# Application of Sorbent-Based Extraction Techniques in Food Analysis
## 3 SPE in Food Analysis--Main Fields of Applications  
### Lipid Analysis  
Lipids form the fundamental constituents of food that perform necessary structural, functional and biological properties. Lipids play an important role in cells of the biological systems but also have considerable influence on eligible food quality factors, e.g., aroma, color, structure, texture, and taste sensation. Commonly, consumer acceptance of food is strictly related to the quality of its lipids ([PERSON] et al., 2018). In general, single or dual organic solvent extraction, liquid-liquid extraction (LLE) and SPE are mainly used to extract, and then fractionate lipids. The main advantage of the utilization of SPE with a properly selected sorbent enables isolating particular lipid classes. A basic scheme for the fractionation of lipid classes proposed by [PERSON] et al. (2018), which utilizes an NH\({}_{2}\) column for the fractionation of a lipid mixture into fractions of different polarity, can be seen in Figure 1. SPE is commonly used for analytes enrichment, especially when they are present in a sample at a low concentration, such as eicosanoids, steroid hormones and fatty acid esters of hydroxy fatty acids. Accurate and proper fractionation of lipid classes in muscle foods analysis is required to achieve an authentic fatty acid profile appropriate for a given fraction. A traditional approach for muscle lipid separation utilized the SPE method; however, the main problem was with the neutral lipid fraction, which contains a certain amount of phospholipids. The further development of that method, changingthe type, proportion, and volume of used solvents as well as the aminopropyl minicolumn size, contributed to reducing phospholipids in the first fraction to almost undetectable. The SPE method is also supportive in the determination of free fatty acids (FFAs) in beer in a reliable and simple way. The presence of FFAs in beer considerably affects consumer acceptance of beer quality, in particular the foam stability and flavor [25]. Analysis of FFAs by chromatographic methods (GC, HPLC) requires a sample pre-treatment step. SPE with aminopropyl cartridges, in comparison to the previously used method (steam distillation and LLE) is faster, more reproducible, less complicated and demands a minor volume of organic solvents. Moreover, the emulsion creation issue resulting in analyte loss is eliminated. Additionally, the SPE technique is appropriate to determine a wide variety of fatty acids (differing with the number of carbons in the chain) in contrast to other extraction methods like SPME or SBSE [25]. Lipid autoxidation is a chain reaction which consists of three stages: initiation, propagation, and termination. The compounds obtained are chemically active and participate in further free radical processes [26]. SPE is commonly used to control the content of potentially harmful compounds in the oxidation process. Regardless of the analytical technique used (GC-MS, HPLC-MS, HPLC-MS/MS) for monitoring the deterioration of lipids, the sample preparation step is required and relies on a double SPE process which applies C18 and silica cartridges. Determination of 3-monochloropropane-1,2-diol and its esters,  
Figure 1: Fractionation of lipid classes using NH\({}_{2}\) SPE columns. Based on [24]. not only in edible oils but also in fats and food with high content of fats, is performed using SPE [27]. The new approach assumes the application of d-SPE with Si-SAX-PSA and Z-sep+-PSAorbents as a sample pre-treatment step. d-SPE has many advantages compared to solvent-based methods--for instance, shorter clean-up times, lower costs, lower amounts of solvents required, and less waste. Further, utilizing optimized d-SPE enhances precision and recovery and reduces matrix effects [27]. Phytosterols are found in cells of plant material and play a crucial structural function in maintaining the stability of biological membranes.