Pro Tip: Using extruders as chemical reactors, instead of a finished goods process, can help functionalize protein. 

Processing protein and grains into finished goods by extrusion is fairly common, since extrusion can create unique shapes and textures that are not achievable by other production methods. This includes crispy puffed grains, chewy proteins and unusual shapes of pasta or cereal. These novel shapes and textures form because extrusion brings together several of the levers of denaturation discussed in previous Pro Tips, including pressure, shear and temperature. 

As a protein (or a mixture of protein and other ingredients) moves down the length of an extrusion barrel, large shear forces occur between the material and walls of the extruder, or between the screws in the case of twin-screw extruders. As discussed in the shear Pro Tip, this leads to more β-Sheet structures causing chewier products. Simultaneously, the material is often heated to temperatures well above the boiling point of water, made possible by large amounts of pressure that build up during extrusion. This combination of forces causes protein to unfold and form new bonds, establishing new textures in the finished products. 

During grain extrusion, super-heated water moves from a high-pressure environment in the extruder to regular atmospheric pressure as it exits, causing the water to evaporate quickly and creating puffed cereals and chips. However, the material out of the extruder does not have to be a finished product. 

It is becoming increasingly common to use extruders as one part in a series of processing steps aimed at achieving certain functionality from protein. This approach uses an extruder as a chemical reactor, where the goal is to modify the protein shape in a targeted way, rather than produce a finished product. The material that comes out of the end of the extruder will have very different handling properties than what went in, and this extrudate can be chopped, ground or processed into functional ingredients to use in other applications. 

In protein, the denaturation that occurs during extrusion may change the structure of the protein in ways that exposes more hydrophobic surface area. This opens up the possibility of new emulsifiers made from protein that has been extruded and made into powder. It may even be possible to use the output of one extruder as an input for a different extruder, using each machine to target different parts of the protein’s structure. In this way, new products can be made that have superior texture and functionality, including better gels and emulsions.  

Harrison Helmick is a PhD candidate at Purdue University. Connect with him on LinkedIn and see his other baking tips at BakeSci.com.

His research is conducted with the support of Jozef Kokini, Andrea Liceaga and Arun Bhunia.