# Applications of Plant Polymer-Based Solid Foams: Current Trends in the Food Industry
## 2 Food Industry Applications
### Plant Polymer-Based Foams as Edible Materials
#### 2.1.5. Starch  
Starch is a low-cost, biodegradable, non-toxic, and readily available organic polysaccharide. As such, it is a valuable and convenient resource for the food industry. However, its use in edible solid foams is still not well explored. One of the techniques that has been studied is that used in the production of extruded starch-based snacks. The extrusion process produces foam by the expansion of the ingredients' melted mixture. The expansion phenomenon occurs in different stages. The first stage is the expansion of the melt, which is often followed by a shrinkage phase before the melt solidifies. Then, the melt exits the die, followed by its expansion due to the sudden pressure drop. Internal moisture migrates quickly towards the surface, which drastically decreases the moisture content and temperature of the melt. The melt continues to expand until the melt temperature decreases below its glass transition temperature (Tg), where expansion ceases and the structure of the extrudate sets [33]. In Martinez-Sanz [34], foams were produced by extrusion cooking using Spirulina at 0, 1, 5, and 10 wt.% mixed with corn starch. During extrusion, starch granules are ruptured as they are subjected to high shear, pressure, and temperature. As a result, the crystalline regions of the granules are melted. A polymerization and rearrangement of amylose and amylopectin chains occurs before leaving the extruder die nozzle. Shorter amylose and amylopectin chains are associated by hydrogen bonds after exiting the nozzle if water or some other plasticizer is present [35]. The gelatinization of starch was produced during extrusion by raising the moisture content up to 30 wt.% and letting the samples equilibrate for 24 h before processing. As a result, starch foams were very amorphous, while Spirulina-starch foams showed a slightly more crystalline structure than the pure extruded starch, due to Spirulina complexation with amylose. Thus, Spirulina-starch foams showed more densely packed and well-connected porous structures, making the foam texture harder as Spirulina content increased. During storage, the free fatty acidsfrom Spirulina re-crystallized and the resistant starch content in the 10% Spirulina foam rose. Nutritionally speaking, this result might be interesting for the food industry, which is always looking for a way to produce new healthy snacks. Several extruded starch-based snacks have been studied in this category. For example, corn starch-based snacks with amaranth, quinoa and kariwa [66]; rice starch and pea protein snacks [33]; and corn starch based-snacks with common bean [67]; among others. According to [PERSON] et al. [68], starch is suitable for this kind of application, since granule degradation by gelatinization allows the formation of a stable, expanded structure. Regarding proteins, during the extrusion process they are unfolded, realigned, hydrolys, and can cross-link with other ingredients like starch. In addition, proteins affect extrudate expansion through their ability to influence the water distribution within the extruded melt. The presence of protein can disrupt the pore walls of the expanding melt, which results in reduced expansion and smaller sized, denser cell structures. On the other hand, protein content can increase the number of pores of starch-based extrudates at certain levels, as a result of a greater number of nucleation sites [34].