## 1 Introduction  
Apples (_Malus domestica_ Borkh) are one of the most often cultivated and consumed fruits worldwide. Considering the annual production, only watermelons and bananas are more popular [1]. Poland is the biggest apple exporter in the world and the fourth largest producer of apples, right after China, Turkey, and the USA. Over 7500 apple cultivars are already known [2], and some authors estimate that even tens of thousands can be found all over the world [3]. However, the vast majority of known cultivars do not have economic significance. Throughout the years of selective breeding and genetic modification, the aim was to obtain apple cultivars that would meet consumers' as well as producers' expectations. For the majority of consumers, the most important aspect is an attractive look. On the other hand, for producers, the ultimate apple cultivar should be immune to diseases, mature quickly, endure the conditions of transshipment and transport after harvesting, and have a long shelf life, meaning the time it can be on a store shelf with no signs of aging. The best example is the 'Red delicious' cultivar, which was considered one of the less tsaty apples (described as uncomfortably dry). However, for many years, it was the most popular variety in the USA because of its perfect look and because it turned red before any other cultivar, so it could be picked earlier and stored longer. Yet no one considers how these breeding programs have a strong impact on volatile organic compound profiles (aroma), which are strongly associated with taste sensation [4; 5]. Aroma has always been the basis for people to assess the suitability of food for consumption. A pleasant, sweet aroma suggests that the fruit is already ripe and will be tasty, sweet, and therefore nutritious. The newest research evidenced that approximately 1/3 of perceived sweetness can be explained by the appropriate composition of volatile organic compounds (VOCs) [6]. In the last decade, there has been growing interest in food products produced in an ecological and environmentally friendly way, including fruits such as apples. In particular, attention is paid to the ancient cultivars of apples that are more resistant to diseases or have improved sensory properties compared to modern, commercially grown cultivars [7]. Moreover, regional and classic cultivars are a particular crop gene pool that needs to be preserved and promoted as a gene bank to maintain biodiversity. In apples, there is an extremely high number of up to 500 different VOCs [8]. The composition of apple VOCs has been the subject of numerous studies (Table 1). Some studies focused mainly on comparing as many varieties as possible (42 varieties in [2] and 40 in [9]), while other researchers tried to determine as many compounds as possible (498 in [8] and 399 in [10]). Taking into account the diversity of varieties and the number of compounds determined in them, this is still an insufficient amount of knowledge. As far as extraction and analysis methods are concerned, it can be generally assumed that the method using solid-phase microextraction (SPME) is the most sensitive and the most developed. Additionally, the quality of analyses is improved by the use of two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC/GC-ToF-MS). The literature provides the frequency of 23 aroma components, mainly esters, which have a significant impact on the sensory description of the aroma of apples [17]. As indicated in Table 2, the most important compounds that affect the aroma of apples differ in terms of odor detection threshold by several orders of magnitude. In addition, the smell of some of them is not at all similar to the smell of apples. Also, in [17], it was pointed out that there is no key characteristic compound for any given cultivar.