Understanding extrusion technology for cereal–pulse blends: A review

Abstract

In recent times, extrusion cooking is being used to develop nutrient-dense products with an aim to benefit humans and to cater their nutritional requirements. The present paper discusses various aspects of twin-screw extrusion cooking and behaviour of its process parameters in relation to cereal–pulse blend. Profiling of raw material, i.e. moisture, protein, lipid, and starch, along with the assessment of product parameters including expansion ratio, bulk density, water absorption index, and water solubility index has been extensively reviewed to develop a desirable product. Therefore, in the present review paper, extrusion cooking has been highlighted and discussed in detail as a suitable technology for the development of cereal–pulse-based snacks/products.

Keywords:

• twin-screw extrusion
• expansion ratio
• bulk density
• water absorption index
• water solubility index

1. Introduction

The extrusion technology came into existence in order to manufacture lead pipes. Joseph Bramah in 1797 patented the process. Interestingly, the technology was extended by Fellow and Bates in 1870s to extrude sausages. In early 1930s, corn was commonly used as a feed mixture, which provided a unique taste and texture to consumers. Thereafter, extrusion cooking became an important technology to develop products such as breakfast cereals, snacks, corn curls, pasta, chewing gums, Ready-to-Eat/Use products (RTE/RTU), confectionary, texturized meat substitute, etc. Extrusion is a simple continuous approach, generally known as high temperature and short time. Products developed are versatile and economical, making them popular among consumers. The feed mixture’s macronutrient and micronutrient profiles significantly determine the extent of structural and chemical modifications under given extrusion process parameters. The benefits of utilizing extrusion technology are its automated control, low cost per part, operation flexibility, continuous operation, high production volumes, adaptability with different raw material blends, constant mixing during the process, versatility, energy efficiency, and no effluent generation. It also aids in the design and development of a new product, unique shape and characteristics, and high product quality. Extrusion cooking improves nutritional characteristics such as protein quality and quantity, soluble fibre content as well as product’s solubility, swelling power, water hydration viscosity, water-holding capacity, colour, texture, in-vitro protein digestibility, reduces anti-nutritional compounds, etc. Extrusion technology also has few demerits such as high cost of initial setup and product limitation because only one type of cross-section can be obtained at a time. However, extruded products have gained utmost consumers attention, which has intrigued researchers to explore every aspect in detail (Alam et al., 2016; Obatolu Veronica et al., 2006; Pastor-Cavada et al., 2011).

2. Improving cereal extrusion: Introduction to legumes

The notion of legumes as a novel ingredient to a well-established cereal-based extrusion industry was introduced in order to extend its nutritional functionality. Pulses are grown locally, with nutrition-appealing benefits such as being rich in protein and fibre and low fat content. They also make a good alternative to animal-based proteins, thus establishing its vitality in extrusion industry. Several surveys have shown that the consumer acceptance for cereal–pulse-based extruded snacks are because of its high nutrient density and health benefits. Snacks have always been everyone’s favourite because of their unique texture. The cereal–pulse-based mixture develops microstructure, resulting in desirable crunchiness and enhanced flavour, which offers an interesting mouth feel and after taste. With such popularity, snacks are now being extensively researched. The process of snack development has been enhanced with concepts like value addition of micronutrients. The need to understand cereal–pulse-based extrusion cooking will aid in the development of such novel sustainable products.

3. Extruder: Basic understanding

Twin-screw extruder mainly consists of a metal-barrel-like chamber enclosing two screws, an inlet for feeding the raw materials, and an outlet consisting of a die, from which the final product is collected. The raw materials (feed mixture) are fed into the extruder in which processes like mixing, dissolving, forming, shearing, and cooking are acted upon simultaneously, then leaving the chamber through a specific die size, forming a definite shape (Ding et al., 2005). The development of cereal–pulse-based extruded products requires a basic understanding of the raw material and process parameters. For many decades, researchers have tried to understand what goes inside an extruder bioreactor. The science behind the interactions, dependence, and relations among added feed mixture is dynamic. The starchy and proteinaceous nature of raw material aids into puffed and plasticised mass. These molecular alterations lead to physical (granular and crystalline) and chemical changes, such as the formation of complex linkages between ingredients, which significantly alters the matrix and its digestible value (Arêas, 1992; Camire et al., 1990). These variables are potent for making the technology flexible yet complex, which requires a close control. Researchers found that the composition of feed mixtures like cereals and pulses undergoes physio-chemical changes such as gelatinisation of starch and denaturation of proteins. Also, the enzymes and heat-sensitive non-nutritive factors are inactivated (Frías et al., 2011). Molecular disruptions due to thermal action greatly influence inactivation of enzymes inhibitors, which exposes proteins to digestive enzymes, leading to enhanced digestibility (Wang et al., 2020). The extent of these changes requires close monitoring at each step for a desired product.

