Insights into the material and 3D printing behaviour of fiber-enriched protein gels

One of the widely used materials in food printing is soy protein isolate (SPI) due to its functional and nutritional properties. However, a major printing drawback of SPI gels is network brittleness due to extensive aggregation leading to rheological properties that are unsuitable for printing. In this study, 0.4 %, 0.8 %, 1.2 %, 1.6 % and 2.0 % w/w milled psyllium husk and apple fibers were integrated into 20 % w/w SPI inks to improve the network properties and printability. Microstructural, textural, rheological properties and printability were investigated by microscopy, texture profile analysis, rheometry and image analysis. Incorporating psyllium husk fibers resulted in an interpenetrating protein-fiber network. This led to an increase in the network strength from 2155.8 Pa to 4228.15 Pa, relative to soy protein inks only, making them less susceptible to deformation during extrusion. Additionally, the geometrical deviation of the printed cubes decreased from 37.00 ± 4.55 % (length) and 24.00 ± 4.45 % (height) in the control inks to 13.86 ± 1.61 % and 24.86 ± 3.17 % respectively at psyllium husk concentration of 2.0 w/w %. The results showed that psyllium husk improved ink flexibility due to the high water-holding capacity of the fibers while maintaining structural integrity. This study revealed that the interpenetrating network effect of soluble dietary fibers in SPI inks improved printability while apple fibers with a high fraction of insoluble fibers embedded in a soy protein ink network caused printing defects. The findings highlight the potential to understand the influence of dietary fiber with varying physicochemical properties on 3D food printing of protein inks.