Nutritional, Functional and Physical Properties of Extrudates from Blends of Cassava Flour with Cereal and Legume Flours

Abstract

Low protein and poor functionality limit the use of cassava flour in snack foods, which were modified using blends with cereal and/or legume flours. Native, malted (using alpha-amylase) as well as malted and pre-gelatinized was blended with cereal (finger millet and whole wheat flours) and/or legume (chick pea flour). Extrudates were prepared at a screw speed of 100 rpm and die temperature of 180 °C. Malted flour based extrudates had lower starch content than native flour. Gram malted cassava based blends gave products with the highest protein. In vitro starch digestibility was the highest for pre-gelatinized flour based mixes. Extrudates with low fat and energy have scope as low calorie snacks for obese and diabetic people.

Keywords:

• Cassava
• Composite flours
• Extrudates
• Nutritional and functional properties
• Expansion ration

INTRODUCTION

Extrusion cooking is a high temperature short time (HTST) process that is widely popular in the food and feed industries, due to several of its advantages like faster processing time, considerable saving in energy leading to lower price of the products, possible production of oil free puffed products, etc. It comprises a substantial fraction of the snack food market ranging from breakfast cereals, pasta and noodles, vermicelli, confectionaries expanded crunchy snacks, etc., besides pet foods and modified starches.[1,2] Extrusion cooking is used to make products having better flavour, digestibility, storage life, and safety.[3] Several ready-to-eat snack foods have been reported exclusively from cereals or their blends with legume protein sources, etc.[4–6] Cereal flours are generally low in protein content, but high in sulfur containing amino acids, while legumes are rich in protein with a high proportion of lysine and only small quantity of sulfur containing amino acids.[7–11]

Cassava (Manihot esculenta Crantz) is a root crop with starchy tubers and is cultivated and consumed in approximately 102 countries.[12] Nevertheless, the flour is low in protein, which is a disadvantage in its wide scale use in bakery and snack foods. Fortification with protein sources, cereals and legumes has been attempted by several workers to expand the spectrum of cassava flour utilization in the food industry.[13–18] The extrusion behaviour of cassava for making snack foods was studied by some workers.[20–24] Earlier studies in our laboratory have shown that the nutritional and functional properties of cassava flour could be beneficially modified through pre- treatment with amylolytic enzymes as well as by blending with cereal and/or legume and bran sources.[18] The use of such modified composite flours for making baked goods having high protein and high dietary fiber has been also been investigated.[25] Unmalted (native) cassava flour, malted (using termamyl and green gram amylase) cassava as well as malted and pre- gelatinized cassava flour were prepared as described elsewhere[18] and these were blended with cereal and/or legume flours and used for making extrudates.

MATERIALS AND METHODS

Samples

Edible grade cassava flour was prepared from the cassava variety M4, by washing, peeling and slicing into 0.5-cm chips, followed by sun drying and powdering. The flour was sieved (30-mesh size) to obtain fine grade sample. Chick pea (Cicer arietinum; CP) flour and whole wheat flour (WWF) were procured from the local market. Finger millet (Eleusine coracana), procured from the local market was washed cleaned free of dirt, sun- dried, powdered and sieved through the same mesh size sieve to obtain fine flour (FF).

Malting and Pre-gelatinization

Malted cassava flour was prepared by two methods viz., using termamyl (commercial α-amylase from M/s Novo Industries, Denmark) and germinated green gram amylase, as per the methods described earlier.[18,19] Part of the malted flour was pre- gelatinized by hydro-thermal cooking in a steam cooker for 30 min.[19] The cooked flour was dried, powdered, and used for the study.

