Apple is ranked as the fourth most consumed fruit crop in the world (Montañés et al., 2018). Besides the consumption of fresh fruit, several processed foods are marketed from apples such as purees, juices, fresh cut snacks or canned apples. From the apple processing industries, massive amounts of waste are then generated with high costs of disposal. As an example, in apple juice production nearly 25% of the fruit is discarded (Mirabella, Castellani, & Sala, 2014). This by-product is commonly called apple pomace and is composed mainly of peel, stems, seeds, calyx, and core (Ferrentino, Morozova, Mosibo, Ramezani, & Scampicchio, 2018). As for now, apple pomace is mainly used for extraction of pectin and for animal feed production (Walia, Rawat, Bhushan, Padwad, & Singh, 2014). Indeed, it is a valuable source of pectin, proteins, enzymes, organic acids, antioxidants, vitamins, minerals, aroma compounds, and simple sugars (glucose, fructose, and sucrose) and fibers (Mirabella et al., 2014; Walia et al., 2014).

Apple seeds represent up to 0.7% of the fresh fruit and 4–5% of total apple pomace (Fromm, Bayha, Carle, & Kammerer, 2012; Walia et al., 2014). They are rich in vitamin E and fatty acids, mainly unsaturated such as palmitic, linoleic and oleic acids (Górnaś, Rudzińska, & Segliņa, 2014; Rodríguez Madrera & Suárez Valles, 2018). The oily part can be extracted from the seeds with yields ranging from 15 to 22% reaching values up to 29.4% (Fromm et al., 2012). Several studies showed that apple seed oil has potential antioxidant, antiproliferative, and antimicrobial activity. However, more investigation is still required for its possible use as a food ingredient. Indeed, currently apple seed oil is used in cosmetic industries for hair and skin care (Montañés et al., 2018).

Different extraction techniques can be applied to obtain oils from seeds. Some of them are based on the use of organic solvents such the Soxhlet extraction, which showed to have the best extraction yields with respect to other traditional techniques (i.e., maceration), although it requires long extraction times. Moreover, it is not suitable for the extraction of thermolabile compounds (Wang & Weller, 2006). Among the alternative techniques, two methods are receiving more attention in recent years: the ultrasound assisted extraction (UAE) and the supercritical fluid extraction (SFE). The UAE is used for the extraction of bioactive compounds from food matrices, and recently it was introduced also for the oil extraction from seeds (Hernández-Santos et al., 2016). Ultrasound waves travel in the matrix generating cycles of compression and expansion that favor the solvent penetration improving the extraction. The mechanical effect induces biological cell walls disruption making the release of compounds easier and resulting in a more effective mass transfer. The ultrasound waves can be generated in a water bath or by using a probe immersed in the sample. During the process, it is possible to monitor the temperature to avoid the deleterious effects on thermolabile compounds. As technique, it is less expensive compared to other extraction methods although its efficiency is strongly correlated to the food matrix (Wang & Weller, 2006). UAE has been already applied for the extraction of bioactive compounds from apple pomace (Egüés, Hernandez-Ramos, Rivilla, & Labidi, 2021). Moreover, it has been tested for the extraction of oil from apple seeds in few studies focused on the identification of tocopherols and antioxidant compounds (Górnaś, Soliven, & Segliņa, 2015). However, no studies have been performed so far to optimize the UAE processing conditions for the recovery of oils from apple seeds.

Supercritical fluid extraction uses a supercritical fluid as a solvent. The most common one is carbon dioxide, which is selected for its apolar properties. It is gaining high popularity as a green technology, since the solvent is non-toxic, non-flammable, safe and it is easily removed after extraction with decompression. With respect to other techniques, SFE solvent has a dissolving power that can be modulated by changing the pressure and the temperature. Moreover, the supercritical fluid has a higher diffusion coefficient and lower viscosity and surface tension than a liquid solvent, thus favoring the mass transfer (Wang & Weller, 2006). This technique was recently introduced for the extraction of oil from apple seeds (Ferrentino et al., 2020; Montañés et al., 2018) and for the extraction of polyphenols from apple pomace (Ferrentino et al., 2018).

Accordingly, this work aimed to recover oils from apple seeds by applying three different technologies: ultrasound, supercritical fluid and Soxhlet extraction. Indeed, the first objective was to optimize the UAE processing parameters by using the response surface methodology to obtain an oil from apple seeds with the highest yield and rich in fatty acids components. In a second step, the oil obtained at the UAE optimized processing condition was compared with the ones extracted by supercritical fluid and Soxhlet techniques.