## 1 Introduction  
Apples (_Malus domestica_ Borkh.) are a seasonal fruit and, in the northern hemisphere, they are typically harvested from late summer to end of autumn. With the technological advancements of the past 100 years, it is now normal for both apple producers and consumers to have good quality apples available throughout the year. The main goal of any storage technique is to slow down fruit metabolism and delay loss of quality and senescence. Slowing down fruit metabolism significantly impacts biochemical reactions in the fruit. Depending on the fruits' response to the taken measures, the implication on the fruit quality will be different. The flavor of fruits represents an essential quality criterion as it is directly perceived by the consumer upon consumption. The perception of flavor is a multisensory experience including the perception of taste, texture as well as the ortho- and retronasal odor perception [1]. When investigating the impact of different storage techniques on apples, different aspects of fruit quality need to be considered including physicochemical parameters such as texture, juiciness, sugar concentrations and acidity. Furthermore, it is necessary to investigate the aroma of the produce. Fruit volatiles are basically divided into two groups: (i) primary aroma compounds that are formed and emitted by the intact fruit during maturation and ripening and (ii) secondary aroma compounds that are formed after injury of the fruit (e.g., in course of peeling, cutting or pressing). The formation of the latter groups is based on release of enzymes from the injured cells followed by enzymatically catalyzed reactions in the presence of atmospheric oxygen [2; 3]. The presence and the respective enzyme activity and, thus, the formation of primary and secondary apple volatiles is strongly dependent on the apple cultivar as well as on the maturity/ripeness of the fruit [4; 5]. The investigation of the fruit volatiles does not only give information on the impact of the storage condition on the flavor formation, but allows us to draw indirect conclusions on how the technical measures influence the biochemical reactions in the apple. Over the years, a range of different storage techniques have been developed, many of them relying on similar mechanisms [6]. In general, refrigerated storage under regular atmosphere (RA) is the easiest and least expensive method to handle fresh fruits and vegetables. Depending on the sensitivity of each cultivar, apples can be stored between 0-4 \({}^{\circ}\)C for periods up to 6 months [7]. The storage period can sometimes be prolonged to 12 months by close monitoring of the produce. The lowered temperatures slow down biochemical degradation and maintain apple quality over a certain period of time [8]. Another well-established storage technique is the so-called controlled atmosphere storage (CA) to extend apple shelf-life while maintaining quality. In addition to low temperatures (\(-1\)-\(6\)\({}^{\circ}\)C), the surrounding atmosphere of the stored product is altered and controlled. While oxygen concentrations are kept low (2-3%), carbon dioxide levels are increased (1.5-5%) and relative humidity is also kept at high levels (>92%) [9]. Ethylene production is reduced, the respiration rate is decelerated and the enzymatic oxidation rates decrease due to the lower availability of oxygen [10; 11]. Moreover, color changes and chlorophyll degradation are reduced while fruit firmness, sugars, organic acids and vitamin concentrations are better maintained [12]. Furthermore, storage disorders such as superficial scalp are reduced while senescence is delayed [13]. However, long-term storage under air or controlled atmosphere may lead to fruit disorders and is described not only to be a function of the storage conditions but also the cultivar and the maturity of the fruits [14; 15]. Another technique used to delay fruit senescence is the use of ethylene-receptor inhibitors such as 1-methylcyclopropene (1-MCP) [16].