Flour Mixes

The treatment combinations described below, which yielded good quality biscuits in previous studies were only tried for extrusion cooking. These were unmalted cassava-finger millet flour (FF) (70: 30), [Group I], termamyl malted cassava blended with WWF and CP (70: 20: 10) as well as with FF (70: 30) [Group II a], gram malted cassava blended with WWF and CP (70: 20: 10) [Group II b], termamyl malted and pre- gelatinized cassava blends with WWF and CP (Group III a) and gram malted and pre- gelatinized cassava blends with WWF and CP (Group III b). The flour mixes were uniformly blended and their moisture contents were adjusted to 15% by adding calculated amount of water, sealed in polythene bags and stored at 5°C for 10 days for conditioning, with frequent mixing to ensure uniform distribution of water within the mix.

Extrusion Cooking

The flour mixes after low temperature conditioning was kept at ambient temperature, to reach the equilibrium temperature. Extrusion experiments were carried out in a laboratory model single screw extruder (Model KE 19, Stand alone extruder, M/s Brabender Measuring and Control Systems, Germany). The extrusion parameters used for the study are presented in Table 1.

Table 1 Extrusion parameters for the study

Extrusion Parameters	Values
Feed zone temperature	80°C
Compression zone temperature	100°C
Metering zone	120°C
Die temperature	180°C
Extruder screw speed	100 rpm
Dosing screw speed	*10,20 and 25 rpm
Compression ratio of the extruder	1:01
Moisture content of the blend	15%
*Group IIIa combination was extruded at a dozing speed of 10 rpm since the torque on the machine was high.

Nutritional Analysis

The extrudates were analysed for the nutritional profile such as starch, total sugars, fat, crude protein (N × 6.25) as well as energy value as per the standard procedures described elsewhere.[18,19]

Functional Properties

The in vitro starch digestibility of the extrudates was determined by the method of Singh et al.[26] In vitro protein digestibility was estimated by the AOAC method[27] with the modification that a combined digestive enzyme tablet, Panzynorm-N (German Remedies Ltd., Mumbai, India) was used for the study and the value expressed as g amino acid released/h/100 g flour mix.

Physical Properties

The physical characteristics of the extrudates like diameter (m), length (m), weight (kg) were measured and volume (m³) was computed. The water solubility index (WSI) was measured according to the method of Machado et al.[28] The water absorption index (WAI) was measured as per the method described by Anderson et al.[29] The expansion ratio is a measure of the increase in volume of the product due to extrusion cooking. It was computed as the ratio between the extrudate diameter and the diameter of the die.[30] The diameter of extrudates was measured using a digital vernier caliper.

The bulk density of the extrudates was measured by weighing 0.1-m long cylindrical section and their diameter by a digital vernier caliper.[31]

• Bulk density (kg/m³) = Weight

• π x diameter² x 0.1

RESULTS AND DISCUSSION

Nutritional Properties

The starch, total sugars and fat content of the extrudates from the three groups of composite mixes are presented in Table 2. Highest starch content of the extrudates was observed from unmalted cassava—FF combination (72%), because native flour used in Group I mix. Termamyl and gram amylase malting hydrolyzes part of the starch to dextrins and hence the resulting flour was found to contain less starch content than the native flour.[18] This was reflected in the low starch content of the extrudates from Group II (a and b), as well as those from the malted and pre- gelatinized cassava-based mixes (Group III a and b) (Table 2). Prasad et al.[32] reported a carbohydrate content of 73% in sorghum extrudates, while in the sorghum- soy (80: 20) extrudates the carbohydrate content was only 69%.

Table 2 Starch, sugar, and fat content* of extruded snack foods from cassava-based composite flours

Treatment combinations	Starch (%)	Sugar (%)	Fat (%)
Unmalted CF based (Group I)
Cassava:FF	72.01 ± 0.29	5.15 ± 0.12	2.05 ± 0.05
Termamyl malted CF based (Group IIa)
Cassava:WWF:CP	67.66±0.39	5.40 ± 0.10	1.20 ± 0.06
Cassava:FF	62.06 ± 0.56	5.32 ± 0.09	2.15 ± 0.04
Gram malted CF based (Group IIb)
Cassava:WWF:CP	68.76 ± 0.54	6.15 ± 0.94	1.25 ± 0.08
Termamyl malted CF based (pre-gelatinized) (Group IIIa)
Cassava:WWF:CP	64.29 ± 0.45	8.3 ± 0.15	1.22 ± 0.05
Gram malted CF based (pre-gelatinized) (Group IIIb)
Cassava:WWF:CP	63.06 ± 0.68	6.25 ± 0.14	1.26 ± 0.04
*Expressed on a dry weight basis.

