# Applications of Plant Polymer-Based Solid Foams: Current Trends in the Food Industry
## 2 Food Industry Applications
### Plant Polymer-Based Foams as Edible Materials
#### 2.1.3 Soy Protein  
The quaternary and tertiary structures of native soy protein limit and hinder foaming properties for food applications because of the large size of the molecules and their compact tertiary structure. Thus, some treatments that modify structure, such as heating and hydrolysis, must be applied to allow soy protein to be used as a foaming agent [25].  
Soy protein isolate (SPI) was utilized by [PERSON] et al. [26] to prepare a solid foam from freeze-dried O/W emulsions containing bacterial cellulose (BC) as Pickering particles. Using different oil fractions, the researchers modified pore size and density. Increasing the amount of oil, SPI-BC solid foams were produced, which exhibited uniform and smaller pores that displayed an open-cell structure with pore sizes of several dozen micrometers (\(<\)50 \(\upmu\)m). This is likely because emulsion droplets gradually became smaller and more uniform, contributing to the construction of a denser network and increased viscosity to prevent droplet accumulation. Thus, the physical stability of the prepared emulsions was high before freeze-drying. Along with this tunable structure, SPI-BC solid foams showedimproved mechanical properties, no cytotoxicity, and great biocompatibility, with potential for food industry applications [27].  
Another way of using SPI as a foaming agent was tested by [PERSON] et al. [28] to produce a banana snack. SPI banana foam had a dense porous structure that was crispier than foams produced by fresh egg albumin (EA) or whey protein concentrate (WPC). It is probable that SPI could not be well dispersed in the banana puree during whipping and that the final interfacial tension at the air/liquid interface might not be low enough to produce a significant foaming of the banana puree. WPC and EA banana foams underwent less shrinkage because SPI-banana foam was less stable during drying, so its structure collapsed. Also, WPC and EA banana foams had fewer volatile substances due to shorter drying times.  
A similar approach was attempted by [PERSON] et al. [29] using a combination of soy protein as a foaming agent and methyl cellulose as a stabilizer to produce a foamed mango pulp by the foam mat drying method. To obtain the same level of foam expansion, the optimum concentration of soy protein as foaming agent was 1% compared to 10% of egg albumin. Although biochemical and nutritional qualities in the final product were better when using egg albumin, the much lower concentration required for soy protein would be beneficial in terms of cost. It would be interesting to understand how the soy protein and methyl cellulose combination contributed to the positive results in foam expansion; however, this effect was not studied.  
Similarly, blackcurrant berry pulp was foamed using SPI and carboxyl methyl cellulose (CMC) as foaming and stabilizer agents, respectively. In this study, [PERSON], [PERSON], and [PERSON] [30] tested the effect of microwave-assisted foam mat drying on the vitamin C content, anthocyanin content, and moisture content of SPI blackcurrant foam. Several parameters of the microwave drying process, such as pulp load and drying time, had positive effects up to a certain level and then showed a negative effect on the content of both vitamin C and anthocyanin in blackcurrant pulp foam. At the lower pulp load condition, microwave power causes the temperature of blackcurrant pulp foam to rise, which resulted in vitamin C degradation. As for drying time, the longer the drying time, the greater the degradation of vitamin C in the blackcurrant pulp. The quality attributes in terms of color and appearance were better than in samples treated under traditional drying conditions.