*4.3. Headspace Volatile Analysis*

The greater accumulation of MHO observed for RA treated peels compared to RLOS (ULO and CA) and DCA-CF stored apples was consistent with results reported by Mditshwa et al. [2] for 'Granny Smith' apple peels subjected to repeated DCA-CF storage for up to 6 w at −0.5 ◦C and 95% relative humidity (RH). Additionally, Ramokonyane [14] outlined that optimally harvested 'Granny Smith' apples stored in RA at 0 ◦C for up to 7 months had markedly higher MHO concentration compared to DCA-CF and CA, preceded by initial low oxygen stress (ILOS + CA) stored fruit. Studies have reported that superficial scald is related to the extent of α-farnesene oxidation in susceptible apple cultivars [1,2]. Therefore, the accumulation α-farnesene in apple peels is expected to vary with storage duration as oxidative products increase.

Alpha-farnesene was detected at harvest, which confirms that it is a naturally occurring volatile in 'Granny Smith' apples [4]. Accumulation of α-farnesene in apple peel is reduced by low oxygen storage by applying anaerobic treatments, CA, or hypobaric storage [48,49]. Moreover, Sabban-Amin et al. [50] reported reduced α-farnesene production for 'Granny Smith' apples subjected to <0.5% O2 followed by RA at 0 ◦C for 24 w. Likewise, Mditshwa et al. [2] observed substantially low α-farnesene content for 'Granny Smith' apples subjected to repeated DCA-CF treatment with an intermittent RA period in comparison to RA stored fruit for 24–70 d storage. It is generally accepted that the concentration of α-farnesene increases when apples are transferred into cold storage, particularly in RA, reaching a maximum between 8 and 12 w and subsequently declining [51–53]. However, dynamic changes in α-farnesene content in apple peel varies with storage temperature and atmosphere. For instance, Mditshwa [54] reported that α-farnesene concentration in 'Granny Smith' apples was highest after 12 weeks of storage in RA at 0 ◦C. This is contrary to Ramokonyane [14], who only observed significant differences in α-farnesene concentration at shelf life and not during storage for 'Granny Smith' apples subjected to DCA-CF at 0 ◦C. In this study, the delay in α-farnesene accumulation could suggest suppression of oxidative processes in peels, particularly for RLOS (ULO and CA) and DCA-CF treatments. Subsequent decreases in α-farnesene may also indicate the accumulation of MHO, its oxidative product [4,55].

#### *4.4. Biochemical Analysis*

#### 4.4.1. Total Phenolic Content

Slight changes in TPC observed under all treatments during storage could be associated with the respiratory and ethylene climacteric in apple fruit, which initiates ripening, particularly for fruit subjected to RA storage regimes [4,9]. Research work has suggested that the production of phenolics in apple peel during storage is an ethylene-independent process, and phenolics exhibit an ethylene-dependent regulation when ethylene biosynthesis is suppressed [56]. The behavior of RLOS (ULO and CA) and DCA-CF treated peels resembles 1-MCP treated 'Granny Smith' apples, as reported by Shaham et al. [57], in which no significant decline in phenol concentration were observed when fruit was stored at 0 ◦C. However, Leja et al. [58] observed a significant increase in total phenolics for 'Jonagold' and 'S'ampion' during cold storage at 1 ◦C. Studies have demonstrated various patterns of change exhibited by phenolics during storage in several apple cultivars. For instance, Tarozzi et al. [59] reported that, during storage of 'Golden Delicious' apples at 0 ◦C, total phenolic content decreased in fruit peels after 3 months of cold storage. However, Shaham et al. [57] did not observe a distinct pattern of change in phenolics during storage of optimally harvested 'Granny Smith' apples stored at 0 ◦C for up to 6 w after pre-treatment with 1-MCP or heat. Golding et al. [8] reported that phenol concentration was generally stable, though simple phenols decreased in optimally harvested 'Granny Smith', 'Lady Williams', and 'Crofton' apples stored in air at 0 ◦C for 9 months.
