Apple by-product could be defined as the remaining solid of apple processing in order to obtain juices or cider, among other products. It is mainly constituted of apple peel, seeds, stems, and pulp. Regarding the proximate composition of apple by-product (Table 1), the main element, overall, is dietary fibre, followed by non-fibrous carbohydrates, lipids, proteins, and ashes. Apple by-product has been recognised as a health-promoting ingredient by numerous authors (Table 2) and due to its composition, it has been determined as a potential source of bioactive compounds, such as dietary fibre, and also phenolic compounds, triterpenoids, or fatty acids (Figure 1) [24,30,35]. Polyphenolic antioxidants in apples are predominantly located in the skin, thus, most polyphenols persist in the apple pomace [30]. Phenolic compounds found in apple by-product include flavanols as the major class of apple polyphenols, followed by hydroxycinnamic acid derivatives, flavonols, di-hydrochalcones, and anthocyanins [24]. They have exhibited a high scavenging activity, indicating apple by-product as a potential source of dietary antioxidants [36]. Besides, triterpenic acids are abundant in many plants, including apples [37]. Particularly, apple terpenoids might be found either in the peel epidermal cells or epicuticular wax, which are often discarded as food processing industry waste [38,39]. Triterpenic acids have demonstrated various pharmacological properties and health effects, such as antioxidant, anti-inflammatory, or antimicrobial effects, increasing the attraction to producing healthcare products with multifunctional values [37,40]. Regarding fatty acids, recent studies have verified the presence of unsaturated fatty acids in the apple seed and apple peel. Particularly, oleic, linoleic, and linolenic acids have been described as the main components of this lipid fraction [35,41]. The contribution of the fatty acids to the potential antioxidant, antimicrobial, and antiproliferative capacities of apple seed oil has been proved [42,43,44].

Dietary fibre represents the highest contribution in the proximate composition of apple by-product (55.48 ± 0.7 g/100 g dry matter). Insoluble dietary fibre (IDF) accounts for the greatest proportion (43.58 ± 0.6 g/100 g dry matter) whereas soluble dietary fibre (SDF), represents a lower percentage (11.06 ± 0.1 g/100 g dry matter) [45]. Besides, apple by-product’ dietary fibre is mainly constituted of non-starch polysaccharides, including a high range of molecular sizes, structures, and monomeric composition. De la Peña-Armada and collaborators [60] determined the configuration of the total dietary fibre indicating uronic acids and glucose as principal components. In addition, arabinose, xylose, and galactose were also remarkable, suggesting the presence of pectin substances made of arabinans, arabinoxylans, homogalacturonans, arabinogalactans, or galactans as side chains.

Pectins are present in the cell walls and intracellular tissues of fruit and vegetables [61]. They are mainly composed of numerous chains of galacturonic acid and are characterised by being resistant to human digestive enzymes. Pectins can be fermented along the intestine favouring the growth of Lactobacillus or Bifidobacteria, therefore promoting the release of acetate and propionate, and enhancing gut health [62,63]. In fact, apple by-product potential as a prebiotic ingredient has been demonstrated and mainly attributed to its pectin content [33,34,45,62].

Gibson and collaborators [64] proposed the following definition of a prebiotic: “A substrate that is selectively utilised by host microorganisms conferring a health benefit.” Certain fermentable carbohydrates have been described to promote a prebiotic effect. However, the most widely documented dietary prebiotics are the non-digestible oligosaccharides fructans and galactans [65]. Prebiotics have the potential to improve digestive function, modulate the immune system, improve mineral absorption, or help glucose metabolism. Furthermore, prebiotic compounds can promote metabolic health, including preventing insulin resistance and promoting healthy blood lipid levels [66,67]. The main derived products of bacterial prebiotic metabolism are the short-chain fatty acids, namely, acetate, butyrate, and propionate. These compounds are known to interact with the host systems, leading to health benefits in the gut microbial community and consequently elsewhere in the body. Thus, short-chain fatty acids have been suggested as an interesting strategy for gastrointestinal dysfunction, obesity, and type 2 diabetes mellitus prevention [68]. Besides, the feasibility of recovering prebiotic substrates from novel sources, such as food by-product, has been recently proposed for addressing the economic and environmental current needs [69].

Therefore, efforts of the juice and cider industry should be focused on the valorisation of the apple by-product for the development of functional foods, dietary supplements, and/or food additives [34].

