1. Introduction

The accumulation of plastic wastes is a globally recognized problem that involves an extremely negative impact on the environment [1]. The exceptionally low biodegradability of fossil-based plastics, together with the massive production scale associated with the plastic market over the past 60 years, has generated a huge accumulation of plastics in landfills and the oceans [2]. To illustrate the magnitude of the problem, considering that almost 400 Mt of plastic waste is generated every year [3], there is currently more than 1 ton of plastic/person alive in the world. However, in spite of the recent efforts made in this field to shift from a fossil-based to a bio-based circular economy model, only 20% of plastic is collected for recycling, of which only 3% is reused [4,5]. The rest is incinerated, landfilled, or disposed of into nature, a large part of which is ending up in the oceans [6]. In this sense, European Union Directive (EU) 2019/904 aims to prevent and reduce the impact of certain single-use plastic products on the environment, especially the marine environment, and on human health. Consequently, the future of the plastics industry needs to be driven by sustainability issues, where the bioplastic sector is a crucial building block for a circular economy scenario [7,8].

The most accepted definition of the term bioplastic, which has been controversial among plastic industrial associations and environmental organization, is given by European Bioplastics [5]. According to this association, any plastic material can be denoted as a bioplastic if it is either bio-based, biodegradable or displays both properties. Consequently, bioplastics embrace a whole family of materials with different properties and applications, ranging from biodegradable fossil-based polymers, such as poly(butylene adipate-co-terephthalate) (PBAT) or polycaprolactone (PCL), to non-biodegradable bio-based polymers, such as bio-based polyolefins (e.g., bioPE and bioPP) or polyesters (e.g., bioPET) [9]. However, the ecofriendliest bioplastic group is formed by biodegradable and bio-based polymers. This group comprises biodegradable aliphatic polyesters produced by fermentation of biomass, including polylactates (PLA), polyglycolates (PGA), polyhydroxy butyrate (PHB), polyhydroxy valerate (PHV), etc., and polymers extracted from renewable sources, also known as agropolymers, which include polysaccharide-based polymers (e.g., starch, cellulose, and cellulose derivatives) and protein-based polymers that can be extracted from animal or plant sources [10,11]. Currently, a big amount of the food produced worldwide (~30%) is discarded by the agri-food industry, being considered as byproducts or wastes [12]. These food biowastes could be reused as raw materials for the emerging bioplastics sector since their proteins, carbohydrates, lipids, and other compounds can be used for this application [13]

Agropolymers are considered the most eco-efficient bioplastic source in terms of the ratio between the added value of their potential applications and the environmental impacts associated with them [14]. They consist of a carbon backbone with different side groups that can form inter-/intra-molecular H-bonds. It is precisely the ability to temporarily disrupt these H-bonds and cause flow into new material shapes that allows forming plastic materials by conventional polymer processing techniques (e.g., casting, thermoforming, compression molding, extrusion, and injection molding) [15]. However, despite the unquestionable importance of bioplastics for enabling a more sustainable circular economy [16], they only cover approximately 1% of the global plastic market, accounting for 2.11 Mt in 2020 [5]. About 60% of the bioplastic market corresponds to biodegradable polymers and 20% to agropolymers (over 420 kt). Among them, starch and cellulose are abundant and low-priced raw materials [6]. Unfortunately, they typically require complex processing before they can be properly used as bioplastics. These processes, including fermentation or functionalization, typically increase their costs and, as a result, reduce their efficiency in the replacement of conventional plastics.

In contrast, an emerging ecofriendly and cost-efficient alternative to plastics is based on the use of protein which can be easily processed for many applications [17,18]. Moreover, protein may be inexpensively extracted from many sources that are also abundant in nature. Interestingly, global food biowastes represent about 1300 Mt/year, according to the Food and Agriculture Organization (FAO) of the United Nations [19]. This biomass may be regarded as a potential source that can be used in the protein-based bioplastic sector, competing with other uses (e.g., biofuel). However, some problems related to the collection of food biowastes, due to their extremely wide dispersion, still impose a barrier to their efficient application at large scales [20]. Other more interesting alternatives are currently being considered for the valorization of proteins, such as the use of agri-food co-products from the starch, oil, or biofuel industries; the extraction from industrial biowastes such as blood, bones, feathers, wool, hair, nails, etc., from poultry or cattle slaughterhouses; or microalgae from sewage plants [20,21]. However, the commercial use of protein-based bioplastics in 2020 is still residual (with an output lower than 30 kt) as compared to other agropolymers, particularly starch which accounts for almost 400 kt in 2020 [5]. Some authors have indicated that plastics production from proteins is economically feasible, reducing the wastes associated with industrial products [22].

As for the portfolio of bioplastic applications, food packaging remains the widest segment of the whole bioplastic family, with an output of almost 1 Mt in 2020, representing nearly half of the total bioplastics market. The other half is largely diversified finding applications as consumer goods, or in the textile, agriculture, automotive, and construction field, among others [5]. In particular, protein-based bioplastics may also accomplish some of those applications, mainly in the fields of food packaging and plastics for agriculture. Moreover, they may also be used in more specific applications, such as in the development of absorbent and superabsorbent materials, in the controlled release of active agents (e.g., drugs, antimicrobial agents, nutrients, etc.), or biomedical applications (e.g., as scaffolds for tissue engineering) [23]. Therefore, despite the many advantages associated with the use of protein as bioplastics for a wide variety of applications, its high potential for the replacement of conventional plastics has not yet been sufficiently explored [20].

This review is focused on the potentials of protein co-products of the agri-food industry or protein fractions extracted from agri-food industrial biowastes, as well as their applications as substitutes for conventional non-biodegradable fossil-based plastics. The characteristic of these protein-based materials must be analyzed to assess the functionality required for each application. These properties can be typically divided into mechanical properties, thermal properties, and optical properties, correlating them to the microscopic (even molecular) structure of the materials [24].

2. Proteins from Industrial Biowastes and Co-Products

Every year, around one third of all food produced worldwide, is either lost or wasted [25]. In Europe, that amount is reduced to one fifth, being 19% obtained from food processing and 11% from primary production [12]. Food biowaste is mainly composed of carbohydrates, proteins, lipids, and other compounds with great potential for high-value applications [13]. In this section, the main protein-rich biowastes and co-products from the agri-food industry that have been used in the development of plastic materials are presented. It should be highlighted that depending on the application pursued, proteins should be previously extracted and/or concentrated from the biowaste. Extraction can be carried out either through dry (e.g., air classification) or non-dry (e.g., chemical treatment) conditions. Among the concentration procedures for obtaining protein concentrates or isolates are isoelectric precipitation or ultrafiltration. These preparation techniques are outside of the scope of this study, and readers interested in their description are referred to a recent review on this topic [26].

3. Processing of bio-based materials