**1. Introduction**

'Granny Smith' apples (*Malus* × *domestica* Borkh.) are susceptible to superficial scald, a serious postharvest physiological disorder adversely affecting fruit quality and marketability [1–3]. Superficial scald is observed as black or brown patches on fruit skin during cold storage and is associated with cell death or necrosis in hypodermal cortical tissue [4]. Although internal quality is usually not affected, development of superficial scald renders fruit unmarketable because of reduced appearance quality [5]. The autooxidation of naturally occurring sesquiterpene α-farnesene volatile to conjugated trienols and 6-methyl-5-hepten-2-one (MHO) is probably the main reaction resulting in the manifestation of symptoms of superficial scald [2,6]. However, the loss of natural antioxidant metabolites (tocopherol and phenolic compounds) and enzymes, which prevent cell damage by reactive oxygen species, contributes to the development of scald symptoms [5,7]. In addition, low phenolic content in apple peel has been correlated with high superficial scald incidence [8]. Both lipophilic and hydrophilic antioxidants may be involved in superficial scald prevention; however, no specific antioxidant has been consistently linked to α-farnesene, MHO, or superficial scald [4,7,9]. Additionally, pre-harvest factors such as cultivar, maturity, rootstock, and seasonal differences determine susceptibility to scald development [4].

**Citation:** Kawhena, T.G.; Fawole, O.A.; Opara, U.L. Application of Dynamic Controlled Atmosphere Technologies to Reduce Incidence of Physiological Disorders and Maintain Quality of 'Granny Smith' Apples. *Agriculture* **2021**, *11*, 491. https://doi.org/10.3390/ agriculture11060491

Academic Editor: Dirk E. Maier

Received: 24 April 2021 Accepted: 18 May 2021 Published: 26 May 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

In an attempt to control superficial scald, several methods have been applied to fruit that were either chemical or non-chemical in nature. Of note was diphenylamine, a synthetic antioxidant that successfully inhibited superficial scald until it was banned in Europe because of consumer safety concerns [10]. Low oxygen stress (LOS) storage of apples has been used for many decades as a non-chemical storage technology alternative for apples [3,4]. For instance, initial low oxygen stress (ILOS), anaerobic treatment for 9–14 d, before controlled atmosphere (CA), ultra-low oxygen (ULO), or regular atmosphere (RA) has been reported to effectively prevent the development of superficial scald in 'Granny Smith', Starkimson', 'Delicious', and 'Royal Gala' apples [11–14]. The mechanism of action is not fully understood. However, hypoxic conditions during storage lead to stimulated and rapid ethanol production in the fruit pulp, which presumably limits the oxidation of α-farnesene in the peel [4,15]. Ethanol vapors have shown inhibitory effects against superficial scald when exposed to various apple cultivars, further supporting this hypothesis [16,17].

In recent years, CA technologies have gained a lot of attention, particularly dynamic controlled atmosphere (DCA) [3]. The storage technology regularly adjusts gas composition during storage using biosensors, namely chlorophyll fluorescence (DCA-CF), respiration quotient (DCA-RQ), and ethanol (DCA-ET) [2,3]. Research studies by Mditshwa et al. [2] demonstrated the efficacy of repeated application of DCA-CF to control superficial scald (2%) on 'Granny Smith' apples when stored for 16 w in DCA-CF with a 14 d of interruption with regular atmosphere (RA) at −0.5 ◦C and 95% relative humidity (RH). Similarly, research work has also shown that DCA-CF storage maintains fruit firmness, inhibits the development of decay, and preserves Gala's internal quality [18] and 'Granny Smith' apples [19].

Despite the demonstrated efficacy of DCA-CF, more studies are still relevant to developing cultivar-specific storage protocols and validating existing results. For example, no evidence of a substantial difference between the sensory parameters of 'Greenstar' apples stored in DCA-CF at two oxygen concentration regimes (0.4 and 0.7%) (1.2 ± 0.2 ◦C) was observed after storage for 10 months, which suggests the need for further optimization studies [20]. Additionally, DCA-CF is subject to errors in determining the low oxygen limit of fruit because chlorophyll fluorescence depends on the metabolic activity of fruit [21,22]. For example, Feng et al. [23] reported variations in metabolic activity of fruit depending on canopy position in a tree. For the three apple cultivars 'McIntosh', 'Gala', and 'Mutsu', the sun-exposed side exhibited elevated rates of metabolism (higher soluble sugars, sugar alcohols, ascorbic acid, and succinic acids in the peel) compared to the shaded side.

Recent studies, according to Bessemans et al. [24], showed better fruit quality in 'Granny Smith' apples subjected to DCA-RQ (0.25–0.4 kPa O2) compared to standard CA storage at low ethanol concentration (<0.028 g L<sup>−</sup>1) in the fruit pulp. The quality of the fruit resembled that of 1-Methylcyclopropene (1-MCP) treated (preceded with CA) apples after 7 d at 18 ◦C. In a study on 'Royal Gala' apples, Weber et al. [22] showed that fruit stored in DCA-RQ had superior quality (less flesh breakdown) compared to static CA after 8 months of cold storage (1 ◦C). Despite the notable benefits in adopting DCA-RQ, the use of RQ is usually feasible under strict and gas tight conditions, unattainable in most CA rooms due to leakage [25]. In addition, this storage technique is feasible in controlled laboratory conditions with sensitive instruments that can accurately measure oxygen consumption rates and CO2 production [22,24].

Application of DCA-ET, also known as repeated low oxygen stress (RLOS), is based on determination of low oxygen limit (LOL) through either the destructive measurements of ethanol content from fruit pulp (estimated to be 1 ppm) or headspace analysis with sensors, notably DCS™ (Storex, Gravendeel) [3]. Few studies have reported the commercial application of DCA-ET beyond the ongoing research work in the Netherlands on different apple cultivars ('Elstar' and 'Jonagold') [21]. The study by Veltman et al. [26] investigated the effects of DCA-ET using Chrompack gas chromatography to regulate ethanol levels in the fruit pulp of 'Elstar' apples during cold storage (1 ◦C). The results showed that,

in addition to less skin spot development, the fruit had better color and firmness retention than standard CA (1.2% O2 and 2.5% CO2). However, there is limited information available on the application of the RLOS technology in other important apple cultivars such as 'Granny Smith', and the mechanism of action of RLOS is not clearly understood.

This study will further evaluate RLOS phases' effects on the incidence of physiological disorders and internal quality of the 'Granny Smith' apples during long-term storage. The study also assessed changes in radical scavenging activity, total phenolic content, and selected volatiles of 'Granny Smith' apples subjected to DCA-CF and RLOS storage technologies.
