## 2 Results and Discussion
### Optimization of SPME-Arrow Conditions  
The experimental variables included in BBD experimental design were chosen by taking into account that these variables could greatly influence the equilibrium of the volatile compounds in the headspace of the sample [19]. For instance, it is known that an increase in extraction temperature can release more volatiles to the headspace due to the effect on the vapour pressure and enhance the overall extraction procedure [28]. In addition, extraction time is an important parameter because headspace SPME is an equilibrium process of analytes between fibre and vapour phase [32]. In this study the evaluated experimental factors were temperature (40, 50, 60 \({}^{\circ}\)C), incubation time (10, 20, 30 min), exposure time (30, 45, 60 min), and desorption time (5, 7.5, 10 min). The BBD experimental design generated 27 experiments, including three central points (C). The experiments were randomly performed. To evaluate the significance of each of the studied factors, the area of the responses based on the sum of the peak areas was used. This is one of the most useful parameters for the optimization of the SPME conditions [29; 33]. Fitting the data with various models showed that the content of aldehydes, alcohols, ketones, acids, monoterepenes and sesquiterpenes were best described with a quadratic polynomial model. For models, analyses of variance (ANOVA) parameters are presented in Table 1. The determination coefficients (R\({}^{2}\)) are in range from 0.9379 to 0.9919, while \(p\)-values are in range from \(<\)0.0001 to 0.0332 indicating that models are highly significant. Models also showed statistically insignificant lack of fit because all determined \(p\)-values are higher than 0.4500. Parameters of quadratic polynomial equations for all responses are also presented in Table 1.  
Figure 2: Number of identified volatile compounds (**A**) in skins of cv. Merlot, and the absolute peak areas (**B**) using five different SPME Arrow fibre coatings. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Aldehydes**} & \multicolumn{2}{c}{**Alcohols**} & \multicolumn{2}{c}{**Acids**} & \multicolumn{2}{c}{**Acids**} & \multicolumn{2}{c}{**Ketones**} & \multicolumn{2}{c}{**Monoterpenses**} & \multicolumn{2}{c}{**Sesquiterpenpenses**} \\
**Cers** & **Coefficient** & \(p\)**-Value** & **Coefficient** & \(p\)**-Value** & **Coefficient** & \(p\)**-Value** & **Coefficient** & \(p\)**-Value** & **Coefficient** & \(p\)**-Value** & **Coefficient** & \(p\)**-Value** \\ \hline \multicolumn{1}{c}{Model} & & & & & & & & & & & & \\ Lack of Fit & 0.5139 & & & & & & & & & & \\ \multicolumn{1}{c}{} & & & & & & & & & & & \\ \multicolumn{1}{c}{} & & & & & & & & & & & \\ \multicolumn{1}{c}{} & & & & & & & & & & & \\ \multicolumn{1}{c}{} & & & & & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Coefficients of the second order polynomial equation and regression coefficients of response and ANOVA parameters for obtained models for free volatiles. The results obtained in this study indicate that the most significant factors were extraction temperature (\(p\)-values for almost all group of analyzed VOCs are lower than 0.0001) and exposure time (\(p\)-values for all group of analyzed VOCs are in the range from 0.0002 up to 0.0412) (Table 1). An increase in extraction temperature positively influenced the extraction efficiency for aldehydes, acids, alcohols, monoterpenes, and sesquiterpenes. The rising temperature from 25 to 60 \({}^{\circ}\)C exponentially increased vapour pressure and thereby enhanced the volatilization. Based on the obtained results, the optimum extraction temperature is 60 \({}^{\circ}\)C. Similar results were obtained by [PERSON], et al. [11], who also found that extraction at 60 \({}^{\circ}\)C gave the best extraction efficiency for VOCs from grape skins. Ketones were the only class of VOCs whose extraction efficiency was reduced by increasing temperature.