4. Cereal–pulse profiling

Extrusion industry survives on consumer-based popular ingredients. Many researches have been done in scanning, analysing, and profiling of cereal and pulses of different varieties and origins. The selection of raw materials with a specific proximate composition is primarily dependent on the certainty of the researcher’s objective(s). A clear composition aids in determining the process and product characteristics. Table 1 briefly highlights the research aimed to develop cereal–pulse-based extruded products and their initial composition. These parameters are needed to define the processing conditions which influences the product parameters. The details of the influence and effect observed by these researchers are given in Table 2.

Table 1. Summary of raw material proximate composition before extrusion

Type, Source	Moisture (%)	Protein (%)	Fat (%)	Carbohydrate (%)	Fibre (%)	References
Pulses
Full fat soy flour	–	45.7	20.9	–	–	Chauhan and Bains (1988)
Soybean Fibre (Fibrim)	7.0	12.0	0.2	–	75.0	Jin et al. (1995)
Green gram, Flour (Procured from Local market, Mysore)	10.7 ± 0.2	22.8 ± 0.5	1.3 ± 0.1	63.9	0.9 ± 0.1	Bhattacharya (1997)
Washed Mung Bean	10.21 ± 0.02	22.34 ± 0.28	1.31 ± 0.03	62.01 ± 0.24	0.95 ± 0.16	Jain et al. (2022)
Cereals
Jaya Rice	–	–	–	–	–	Chauhan and Bains (1988)
Yellow Corn Meal	12.0	7.0	0.7	–	0.5	Jin et al. (1995)
Rice (Procured from Local market, Mysore)	12.9 ± 0.2	6.6 ± 0.2	0.4 ± 0.0	79.6	0.2 ± 0.0	Bhattacharya (1997)
Oats (Broken)	7.94 ± 0.14	8.23 ± 0.08	5.6 ± 0.27	68.9 ± 0.25	9.1 ± 0.11	Jain et al. (2022)
Cereal-Legume Blend Composition
Rice + *LA	8.1 ± 0.11	10.50 ± 0.53	0.54 ± 0.14	79.70 ± 0.49	7.51 ± 0.16	Pastor-Cavada et al. (2011)
Rice + *LC	7.9 ± .0.07	10.09 ± 0.16	0.81 ± 0.00	79.38 ± 0.14	7.96 ± 0.01
Corn+ *LA	7.1 ± 0.17	11.25 ± 0.36	1.65 ± 0.09	72.99 ± 0.42	11.95 ± 0.13
Corn+ *LC	7.3 ± 0.05	12.13 ± 0.0007	1.12 ± 0.15	72.89 ± 1.40	11.86 ± 1.53

*LA-L. annuus; *LC- L. clymenum.

Table 2. Brief description of product parameter and dependent factors during extrusion

Parameter	Value	Dependent upon other parameters	Reference
Expansion Ratio	1.7–2.5	+BT	Chauhan and Bains (1988)
	1.31–2.53	+ BT -BSS	Bhattacharya (1997)
	2.59–3.22	-Fat & legumes	Pastor-Cavada et al. (2011)
	2.60–3.82	+ BT + Moisture Content	Jain et al. (2022)
Bulk Density	0.42–0.78g/ml	-Feed Rate	Chauhan and Bains (1988)
	149–1088 kg/m^3	BT	Bhattacharya (1997)
	0.126–0.147 g/cm^3	+Protein & fibre	Pastor-Cavada et al. (2011)
	0.021–0.79 g/cm^3	+ BT + Moisture Content	Jain et al. (2022)
Water Absorption Index (WAI)%	2.5–7.8	+BT	Chauhan and Bains (1988)
	3.27	-BSS	Jin et al. (1995)
	4.86–7.53%	+ BT + Moisture Content	Jain et al. (2022)
Water Solubility Index (WSI) %	5.8–20.5	+BT	Chauhan and Bains (1988)
	53.47	+BSS	Jin et al. (1995)
	18.3–25.2%	+ BT + Moisture Content	Jain et al. (2022)

