## 1 Introduction  
Aroma, which is one of the most important quality indicators for fruits, has a great influence on the overall flavour and consumer acceptance [1]. It is generally a complex mixture of volatile compounds whose composition and concentrations are specific to the species, and often the variety, of fruit [2, 3]. Volatile compounds, which determine the aroma profile of fruits, directly contribute to perceived odour and flavour attributes. Knowledge of these volatile compounds forms the basis of breeding programs aiming to improving fruit aroma. As an important trait of fruit quality, more attention has been paid to the study of aroma volatiles in recent years.  
Apples (_Malus\(\times\)domestic_ Borkh.) are one of the most widely cultivated and frequently consumed fruits in the world [4]. Aroma is an important standard for evaluating the quality and characteristics of apples, and the aroma volatile compounds in apples have been studied for more than 50 years. Although more than 300 volatile compounds have been identified in apples, including alcohols, aldehydes, acids, ketones, terpenoids, sesquiterpenenes, and esters, only a subset of 20-30 compounds significantly contribute to the typical apple aroma [5, 6]. Among these, esters are the most abundant compounds. The esters, especially those with even-numbered carbon chains including combinations of acetic, butanoic, and hexanoic acids with ethyl, butyl, and hexyl alcohols, are the major contributors to apple volatiles. Butyl acetate, hexyl acetate, 2-methylbutyl acetate, and ethyl 2-methyl-butanoate are the crucial esters due to their high content and impact on the aroma of several apple varieties [7]. Alcohols are another important group of compounds, after esters, which affect the aroma of ripe apples, with the most abundant being 2-methyl-1-butanol, 1-butanol, 1-hexanol and 1-propanol [8; 9]. Aldehydes are abundant in pre-climacter apples, but after ripening, some aldehydes become almost imperceptible [10]. More than 25 aldehydes, mostly hexanal, trans-2-hexanal, and butanal, have been identified in apples [8]. During apple ripening, the volatile compounds are converted from aldehydes to esters to such an extent that esters can account for more than 80% of all aromatic compounds in some cultivars, such as Golden Delicious and Golden Reinders [9; 11]. Aroma is cultivar-specific; therefore, study of the volatile profile at the variety level is necessary. Volatile compounds have been investigated at the germplasm level for peach (_Prunus persica_), pear (_Pyrus usuriensis_), and melon (_Cucumis melo_) [12; 13; 14]. However, there are few studies on the comparative analysis of volatile compounds in a number of apple cultivars.  
There are some microextraction techniques for the determination of volatile compounds, such as continuous sample drop flow microextraction [15], dispersive liquid-liquid microextraction [16] and solid-phase microextraction (SPME) [17]. The determination of volatile compounds in apples requires a suitable selective, sensitive analytical method. Although the lifetime of the microfiber is short, SPME, a simple, solvent-free method for the extraction of volatile compounds, combined with gas chromatography-mass spectrometry (GC-MS), has been widely used for the qualitative and quantitative analysis of volatile compounds in apple fruit [18; 19].  
In this study, HS-SPME combined with GC-MS was used to determine the composition and concentration of the volatile compounds in 40 apple cultivars. This work evaluated the aroma profiles of apple peels at cultivar levels, and these results could be valuable for future breeding programs, aiming to produce apple cultivars with enhanced aroma quality.