# Application of Sorbent-Based Extraction Techniques in Food Analysis
## 4 SPME in Food Analysis--Main Fields of Applications
### Flavor Compounds  
The term flavor is assumed to be the interaction of the taste, odor and textural feeling of a food product. It is perceived mostly by the sense of smell and taste. Therefore, flavor compounds can be divided into two classes: compounds responsible for taste and aroma; however, there are substances providing both sensations. Taste compounds are generally non-volatile, whereas aroma compounds are always volatile. Odors are characterized by relatively high vapor pressures and their molecular weight does not usually exceed 300 Da to be detectable by the odor receptor sites in the nasal cavity. SPME is a technique most often used for the analysis of VOCs, as well as many studies on flavor for aroma compound analysis. The number of VOCs detected in foods exceeds 10,000--among them, only a few hundred have aroma activity [108]. They usually occur at extremely low amounts in food matrices and consist of many various organic compounds possessing different polarities and reactivities. Moreover, food flavor compounds are challenging to analyze due to their very low concentration and bounding into the food matrix. Commonly used exhaustive extraction methods such as solvent extraction, steam distillation extraction (SDE) or SAFE require a long preparation process before chromatographic analysis. The aroma compounds are also analyzed by direct headspace sampling or purge and trap methods. However, those techniques are expensive and often suffer from low sensitivity. Certain applications of SPME in flavor research can be distinguished: comparison of different plant cultivars, profiling of particular food products, analysis of compounds important for food quality, monitoring of technological processes on aroma, monitoring of chemical/biochemical processes on flavor substance degradation and use of SPME for GC-olfactometry purposes [109]. Different matrixes were analyzed using SPME, including plant [110], meat products [111], dairy products [112] and beverages [113]. The first attempt at SPME application in flavor analysis was made by [PERSON] and [PERSON] in 1994 [114]. They tested SPME for 25 common flavor compounds in water solutions as well as for authentic samples like juice or coffee. It was proved that any changes in extraction conditions strongly affected the absorption distribution. SPME was found to be a useful technique in flavor analysis, mostly as a complementary tool to traditional extraction techniques. According to the authors, the main disadvantage of this technique was problematic quantitative determination [114]. Nowadays, the development and availability of isotope-labelled standards enables overcoming those limitations. However, to quantify VOCs, the appropriately optimized method and appropriate internal standards have to be selected [86; 115]. In a study conducted by [PERSON] et al. [116], different SPME fibers were compared for their utility for aroma compound analysis. PDMS/DVB fiber was the most sensitive for heavier polar odorants like vanillin. Carboxen/PDMS was the most suitable for small molecules such as 2-methylpropanal and acetaldehyde. Carbowax/DVB was found to be adequate for organic acids. Non-polar substances were detected at the ppb level, while polar substances were more problematic in analysis. Polar compounds of low volatility such as sotolon and vanillin had a meaningful impact on the overall aroma. It was observed that SPME has difficulties detecting those aromas below the ppm level. Off-flavor analysis is a field of high interest to scientists and food producers. Compounds that caused off-flavor or food taints mostly belong to the following groups: sulfur compounds, alcohols, fatty acids, haloanisoles, chlorophenols, phenolic compounds, esters and amines.