The Development of Expanded Snack Product Made from Pumpkin Flour-Corn Grits: Effect of Extrusion Conditions and Formulations on Physical Characteristics and Microstructure

Abstract

Pumpkin products confer natural sweetness, desirable flavours and β-carotene, a vitamin A precursor when added as ingredients to extruded snacks. Therefore, a potential use for dried pumpkin flour is as an ingredient in ready-to-eat (RTE) snack foods. Growth in this market has driven food manufacturers to produce a variety of new high value snack foods incorporating diverse ingredients to enhance the appearance and nutritional properties of these foods. Ready-to-eat snacks were made by extruding corn grits with 5%, 10%, 15% and 20% of pumpkin flour. Snacks made from 100% corn grits were used as control products for this work. The effect of formulation and screw speeds of 250 rpm and 350 rpm on torque and specific mechanical energy (SME, kWh/kg), physical characteristics (expansion ratio, bulk density, true density and hardness) and the microstructure of the snacks were studied. Increasing the screw speed resulted in a decrease of torque for all formulations. When pumpkin flour was added the specific mechanical energy (SME) decreased by approximately 45%. Increasing the percentage of pumpkin flour at the higher screw speed resulted in a harder texture for the extruded products. X-ray tomography of pumpkin flour-corn grit snacks showed that increased levels of pumpkin flour decreased both the bubble area and bubble size. However, no significant differences (p > 0.05) in bubble wall thickness were measured. By understanding the conditions during extrusion, desirable nutritional characteristics can be incorporated while maximizing expansion to make a product with low bulk density, a fine bubble structure and acceptable organoleptic properties.

Keywords: extrusion; feed rate; physical characteristic; pumpkin; screw speed.

Figure 1

Schematic diagram of screw configuration of the extruder, the angle on the schematic represents screw pitch from the feed at the left hand side to the die on the right hand side.

Figure 2

Effect of screw speed and the proportion of pumpkin flour on torque during extrusion.

Figure 3

Effect of screw speed and the proportion of pumpkin flour on the specific mechanical energy (SME (kWh/kg)). Each value was an average of duplicate samples.

Figure 4

Effect of screw speed and the proportion of pumpkin flour on the expansion ratio. Error bars represent the standard deviation.

Figure 5

Effect of varying pumpkin flour percentage and screw speed on the hardness of extruded product.

Figure 6

Effect of varying pumpkin flour percentage and screw speed on the bulk density of extruded product. Each value as an average of duplicate samples.

Figure 7

Effect of varying pumpkin flour percentage and screw speed on the true density of extruded product. Each value was an average of duplicate samples.

Figure 8

Effect of varying pumpkin flour at constant screw speed (250 rpm) on bubble area distribution at the upper end and centre of the extruded expanded snack.