2.1.3. Biopolymers

Biopolymers are natural, biocompatible, highly biodegradable and environmentally friendly ("green") alternatives to widespread synthetic plastics. Biopolymers can be obtained/extracted by the following means: *i.* from natural sources (e.g., agricultural waste); *ii.* via direct biosynthesis by microorganisms; and *iii.* through chemical synthesis [58]. AP is a promising raw material for the production of biopolymers due to its high sugar content. It is estimated that the dry mass of AP contains 7–44% cellulose, 14–17% starch, 15–20% lignin and 4–14% pectin, which can be used for the production of biopolymers [19].

There are several recently published literature reports regarding the production of sustainable biomaterials from AP [12,13,59–64]. In the study of Gustafsson et al. [13], AP was used for the production of 3D objects (fibreboards) and biofilms. Solution casting was used to form fibreboards, while film casting was used to produce biofilms. The obtained structures were tested for tensile strength (TS) and elongation at max (EAM). The results showed that the highest value of TS (5.79 MPa) and EAM (1.54%) was reached by the biopolymer made from AP with 30% (*w*/*w*) glycerol. For comparison, the biopolymer produced only from AP showed significantly lower values of TS = 3.71% and EAM = 1.56%. The biopolymer prepared from AP with 7% (*w*/*w*) glycerol had three-times-lower flexibility (EAM = 10.77%) and a four-times-higher TS value (TS = 16.49 MPa) than biofilm prepared from AP without glycerol using values of EAM = 37.39% and TS = 4.20 MPa [13]. In another work [59], AP-derived bioplastic was used in the production of cups. The mechanical properties of bioplastic were measured, and the results showed that the highest values of TS and EAM were also reached using a mixture of washed AP with 30% (*w*/*w*) glycerol content. However, other important parameters, such as water resistance, exposure to environmental factors (e.g., light), or biodegradability, were not investigated in this study. AP-derived biopolymer could be an environmentally friendly replacement for synthetic plastic tableware or additives for the production of structural or building elements (e.g., bricks) [59].

In the work of Liu et al., AP was characterised as a potential source of biopolymers— PHAs (poly-hydroxyalkanoates) [12]. PHAs are biosynthesised by a wide range of Grampositive and Gram-negative bacteria (e.g., *Azotobacter*, *Clostridium*, *Alcaligenes latus* and *Cupriavidus necator*) and serve as an energy and carbon storage source [60]. Generally, the production of PHAs (2.4 and 5.5 US\$/kg) generates much higher costs as opposed to conventional synthetic plastics (1.2 US\$/kg) [61]. However, by changing the carbon source used in the production of PHAs to inexpensive agricultural waste (including AP), the production costs can be significantly reduced (up to 50%), which has a great impact on PHAs' applicability in many industries [12,62].

In the work of Pereira [63], *Pseudomonas chlororaphis* sub-sp. *Aurantiaca* was used to produce medium-chain-length PHAs (mcl-PHAs) from apple waste. The obtained mcl-PHAs consisted of, i.e., 3-hydroxydecanoate (42.7 ± 0.1 mol%), 3-hydroxyoctanoate (17.9 ± 1.0 mol%), 3-hydroxybutyrate (14.5 ± 1.1 mol%) and 3-hydroxytetradecanoate (11.1 ± 0.6 mol%) with a yield of 49.25 ± 4.08%. The obtained mcl-PHAs biofilms showed attractive mechanical properties (TS = 5.21 ± 1.09 MPa, EAM = 400.05 ± 55.8%) [63]. Rebocho et al. [64], in their study, used apple waste as a feedstock for the production of mcl-PHAs using *Pseudomonas citronellolis*. The major components of the obtained biopolymer were 3-hydroxydecanoate (68% mol) and 3-hydroxyoctanoate (22% mol) with a total yield of 1.2 ± 0.05 (g/L). *P. citronellolis* mcl-PHA films showed high tensile strength (TS = 4.9 ± 0.68 MPa) and thermal stability [64]. The above research confirms the potential possibilities of using apple waste for the production of biopolymers that could be used as, e.g., packaging materials in many industries [63,64].
