Aroma is one of the most important sensory qualities of wine, determined by the grape variety, origin, and wine-making process. The volatile compounds derived from fermentation constitute the largest percentage of the total aroma composition of wine. Therefore, understanding the composition and formation of aroma is crucial to optimize the wine-making process and improve wine quality. Headspace solid-phase micro-extraction (HS-SPME) coupled with gas chromatography-mass spectrometry (GC-MS) is widely used in the identification of wine aroma compounds. Compared with conventional techniques, HS-SPME offers superior enrichment, is simple, inexpensive, and fast to operate, and requires no solvent (Lancioni, Castells, Candal, & Tascon, 2022).
SPME uses a fiber coated with an extraction phase comprising a pure polymer or adsorptive particles embedded in a polymer. The fiber coating absorbs (extracts) the volatile organic compounds (VOCs) from the sample. The SPME fiber is then inserted directly into the GC injector for thermal desorption and analysis. Selecting an appropriate SPME fiber is essential because the coating determines the absorption selectivity of VOCs. The divinylbenzene/Carboxen/polydimethylsiloxane (DVB/CAR/PDMS) coating is widely used in the identification of wine VOCs owing to its broad range of analyte detection. This mixed-phase sorbent extracts analytes based on van der Waals, π-π, or hydrogen bonding interactions (Lancioni, Castells, Candal, & Tascon, 2022).
In VOC extraction by HS-SPME, multiple factors influence the analysis results, such as the sample volume, SPME fiber coating, extraction time, extraction temperature, stirring, ionic strength, pH of the samples, and fiber-sample distance (Azzi-Achkouty, Estephan, Ouaini, & Rutledge, 2017; Kamgang Nzekoue, Angeloni, Caprioli, Cortese, Maggi, Marconi, et al., 2020). (Azzi-Achkouty, Estephan, Ouaini, & Rutledge, 2017) reported different HS-SPME methods to identify VOCs in wines. Previous studies applied an extraction temperature of 25β~70β, an extraction time of 10β~β120 min, a salt concentration of 7%~30%, and a sample volume (Vsample/Vvial) of 5%~75%.
The extraction process consists of two stages, namely the equilibrium and absorption stages. Firstly, the VOCs are released from the sample into headspace until saturation. Since this is an endothermic process, increasing extraction temperature can shorten the equilibrium stage. Secondly, the fiber is inserted into the headspace, and VOCs are absorbed on the fiber. In contrast, this process is exothermic, and heightened temperature decreases absorption. (Marinaki, Sampsonidis, Lioupi, Arapitsas, Thomaidis, Zinoviadou, et al., 2023) demonstrated that the total area of VOCs initially increased with temperature until reaching a maximum and then decreased (30β~70β). However, (Rossi, Foschi, Biancolillo, Maggi, & DβArchivio, 2023) reported an increase in total area as the temperature was increased (30β~50β). The disparity in their results may arise from the different temperature ranges used. In addition, for some specific applications, SPME is run at lower or moderate temperatures (12β or 37β) to simulate the fermentation temperature of white wines or mouth temperature during tasting, respectively (Comuzzo, Tat, Tonizzo, & Battistutta, 2006). Although some researchers suggest not exceeding an extraction temperature of 45β to avoid modifying the original composition of the wine matrix, the low temperature might negatively affect the sensitivity of low-concentration analytes.
Since only the neutral/undissociated compounds are adsorbed/absorbed from the headspace by the SPME fiber, adjusting the pH may improve the sensitivity by converting ions into their neutral forms, thereby decreasing their solubility in hydrophilic matrix and increasing their release (Schmidt & Podmore, 2015; Zhang & Zhang, 2008).
Furthermore, adding salt may enhance the extraction of HS-SPME due to a βsalt-outβ effect (Schmidt & Podmore, 2015). This effect drives hydrophobic analytes into the headspace by decreasing their solubility in the aqueous phase. However, an opposite effect may be observed at higher salt concentrations. The concentration of analyte compounds in the headspace could be reduced by electrostatic interactions between analyte molecules and ionic salt molecules, and increased viscosity and density of the aqueous phase (Zhang & Zhang, 2008). In general, salt addition increases the extraction of polar compounds, but has no significant effect on non-polar compounds.
The headspace volume also affects the sensitivity of HS-SPME. Under constant sample volume, a lower headspace volume results in higher HS-SPME sensitivity. With a larger headspace volume, a higher proportion of analytes is distributed into the headspace, while a lower proportion is absorbed onto the fiber. Therefore, a small headspace volume could reduce the equilibration time (Yang & Peppard, 1994).
A previous study showed that the distance between the fiber and the sample during extraction significantly affects the quantity of absorbed VOCs. Moreover, the lower the time of extraction, the stronger the dependence on the fiber-sample distance (Kamgang Nzekoue, et al., 2020).