Journal of Energy Bioscience 2024, Vol.15, No.5, 289-300 http://bioscipublisher.com/index.php/jeb 291 2.3 Variability in waste components (fats, oils, and grease content) The composition of kitchen waste, particularly the FOG content, can vary significantly depending on the source and type of waste. For example, post-consumption food wastes have higher dry matter (>26%) and fat content (>13%), making them more reliable feedstocks for biodiesel production due to their lower temporal variability (Ho and Chu, 2018). The levels of free fatty acids (FFAs) and moisture in waste cooking greases can also vary widely, with FFAs ranging from 0.7% to 41.8% and moisture from 0.01% to 55.38% (Çanakçı, 2007). This variability necessitates efficient pretreatment methods to optimize the conversion of these wastes into biodiesel (Çanakçı, 2007; Abomohra et al., 2020). 3 Microbial Conversion Process Overview 3.1 Introduction to microbial fermentation in biodiesel production Microbial fermentation is a pivotal process in the production of biodiesel, particularly when utilizing waste materials as feedstock. This biological method leverages the metabolic capabilities of microorganisms to convert organic substrates into valuable biofuels. Unlike traditional chemical methods, microbial fermentation can operate under milder conditions and can process a variety of feedstocks, including lignocellulosic biomass and industrial organic wastes (Zhang et al., 2020; Adegboye et al., 2021). The process typically involves the breakdown of complex organic materials into simpler compounds, which are then converted into lipids by oleaginous microorganisms. These lipids can subsequently be transesterified into biodiesel (Wahlen et al., 2011; Carmona-Cabello et al., 2020). 3.2 Types of microorganisms involved in the process A diverse array of microorganisms is employed in the microbial fermentation process for biodiesel production. These include bacteria, yeasts, and microalgae, each contributing uniquely to the conversion of substrates into lipids. For instance, oleaginous yeasts like Rhodosporidium toruloides are known for their high lipid accumulation capabilities (Carmona-Cabello et al., 2020). Bacterial species such as those from the families Clostridiaceae and Ruminococcaceae play significant roles in the anaerobic fermentation process, aiding in the efficient breakdown of complex organic materials (Martínez-Burgos et al., 2020). Microalgae, such as Nannochloropsis sp., are also utilized due to their high lipid productivity and ability to grow on non-arable land (Wahlen et al., 2011; Shi et al., 2021). 3.3 Metabolic pathways for lipid production from waste The metabolic pathways involved in lipid production from waste materials are complex and involve several key steps (Figure 2). Initially, the organic waste is hydrolyzed into simpler sugars and fatty acids through enzymatic actions. These simpler compounds are then taken up by microorganisms and funneled into metabolic pathways such as glycolysis and the tricarboxylic acid (TCA) cycle. The intermediates from these pathways are subsequently directed towards lipid biosynthesis. Metabolic engineering can further enhance these pathways to increase lipid yield and productivity (Zhang et al., 2020; Adegboye et al., 2021). For example, genetic modifications can be made to increase the flux through the acetyl-CoA pathway, a critical precursor for lipid biosynthesis (Adegboye et al., 2021). 3.4 Advantages of Using Microbes Over Conventional Methods Using microbes for biodiesel production offers several advantages over conventional chemical methods. Firstly, microbial processes can utilize a wide range of feedstocks, including waste materials, which reduces the overall cost and environmental impact (Carmona-Cabello et al., 2020; Zhang et al., 2020). Secondly, microbial fermentation operates under milder conditions, which can lower energy requirements and improve safety (Adegboye et al., 2021; Nanda et al., 2023). Additionally, the use of engineered microorganisms can lead to higher yields and more efficient conversion processes (Adegboye et al., 2021). Microbial methods also allow for the production of biodiesel with favorable properties, such as a high content of saturated fatty acids, which can enhance fuel stability and performance (Wahlen et al., 2011; Shi et al., 2021). Finally, the integration of microbial processes into a biorefinery approach can facilitate the production of multiple biofuels and biochemicals,
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