# 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
#### 2.2.2 Cellulose  
Cellulose materials are appropriate for the development of biopolymer-based foams due to their biodegradability and low environmental impact but also because of their low density, high aspect ratio, large surface area, and non-toxicity [7]. In general, cellulose nanofiber-based solid foams can be made using various procedures and these usually comprise three steps: (i) the preparation of a gel, (ii) the creation of the 3-D structure via foaming in the presence of surfactants, and (iii) the removal of the solvent. The subtraction of the solvent can be performed using several techniques, such as, supercritical drying, freeze-drying, oven-drying or ambient conditions. Varying the processing route will impact the nano- or macrostructure of the final product, which subsequently will have an effect on the properties of the solid foam, such as porosity and its mechanical and barrier properties [73]. Cellulose nano- and microfibrils, especially, have been utilized in the production of low-density porous materials that display high specific surface areas, low thermal conductivity, and low dielectric permittivity [70]. Because of their distinctive mechanical and morphological characteristics, the cellulose nano- and microfiber-based foams have attracted industrial interest over the last 20 years [1]. For example, [PERSON] et al. [74], created a lightweight and strong porous matrix by drying aqueous foams stabilized with surface-modified nanofibrilated cellulose (NFC). The innovation in that study was that they use cellulose as foam-stabilizing particles. As shown by confocal microscopy and high-speed video imaging, NFC nanoparticles stopped the air bubbles from collapsing or coalescing by arranging themselves at the air--liquid interface. Stability was achieved at a solids content around 1% by weight. Careful foam drying resulted in a cellulose-based porous matrix of high porosity (98%), low density (30 mg/cm\({}^{3}\)), and with a Young's modulus higher than porous cellulose-based materials made by freeze drying. The size of the pores was in the range of 300 to 500 \(\mu\)m.  
Similarly, [PERSON] et al. [75] reported the effect of cellulose nanofibers (CNFs) on thermoplastic starch (TPS) foamed composites. The analyses were focused on the thermal, dynamic mechanical analysis (DMA), density, and water uptake. The results revealed that thermal stability, storage modulus (E\({}^{\prime}\)), loss modulus (E\({}^{\prime\prime}\)), and damping factor (tan \(\delta\)) increased for all TPS/CNF samples compared to the pure TPS-foamed composites, while apparent density and water absorption of foams decreased when composed with CNF. Additionally, incorporation of CNFs caused an increase in the glass transition temperature (Tg) of the foams. Moreover, 1.5 (wt.%) CNF concentration gave superior resistance or stability with respect to heat compared to its counterparts. An interesting feature shown by the foams was revealed by SEM images of composite foams containing 1.0 or 1.5 (wt.%) CNF: the size of the cell decreased while density increased as a result of CNF acting as the nucleation agent. CNF favored the formation of the cell nucleation sites and the bubble heterogeneous nucleation during the foaming process. In the study of [PERSON] et al. [70], various types of isolated lignin-containing cellulosic nanofibrils (LCNF) were used to reinforce waxy corn starch-based biofoams. The addition of LCNF increased the Young's modulus and yielded stress in compression mode by a factor of 44 and 66, respectively. In addition, the water sorption of the foams was decreased by adding LCNF due to relatively lower hydrophilicity of residual lignin. The optimized foams exhibited mechanical properties similar to those of polystyrene foams. Based on the results, cellulose reinforced foams might potentially become a sustainable and biodegradable alternative for packaging and insulation materials. Using similar components but a different approach, [PERSON] et al.