Sustainable lipid extraction: green solvents and hydrotalcite as alternatives to conventional methods for measuring fatty acids in fat, oil and grease†

Fat, oil, and grease (FOG) deposits, commonly found in urban wastewater systems, primarily originate from food service establishments, households and industrial activities. These deposits consist predominantly of fatty acids (FA) and present significant challenges for wastewater management. Accurate measurement of FA within these deposits is essential for designing targeted solutions that can effectively prevent FOG accumulation, thereby mitigating wastewater management challenges. Traditional extraction methods for measuring FA concentration, such as using n-hexane and dichloromethane, not only pose environmental and health hazards but also tend to overestimate FA content due to inadequate separation from other lipid components. This study introduces a novel green solvent-based method for FA separation and quantification from FOG deposits, utilizing d-limonene, p-cymene, and methyl tetrahydrofuran and provides a comparative analysis against conventional methods, including Association of Official Analytical Chemists (AOAC) 996.06, transesterification, and the hexane extractable material method. A double extraction method employing hydrotalcite and NH₄OH achieved 99% FA purity, as confirmed by nuclear magnetic resonance spectroscopy, avoiding the overestimation seen in conventional methods that fail to separate FAs from other lipids. d-Limonene extracted a total FA content of around 97% which is much lower than the 128% obtained through transesterification. This difference is attributed to transesterification's conversion of both FAs and glycerides into fatty acid methyl esters, leading to an overestimation of the FA content. Notably, d-limonene demonstrated superior selectivity for saturated fatty acids (SFAs), extracting 51.1% SFAs outperforming n-hexane (47.9%), AOAC 996.06 (30.5%), and transesterification (32.4%). With a high lipid extraction efficiency of 98.3% and reduced environmental impact, d-limonene offers a more accurate and sustainable approach to FA quantification, underscoring its potential as a viable solvent for FOG analysis and improved wastewater management.