4.1. Moisture

It is a known index for efficient uninterrupted process and storage stability. The moisture content is taken as an operating variable. Water acts like a plasticising solvent and a viscosity modifier. The role of moisture is clearly responsible for the modification of elastic properties of amylopectin network, which dictates the diametrical expansion of the extrudate. Moisture content determines the viscosity and demanding effect of the dough, depending on the pre-conditioning of the raw material. Different experimentation has shown that the moisture content aids to reduce die pressure, improves texture, expansion, and modulates storage life. In 1986, Bhattacharya studied corn gluten meal-soy protein concentrate extrudates and made an observation that high moisture conditioning results in less viscous dough formation. The tension between extruder die and atmosphere develops low pressure difference, resulting in less puffed products (Bhattacharya et al., 1986). A research conducted by Omueti and Morton (1996) on soya-and-maize-blended snack highlighted a significant relationship between feed moisture and extrudate shelf life stability. They stated that 20% feed mixture was recommended to develop a product with 3–6% less moisture. Extrudate moisture as high as 10% will be achieved if the feed mixture was increased in the range between 20% and 35%.

4.2. Proteins

Proteins are necessary for human bodily functions. However, they are one of the most unavailable nutrients in plant-based food (Pastor-Cavada et al., 2011). The amino acid profiling has shown that the cereals are insufficient in lysine and threonine, whereas legumes are limited in methionine and cysteine. However, researches have shown that such limitations can be significantly improved by complementing them with one another, known as mutual supplementation. Alvarez‐Martinez et al. (1988) showed a model given by Harper in 1986 which described the interaction between starch and protein molecules. These interactions significantly affect the elastic matrix formation, reducing the expansion and textural properties (Alvarez‐Martinez et al., 1988). A research by Chaiyakul et al. (2009) showed that increased amounts of protein have a significant effect on the process and product parameters. Under thermal and shear environment, water–protein complex formation is seen, which increases the pressure at die head.

4.3. Lipid

The nature of the lipid (hydrophilic–lipophilic balance) and the added quantity determine the effect on the extrudates properties. Researchers have reported significant differences in product parameters, such as bulk density, puff ratio, and shear strength, when experimented with different proportions of lipid (-Bhattacharya et al., 1986; Bhattacharya & Hanna, 1988). However, such differences can also be associated with lipid–protein interactions because of the differences in protein type from different sources. The interaction of lipids with starch affects swelling and hydration capacity. Camire et al. (1990) observed that the level of gelatinisation was increased using short chain polar lipids. Lipase and lipoxygenase are reduced due to high barrel temperature, resulting in negatively affecting the production of free fatty acid and their oxidation. Therefore, product attributes such as foul odour, colour change, and storage life are preserved.

Lipids as fat or oil when used during extrusion act in various forms such as plasticiser, lubricant, etc. They improve the increased melt viscosity, mobility of feed mixture in the extruder barrel, reduce shearing and friction of biopolymers, such as starch and protein, and reduce the dissipation of resulted mechanical energy. It helps in lowering glass temperature, which directly influences the structure of expanded product at the die. Lipids when added, generally 0.5–1%, during the extrusion process provide stabilisation and normalisation, which finally improves upon expansion, texture, and other properties. It also protects starch granules from degradation at low moisture extrusion. However, lipids when added up to 2–3% aids in the prevention of starch dispersal and decreases melt viscosity sharply. It has been observed that lipids when added more than 3% results in low effect on the extrudate expansion, whereas when added over 5%, reduces extrudate expansion (Ilo et al., 2000).

