## 4 Valorization Technologies of Avocado-Derived Biomass through Obtaining Bioenergy and Biofuels
### Biogas through Biochemical Conversion  
As commented on before, there are biofuels in the three states of matter: in the solid state are wood and chips, coal, pellets, and biochar; in liquid form, bioethanol, butanol, biodiesel, etc.; and in the gaseous state, biogas and synthesis gases [44]. In particular, biogas is a mixture of 55-70% (_v/v_) methane (17-25 MJ/m\({}^{3}\)), 30-45% (_v/v_) carbon dioxide, and some other gases in traces. It is produced by anaerobic fermentation of the organic matter and thus in an oxygen-free environment [45; 46]. In addition to being a renewable energy source, biogas does not have any geographical limitations nor does it require advanced technology for producing energy [47]. Overall, the share of biogas supply in the bioenergy sector accounts for only about 2%, but it has the potential to contribute much more since it can be supplied to a variety of markets, including electricity, heat, and transportation [48]. Biogas production can therefore provide a sustainable alternative fuel via utilization of agricultural organic wastes with a high percentage of humidity content [49], which is the case of avocado by-products.  
Biogas production depends on factors such as the nature of the substrate, temperature, pH, loading rate, C:N ratio, retention time, and alkalinity. Besides that, the organic material added as inoculums into the organic substrate and the size of the inoculums affected the rate of gas generation [50]. Research was conducted on the production of biomethane from organic waste based on its sugar, starch, lipid, protein, and fiber content. According to their composition, avocado residues were chosen for its high lipid content. The results indicated that the biomass with a high lipid content decreases the methane content, probably owing to low C/N ratio below 20:1. Moreover, the accumulation of lipids could inhibit the production of methane thanks to the formation of long-chain fatty acids [51]. Considering this result, it would not be advisable to use residual avocados (overripe and rotten avocados that are generally discarded) as raw material in biogas production. Alternatively, [PERSON] and coworkers [52] evaluated the potential of avocado stones (10% dry matter) after milling (micronized) and steam-alkaline pretreatment (with or without the addition of cellulolytic enzymes) at the laboratory scale. The best results were achieved using the latter conditions: from 105.7 NmL/g (vs. 87.7 NmL/g biomass d.m.) (micronized biomass) to 214.2 NmL/g (vs. 177.8 NmL/g biomass d.m.). These values are in the range of other agro-industrial by-products, which varied from 78 (cottonseed cake) to 421 mL/g (soybean oil cake) [45].  
Other studies investigated the production of biogas from mixtures of organic matter of animal and vegetable origin, incorporating avocado residues. For example, [PERSON] and colleagues studied the efficiency in the production of biogas from a mixture of vegetable waste, including avocado residual peels, which were mixed with cow manure; this resulted in 37 mL/g, without the need of nutrients or chemical addition to the system [53]. In a similar study, [PERSON] and coworkers investigated the ideal conditions for the generation of biogas from the avocado peel. These authors reported that the maximum production occurred at 25 \({}^{\circ}\)C, neutral pH, and 0.5% salt, and that it can be optimized by the co-digestion of the avocado waste with ruminal fluid and animal manure (up to a cumulative production of 453 mL within four days of incubation for 75% avocado peel + 25% poultry manure) [47].  
In addition to these laboratory studies, an avocado oil production plant for export is located in Kenya that uses scraps from the process as raw material in two digesters, where up to 5000 m\({}^{3}\) of biogas (66% methane) is generated per day. Purified biogas (94% methane) is achieved with a potential around 300 KW, which is enough to power the factory, and the excess energy is used to power the tractors of the local farmers [54].