Abstract
This PhD research investigated the use of pea protein isolate (PPI), an alternative to gluten or animal protein, to improve the nutritional value of extruded rice snacks. PPI and rice starch or rice flour were used as model ingredients. The effect of PPI content on the rheological behaviour as well as physical, structural and sensory characteristics of extruded rice snacks was studied under selected extrusion processing conditions.
The addition of up to 20 % PPI to rice starch or flour formulations resulted in enhanced expansion of extrudates under the selected extrusion processing conditions (moisture content 21 to 26 %, die temperature 130 to 150 °C and screw speed 300 to 700rpm). Highly expanded extrudates were characterised by low densities and hardness and exhibited large bubbles and a crisp texture. However, fortification with higher PPI contents (30 to 50 %) resulted in limited expansion and these extrudates exhibited small bubbles and thick bubble walls resulting in increased hardness of extrudates. In addition, the use of either rice starch or rice flour as the main ingredient influenced the final properties of extruded snacks when the same total protein content and extrusion processing conditions were used. For example, at a total protein content of 26 %, rice flour-based extrudates generally exhibited higher expansion and more uniform structures than rice starch-based extrudates.
PPI-fortification of rice starch significantly (p<0.05) affected the rheological behaviour and glass transition temperature of the extruded melt. Elevating the PPI content up to 30 % increased the melt viscosity but decreased glass transition temperature, which in turn impacted on the bubble growth in the melt at the extruder die and shrinkage during melt solidification. The highest initial expansion was observed at 30 % PPI content, 23 % moisture content, die temperature of 145 °C and screw speeds ranging from 300 to 700 rpm. However, at 30 % PPI and high screw speeds (≥ 500 rpm), significant (p<0.05) shrinkage of extrudates occurred during melt solidification resulting in non-uniform final extrudate structures. Addition of up to 20 % PPI enhanced initial and final expansion of pure rice starch under these extrusion conditions and no significant (p>0.05) shrinkage was observed during melt solidification.
The accuracy and comparability of online and offline rheometer measurements was assessed by using an extruded PPI-rice starch formulation. Melt viscosities obtained using online slit die and offline capillary rheometers showed similar values and log-linear dependence over the applied shear rate. Offline capillary rheometers offer an alternative to online rheometers, which require high amounts of ingredients and resources. The use of extruded food biopolymers in an offline capillary rheometer is a novel approach to obtain rheological characteristics of materials that have undergone thermomechanical transformation.
The addition of PPI to rice-based formulations highly affected the expansion and texture of extruded snacks, and ultimately, the correlation between mechanical, acoustic and descriptive sensory texture analysis. The mechanical and sensory hardness as well as sensory and acoustic crispness of PPI-fortified rice snacks correlated well (r = 0.98 and 0.88, respectively). However, acoustically determined hardness and crunchiness did not correlate well with sensory perceptions.
The results obtained in this study provide insights into the effects of PPI content on melt viscosity and corresponding extrudate properties. It was shown that PPI can be utilised to improve sensory properties of extruded rice snacks such as crispness and uniformity. However, the addition of PPI at concentrations ≥ 30 % was challenging and resulted in extrudates characterised by limited expansion and poor textural quality. This study demonstrates the importance of linking rheological melt properties with the expansion behaviour, structure and texture of extrudates, which subsequently define the sensory characteristics of extruded snacks. A better understanding of the relationships between the melt viscosity and final extrudate properties as a function of protein content and extrusion parameters can help to identify ingredient formulations and extrusion processing conditions that enable enhanced quality and uniform structures of extruded snacks with high protein contents.