# Variation of the Chemical Composition of Waste Cooking Oils upon Bentonite Filtration
## 3 Results and Discussion  
### Characterization of the Volatile Organic Compounds (VOCs)  
The intense smell typical of WCOs is related to the chemical composition, in particular to the volatile fraction. The VOCs of our samples were extracted by solid-phase microextraction (SPME) and analyzed by gas-chromatography coupled with mass spectrometry. Forty-six components were identified (Table 1). As shown in Table 1, several classes of compounds were detected: aldehydes, alcohols, ketones, hydrocarbons, and acids. Many of the decomposition compounds detected originate from the oxidation of triacylglycerlecrols and from the Maillard process, which consists of the formation of nitrogen-containing derivatives as a consequence of a reaction between a carbonyl unit of a carbohydrate and an amino function of a protein. The presence of some terpen is linked to the leaching from the fried food [1]. Taking into account that commercial vegetable oil is almost odorless; the chemicals responsible for the aroma of the fried oil must be produced during the frying process. The volatile compounds are strongly related to the perceived food quality, as the aroma is one of the most important factors in cooked food. Prolonged use of frying oil can cause these compounds to impart unpleasant flavors to food and can limit the re-utilization of the oil. Previous authors have reviewed the complex chemical processes involving the vegetable oil during frying, and, among others, the main reactions involved in the production of low molecular weight volatile compounds are the Maillard reaction, thermo-oxidation, and hydrolysis [30]. In particular, a consistent reduction of the odor in treated samples is related to the removing of acetic, propanoic, butanoic, and hexanoic acids and hexyl acetate. Alkane compounds such as n-heptane, n-octane, butyl-cyclopentane have been detected and reported to be products of pyrolysis of vegetable oils [31]. Although some terpen, such as camphene and limonene, were reported to be the result of heating treatment in peanut oil [32], isoprene derivatives found in our samples probably arose from spices used as flavoring agents during the frying. Furan derivatives, pyrazines, and, in general, nitrogen heterocycles, such as furan, 2-pentyl-pyridine-ethenyl, or 5-HMF, are a result of the Maillard reaction between food components. Other chemicals deriving from the autoxidation of linoleic acid, which is one of the main precursors of aroma components in fried food, such as dienals (2,4-heptalenal or 2,4-decadienal), and alkenals, such as 2-undecanal, 2-decneal, and 2-ocrenal, were also detected [33]. Chromatographic analysis was carried out on raw samples as well as in samples filtered on a cartridge of bentonite (samples WCO1 and WCO2), and the results were expressed as absolute peak area (\(\times 10^{6}\)) obtained by total ion current (TIC) chromatogram. In accordance with the results of [PERSON] and coworkers, who analyzed WCOs of different origin [34], the major compounds detected in our samples were hexanal, furan, 2-pentyl, 2-heptanal, nonanal, acetic acid, furfural, 2-nonenal, 5-HMF, 2-decneal, furan methanol, 2,4-decadienal, and hexanoic acid. The comparison between crude and filtered samples reveals an important role of the bentonite in reducing the amount of several chemicals contained in the crude waste. As expected, reducing the particle size of the filter caused a larger amount of chemicals to be retained. Further, the GC analysis revealed that the bentonite is sensitive to the specific functional groups: organic acids (such as acetic or hexanoic acid) were almost completely retained, molecules with double bonds were partially retained, and some other compounds (e.g., alkalanals) were not retained by the clay at all. Some compounds, like n-heptane or hexanal, were slightly increased after bentonite filtration. These differences are related to the different competition phenomena to which the compounds are subjected during the fiber extraction.