4.4. Starch

Cereal and pulse comprise the highest quantities of carbohydrates in comparison to other food groups. The structural integrity is modified through the melting of crystalline composition and breaking of fragments. Researches have shown that the extent of such deformation is significantly dependent on starch origin, barrel temperature, moisture, screw speed, and geometry. Amylose and amylopectin, components of starch, undergo chain splitting and degradation because of the applied energy. The ratio of these components affects the flow of the molten mass, thus effectively controlling expansion qualities. The molecular weight and their spatial arrangement of starch determine the mechanical and textural properties of the extrudate (de Mesa et al., 2009; Ramulu & Udayasekhara Rao, 1997). A review by Alam et al. (2016) has also mentioned that insoluble fibre reduces sectional expansion and increases density.

5. Behaviour of process parameters during cereal–pulse extrusion

Generally, twin-screw extruders are preferred over single screw extruder in the production of starch-based products because of its efficient movement of feed mixture. Primarily, the rheology of cereal–pulse blend depends upon their polymer solubility, molecular weight, and hydrodynamic volume. Cereal–pulse mixture along with water forms a molten mass which behaves like non-Newtonian fluid (Akdogan, 1996). The role of screw influences the behaviour of melt characteristics as it is responsible for the development of shear intensity. The temperature during an extrusion run is generally different, as parts of molten polymer may get heated at a different temperature than at an average range. Therefore, the continuous rotation of screw provides uniform mixing with increased temperature throughout different barrel zones, thus eliminating the chances of overheating. Thereafter, it resulted in discolouration or degradation of the extrudate. The effect of shear stress on defatted soy flour showed that the increase in shear stress results in denser products, which have low absorption capacity (Holay & Harper, 1982). Researches have also showed an increased puff ratio with increase in shear stress, which is due to a weakened protein molecule bond (Pastor-Cavada et al., 2011). The bonds are stretched resulting in expanded products.

The role of screw and its speed is an utmost factor during extrusion which affects product characteristics. Feed mixture is moved forward proportionately, as barrel screw speed (BSS) is increased. The viscosity of the molten mass drastically reduces with an increase in BSS, resulting in increase in shear stress (Reference). Researchers have observed that when there is an increase in BSS with low moisture content and temperature, the molten mass becomes viscous, leading to develop denser product (Alam et al., 2016). The mass flow rate in the barrel is known to decrease as temperature increases. Specific mechanical energy (SME) is vital to understand the mechanics of energy consumption during the process. As the BSS increases, the torque and SME decreases. Bhattacharya (1997) developed rice-green gram dhal extrudates. He observed that the torque decreased with an increase in BSS. However, at high temperature with an increase in BSS (over 200rpm), an exception of slight increase in torque was noted. The melt is moved forward by the action of screw with varying viscosities, as encountered by the non-uniform barrel temperature, thus affecting the rate of extrusion. The modified molten mass travels through the extruder and leaves from the die. At this point, due to extreme pressure difference (entering an ambient atmosphere), the water vaporises, resulting in the formation of fibrous and porous network matrix product. The collected mass is dehydrated or kept in ambient temperature to remove excess moisture and extend storage life.

6. Product parameters

Parameters are the technical aspect which are analysed by the researcher, making it effortless for the consumer to differentiate among products. This ensures a great level of quality and safety. Physical parameters, especially the expansion indices, are evaluated using various criteria such as expansion ratio, axial length, specific length, sectional, longitudinal expansion index, etc. These parameters captivate consumer’s interest. Other parameters such as bulk density, water absorption index (WAI), and water solubility index (WSI) provide information of developed products matrix dispersion and functionality. Table 2 briefly summarises the product parameters and its dependent factors during extrusion. The outcome is based on the cereal and pulse blend interactions during extrusion. Raw material composition data have already been highlighted in Table 1.

6.1. Expansion ratio

Formulation based on protein–starch composition comprises an interesting cellular and textural characteristic. Addition of pulses (rich in protein content) is known to restrict expansion. Many researches have been done with varying formulations and process parameters. The phenomena to be understood in order to achieve maximum expansion are dough viscosity and elastic swell (Alvarez‐Martinez et al., 1988).

