Effect of fibre–protein additions and process parameters on microstructure of corn extrudates

Highlights

• The microstructure and physical properties of extrudates were analysed.

• Changes in the blend composition, process parameters affected the microstructure of extrudates.

• Addition of fibre material caused a decrease of the dimensions of air cells.

• A change of blend moisture caused the formation of larger air cells.

• At higher temperature the samples had a larger number of smaller air cells.

Abstract

A study was conducted on the application of high-fibre components (everlasting pea wholemeal, oat bran) for the modification of microstructure and physical properties of corn extrudates. The extrusion was conducted using a single screw extruder type S-45 (Metalchem, Poland). The effect of the material blend composition and of the variable process parameters: material blend moisture (11, 13.5, 16%), barrel temperature distribution profile (120/145/115, 130/155/115, 140/165/115 °C) on the microstructure and the physical properties of the extrudates was analysed. All extrudates obtained were characterised by typical cellular structure and “crunchy” texture characteristic of the “ready to eat” type products. The microstructure of the products obtained was determined both by the material composition of the blend and by the process parameters. The differences observed in the size, number of shapes of air cells and in the cell wall thickness indicate extensive possibilities of modification of the physical properties and sensory traits of extrudates.

Introduction

Extrusion-cooking is a technology commonly used in food processing due to the low costs of operation, high efficiency, versatility, and unique features of the final products (Faraj et al., 2004, Repo-Carrasco-Valencia et al., 2009, Thymi et al., 2005). In the process of extrusion, the material processed is subjected to the effect of high temperature, pressure, and tangential stress, until the mass is liquefied and pressed out under controlled conditions of pressure and temperature (Liu et al., 2011, Matuana and Diaz, 2010, Mazumder et al., 2007, Repo-Carrasco-Valencia et al., 2009, Yağcı and Gőğüş, 2008, Yanniotis et al., 2007, Zarzycki et al., 2010, Zhu et al., 2010). The special importance of the extrusion technology results also from the fact that it is the basic technology in the production of “ready to eat” food products. The sensory requirements of the consumers indicate that extrudates should be characterised by porous structure, a large amount of thin-walled air cells, crunchy texture, high degree of expansion, and low specific density. These specific properties of extrudates can be created through proper selection of microstructure, determined by the parameters of the process of extrusion (profile of barrel temperature distribution, profile of pressure distribution, configuration and speed of rotation of the extruder screws, size and shape of the die aperture, moisture of the material blend), and by the composition of the material blend (content of starch, protein, lipids and dietary fibre) (Ding et al., 2005, Lazou and Krokida, 2010, Liu et al., 2011, Matuana and Diaz, 2010, Mazumder et al., 2007, Rzedzicki and Błaszczak, 2005, Thymi et al., 2005, Wang and Ryu, 2013, Zarzycki et al., 2010, Zhu et al., 2010).

The consumers have a special preference for extruded corn products, due to their attractive appearance, colour and taste (Lazou and Krokida, 2010, Mazumder et al., 2007, Thymi et al., 2005, Wang and Ryu, 2013). Corn meal is also a very attractive raw material for the producers of extrudates, due to its very easy liquefaction in the extruder. However, extensive application of corn meal in the extrusion technology creates specific risks. It is a raw material with very poor chemical composition and a very low content of dietary fibre, obtained from hulled grain of corn with the germs removed. Those treatments result in significant deterioration of the chemical composition and the nutritional value of the material (Kasprzak and Rzedzicki, 2008, Sobota and Rzedzicki, 2009).

Those negative properties of maize raw material assume a special significance in the face of the pandemic of civilisation diseases. It is common knowledge that diet poor in dietary fibre contributes to the incidence of numerous civilisation diseases, such as obesity, cardiovascular diseases, type II diabetes, etc. (Bravi et al., 2009, Mann and Cummings, 2009, Topping, 2007).

Taking into account the high popularity of extruded foods and the recommendations of the prophylaxis of civilisation diseases, modification of the raw material composition of blends subjected to the process of extrusion assumes a downright strategic importance. Blends subjected to extrusion must be characterised by a high nutritive value, increased contents of proteins and dietary fibre. Numerous studies indicate that such components can be leguminous materials and materials originating from non-bread cereals (Kasprzak and Rzedzicki, 2007, Liu et al., 2011, Robin et al., 2011, Sobota and Rzedzicki, 2009, Wang and Ryu, 2013, Zarzycki et al., 2010, Zhang et al., 2011). Legumes are recommended due to their unique chemical composition and their health-promoting effects (Boye et al., 2010, Lazou and Krokida, 2010). In the technology of extrusion, special attention is due to everlasting pea. Seeds of everlasting pea are characterised by attractive taste values, light colouring of cotyledons (which do not darken during thermoplastic treatment), high contents of proteins (even above 30%), dietary fibre, vitamins, minerals, numerous biologically active substances and non-starch oligosaccharides (Kasprzak and Rzedzicki, 2007, Kasprzak and Rzedzicki, 2008, Lioi et al., 2011).

Oat components should also find an extensive application in the production of extruded foods. Their unique chemical composition, and especially their fractional composition of dietary fibre, and their ability for microstructure modification, make them particularly suitable for the production of the common “ready to eat” foods (Rzedzicki and Błaszczak, 2005, Zarzycki et al., 2010). The literature does not provide any data on the simultaneous application of a mixture of corn meal, everlasting pea wholemeal and oat components in the extrusion technology, and therefore this issue has been undertaken in the study presented here.