Effects of starches on the expansion structure and quality properties of peanut protein extrudates during extrusion processing

Peanut protein is one type of plant protein with high nutritional value. However, it is still lacking in the high-value-added products of peanut protein. Extrusion processing can be expected to produce the peanut protein, due to the easy operation, safety, green environment, and continuous output. The puffed products of peanut protein have been produced to enhance its added value using extrusion. Nevertheless, the application of peanut protein has been confined to the low expansion effect and quality properties (texture and rehydration properties). This study aims to investigate the effects of starches on the expansion structure and quality properties of peanut protein during extrusion. Taking the peanut protein as a research object, the starches were also selected for the varying thermal and gelatinization properties. Some property parameters were measured, including the texture, color, rehydration, and rheological properties of peanut protein extrudates. In addition, a systematic analysis was then performed on the starch structure (short-range ordered and long-range ordered structure) and protein structure (protein secondary structure, sulfhydryl group content, and disulfide bond content) during extrusion. The results showed that there was a significant decrease in the parameter values of starch-protein extrudates (P<0.05). Particularly, the ΔE and b values significantly increased (P<0.05). The expansion structure, texture, and rehydration properties of peanut protein extrudates were closely related to the amylose content and thermal properties of the starch. Pea starch had a lower enthalpy (1.83 J/g), a higher amylose content (31.10%), and a lower gelatinization peak temperature (67.88 °C). Peanut protein extrudates were then formed on the porous expansion structure with high texture properties. For example, pea starch significantly increased the expansion ratio of peanut protein extrudates to 2.44, while reducing the density and hardness to 0.28 g/cm3 and 19.18 N, respectively (P<0.05). At the same time, the water-holding capacity of pea starch- peanut protein extrudate (PASE) reached the highest in the experiment group (6.05 g/g), indicating that the pea starch significantly improved the rehydration properties of peanut protein extrudates (P<0.05). In addition, the interaction between pea starch and peanut protein promoted the transformation of protein secondary structure from the order to the disorder, where the short-range ordered and crystal structure of starch was destroyed during production. The conversion of sulfhydryl groups to disulfide bonds increased the degree of protein cross-linking. Pea starch was used to raise the viscosity and elasticity of the extrudates, whereas decreased the fluidity, according to rheological investigation. Therefore, the pea starch can effectively improve the expansion and quality properties of peanut protein extrudates. The finding can provide scientific and technological support to the expansion and quality properties of the peanut protein application in extruded food.