Bioethanol

Bioethanol (ethyl alcohol) is used as an eco-friendly fuel, most often as an additive to gasoline. It is obtained through the anaerobic fermentation of carbohydrates [34]. This process requires a proper pretreatment of the biomass feedstock (e.g., fruits) allowing the release of simple sugars (glucose, xylose, galactose, etc.) contained in cellulose and hemicellulose. This way, a hydrolysate rich in hexose and pentose sugars can be obtained [7]. In the case of anaerobic fermentation of the apple wastes, it is essential to choose an adequate bacterial strain to deal with a wide variety of sugars that are initially present in lignocellulosic hydrolysates. In addition to the commonly used yeast, *Saccharomyces cerevisiae*, there are references in the literature concerning the use of alternative microorganisms such as *Zymomonas mobilis*, *Kluyveromyces marxianus*, *Kluyveromyces lactis* or *Lachancea thermotolerans* for lignocellulosic biomasses such as apple pomace [7,35]. For example, the potential of apple pomace as a feedstock for bioethanol production was demonstrated in a recent study by Molinuevo-Salces et al. [7]. In their research, scientists assessed the effectiveness of selected bacterial strains on the amount of bioethanol produced from the hydrolysate of dry apple pomace obtained after juice extraction. The results showed that the highest bioethanol concentrations were obtained by testing *Kluyveromyces* sp. and *Lachancea* sp. (between 49.9 and 51.5 g L−1). Total sugar consumption was in the range of 74.5 and 80.0, with bioethanol yields from 0.402 to 0.444 g g−<sup>1</sup> [7]. In the work of Demiray et al. [36], the influence of a cheap additive—soluble soy protein (at different concentrations: 20, 40, 80, 160 mg/g cellulose)—on enzymatic hydrolysis of AP was investigated. The results showed that the addition of 80 mg/g cellulose soluble soy protein to AP medium hydrolysed with 60 FPU (Filter Paper Units; enzyme concentration) increased the sugar (by 24.8%) and bioethanol concentration (by 8.28% in the case of *Saccharomyces cerevisiae*, and by 20.9% for *Kluyveromyces marxianus*), which makes the bioethanol production from AP process more efficient and still economical [36]. Kut et al. [37], for the first time, conducted the enzymatic hydrolysis of the liquid fraction of AP for the production of bioethanol using a pentose fermenter yeast, namely *Pichia stipitis*. The results of their research indicate that with a properly optimised process (10% (*w*/*v*) AP loading), about 84.1% of the theoretical ethanol yield can be obtained [37].

## Biogas

Another type of alternative biofuel in gaseous form, biogas, is produced via a sequence of low temperature (ca. between 30 and 60 ◦C) processes by which certain microorganisms break down degradable biomaterials in the absence of oxygen. The produced gas mixture consists predominantly of methane (CH4) and carbon dioxide (CO2) with volumes of about 40–75% and 15–60%, respectively [38]. The production of biogas is not completely free of GHG, but research is underway to reduce the concentration of CO2 in biogas [39]; this increases the energy content of the final gas mixture and would result in increased levels of biomethane. Moreover, photosynthetic plants can absorb the CO2 released via biogas combustion, which results in the emission of less total atmospheric carbon than the classical combustion of coal [38]. In the last decades, the extensive amounts of globally generated apple wastes have received interest in terms of the exploration of their potential as a co-substrate to valorise biofuel production. For example, Olech et al. [8] showed that the anaerobic digestion medium made of apple pomace and corn silage (in the organic mass proportion of 50 to 50%) achieved a satisfactory level of methane yield (about 40%) on the third day of fermentation. The highest daily biogas yield was obtained on the ninth day of measurement and amounted to 4460 Nml [8]. In addition, after processing the fruit residues (with negligible heavy metal content) in biogas production, nutrient-rich organic fertiliser can be obtained [11]. In the study of Claes et al. [40], the influence of biochar and graphene (carbon-based conductive materials) and trace metals supplementation on biogas production from AP was investigated. The results of their study showed that this supplementation significantly improved the biogas production from the AP. At a COD (chemical oxygen demand) concentration of 6000 mg/mL, the addition of (a) trace metals, (b) biochar and (c) trace metals and biochar, increased the production of biogas by 7.2%, 13.3% and 22.7%, respectively, compared to the control (without supplementation). At a COD concentration of 12 mg/mL, the greatest changes in biogas production were observed for graphene supplementation (increase by 27.8% in comparison with the control). Moreover, in most cases, supplementation also improved the methane yield. In this study, the highest obtained CH4 yield (increased by 23.0% in comparison with control) was observed in a case of the reactor supplemented with biochar and trace metals (COD = 6000 mg/mL) (468.0 ± 3.6 mL CH4/g vs. and 286.0 ± 6.2 mL CH4/g COD) [40].
