*3.5. Correlation Analysis*

There was a positive correlation for superficial scald with MHO of r = 0.66103 (*p* < 0.0001). A negative correlation was detected between superficial scald and RSA and between superficial scald and TPC (r = −0.30748 and −0.3527, respectively) (Table 3).

**Table 3.** Correlation of superficial scald and volatiles (α-farnesene and MHO), total phenolic content, and total antioxidant capacity.


\*: NS—no significant correlation between two parameters (*p* ≤ 0.05); r—correlation coefficient; *p*—*p* value at 95% confidence interval.

#### **4. Discussion**

*4.1. Physiological Disorders*

#### 4.1.1. Superficial Scald

In this study, the storage of 'Granny Smith' apples using RLOS + ULO, RLOS + CA, and DCA-CF treatments reduced the development of superficial scald compared with RA. Similarly, several studies have reported the reduction of superficial scald development on 'Granny Smith' apples by low oxygen storage technologies [2,3,9]. For instance, Mditshwa et al. [2] demonstrated that DCA-CF (O2 = 0.3–0.5% and CO2 = 1%) reduced superficial scald to 2% after 16 weeks of cold storage. Likewise, Poirier et al. [32] reported that, over two consecutive seasons, ULO (≤1.0 kPa O2) phase applied before CA (0.5–1 ◦C and 0.5–0.8 kPa O2; 0.5–0.6 kPa CO2) storage controlled the development superficial scald on 'Granny Smith' apples. Studies have suggested that low oxygen technologies minimize the accumulation of ethylene, conjugated trienols, 6 methyl-5-hepten-2-one, and α-farnesene in apple peel to reduce superficial incidence [1,2,33].

The development of superficial scald symptoms is believed to be primarily driven by the oxidation processes of implicated volatiles such α-farnesene, 6-methyl-5-hepten-2 one, and others [4]. Therefore, storing 'Granny Smith' apples in low oxygen technologies such as RLOS + ULO, RLOS + CA, and DCA-CF is expected to reduce superficial scald development. The ULO (0.9% O2 and 0.8% CO2 for 21 d and 0.5% O2 for 7 d) phase exposes apples to lower oxygen concentration than the CA (1.5% O2 and 1% CO2 for 21 d and 0.5% O2 for 7 d); that is, 0.9% O2 for ULO and 1.5% O2 for CA. The assumption would be that the risk of superficial scald development would be higher in the CA phase than in the ULO phase because of the lower oxygen availability. However, in a season with a high risk of superficial scald development, other factors besides oxidation of implicated volatiles become more important, especially factors related to seasonal variations.

Critical factors that vary with harvest season, such as air temperature and light intensity, affect superficial scald susceptibility [4]. In 'Granny Smith', 'Cox's Orange', and 'Pacific Queen' apples, low mean air temperature and high light intensity during a growing season was associated with increased total phenolics and ascorbic acid content, which reduced the risk of superficial scald development [34]. A closer look at the results of apples stored using the RA treatment shows that the 2016 season appeared to have a higher risk of superficial scald than the 2015 season. In the 2016 season, superficial scald was detected earlier at 2 months + 7 days, whereas in the 2015 season, it was observed after 4 months of cold storage. In the 2016 season, the detection of superficial scald in fruit stored using RLOS + ULO and DCA-CF suggests that other factors besides oxygen concentration may have been involved in the development of the physiological disorder. We speculate that these factors are linked to seasonal variation. The seasonal variation in the incidence and severity of superficial scald between the 2015 and 2016 seasons corroborates with previous studies conducted on 'Granny Smith' apples during cold storage [2,14].

#### 4.1.2. Coreflush

Coreflush is a physiological disorder often described as a form of chilling injury affecting the quality of pome fruit [35,36]. The physiological disorder is often observed

when apples are cut open as diffuse browning of cortex tissue adjacent to the carpels [37]. The mechanism of development of coreflush is not clearly understood. However, studies suggest that it is more prevalent in late harvested fruit and is induced by low temperature storage [35,38]. Prevention of coreflush incidence is achieved by slow cooling to 0 ◦C, applying antioxidants before cold storage, and low ethylene storage [38]. In this study, fruit were not step-wise cooled before storage, which could have contributed to the development of coreflush. The results showed that low oxygen technologies were not effective in minimizing the development of coreflush in either harvest seasons.

#### *4.2. Physicochemical Properties*

#### 4.2.1. Flesh Firmness

Loss of firmness is often associated with the ripening of apples during cold storage [24]. During storage, the tissue strength of apple peel decreases due to ripening associated processes such as enzyme mediated increase in soluble pectin, volume, and internal cell spaces and net loss of non-cellulosic sugars, galactose, and arabinose [39]. Based on the results, RLOS (ULO and CA) and DCA-CF treatments retarded the loss of flesh firmness during storage. Studies have found apples stored in low oxygen technologies to be more firm compared with fruit stored at RA [40,41]. Low oxygen technologies slow down ethylene production, respiratory metabolism, and tissue breakdown during the ripening processes, which reduces firmness loss [3,42].
