**1. Introduction**

Effective waste management is one of the greatest environmental challenges the world is facing today. Technological advancement, economic development, urbanization, population growth and consumer habits have significantly contributed to a rapid increase in waste generation. Moreover, there are no signs of deceleration of this trend in the near future [1,2]. The 2018 edition of *What a Waste 2.0: A Global Snapshot of Solid Waste Management to 2050* estimated that the global municipal solid waste production will be 2.01 billion tons per year, and is projected to grow to 3.4 billion tons per year by 2050. The largest amount of waste (approximately 44% of the total amount of waste generated in the world) is food and green waste [3]. Among them, large groups are by-products of the fruit processing industries, such as the apple juice industry, the disposal of the major by-products of which (i.e., apple pomace—AP) can pose serious environmental problems or even public health hazards if incinerated and/or dumped [4]. The global production of AP is estimated at an average of 4 million/year and is expected to have an increase in the future. Unfortunately, the recovery rate of AP is quite low and insufficient. The most commonly applied disposal method for AP is to discard it directly to the soil in a landfill. It may cause serious soil and water pollution because AP is rich in water (>70%), sugars and organic acids, which are susceptible to fast microbial fermentation. The growth in microbial flora may decrease available nitrogen in the soil and affect the C/N ratio. Moreover, some authors mention the potential toxicity of AP because apple seeds contain a cyanogenic

**Citation:** Goł ˛ebiewska, E.; Kalinowska, M.; Yildiz, G. Sustainable Use of Apple Pomace (AP) in Different Industrial Sectors. *Materials* **2022**, *15*, 1788. https:// doi.org/10.3390/ma15051788

Academic Editors: Rossana Bellopede and Lorena Zichella

Received: 13 December 2021 Accepted: 25 February 2022 Published: 27 February 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

glycoside, amygdalin. However, it is unlikely to cause acute cyanide poisoning in humans because this would require the consumption of 800 g of AP [5].

Therefore, safe and efficient treatment and utilization of AP is required to reduce the possible environmental and health problems. Taking into account the huge amount of produced AP, the commercial applications of AP can create, in the future, great economic impacts. So far, different extraction methods that recover active substances from the AP have been applied. These active substances can later be applied as preservatives, antioxidants or anticorrosion agents used in, e.g., construction, civil engineering, environmental engineering and many other sectors (Figure 1). One of the basic techniques used to isolate the active compounds from materials of plant origin is classical extraction. In recent years, considerable attention has been given to the development of extraction methods that will be both efficient and environmentally friendly, e.g., limiting the amounts of solvent used or favouring energy efficiency [6]. Raw AP or the solid extraction by-products can also be utilised as a feedstock for the production of various types of intermediate bioenergy carriers in liquid, solid or gaseous forms, i.e., bioethanol, biodiesel, biomethane [7], biogas [8], biochar [9] or raw material for batteries [10]. Replacing traditional fossil fuels (FF) with AP-originated biofuels may reduce some undesirable aspects relegated to the production and use of FF, including emissions of greenhouse gases (GHG) (e.g., carbon dioxide (CO2) and nitrous oxide (N2O)), which contribute to serious environmental and health problems, and exhaustible resource depletion [11]. AP can also be a potential source of substances for the production of non-toxic and environmentally friendly biopolymers. The literature showed that AP-derived biopolymers were used for the production of biodegradable films, packaging materials, cups, plates and 3D objects [12,13].

**Figure 1.** Application of apple pomace in production of green, non-toxic and biodegradable products with applications in construction and building.

Currently, due to poor waste management and a lack of environmental awareness in many countries, a large amount of apple pomace is treated as waste with no economic value. Besides, there are some technical limitations affiliated with the effective utilization of apple waste, such as the requirement of immediate treatment after obtaining it (e.g., by drying); this is important to prevent the excessive growth of microorganisms (microbiological contamination), and hence, the loss of overall economic value [14]. To

foster sustainability, AP should be treated as a valuable raw material that can be reused or processed. In our work, the sustainable management of AP is based on the recovery and utilization of apple waste, which creates a possibility to reuse it and put it back into the supply chain. Green extraction techniques allow the obtaining of AP extracts that are rich in active compounds in an eco-friendly manner. The solid residues generated during extraction can be stabilised and further transformed into, e.g., alternative energy sources or biopolymers with zero waste. Therefore, sustainable AP management gives the opportunities for reducing environmental pollution and increasing integration into a circular economy [15]. This review presents the scale of AP production, and possible means of its utilization as a source of active compounds and biopolymers, a feedstock for the production of biofuels, and as a source of raw materials that can be utilised in different industrial sectors including the construction and building industry sector, the energy sector, and food or material industries.
