# 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
[76] fabricated biodegradable starch/cellulose composite foams cross-linked with citric acid at 220 \({}^{\circ}\)C by compression molding. Increasing the concentration of citric acid made water absorption capacity decrease, while stiffness, tensile strength, flexural strength, and hydrophobicity of the starch/cellulose composite foams increased. For example, tensile strength, flexural modulus, and flexural strength increased from 1.76 MPa, 445 MPa, and 3.76 MPa, for 0 % citric acid to 2.25 MPa, 601.1 MPa, and 7.61 MPa, respectively, for the starch/cellulose composite foam cross-linked with 5% (\(w/w\)) citric acid. The foams also showed better thermal stability compared to the non-cross-linked composite foam, indicating that composite foams might be used as biodegradable alternatives to expanded polystyrene packaging. In another study, lignin from bioethanol production was employed as a reinforcing filler by [PERSON] et al. [77] to fabricate a soy-based polyurethane biofoam (BioPU) from two polyols (soybean oil-derived polyol SOPEP and petrochemical polyol Jeffol A-630) and poly(diphenylmethane diisocyanate) (pMDI). The results are similar to those obtained in other studies where different cellulose sources were used to reinforce lignin-induced cell structure modifications and thereby increase the density and improve the thermal properties of the foam. The mechanical properties were also improved with the presence of lignin, and the samples with 10% concentration had better mechanical properties over other treatments, with values of 0.46 MPa, 11.66 MPa, 0.87 MPa and 26.97 MPa for compressive strength, compressive modulus, flexural strength, and flexural modulus, respectively. According to the authors, the lignin-polyurethane mixtures are characterized by a complex super molecular architecture due to the specific properties of their components. Polyurethanes produced an interpenetrating polymer network (IPN) structure, whereas lignin acted as an emulsifier for polyurethane soft and hard segments because it was subtly dispersed and integrated into the polymer amorphous phase, thereby improving the mechanical properties of foams. The research assessed the potential utilization of lignin in polyurethane applications, such as fillers and coating. [PERSON] et al. [78], studied the use of different concentrations of cellulose fiber on rigid polyurethane foams (RPFs). Mechanical resistance and thermal stability of the composite foams were not significantly changed by the introduction of cellulose industrial residue fibers, whereas thermal conductivity displayed a minor reduction. Based on those results, cellulose-polyurethane composite foams are potentially useful for applications in thermal insulating areas. Interestingly, the composite foams showed a predisposition to fungal attack in wet environments due to the presence of cellulose fibers. However, in this case, this attribute is appropriate, as it decreases the environmental impact after disposal. Due to the pressure of environmental concerns over the two last decades, considerable research and development in the area of nanocellulose-based materials have been extensively carried out. As a consequence, new products and applications of nanocellulose are steadily emerging as a range of applications of nanocellulose-based biodegradable polymers, thermoplastic polymers, and porous nanocomposites [1].