# Applications of Plant Polymer-Based Solid Foams: Current Trends in the Food Industry
## 2 Food Industry Applications  
Utilization of plant polymer-based solid foams in the food industry can be categorized into two main areas according to their final uses: (i) as an edible material, e.g., food matrix, or (ii) as packaging material. Although their final state is solid, solid foams start as wet foams before being solidified. They can be produced by different mechanical, physical, and chemical foaming processes [10]. In the wet stage, they are air-filled systems, in which foam structure stabilization is the result of molecules acting as surfactants or Pickering stabilizers. The transition from a wet to a solid state is usually accomplished by either cooling, heating, or curing the wet foam [11]. The processing techniques to solidify plant polymer-based foams may include baking, freeze-drying, extrusion, injection molding, or compression molding processes [12]. For packaging materials, supercritical fluids, such as carbon dioxide (CO\({}_{2}\)) or nitrogen (N\({}_{2}\)), can be used with molding techniques, producing stable foams that are lighter and of higher dimensions than their solid counterparts [10].  
Edible solid foams constitute the fundamental basis of several food products, such as bread, meringue, and ice cream [13]. For a solid foam to be considered an edible material, the first consideration, obviously, is to be food grade. Additionally, some important properties to be evaluated are moisture adsorption capacity, mechanical and physical properties, sensory and organoleptic properties, cell morphology, digestibility, loading capacity, etc. The evaluation will depend on the targeted usage of the foam.  
Some of the properties mentioned above will be relevant for foams intended to serve as a structural basis for packaging materials. However, certain mechanical and physical properties will be of special relevance to their suitability for this purpose. Thus, these foams must: (i) provide protection for fragile food products, (ii) show appropriate mechanical and barrier properties, (iii) be lightweight and (iv) non-toxic, and (v) have appropriate moisture adsorption capacity, amongst other things. In addition, the evaluated properties will depend on the food to be packaged, as well as other factors, such as shelf-life, storage conditions, etc. [14; 15].  
The present review article deals with the research and development of solid foams derived from plant polymers with potential or direct applications in the food industry over the last fifteen years. Additionally, this review will highlight details of the micro- and nanostructure of foam, the structure-property relationships between polymers, and the physicochemical characteristics elucidated in the studies consulted. It should be borne in mind that although the rheological properties before solidification are important for the physicochemical characteristics of solid foams, this matter will not be addressed in this review due to the depth of the matter and because it has already been touched upon in several other reviews. Readers are encouraged to seek out more detailed information in the articles by [PERSON] and [PERSON] [16], [PERSON] and [PERSON] [17] and [PERSON] et al. [18].