The molecular degradation depends on SME, influencing the melt temperature and shear rates. In general, high melt temperature results in better expansion. Interestingly, significant dependence on feed moisture, barrel temperature, protein and fat content, and screw speeds has been studied. Bhattacharya (1997) studied green gram flour and rice (1:1) extrudates developed at 100–175°C, with screw speed ranging between 100 and 400 rpm. They observed that the relationship of high temperature (175°C) and varying screw speed (250–400 rpm) significantly increases the ER value, therefore, stating that the minimum temperature (150°C) and screw speed (100–150 rpm) are optimal for green-gram-rice-based extrudate.

6.2. Bulk density

It is strongly known in the literature that experiments conducted with increase in protein and fibre will eventually result in denser products. The matrix formed is due to the changes in cell structure, resulting in the formation of pores and voids. Bulk density of extruded products aids in designing the packaging and determining the storage space. Industries and consumers prefer low-density expanded products. They require easy transportation which are potable. Researches have shown that expansion ratio is proportionally linked to bulk density (Ding et al., 2005). BD defines the overall expansion of the developed product. Temperature and BSS have influential effect on BD of cereal- and pulse-based extrudates. Bhattacharya (1997) research showed that low density can be achieved at temperature >150°C, whereas high density (>700 kg/m³) was achieved at low temperature (l00–110°C).

6.3. Water Absorption Index (WAI) and Water Solubility Index (WSI)

These two parameters predict the nature of material, if processed any further (Ding et al., 2005). Applications of such products can be easily seen as a binder, a stabiliser, health and nutrition bars, dairy, baked, etc. Starch is known to swell in aqueous dispersion and gelatinised in high-heat treatment. The power to measure dispersed starch is known as water absorption index (WAI). It majorly depends upon feed moisture proportionally and barrel temperature inversely which results in fragmentation of starch. High amount of starch fragmentation due to an increase in barrel temperature and screw speed results in an increase in WAI. This increase can be attributed to high mechanical shear and higher expansion due to gelatinisation as well as increase in dextrinisation at higher temperature, respectively (Pathania et al., 2013).

Water Solubility Index (WSI) determines soluble starch components released from starch in extruded products after extrusion. In certain extrusion conditions, it has been observed that higher the presence of WSI is a result of starch dextrinisation. This can be attributed to higher temperature in drier conditions, i.e., low moisture which results in starch dextrinisation. This majorly increases the WSI and decreases the WAI of the extruded product (Gomez & Aguilera, 1983). Increase in BSS results in breakdown of complex starch into smaller fragments and results in increased solubility. Screw speed, feed moisture, and barrel temperature significantly regulate these parameters (Ding et al., 2005; Pathania et al., 2013).

7. Summary

The present paper highlights various researches to understand the significance and relations, interactions and dependence among cereal–pulse blend, extruder variables, and product characteristics. This involves understanding of variables such as composition of raw material(s), extruder process parameters, and their analytical methods. The science behind plant protein extrudates has been a real challenge. Therefore, in the present review, a clear picture of these variables helps the researcher to optimise and standardise extrusion science. The need to accept plant-based protein has become a sustainable global concern. Extrusion provides low cost, long shelf life, unique texture, and taste with nutritional benefits as an alternative to our future needs.

Correction

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Notes on contributors

Radhika Jain

Dr. Radhika Jain (Ph. D) is a research scholar (2017-2022) in the Department of Food and Nutrition, Lady Irwin College, University of Delhi, India. Her research work lies at the intersection of nutrition and food technology, with a strong commitment to environmentally sustainable practices (www.linkedin.com/in/dr-radhika-jain-36a698114). Her interest includes development of innovative food products, nutrient retention, proteins, amino acids, extrusion, starch rheology, texture, packaging etc. She has been a recipient of many awards for oral and poster presentation at national/international platforms. She has lectured courses on nutrition, food science and technology to undergraduate/graduate students and guided research theses work.

Sangeeta Goomer

Prof. Sangeeta Goomer is a distinguished food technologist (1995 to present) in the Department of Food and Nutrition, Lady Irwin College, University of Delhi, India. She has been a researcher, innovator, mentor, and lecturer. With an illustrious career spanning over two decades, she has demonstrated unwavering commitment to the field and guided/co-guided graduate, doctoral and post-doctoral research scholars.