**1. Introduction**

Pears are the most cultivated pome species in the world after apples [1]. Pear cv. 'Conference' is the most important cultivars in Europe with a yearly production of around 1 million tonnes [2]. It is also one of the most commonly stored pear cultivars. The storability of pear, which is a typical climacteric species [3], depends on various factors [4], most notably, the optimal harvest date [5], the fruit cooling rate after harvest [6], the degree of pollination and rootstock [7], storage conditions [8], fertilization, and health [9]. For example, studies conducted on the influence of pollination on the quality properties of 'Conference' pears showed that the number of seeds was positively correlated with fruit mass and calcium content, but was negatively correlated with total soluble solids and firmness [7], and the initial TSS value and firmness are crucial for storability assessment.

Sometimes, however, as in the case of apples, other factors may contribute, such as the rootstock used, which, by affecting the nutrition of trees, can influence the properties of the stored fruit [10]. Temperature is a factor that crucially influences the rate of any reaction, in particular, of respiration-related reactions [11]. The optimal fruit storage temperature depends on the species, and sometimes on the cultivar [12]. Pears belong to the few fruit species which suffer no damage if stored at a temperature below zero. The optimal temperature is between −1 and 0 ◦C, with a clear preference towards the negative temperature [13]. As early as 1964, Porrit found out that the storage life of 'd'Anjou' and 'Bartlett' pear was, respectively, 35% and 40% longer at −1 ◦C than at 0 ◦C [14].

Controlled atmosphere (CA) storage significantly extends the storability of pears compared to normal atmosphere (NA) [4]. CA storage delays ripening and preserves fruit

**Citation:** Łysiak, G.P.; Rutkowski, K.; Walkowiak-Tomczak, D. Effect of Storage Conditions on Storability and Antioxidant Potential of Pears cv. 'Conference'. *Agriculture* **2021**, *11*, 545. https://doi.org/10.3390/ agriculture11060545

Academic Editor: Dirk E. Maier

Received: 19 May 2021 Accepted: 11 June 2021 Published: 13 June 2021

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**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/).

quality [15], but it may cause a decrease in the production of aromatic compounds [16]. The very low oxygen content (ULO) commonly used for long-term storage of apples cannot be used for pears due to their greater susceptibility to damage due to oxygen deficiency [17]. Low oxygen levels trigger anaerobic respiration resulting in the accumulation of alcohol in pears, which is directly responsible for damage to the flesh [18]. This can be prevented by the monitoring of changes in the alcoholic respiration in the atmosphere [19]. However, in controlled atmosphere, it is the internal disorders, such internal browning, which are a major limiting factor [20]. In addition, high carbon dioxide content may cause internal browning in many cultivars, and 'Conference' pears are considered to be sensitive to high levels of this gas [21].

Another method used in recent years to improve storability is to use substances that limit ethylene production in climacteric fruit species [22]. One such substance is 1-methylcyclopropene (1-MCP), which has been used for nearly two decades [23]. In both NA and CA cold storage, 1-MCP can be applied to maintain the quality attributes of fruit, especially firmness. 1-MCP, a gaseous ethylene binding inhibitor, has proven useful in preventing the formation of ethylene in fruit, thus increasing its shelf life after harvest and enabling greater flexibility in distribution and retailing [24]. However, it has been shown that, unlike in apples, low doses of 1-MCP do not completely inhibit ethylene production in pears. 1-MCP interacts with fruit in manifold ways. It can limit the development of fungal diseases. It can also reduce the occurrence of superficial scald [25], but it may also cause an unexpected increase in the incidence of this physiological disorder [26]. However, despite the risk of this negative effect, the benefits of 1-MCP application for other quality parameters and storability are significant and can be observed in all climacteric fruit species [24]. The most important of them include the limitation of the incidence of fungal diseases strictly associated with senescence, and the reduction in transpiration and respiration, which translates into lower storage costs, as the fruit produces less heat at the same temperature. Other important advantages of using 1-MCP include the limitation of: vitamin C losses during storage, the incidence of chilling disorders in tropical fruit species (avocado, mango, papaya) and unwanted changes in flesh structure, such as woolliness (mealiness) and internal breakdown in peaches and nectarines [24]. However, the occurrence of disease is specific to particular fruit species and the conditions in which they are grown and stored, and therefore more detailed studies are still needed in this area [24,27].

Pears have moderate antioxidant activity, but because of relatively high consumption of pears in Europe [1], they are an important source of health-promoting compounds [28]. Pears owe their antioxidant potential to such bioactive compounds as polyphenols, triterpenoids, carotenoids, and chlorophylls, and also have anti-inflammatory and anti-proliferative properties [29].

The antioxidant capacity of food, including fruit and vegetables, depends on the presence of complex bioactive compounds which differ considerably not only in terms of compound class and chemical makeup, but also bioavailability, due to the complex composition of food and the interaction between individual nutrients [30]. The literature provides ample information on the complex nature of the antioxidant activity of food products [31]. In this context, a distinction is often made between extractable and nonextractable antioxidants, the presence (or absence) of which determines how nutritional or healthy a food product is [31]. Residues left after the extraction of bioactive compounds from fruit still show antioxidant activity, so various complex extraction methods are applied to determine more precisely the total antioxidant capacity of food, including that of compounds linked by covalent or hydrogen bonds or forming hydrophobic interactions with other nutritional components such as carbohydrates or proteins. Antioxidant capacity is typically measured in water–alcohol or acidic extracts; however, in complex systems, such as food products, other hydrolysis and extraction methods are also needed to determine the total value of this parameter. A method commonly applied in science to determine the

antioxidant capacity of food is to measure the ABTS\* cation radical scavenging activity of 70% methanol extracts (*v*/*v*) from food samples [32].

The objective of this study was to assess (1) the impact of storage conditions on monthly qualitative changes in 'Conference' pears during six months of storage, (2) the effect of the rootstock on the storability of 'Conference' pears, (3) the impact of 1-MCP application on the storability of 'Conference' pears, and how all these factors translate into revenues from the sale of pears after storage.

#### **2. Materials and Methods**

The experiment was conducted in the experimental orchard and laboratory of the Department of Pomology of the University of Life Sciences in Poznan (52◦31 north latitude and 16◦38 east longitude). 'Conference' pears were collected from trees planted in spring 2002 at spacing of 4 × 1.5 m. Pears were grafted on three different rootstocks: Pyrus caucasica Federov, Pyrodwarf, and Quince S1. There were 64 trees on each rootstock. The pear orchard was maintained according to the standard commercial practice for integrated fruit production.

#### *2.1. Sampling*

The experiment was carried out from autumn 2011 to spring 2014. The harvest occurred on dates determined as the optimum harvest dates (OHD), using the starch test [33] and the sum of active temperatures (growing degree units) method proposed by Łysiak [34]. The sum of active temperatures determined according to the latter method was 2580 degrees.

After harvest, pears intended for storage were graded to eliminate those not meeting the highest commercial quality standard applicable in OECD countries [35]. According to those standards, pears of superior quality ("extra"), have to be intact, sound, clean, and free of any damage. The experiment was carried out using 20 boxes of 15 kg of graded pears. Each box contained about 75 pears.
