## 4 Conclusions  
According to the experimental design, we concluded that the proposed chemometric methodology is well-suited for the optimization of the isolation of volatile compounds from monoflolar thyme honey. SPME-GC-MS in combination with RSM led to preposition of the optimum conditions (A: 60 \({}^{\circ}\)C, B: 15 min, C: 30 min, D: 700 rpm, E: 6 mL, F: 1:3 (\(v/w\))), desirabilities, and predicted mean for each volatile response. The temperature, extraction time and ratio of water: honey (\(v/w\)) were the most significant factors with a positive impact on the recovery for most of the volatile compounds. Equilibration time ensured the qualitative and quantitative repeatability, while sample volume was related to the isolation of minor compounds. Although the effect of magnetic stirrer velocity has not been evaluated as a factor affecting extraction efficacy, it was found to be quite important under predictive models, especially when combined with other factors. In some cases, magnetic stirrer speed can act synergistically in the optimization of responses. As it emerged by evaluation of factors, the optimum conditions of some of them depend on the nature of the honey. In addition, a test set of 80 monofloral thyme honey validated these results and reinforced the proposed optimization model. It is useful to remark that this methodology can be applied to highlight the dominant volatiles of thyme honey produced from Greek _Thymbra capitata_ L., thereby making known their extraordinary aroma profile. As arisen from the 80 validation test samples, the proposed optimization methodology increased the sensitivity of isolated volatile compounds in combination with the rapidity of the method. This study could be a good basis for the development of novel RSM for other monofforal honey samples that contain volatile compounds that belong to different categories of chemical compounds with different properties and volatility, and probably different optimum conditions.  
The following are available online. Figure S1: Box-cox plot of benzaldehyde. Figure S2: Box-cox plot of 2-phenylacetaldehyde. Figure S3: Box-cox plot of undecane. Figure S4: Box-cox plot of nonanal. Figure S5: Box-cox plot of 2-phenylletan-1-ol. Figure S6: Box-cox plot of 2-phenylacetonitrile. Figure S7: Box-cox plot of 1-phenylbutane-2.3 dione. Figure S8: Box-cox plot of methyl nonanoate. Figure S9: Box-cox plot of 3-hydroxy-4-phenyl-2-butanone. Figure S10: Box-cox plot of (\(Z\))-3-hydroxy-4-phenylbut-3-en-2-one. Figure S11. A characteristic chromatograph from blank sample. Table S1: Volatile compounds isolated from the headspace of thyme honey samples used for validation test. Table S2: Design layout runs.  
Conceptualization, M.X.; methodology, M.X., E.A., and C.S.P.; software, M.X., A.S., and P.A.T.; validation, M.X., and A.S.; formal analysis, M.X., A.S., and P.-K.R.; investigation, M.X., and P.-K.R.; resources, E.A., and P.-K.R.; data curation, M.X., and A.S.; writing--original draft preparation, M.X.; writing--review and editing, P.-K.R., E.A., C.S.P. and P.A.T.; visualization, M.X.; supervision, C.S.P.; project administration, P.A.T.; All authors have read and agreed to the published version of the manuscript.  
This research has been co-financed by the European Regional Development Fund of the European Union and Greek national funds through the Operational Program Competitiveness, Entrepreneurship and Innovation, under the call \"RESEARCH-CREATE-INNOVATE\" (Project code: T1 EAK-05678, by the acronym \"QuaAuthentic_GR\") awarded to P.A.T. The funders had No role in the study.  
Not applicable.  
The authors declare no conflict of interest.