## 1 Introduction  
Food waste is a recent and global problem. It is estimated that one third of food is lost along the value chain. In Switzerland, food waste amounts to around 2.8 million tons in one year [1]. High percentages of food waste are caused by the households, with fruits and vegetables representing an important group [1, 2]. This group is highly susceptible to microbial decay and other forms of alteration, such as loss of texture, taste, and aroma [3, 4]. The food waste of fruits and vegetables can be reduced in different ways. One aspect is to prolong their shelf life. This can be achieved by means of storage under controlled temperature and atmosphere [5]. A further possibility is an edible, protective coating on the fruits that reduces external influences and fruit transpiration [6]. A third possibility is the reduction in microbial load in the beginning of the storage to delay alteration through microbial decay. This can be achieved by methods such as radiation or the washing of the fruits [6, 7, 8]. The fruits are often washed with water that contains additives, such as chlorine, peracetic acid, and hydrogen peroxide [6, 7, 8]. Alternatively, the fruits can be exposed to ozone [8]. Ozone is an oxidizing agent with well-known antimicrobial effects that has been used for water treatments for a long time [7, 9, 10]. It can be generated from oxygen and decomposes again to oxygen very rapidly after application [7, 10, 11]. One major advantage is that no residues are left on the fruits, which makes it a harmless and non-toxic method in comparison to other methods, such as, for example, washing with chlorine [7; 10; 11]. Different forms of treatment exist: the food can be exposed either to a gaseous ozone atmosphere or it can be washed with ozonated water [7; 9]. The exposition to gaseous ozone can be applied once at the beginning of storage or either regularly or continuously during storage [8]. The effect of ozone on different foods has been the focus of many studies. In addition to effectively reducing bacterial and fungal count, ozone is also able to degrade mycotoxins, pesticides, and chemical residues [9; 10]. Many studies further investigated the influence of ozone on other quality parameters, such as sugars, acids, colour, firmness, weight loss, vitamins, phenolic compounds, and other important food constituents [7; 9; 12; 13; 14; 15; 16]. Most studies reported an improved shelf life and only slight effects on other quality parameters. The influence of ozone on the aroma has only been studied partly in the form of sensory evaluations [13; 14; 17; 18]. Different results were reported ranging from only minor changes to an improvement in the aroma by the ozone treatment during subsequent storage. However, the impact of ozone on the aroma properties of fruits and vegetables has not yet been studied at a molecular level. Although different studies analysed some volatile compounds [15; 17; 19], mostly semi-quantitative methods were applied and the impact on the aroma properties was not clearly demonstrated in all studies. For this reason, this research investigates the impact of ozone on key aroma compounds in oranges and bananas at a molecular level. The effect of ozone on selected aroma active compounds in oranges and bananas has, to the researchers' knowledge, never been reported before. The odorants were quantified using HS-SPME-GC-MS and isotopically substituted odorants as internal standards [20]. To investigate the impact of ozone on other important chemical constituents, ascorbic acid was quantified using HPLC-UV and the amount of titratable acids was determined. Additionally, the effect of ozone treatment on the growth and sporulation of artificially inoculated mould species was tested. The reason for performing the previously mentioned experiments was to demonstrate whether ozone had a significant effect on the aroma properties, further parameters relevant for the quality, and on the shelf life of the investigated bananas and oranges.