## 4 Discussion
### Effect of 1-MCP, Storage Atmosphere, and Juice Processing  
According to the results presented, the content and composition of volatile compounds from clear and cloudy juice were strongly influenced by the different combinations of 1-MCP treatment, storage atmosphere, harvest maturity, and juice type. The subsequent impact of the levels of esters and aldehydes on the juice odor depends on its odor threshold value (i.e., the detection or recognition values, above which the compound can be detected by smell) and concentration [21]. Based on the threshold values of volatile compounds summarized from the literature (Table 2), esters and aldehydes have considerably lower threshold values as compared with alcohols. This means esters and aldehydes may have a key role in influencing the odor of the juice even at low concentrations. On the other hand, volatile compounds with higher threshold values (notably ethanol, Table 2) might not have a large impact on the odor of apple juice. Hence, our discussion will focus on aldehydes and esters. In McIntosh juices, whether it is clear or cloudy, our results indicated a remarkable reduction of all types of esters, aldehydes, most alcohols, and total volatile compounds when juices are extracted from 1-MCP-treated fruit stored in CA or RA. This is consistent with previous studies that found a substantial suppression of volatile aroma compounds in several apple cultivars that had been treated with 1-MCP before storage in RA or CA [6; 7]. Unlike McIntosh, 1-MCP treatment alone (1-MCP + RA) in Honeycrisp apples did not alter the content of most volatile compounds except ethyl acetate and ethanol, which were substantially suppressed by the treatment. This effect might be attributed to the unusual response of this cultivar to 1-MCP treatment. In our previous study (unpublished) while 1-MCP + RA treatment in McIntosh apples inhibited ethylene production better than the control + CA/RA treatments, the same treatment in Honeycrisp produced the highest ethylene (49.03 \(\upmu\)L kg\({}^{-1}\) h\({}^{-1}\)) level, which was present at higher levels than in control fruit (17 \(\upmu\)L kg\({}^{-1}\) h\({}^{-1}\)). As reported in other cultivars [6; 7] elevated ethylene production is usually accompanied by an increased level of volatile compounds and vice versa. Nevertheless, this trend did not occur in Honeycrisp apples. As there is no published information regarding the volatile profile of Honeycrisp fruit or juice, especially none focusing on 1-MCP treatment, it is difficult to account for the unusual response of this cultivar to 1-MCP treatment. The observed inhibitory effect of CA storage on the content of volatile compounds is consistent with previous studies in different apple cultivars [29; 30]. Reduced sensitivity to ethylene [31] or suppressed ethylene production of CA-stored fruit [31; 32] has been suggested as a mechanism by which volatile production could be inhibited in CA-stored apples. The biosynthesis of volatile compounds via beta-oxidation or the LOX pathway needs oxygen, and therefore their production could be slowed down by CA condition where the oxygen level is much lower than the RA atmosphere [5]. Even though CA storage suppressed the content of most volatile compounds, it also enhanced some branched-chain esters detected from intact Honeycrisp apples (3-methyl-1-butyl acetate) as well as from Honeycrisp juices (2-methyl butyl acetate). In agreement with our observation, other studies also reported the increased level of branched-chain acetate esters in Delicious [33], Gala [30], and Fuji [21] apples that were kept under low oxygen storage conditions. A study in pear fruit found an increased level of branched-chain esters associated with a higher level of amino acids, which are the main precursors of branched-chain esters [34]. The higher concentration of (E)-2-hexenal in juices from 1-MCP and/or CA-treated Honeycrisp apples might be attributed to the suppressed ripening of the apples associated with lower ethylene production [30].