**2. The Recovery from Apple Pomace Dried and Powder**

*2.1. The Pretreatment of Apple Pomace*

#### 2.1.1. Biofuels

Due to serious concerns affiliated with climate change, the shift from fossil-based energy production options to various renewable-based alternatives has been a trending topic in the world. The most common renewable energy sources are wind, solar, hydropower, tidal, geothermal and biomass [29]. The latter involves organic materials such as agricultural residues, forestry residues, agri-food industry by-products (e.g., apple pomace), animal wastes, etc. Through various mechanical, biochemical or thermochemical processes, biomass can be transformed into fuels and/or fuel intermediates in solid (e.g., biochar), liquid (e.g., bioethanol, pyrolysis oil, etc.) and gaseous (e.g., biogas/biomethane, etc.) forms. Up to 80% of the organic matter in the pomace can be transformed into biofuel with an energy value of 10–30 W·m−<sup>3</sup> [30]. Generally, production residues from the food industry are characterised by a low concentration of heavy metals and are good raw materials for biofuel production [31]. AP is rich in fermentable sugars (for example, it has contents of 19.2% fructose and 1.0% sucrose [32]) and is characterised by a low concentration of heavy metals; therefore it could be applied alone as a raw material for biofuel production. However, in practice, installations that use a mixture of several substrates are the most often used. Diversification of substrates favours the obtaining of better biofuel parameters and increases the safety of raw material supplies. Co-fermentation increases the efficiency of the process while reducing the costs incurred by the biogas plant for the purchase of raw material. Batches for energy production should be selected on account of, e.g., the maximization of energy yields, the stability of the fermentation process and the possibility of using the post-fermentation mass. In the study of Olech et al., the results of the analysis of the fermentation medium made of corn silage and apple pomace (in a proportion of 50/50%) showed that the highest methane generation was 61% [8]. The sample efficiency achieved a value of 4460 Nml. It was also demonstrated that olive and apple pomace are good co-substrate in the fermentation of cow slime (excrement) [33].
