**1. Introduction**

#### **1.1 Application of biodegradable films**

In 2016, 335 million tons of plastics were produced globally, of which the majority were single-use plastics. Approximately 40% of the produced plastics were used for packaging [1]. This large volume of plastics leads to several problems: most oilbased plastics are highly resistant to biodegradation, which means if they enter the

environment, they can accumulate, and this leads to adverse environmental consequences [1]. These negative side effects could be reduced with biodegradable plastics. Since biodegradable plastics can be produced from fruit and vegetable wastes, the use of biodegradable plastics also helps valorize food wastes. Creating biodegradable plastics from food wastes could reduce the carbon footprint and the adverse environmental effects of oil-based plastics. Packaging is an essential step in maintaining food quality during manufacturing and shelf life. Depending on the packaging properties, it can also protect from microorganisms and moisture. With proper packaging, shelf life can be extended, and food waste can be reduced [2]. Due to the crucial role of packaging in this context, it is essential to develop new biodegradable materials with desired properties. An interesting application would be a direct coating of a product with a thin layer of edible plastic. The coating could be enriched with antimicrobial substances or antioxidants, which protect the product directly [3].

#### **1.2 Pectin from fruit wastes as an ingredient of edible films**

Some biopolymers, such as casein, alginate, and pectin, are increasingly gaining attention because of their inherent biodegradability. Biopolymers made from polysaccharides have a higher thermostability compared to biopolymers based on proteins, such as casein [4]. To improve the properties of the bioplastic, combinations of different polysaccharides or additives can be used. An interesting polysaccharide that could be taken advantage of to form bioplastic is pectin. Pectin is one of the main structural polysaccharides in dicot plants and consists mainly of galacturonic acid units as sugar backbone [3]. The carboxyl groups of the uronic acid residues can be present in different degrees of methylesterification, influencing the processing properties [5]. Pectin is classified as high methoxyl pectin if more than 50% of the hydroxyl groups are esterified, and low methoxyl pectin if less than 50% of the hydroxyl groups are esterified [6]. A significant percentage of pectin is found in fruit peels that are often discarded. It is possible to use the discarded peels to create edible films, which could be applied as coating for fruits and vegetables. Additionally, unused fruit peel wastes can be reused by creating biodegradable plastic [4]. Pectin edible films might be used for the protection of food with low moisture content due to their low resistance to humidity. They also provide an excellent barrier to aroma compounds and oxygen [7]. Their poor water vapor barrier properties could be explained by the hydrophilic nature of pectin [7]. A plasticizer and polyvalent cations are often added to edible pectin film formulations [3]. Plasticizers are used to reduce the brittleness; thus, improving the mechanical properties of the film [8]. Polyvalent cations such as calcium can be applied to crosslink the pectin chains and form a gel [3].

#### **1.3 Extraction methods for pectin from fruit substrates**

There are multiple methods for extracting pectin from fruit wastes: for example, enzymatic, acidic, and microwave-assisted extraction. These methods differ in the yield of resulting pectin and its properties. One possibility is the application of an enzymatic preparation like Celluclast® 1.5 L. The enzymes partly degrade the plant cell wall components and enable the subsequent extraction of pectin [9]. The advantage of this method is that no strong acids are used; thus, no acidic wastes are produced. Microwave-assisted extraction is another way to extract pectin, characterized by its short processing time and improved yields. The yield of the extracted pectin

varies with the microwave power used. The higher the applied microwave power, the higher the extraction yield [10].

#### **1.4 Kabog millet flour as valuable ingredient of edible films**

Kabog millet is an ecotype of *Panicum miliaceum* L. and grows only in the Philippines. The grain is rich in dietary fibers, proteins, and antioxidants, such as carotenoids and tocopherols [11]. Due to its high nutritional quality, the addition of whole-grain kabog millet flour to edible films is a promising option to improve their nutritional profile. In whole-grain kabog millet flour, lipids are also present, which may improve the hydrophobicity of the biopolymers [12]. Therefore, it may be a solution to one of the most common problems for bioplastics: low water resistance [13]. Whole-grain kabog millet flour has some promising properties that can be integrated into edible bioplastics formulations.

With these concepts in mind, the aims of this study were to 1) extract pectin from orange peel wastes by microwave-assisted extraction and enzymatic extraction; 2) characterize the extracted pectin by Fourier-transform infrared spectroscopy (FTIR) for the degree of esterification, by ion-exchange chromatography for the monosaccharide composition in pectin, and size-exclusion chromatography (SEC) for the polymer structure in water and its molecular weight; 3) create edible films containing kabog millet flour with the extracted and characterized pectin from different extraction methods; 4) characterize the produced films by their mechanical properties and their water contact angle; and 5) observe the effect of adding kabog millet flour on the properties of the films.
