## 2 Results and Discussion
### Application of the HS-SPME/GC-MS Method to Strawberry Fruit Samples
#### 2.2.1. Esters  
Esters are the characteristic volatile compounds that define strawberry volatiles, and they represent the most abundant volatile compounds in strawberries, with as many as 131 different types, comprising 25% to 90% of total strawberry fruit volatiles [25]. Esters are also the major source of fruity and floral odors in strawberries, and the content of esters may be the basis for classifying different volatile patterns [3; 26; 27]. The most important odorants of \(F\). \(\times\)_ananassa_ are methyl butanoate, ethyl butanoate, ethyl hexanoate, methyl hexanoate, hexyl acetate, and 2-methylbutanoate [3; 28]. In the present work, total esters  
Figure 2. HS-SPME/GC-MS chromatograms of four different samples of strawberry: (**A**) fresh; (**B**) shade air-dried; (**C**) microwave-dried; (**D**) Jam 3.  
comprised 0.35%, 17.48%, 19.0%, 24.05%, and 34.34 % of total volatiles in microwave-dried, oven-dried (60 \({}^{\circ}\)C), oven-dried (45 \({}^{\circ}\)C), freeze-dried, and shade air-dried strawberry samples, respectively, with respect to 36.96% in fresh samples. Shade air-drying was the best method of drying in terms of total ester content, which was only slightly decreased with respect to the control (fresh strawberry). The most abundant esters present in the fresh fruit were _trans_-2-hexenyl acetate (13.70%), hexyl acetate (12.68%), ethyl acetate (2.56%), ethyl hexanoate (2.17%), _cis_-2-hexenyl acetate (1.52%), octyl formate (1.51%), octyl acetate (0.99%), hexyl butanoate (0.48%), methyl butanoate (0.24%), methyl hexanoate (0.12%), and ethyl butanoate (0.11%); these finding are consistent with the literature [3; 18; 23]. These compounds have aroma properties with fruity notes, and most of them are among the most important flavor notes in terms of the aroma values [11]. The most abundant esters present in the microwave-dried, oven-dried (60 \({}^{\circ}\)C), oven-dried (45 \({}^{\circ}\)C), freeze-dried, and shade air-dried strawberry samples were (\(E\))-2-hexenyl hexanoate (0.16%), ethyl hexanoate (8.91%), ethyl hexanoate (12.89%), _trans_-2-hexenyl acetate, and ethyl hexanoate (20.54%), respectively. Microwave-drying, oven-drying (60 \({}^{\circ}\)C), oven-drying (45 \({}^{\circ}\)C), and freeze-drying led to a significant reduction in fruity ester notes (105.60-, 2.24-, 1.76-, and 1.54-fold less, respectively) compared with the control, indicating microwave-drying as the worst method, which showed a drastic reduction in ester content (total esters; 0.35%). In contrast, the fruity compounds were well retained in the shade air-dried sample, followed by freeze-dried strawberry product. In addition, frozen strawberry sample (1 week freezing) showed a moderate reduction in ester content (21.38%) compared with the control (36.96%), and this is consistent with the literature [28]. It has been reported that concentrations of esters present in fresh strawberries are superior to those in frozen strawberries, and there was a decrease of 80% in ester content after 1 day of freezing [29; 30]. In contrast to this, it was found that the levels of ethyl butanoate in frozen strawberries was higher than that in fresh strawberries [30; 31], and these findings totally agree with our results. The reason for this phenomenon is the oxidation of acids following the thawing process of the frozen strawberries. It can be noted that the ethyl acetate content decreased in the microwave-dried, oven-dried (60 \({}^{\circ}\)C), oven-dried (45 \({}^{\circ}\)C), and freeze-dried samples compared to the fresh control, but air-dried samples showed an equal amount to the fresh fruits. Ethyl acetate was identified as an off-flavor note in strawberries [11].