## 4 Discussion  
These results show that the exposition of ozone to oranges and bananas with 81 ppm for 20 min either once or three times a day did not negatively affect the concentration of key odorants, total tittatable acidity, ascorbic acid content, and weight loss. In addition, the ozone treatment during storage led to the concentration-dependent delay of mould growth, depending on the fruit and mould used. Consequently, the tested ozone treatment slightly improved the shelf life of the fruits without significantly affecting their quality in terms of aroma and health aspects. The effect of ozone on odorants in bananas and oranges has not yet been investigated. However, the influence of ozone on volatiles in orange juice has been studied [19]. Juice was exposed for a period ranging from 1 to 6 min to gaseous ozone. A clear impact of ozone on the volatiles in orange juice was observed, especially after longer exposition times [19]. The relative amounts of limonene decreased with higher exposition times, while other compounds, such as \(\beta\)-mycrene and \(\alpha\)-terpineol, increased. The researchers suggested oxidation and hydrolysis of and reduction in terpen to be responsible for the changes in orange volatiles at longer ozone exposition times. [PERSON] et al. [16] reported a degradative effect of ozone on the ascorbic acid content in orange juice. However, the total tittatable acidity was not affected by the ozone treatment [16]. These aforementioned observations were mostly contrary to the results discussed in this study. One reason for this might be the difference in the application of ozone on whole oranges in contrast to orange juice. When applying ozone on whole oranges, the ozone molecules might only react with the peel constituents and not influence the inner part of the fruit, which was analysed in this case. The peel might have a protective function for valuable orange constituents, such as odorants, acids, and vitamins. The same effect might have occurred in bananas and could explain why key odorants, total tittatable acids, and ascorbic acid were not affected by ozone in this study. On the contrary, a slight effect was observed on fungal growth, which occurs on the fruit peels. Further studies will be necessary to clarify a possible protective function of orange and banana peels and how different parts of the fruits are impacted by the application of a gaseous ozone atmosphere. Furthermore, the effect of ozone on the shelf life and quality parameters has been studied in other fruits than oranges and bananas before. Ozone treatments ranging from 0.5 to 1.5 mg/L for 60 min before the refrigerated storage of apricots reduced fungal decay significantly during storage time [14]. However,  
Figure 9: Ascorbic acid mean concentrations with standard deviations in bananas and oranges without (control) or with ozone treatment three times a day. Different letters after the value indicate a significant difference between the samples for the compound. the researchers observed a significantly higher weight loss in the ozone-treated fruits. Interestingly, the ozonated apricots kept their sensory properties for a longer storage time than the not ozonated ones. Ozone application on tomatoes (25 or 45 mg/m\({}^{3}\) for 2 h/day for 16 days) showed no significant influence on the titratable acidity [31]. The ozone treatment improved the weight loss during storage. Until 13 days of storage, the ascorbic acid content was not significantly affected by the ozone treatment. Lower ascorbic acid concentrations in the ozone-treated samples were observed after 16 days of storage. The authors explained this by the oxidizing effect of ozone that leads to a loss in antioxidant compounds. They explained the low effect of ozone on the ascorbic acid content by the action of the tomato pericarp as a physical barrier for the ozone. Ascorbic acid is concentrated in the aqueous bulk of the tomatoes and, therefore, not affected by the treatment [31].