent chains. This phenomenon, called retrogradation, is enhanced by the presence of other polar components such as random coiled proteins (ie heat-denatured proteins), fatty acids or monoglycerides. This is evident from the stability to retrogradation of starch flours that have high amylopectin content or low protein content. Similarly the ability of a type of starch to resist shear depends on the shape of the molecules. Molecules that are more globular (amylopectin) will show a more Newtonian behaviour and lower resistance to shear than linear molecules such as amylose. Another explanation is that certain portions of the linear amylose molecules tend to associate to form intermolecular crosslinks and lead to a rigid gel on cooling, thus resisting shear. Although flours that have potential for use in making good keropok have lower resistance to shear when cold, they have a high hot-paste viscosity. This is because at the higher temperatures amylopectin swells and imbibes considerable amounts of water whereas amylose molecules dissociate from each other due to the weakening of the hydrogen bonding and increased motion of the molecules. The presence of protein appears to reduce the hot-paste viscosity of flours that have similar amylose/amylopectin ratios, probably by inhibiting the swelling of amylopectin molecules.

Good correlations were obtained between the amylopectin content in the whole flour with oil absorption ( r2 = 0.97), linear expansion ( r2 = 0-99) and elasticity of the keropok (r2=0.97) following the formula y=0.15x-74, y=0.35x- 162 and y = 0.0008~ - 0.41 respectively (Fig 3) . The decrease in expansion with increasing

c g 1201 a, 1101

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Fig 3. Effect of amylopectin content of the whole A w y flour on x elasticityx 100(Nrn-'), + oil

absorption (x ) , and linear expansion ( x ) of starch-based crisps. Experimental values vs

% omyiopectin content in whole flour calculated best fitted line.

376 S Mohunwd, N Abdiilluh. M K Mdlr

protein content (Siaw et a1 1979; Yu et a1 1981) is more likely to be due to the reduction of the overall carbohydrate content and consequent reduction of the total amylopectin content in the keropok. Work relating the composition of starch to the property of extruded starch-based snacks led to the conclusion that high amylopectin content starches tend to give fragile products of low density, where the extent of puff and the texture of the finished snack are influenced by the amylose] amylopectin ratio (Matz 1984). Some amylose is needed to give adequate resistance to breakage and acceptable textures. A starch system containing 5&200 g kg- ' amylose was suggested as being the most suitable (Feldberg 1969). However, there was no mention of any relationship between the total amylopectin content of the whole flour and the properties of the finished products.

Mung beanltapioca crisps

Mung bean/tapioca crisps were made with combinations of mung bean and tapioca flour in the weight ratio range 30:70 to 80:20. The amylopectin and protein contents in the whole flour were calculated from the amylopectin and protein contents of the individual flours (Table 2). Preliminary organoleptic evaluation showed that the 50:50 combination was most liked and the 40:60 and 60:40mung bean/tapioca combinations were fairly well accepted. The addition of mung bean flour to the tapioca flour helped to impart some flavour to otherwise bland crisps, besides improving their nutritional value. The 40: 60 mung bean/tapioca flour keropok made crisps that were very well expanded but slightly lacking in flavour, whereas the 60:40 combination was more tasty than the 50: 50 combination but lacked good expansion. The 50: 50 combination represents a compromise between optimum flavour, moderate oil absorption and optimum expansion. The predicted and experimental values of the linear expansion of keropok made from mung

TABLE 2 Composition of mung beanltapioca crisps (g kg - ')

Mung beunltapioca (w /w) 30:70 40:60 50:50 60:40 70:30 80:20 ~

Protein 72 92 113 134 154 175 Amylose in starch 193 200 208 215 223 230 Amylopectin (flour) 654 625 597 568 539 510

Fig 4. Linear expansion of mung bean/tapioca crisps with varying amylopectin content. -

Experimental vs theoretical. 1 o i I

51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 % arnylopectin content in whole flour

Phvsical proprrrirs q / krropok 377

bean/tapioca flours are shown in Fig 4. The experimental values for expansion were very close, r2 =O.99, to the values predicted by the equation y = 035x- 162, where y is the linear expansion and x is the amylopectin content of the combined flour.

CONCLUSIONS

The main conclusions from the above experiments are that linear expansion, oil absorption and elasticity of keropok (fried crisps) are positively correlated to the amylopectin content of the whole flour, at least for the range studied, and that the lines of fit for the prediction of linear expansion, oil absorption and elasticity are y=O.35x- 162, y=O-15x-74 and y=O~OOO8s-O-41, respectively, where .Y is the amylopectin content of the whole flour. The initial amount of water used to mix the dough and assist gelatinisation of the starches was found to be very important because it affected linear expansion, oil absorption, crunchiness and elasticity in the final product.

Good keropok can be made from any starch-rich flour that contains a high amylopectin content. These starches can be recognised physically by their clarity on gelatinisation, being fairly stable to retrogradation and having low resistance to shear when cold but high paste viscosity.

REFERENCES

.40AC 1980 Of$cial Methods of Analysis (13th edn). Association of Official Analytical

Feldberg C 1969 Extruded starch-based snacks. Cereal Sci Today 14 21 1-214. Matz S A 1984 Snack Food Technology (2nd edn). AVI Publishing, Westport, CT, p 153. Morrison W R, Laignelet B 1983 An improved colorimetric procedure for determining

apparent and total amylose in cereal and other starches. J Cereal Sci 1 9-20. Pomeranz Y 1985 Functional Properties of Food Components. Academic Press, New York,

p 29. Siaw C L, Yu S Y, Chen S S 1979 Studies on Malaysian fish crackers: effect of sago, tapioca

and wheat flour on the acceptability. Symp Food Nutr Biochem in Asia and Oceania, Kuala Lumpur.

Siaw C L, Idrus A Z, Yu S Y 1985 Intermediate technology for fish crackers (keropok) production. J Food Technol 20 17-21.

Southgate D A T 1976 Determination of Food Carbohydrates. Applied Science, London,

Yu S Y, Mitchell J R, Abdullah A 1981 Production and acceptability testing of fish crackers

Chemists, Washington, DC.

pp 128-129.

(keropok) prepared by extrusion method. J Food Technol 16 57-58.