Sustainable chemistry has suggested that food by-products are rich sources of bioactive compounds [70]. The convenience of improving this raw material’s functionality, namely, increasing the accessibility to the health-promoting components, suggests the possibility to design different transformation strategies. The enhancement of the nutritional value of by-products is of great relevance due to its human health, economic, and environmental benefits [16]. In this regard, green processes are based on methodologies with low energy consumption, use Generally Recognised as Safe (GRAS) solvents, and ensure a high-quality and safe product [71]. The main objective of those procedures consists of developing new natural resource products with added value while decreasing waste generation. Emerging technologies, such as ultrasound, enzyme digestion, or pulsed electric field, have been developed over the last fifty years. However, these innovative alternatives have been greatly implemented in the food, pharmaceutical, and cosmetic sectors [71,72]. Indeed, numerous studies are focused on the application of microwave, ultrasound, enzymes, high hydrostatic pressure, and supercritical fluid extraction, among others, for food by-product valorisation (Figure 2) [72].

Microwave extraction has developed an important role in food science and technology over the last two decades. Microwaves generate heat after their contact with cellular polar compounds. The heat disrupts the cell walls and therefore enhances the availability of certain components [73]. Microwave-assisted extractions are generally performed in polar media such as water, acetone, alcohol, or mixtures. The reaction typically requires between seconds and a few minutes [74]. Furthermore, microwaves have been studied in combination with different processing technologies. In fact, the microwave-assisted ultrasound extraction technique has been described as a novel method for fast and efficient extraction. It has great potential for commercial applications due to the fast sample preparation and the appropriate balance between costs and effectiveness [75]. On the other hand, ultrasound is based on sound waves, i.e., mechanical vibrations, which, when applied to plant-based matrices, can collapse cavitation bubbles close to cell walls, producing a cell disruption. Thus, this methodology enhances the extraction processes [73]. The spectrum of ultrasonic wave frequency varies in the range from 20 kHz, considered the audible range, up to 10 MHz [76].

Enzyme-assisted extraction methods are becoming increasingly popular [77]. Enzymes, which can be found in nature, have been used for thousands of years to produce different food products. However, the latest development within biotechnology enables the creation of tailor-made enzymes, which display new activities, and can be adapted to diverse process conditions [78]. This enzymatic non-conventional methodology provides the opportunity of processing foods in a short time and at a low temperature, achieving high extraction yield in the industry [79]. The substrate specificity favours the extraction of a great number of bioactive compounds from the food matrix [77]. Therefore, according to the desired results after the application of this methodology, the enzyme utilised for the procedure will vary. For instance, if the main goal is cell wall disruption, degradation is generally carried out by complex cellulolytic enzyme preparations, which contain a mixture of several cellulase kinds [80]. Thus, inaccessible bound forms, contained within cellular walls, can be released and their availability increased [77]. Enzyme-assisted extraction is usually considered within the so-called bioprocessing. Bioprocessing includes numerous methodologies in which microbial fermentation is notable. Fermentation requires the contribution of living microorganisms for the generation of new products. In addition, improves organoleptic properties, increases the product shelf-life and modifies the nutritional profile of food. The fermentation valorisation process has been demonstrated to be a feasible method to improve bioactive component content in food by-products [81,82,83,84,85].

High hydrostatic pressure (HHP) treatment is a cold pasteurisation technique in which products are subjected to a high level of isostatic pressure transmitted by water. The application of this technology destroys pathogens, extends product shelf-life and only requires water and electricity. Furthermore, numerous studies have concluded that HHP is not only a conservation technology but also enables the extraction of bioactive compounds [74,86]. HHP may increase the extractability of phenols, carotenoids, and anthocyanins, and also the bioavailability of minerals, antioxidants, and starch [74]. Besides, HHP could be relevant for food by-product valorisation, since these derived products have been over-processed, and further treatments, especially thermal procedures, could cause an excessive loss of their bioactive compounds’ functionality [87]. Additionally, HHP has been combined with different methodologies, including the incorporation of food-grade enzymes into the process for a potential application in the enhancement of the health benefits of food products [86,88]. Particularly, HHP assisted by certain enzymes has resulted in an increase in the enzymatic activity that may be due to a change in the active site or in substrate specificity [89].

Another alternative green process for food by-product valorisation is supercritical fluid extraction (SFE). The supercritical technique has been used since 1882 and its main application fields are food and flavour, pharmaceuticals, and various biochemical sectors [90]. Supercritical carbon dioxide enables a clean extraction to separate different components from food matrices, producing a pure and safe product [91,92]. Particularly, CO₂ is a non-polar solvent which facilitates lipophilic compound obtention [42,93]. In addition, this procedure has other major advantages, such as the low critical temperature required for it to be conducted, or the absence of light and air during the process, which reduces the risk of recovered compound degradation [94].

Emerging technologies present various applications forms, including small-scale and large-scale, entailing industrial uses, such as food or nutraceutical processing [77]. Hence, in order to reintroduce the apple by-product back into the food chain, these technologies represent an interesting option for the present and future time.