# Applications of Plant Polymer-Based Solid Foams: Current Trends in the Food Industry
## 3 Conclusions  
Applications of plant polymer-based solid foams in the food industry are mainly focused in two areas: edible foams and packaging materials. In these areas, there are many plant polymers that are utilized. However, most of the studies focused on the utilization of starch and cellulose, due to their availability and production costs. Still, it is observedthat starch is not more widely studied in the field of edible foams, being a very common by-product of the agriculture and food industry. This is likely because pure starch makes weak and high water absorption foams, so starch must be modified, or other compounds must be incorporated, in order to strengthen the foams and reduce their water absorption. However, these increase the cost of the final product. In addition, a deep knowledge of starch behavior in the presence of other components is required to overcome some disadvantages, such as brittleness and high water absorption capacity.  
In this context, most research aims to improve physical characteristics of solid foams, especially the mechanical and thermal properties which are generally impacted by the conditions of the foam process. However, creating a solid knowledge and comprehension of micro- to macrostructure modification will allow for more adequate management and design of processing conditions. Considering the above, there are two kinds of studies to perform. First, those in which researchers evaluate different properties or characteristics and link them to structural changes without further discussion. The second type goes beyond that and explains the changes based on phenomena produced by the several physical and chemical interactions among the components of the foams. This knowledge is crucial in order to tune the structural properties of solid foams through the control of the properties of the liquid foam (bubble size distribution, pore opening, foam density, etc.), which is a big challenge. However, research involving plant polymers in this area is still lacking.  
Along with this, the successful application of solid foams depends on other aspects related to foam creation that are very challenging; for instance, the preservation of the liquid foam structure throughout the transition process from liquid to solid foam, and the timescale pairing between the stability of the liquid foam and the solidification. Thus, one of the major drawbacks is rooted in the internal structure of the plant polymer-based foams, which will probably collapse due to poor mechanical properties. Considering this, in the field of edible solid foams, the utilization of functional proteins as foaming agents, e.g., soy protein and lentil protein, helps keep internal structure to an extent. Still, stabilizers are needed, e.g., to act as Pickering particles or to increase viscosity. Other compounds also might help to reinforce the structure, e.g., cellulose, even though this has not been widely explored in this field. In another vein, the incorporation of natural fibers, such as cellulose nanofiber (CNF) or microfibrillated (CMF), has improved the mechanical properties of plant polymer-based solid foams, helping to overcome their natural lack of strength by reinforcing the structure to endure the foam drying process. Other serious limitations arise from the high hydrophilicity of plant polymer compounds in wet conditions and limited thermal resistance. These drawbacks are also addressed by adding cellulose fibers, especially lignin, which may provide the system with better water resistance.  
Conceptualization, M.J.-P. and L.C.; methodology: M.J.-P. and L.C.; investigation, M.J.-P.; resources, M.J.-P.; writing--original draft preparation, M.J.-P.; writing--review and editing, M.J.-P. and L.C.; project administration, M.J.-P.; funding acquisition, M.J.-P. All authors have read and agreed to the published version of the manuscript  
This research and the APC were funded by Agencia Nacional de Investigacion (ANID) de Chile, FONDECYT Iniciacion, grant number 11180139.  
The authors declare no conflict of interest.