## 4 Discussion
### Impact of Hot-Water Treatment on the Release of VOCs, Ethylene and CO\({}_{2}\)  
Even the sHWT at 65 \({}^{\circ}\)C did not alter the emission of most VOCs or pronouncedly impacted VOCs profiles but only marginally reduced that of few esters. In contrast to cutting, sHWT did obviously also not affect the synthesis of acetate-esters. This is surprising because a high number of VOCs emanate from the epidermal layers of apples, while only a lower proportion derived from the pulp tissue [45]. However, dipping intact apples for 30 s in hot-water at 55 \({}^{\circ}\)C exclusively heated the epidermis and few hypodermal cell layers [17]. Furthermore, although treatments at higher temperatures (i.e., \(>\)55 \({}^{\circ}\)C) resulted in faster increase in tissue temperature of deeper cell layers, only 70 \({}^{\circ}\)C pronouncedly damaged the epidermis [17]. Thus, sHWT, in the temperature range used, did not decrease the emission of important \"character impact\" compounds [29] or increase that of VOCs associated with off-odor. Therefore, sHWT did not reduce the aroma quality. Similarly, sHWT (of up to 65 \({}^{\circ}\)C) did not adversely affect color attributes, tissue strength or other important quality parameters of fresh-cut apple slices [18]. Although the increased ethanol emission may be assumed as critically, the aroma-relevant threshold for this compound is much higher than for other important VOCs (Table 3). In addition, the ethanol concentrations in total were very low. The observed increase in ethanol emission, therefore, did not negatively affect the aroma quality at all. It may, nevertheless, indicate some sHWT-induced heat stress because ethanol and ethyl-acetate emissions are known to increase in response to and may be used as indicators of this stressor [46; 47]. Heat stress was reported to alter the glycolytic pathway by disturbing the mitochondrial electron transport, which, similar to reduced O\({}_{2}\) availability, results in increased ethanol formation [48]. This may also be reflected by the marginally higher respiratory activity observed at the end of storage. Heat treatment-enhanced respiration was also reported earlier for intact 'Granny Smith' and 'Anna' apples [49]. In addition, the increased D-limonene emissions of acid-treated sHWT samples may be attributed to major perturbations of the cellular metabolism that result in the expression of multiple genes and, finally, in enhanced terpene emissions [50; 51]. Omitting the post-processing acid treatment increased the ethanol emission, which, however, did not negatively affect the aroma (see above). Hot-water treatments without post-processing acid application only slightly enhanced the alteration of the VOCs profile typical for intact apples in comparison to those with additional acid dipping. Interestingly, sHWT at 55 \({}^{\circ}\)C without additional acid treatment clearly intensified the storage-induced physiological responses of fresh-cut apple slices, i.e., it slightly enhanced their respiration activity and ethylene release. This may indicate a direct response of the treated apples to moderate but not to excessive heat or to acid pretreatment [52; 53]. Additionally, the emission of ethyl esters is further intensified in sHW-treated (55 \({}^{\circ}\)C) apple slices. This is especially obvious for ethyl acetate and ethanol, which, however, both increased in all samples during storage (see Section 4.3). As ethylene may directly regulate the synthesis of VOCs [44; 54; 55], the increase in ethyl 2-methylpropanoate, ethyl butyrate and ethyl 2-methylbutanoate may be directly related to the enhanced ethylene tissue concentrations. Ethyl acetate correlates with pronounced off-odor [56], while the \"character impact compounds\" [29] ethyl-butyrate and ethyl 2-methylbutanoate are generally associated with fruity and apple-like aroma [57]. The increase in the head space concentrations of the latter compounds, which own aroma thresholds much lower than ethyl acetate [29; 30], may thus even improve the overall aroma of the apple samples (Table 3).