White Paper: Fats, Oils, And Grease (FOG) Microbiology; Metabolic Pathways Of FOG Removal Potential
•White Paper: Fats, Oils, And Grease (FOG) Microbiology; Metabolic Pathways Of FOG Removal Potential
Fats are esters of fatty acids and glycerol which are normally solid at room temperature. Oils are esters of fatty acids and glycerol which are normally liquid at room temperature. Grease is a general term used to describe a soft or melted animal fat or a lubricant (Campbell, 1999).
The degradation of fats, oil, and grease that collect within the wastewater treatment system is critical for quality of life and economic reasons. Large amounts of FOG are deposited into waste streams on a continuous basis particularly where there are high numbers of restaurants or food processing facilities. When FOG enters the waste stream it tends to congeal at the entrance and within the piping system creating flow problems and odors due to the property of being poorly soluble in water. Some microbes are particularly useful in biodegrading most of the fats, oils, and grease deposited in the waste-stream.
Degradation of FOG begins with the breakdown of the complex molecule by extracellular enzymes produced by microorganisms. Microorganisms produce many different classes of lypolytic enzymes including true lipases and esterases (carboxylesterases). Lipases display the most activity towards water-insoluble long-chain triglycerides while esterases degrade smaller molecules that are at least partially soluble in water. Lipase activity depends on the presence of a substrate/water interface while esterase activity follows the Michaelis-Menten kinetic reaction where maximum activity is reached long before the solution becomes substratesaturated (Jaeger et al., 1994). Bacteria as a group have great diversity in activity of the lipases and/or esterases that they produce. Some of the lipases and esterases are very broad in their substrate activity while others have preferences for specific fatty acids. For example, the Bacillus subtilis lipase attacks fatty acids with chain lengths of 8 carbons found in the 1, 3 position of a triglyceride while the Staphylococcus aureus lipase has a very broad range of substrate specificity (Thomson, 1999).
Other compounds produced by microorganisms and useful in the process of biodegradation of FOG are biosurfactants which are excreted into the environment surrounding the microorganism. The physiological role that biosurfactant production allows has not been defined but it is speculated that it facilitates growth of microorganisms on water-immiscible substrates by reducing the interfacial tension and making the substrate more bioavailable (Maier, 2003). Some of the bacteria genera reported to produce surfactants include Pseudomonas, Rhodococcus, Mycobacterium, Nocardia, Flavobacterium, Corynebacterium, Clostridium, Acinetobacter, Thiobacillus, Bacillus, Serratia, Arthrobacter, and Alcanivorax (Maier, 2003). The type of biosurfactant produced is genus and sometimes even species specific.
After the FOG has been exposed to biosurfactant and degraded by enzymes, the fatty acids and glycerol are consumed by the microorganisms that are capable of utilizing them (pseudomonads, Acinetobacter, various bacilli, and E. coli) (Gottschalk, 1986). The fatty acids are oxidized to acetyl-CoA via a pathway called ß-oxidation. If the fatty acid has an even number of carbon atoms, then the entire chain is degraded to acetyl-CoA. If the fatty acid chain is an odd-chain fatty acid, then the last fragment is propionyl-CoA which is converted to acetyl- CoA through a variety of possible pathways. ß-oxidation of fatty acids, in combination with the tricarboxylic acid cycle and respiratory chain, provides more energy per carbon atom than any other energy source (Zubay, 1996).
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