# Applications of Plant Polymer-Based Solid Foams: Current Trends in the Food Industry
## 2 Food Industry Applications
### Plant Polymer-Based Foams as Food Packaging Materials
#### 2.2.1 Starch
In addition, the thermal stability of the blended starch foam was lower than NS foam, probably due to the presence of ester bonds with low thermal stability, while the stabilizing effect of the higher degree of cross-linking and strong hydrogen bonds in the citric acid-modified starch might explain the significantly slower water evaporation and decomposition rate of NS/CNS blend chains. In the same vein, the morphology and the physical, flexural, and thermal properties of cassava starch foams for packaging applications were researched as a function of cotton fiber and concentrated natural rubber latex (CNRL) content [53]. The main objectives were to solve their two main weaknesses, i.e., lack of flexibility and sensitivity to moisture. Cotton fiber was principally added as a reinforcing material. A comparison among SEM micrographs of starch biofoams, both with and without cotton fiber, showed a sandwich-type structure. However, after the addition of cotton fibers, the foam exhibited denser structures, thicker cell walls, and a lower area porosity (43.37% compared to 52.60%). It seems that cotton fiber presence decreased the chain mobility of starch via hydrogen bonding, resulting in a high viscosity of the starch batter and less expansion of the foam. CNRL helped to control moisture into cassava starch foam. As CNRL content rose, the moisture adsorption capacity of the foam declined (\(-\)73.4% and \(-\)41.78% at 0 and 100% RH, respectively). This may be due to the hydrophobicity increment of the foam. Foam flexural properties were also tuned by regulating CNRL content. For example, with an amount of 2.5 phr of CNRL, the elongation of the biofoam improved by 24%, while the bending modulus decreased by 2.2%. An interesting study carried out by the same research group involved a soil burial test that assessed the biodegradability of the cotton-fiber-reinforcedcassava starch foam. They found that the degradation mainly progresses by hydrolysis and is delayed by the addition of CNRL. Sunflower proteins and cellulose fibers were also added to cassava starch to produce biodegradable food packaging trays through a baking process [55]. The study was focused on the relationship between the proportions of these three components and their effect on microstructure, physicochemical and mechanical properties of the trays. The results showed that increasing the fiber concentration from 10% to 20% (\(w/w\)) raised the water absorption capacity of the material by at least 15%, while mechanical properties were improved. On the contrary, an increase in sunflower proteins up to 20% (\(w/w\)) reduced the water absorption capacity and the relative deformation of the trays to 43% and 21%, respectively. The formulation that exhibited a more compact, homogeneous, and dense microstructure, with maximal resistance (6.57 MPa) and 38% reduction in water absorption capacity, contained 20% fiber and 10% protein isolate. This optimized material presented the best mechanical properties, lower water absorption, a lower thickness, and a higher density. Likewise, [PERSON] and [PERSON] [56] used the baking process to produce biodegradable foam trays by mixing malt bagasse with cassava starch. The concentration of malt bagasse varied from 0-20% (\(w/w\)) and the microstructural, physical and mechanical properties of foams were assessed. The trays had an amorphous structure as a result of a good distribution of the malt bagasse throughout the polymeric matrix. Foams showed a sandwich-type structure with dense outer skins enclosing small cells. The interior of the foams had large air cells with thin walls. They showed good expansion with large air cells. Their mechanical properties were not affected by variation in the relative humidity (RH) from 33 to 58%. However, when the trays were stored at 90% RH, the stress at break decreased and the strain at break increased. This is likely due to the formation of hydrogen bonds with water favored by the hydrophilicity of starch molecules.