The fat content of the extrudates ranged from 1.20–2.15% (Table 2). Among the various combinations, the highest fat content was observed for the extrudates made from the cassava- FF mixes (Ca. 2%). Earlier studies also showed that the cassava- WWF- CP (70:20:10) premix had a total fat content of 1.0–1.5% in termamyl malted cassava-based mix and 1.0–1.95% in gram amylase malted cassava-based mix.[18] Significant reduction in fat in the extruded product from the same mix was not noticed in the present study (Table 2). However, Prasad et al.[32] reported a slight decrease from the premix in sorghum- soy extrudates. Bjorck and Asp[33] observed that there was a possibility of conversion of some of the lipids into bound form during extrusion. The very low fat content of the cassava ready-to-eat extrudates could be considered a positive attribute.

The crude protein content (N × 6.25) of the extrudates ranged from 4.25–6.68%, with the lowest values of 4.25 and 4.61% in the extrudates from the cassava- FF blends in Group I and Group II a, respectively (Table 3). The data on the crude protein content of the extrudates were similar to the values reported earlier for termamyl- malted cassava- WWF- CP mix (5.69%) and gram amylase malted cassava- WWF- CP mix (6.56%).[18] The study showed that extrusion did not alter the crude protein content. Similar results were also reported for sorghum-soy extrudates.[31]

Table 3 Protein, energy content, and digestibility of extruded snack foods from cassava based composite flours

Treatment combinations	Protein (%)	Energy value (kJ/100g)	In vitro starch digestibility*	In vitro protein digestibility**
Group I
Cassava:FF	4.25 ± 0.09	1469 ± 10.0	46.22 ± 0.98	6.56 ± 0.03
Group IIa
Cassava:WWF:CP	5.75 ± 0.14	1377 ± 10.0	35.15 ± 1.02	5.08 ± 0.08
Cassava:FF	4.61 ± 0.08	1298 ± 15.0	50.66 ± 0.92	4.95 ± 0.04
Group IIb
Cassava:WWF:CP	6.68 ± 0.11	1423 ± 15.0	34.24 ± 0.89	3.98 ± 0.11
Group IIIa
Cassava:WWF:CP	5.49 ± 0.12	1322 ± 8.00	52.88 ± 1.36	7.20 ± 0.06
Group IIIb
Cassava:WWF:CP	6.01 ± 0.06	1318 ± 16.0	48.66 ± 1.25	6.15 ± 0.10
*Expressed as mg reducing groups formed / h/g dry sample;**expressed as g amino acid released/h/100g dry sample; treatment details as in Table 1.

The energy value of extrudates from the three groups I, II (a and b) and III (a and b) ranged from 1298–1469 kJ/100 g (equivalent to 310–351 Kcal/100 g) (Table 3). Earlier studies showed that blends of malted cassava with cereal and/ or legume and bran sources reduced the total energy content.[18] This study supports with the earlier results, with energy contents of 1377 kJ/100 g for termamyl malted cassava- WWF- CP based extrudates (1399 kJ/100 g for the respective premix) and 1322 kJ/100 g for the extrudates from termamyl malted and pre- gelatinized cassava-WWF-CP blend (1313 kJ/100 g in the premix). Similar results were also observed for the extrudates from gram amylase malted cassava. The low energy content of the extrudates resulted also from the oil free extrusion process and will have wide scope in the food industry, as low caloric snack foods for obese and diabetic people. Prasad et al.[32] also reported similar energy values (358–361 Kcal/100 g) for sorghum- soy extrudates.

