# 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  
The low cost, availability, and compostability of starch make it of great significance for the packaging industry [70]. There are hundreds of studies utilizing starch as a replacement for synthetic packaging material. Nonetheless, and regardless of the fabrication method or starch source, all show the same drawback related to water absorption or water vapor permeability [71; 72] due to the hydrophilic nature of starch molecules. Therefore, this is one of the biggest problems to solve. One strategy has been to modify the molecular structure of starch to make it more hydrophobic. In a study by [PERSON] et al. [44], potato starch was used to fabricate a thermoplastic starch (TPS) foam that was modified by two methods: (i) acetylation and (ii) esterification with maleic anhydride. Their results showed that non-modified TPS foams absorbed 75 g water/100 g solids, while foams with 13% acetylated starch (TPS-Ac) and 20% esterified starch (TPS-Es) presented lower water absorption (42 g and 45 g water/100 g solids, respectively), improving the foam water resistance. Analysis of pure TPS, TPS-Ac, and TPS-Es foam microstructure showed that they have a sandwich-type structure. This kind of structure is typical in TPS foams produced by a mold compression process or baking process. They consist, roughly speaking, of two sets of layers--outer layers and interior layers. Outer layers have a denser structure, smaller cell size, and less voids than the interior layers which have larger cells and more expanded structures. Additionally, in this study, TPS-Ac presented more or less dense outer layers, depending on the acetylation degree, and a more compact cellular structure than pure TPS foam. Differences in viscosity values of foams might explain distinctive microstructures, as acetyl groups are related to a decline of intermolecular bonds between water and unmodified starch due to their hydrophobicity. If the pastes have low viscosity, they cannot hold vapor bubbles as effectively as more viscous ones during the baking process. Therefore, the lower the viscosity of the paste, the greater the paste expansion, which generates foams with a thinner outer layer and large inner cells. Similarly, in another study by [PERSON] et al. [46], two silanes were used for potato starch silylation: 3-chloropropyl trimethoxysilane and methyltrimethoxysilane. The foams were made using modified starch, gelatinized starch, polyvinyl alcohol, and water. Microstructure analyses showed the typical sandwich structure with denser outer layers of small cells and an inner layer of larger and more expanded cells. This microstructure translates into a more compact structure and thicker outer layers, which can be explained by the higher viscosity of the silylated starch pastes applied to make these foams. High viscosity is caused by silane cross-linking. Additionally, mechanical tests showed that foams become more resistant to cracking and fracture with the addition of silylated starch. This may also be due to the cross-linking of silanes which make starch pastes more viscous. Meanwhile, the silylation modification yielded foams with less water absorption. The improved foam performances make them a potential packaging material for use in the food industry. [PERSON] et al. [47] utilized sweet potato starch and oregano (OEO) or thymre (TEO) essential oils to produce bioactive foams by thermopressing. The essential oils were used at two concentrations (7.5 and 10%). The foams were characterized according to microstructure, mechanical properties, antimicrobial properties, and structural properties. In terms of structure, SEM micrographs revealed that foams presented a sandwich-type structure with two well-defined layers and the presence of air cells. The foam thickness was not significantly affected by the essential oil type and concentration at any level, but the starch-lipid interactions resulted in the formation of amylose-essential oil complexes with lipids localized in the first layer.