Functional Properties

The functional properties like in vitro starch and protein digestibility of the extrudates showed that the lowest starch digestibility was for the extrudates from malted cassava-WWF-CP mixes (Table 3). However, when extrudates were prepared from malted and pre- gelatinized cassava flour, there was a sharp increase in starch digestibility from 34–35 units to 48–53 units. Nawab Ali[2] reported that gelatinization of starch granules could increase their susceptibility to enzymatic hydrolysis. Earlier studies in our laboratory showed that pre- gelatinization of termamyl and gram malted cassava could raise the in vitro starch digestibility of the cassava- WWF- CP mix by 5–7 units only. However, in the present study we observed a greater increase (14–17 units) in starch digestibility of the extrudates from the same mix. This increase would have occurred due to the complete gelatinization of the starch at the high temperature (180°C) in the extruder. It is reported that although complete starch gelatinization requires at least 30–40% moisture at atmospheric pressure, extrusion permits the total gelatinization in the absence of moisture itself.[2] The heat treatment and mechanical shear under the extrusion conditions also disrupts the starch granular structure, thereby exposing more pockets for the alpha-amylase action. Protein digestibility was the lowest for the extrudates from malted cassava-based mixes (Group II a and b) (Table 3). Mild extrusion conditions were reported to increase the digestibility of protein and enhance their nutritional value.[2] However, Prasad et al.[32] reported that the Protein Efficiency Ratio (PER) of extruded sorghum was only 35% of casein. Nevertheless, PER was reported to increase when extrudates from sorghum-soy (80:20) blends were fed to rats. In our study, increase in protein digestibility of the extrudates prepared from malted and pre-gelatinized cassava flour based mixes was evident (Table 3).

The functional properties of the extrudates such as water solubility index (WSI) and WAI of the extrudates showed that WSI was reduced for the extrudates from gram malted cassava-based mix (Group II b) and for the extrudates from Group III (a and b) (Table 4). The crude protein content was higher for the extrudates from these combinations, indicating a negative correlation between crude protein and WSI. Similar negative correlation of crude protein with water solubility was reported for cassava flour extrudates made with soybean flour and wheat bran.[34] Anandh et al.[35] also reported decrease in WSI from 1.12 to 0.95, in the extrudates from buffalo rumen meat and corn, flour, with increase in crude protein from 11.41% to 28.27%. The water absorption index ranged from 660–687 for the extrudates made from Group III (a and b) mixes as compared to 563–596 for the product from Group II (a and b) mixes. Highest WAI was obtained for the extrudate from unmalted cassava –FF mix. Increased starch content in the mix used for extrusion has been reported to increase the WAI of the extrudates, due to increased starch gelatinization.[35–37] High starch content of corn and low fat level have also been reported to lead to extrudates with high WAI.[38] The effect of pigeon pea flour addition to cassava flour on the physicochemical quality of extrudates was investigated by Rampersad et al.,[39] who found out that products with added pigeon pea flour had higher WAI.

Table 4 Water solubility index (WSI) and Water absorption index (WAI) of extruded snack foods from cassava-based composite flours

Treatment combinations*	WSI	WAI
Group I
Cassava:FF	2.70 ± 0.09	707.58 ± 2.23
Group IIa
Cassava:WWF:CP	2.00 ± 0.07	563.58 ± 2.95
Cassava:FF	2.90 ± 0.08	596.50 ± 3.34
Group IIb
Cassava:WWF:CP	1.39 ± 0.05	660.46 ± 3.35
Group IIIa
Cassava:WWF:CP	1.28 ± 0.06	687.48 ± 2.52
Group IIIb
Cassava:WWF:CP	1.48 ± 0.02	663.78 ± 3.55
*Treatment details as in Table 1.

The physical characteristics of the extrudates from the three groups are given in Table 5, and it was found that the diameter of the expanded product was the least from termamyl malted cassava- WWF-CP. However, when extrusion was done using malted and pre- gelatinized cassava-based mix, better expansion was observed. Diameter was also higher for extrudates from cassava-FF mix than the other combinations.

Table 5 Physical characteristics of extruded snack foods from cassava-based composite flours

Treatment combinations*	Diameter (m) × 10^−3	Length (m) × 10^−2	Weight (kg) × 10^−3	Volume (m^3)
Group I
Cassava:FF	9.28 ± 0.65	8.52 ± 0.57	1.72 ± 0.05	576.93×10^8
Group IIa
Cassava:WWF:CP	6.66 ± 0.54	13.65 ± 0.85	2.32 ± 0.01	475.52×10^8
Cassava:FF	11.87 ± 0.85	11.36 ± 0.84	3.02 ± 0.04	1258×10^8
Group IIb
Cassava:WWF:CP	6.82 ± 0.86	14.8 ± 1.10	1.91 ± 0.02	541.92×10^8
Group IIIa
Cassava:WWF:CP	9.58 ± 0.98	11.86 ± 0.74	2.08 ± 0.01	858.37×10^8
Group IIIb
Cassava:WWF:CP	7.31 ± 0.85	9.20 ± 0.96	1.76 ± 0.02	386.74×10^8
*Treatment details as in Table 1.

The degree of puffing of extrudates is described by the two parameters viz; bulk density and expansion ratio.[39] The bulk density of the extrudates was found to be higher in the case of termamyl and gram malted cassava-WWF-CP mixes and for the extrudates from the gram-malted and pre-gelatinized cassava- WWF-CP mix. The expansion ratio of extrudates in these cases was the least (Table 6) ranging from 1.33–1.47. The expansion ratio of the other extrudates ranged from 1.85–2.37. Camire et al.[6] reported that samples with lower bulk density generally exhibit larger expansion ratio and our results support the earlier findings. Park et al[38] observed that these two are not always interrelated. Crude protein content of the extrudates was found to be negatively correlated with the expansion ratio (Tables 3 and 6). Badrie and Mellowes[34] also reported such negative correlation of crude protein with expansion ratio and positive correlation with bulk density, which corroborates with our findings.

Table 6 Bulk density and Expansion ratio of extruded snack foods from cassava-based composite flours

Treatment combinations*	Bulk density (kg/m^3)	Expansion Ratio
Group I
Cassava:FF	2.99×10^3	1.85 ± 0.03
Group IIa
Cassava:WWF:CP	4.89×10^3	1.33 ± 0.01
Cassava:FF	2.40×10^3	2.37 ± 0.02
Group IIb
Cassava:WWF:CP	3.51×10^3	1.36 ± 0.03
Group IIIa
Cassava:WWF:CP	2.43×10^3	1.91 ± 0.04
Group IIIb
Cassava:WWF:CP	4.56×10^3	1.47 ± 0.02
*Treatment details as in Table 1.

CONCLUSION

Nutritional characteristics of the extrudates from native as well as pre- treated (malted or malted and pre- gelatinized) cassava flour showed that the pre-treatment could yield products having lower starch content. Protein fortification to the level of 4.25–6.6% could be achieved with blends of cassava with whole wheat flour or finger millet flour and/ or chick pea flour. The extrudates from malted and pre- gelatinized cassava-based combinations had high starch digestibility, due to complete gelatinization of starch at the high temperature of extrusion. The ready-to-eat extrudates developed from the study had very low fat content and hence ideal as low calorie snack foods.

ACKNOWLEDGMENTS

The first author (S.Jisha) expresses her gratitude to the University of Kerala for the research fellowship awarded to pursue the programme. The authors also acknowledge the facilities and encouragement provided by the Director, CTCRI, Thiruvananthapuram for the study.