## 2 Characteristics of Apples
### Storage and Harvesting Conditions  
The selection of the right temperature and storage in optimal conditions of the gas composition of the atmosphere must meet the requirements of individual varieties [17]. Typically, apples are stored at a temperature of 0 to 4 \({}^{\circ}\)C, which allows them to maintain their quality for several months. The relative humidity should be maintained at 85 to 95%, and the weight loss of apples due to transpiration and respiration should not exceed 3.5% at the end of storage. After harvesting, the fruit is usually placed in large plastic packages (1.20 \(\times\) 1.00 \(\times\) 0.78 m) of approx. 300 kg, is stacked close together and up to a maximum of 12 levels. For this reason, there is a change in the thermal conditions in the storage rooms, caused by differences in air flow, to which the top layers of apples are most exposed. Refrigeration systems in storage chambers that cause temperature fluctuations can cause the temperature to drop below the dew point, resulting in condensation of some of the water vapour in the humid air. Condensed water manifests itself in the presence of single droplets or a film on the surface of apples, hurting the skin, structure, and surface texture and also providing excellent conditions for the proliferation of microorganisms [42]. Each change in temperature by 10 \({}^{\circ}\)C causes a 2- or 3-fold change in the rate of biological reactions [43]. Nevertheless, an important factor during fruit storage is the gas composition of the atmosphere, affecting, among others, changes in the antioxidant activity of apples. Determining the optimal storage conditions allows for maintaining the appropriate quality of apples with minimally changed chemical composition. In the literature, there are many reports on storing fruit in chambers with low oxygen content (approx. 1 kPa) or very low oxygen content (0.5 and 0.7-0.8 kPa), which is associated with slowing down the metabolism, production of ethylene, and reduction of fermentation processes [44]. Ethylene (C\({}_{2}\)H\({}_{4}\)) is a simple hydrocarbon that occurs in gaseous form. It is also a naturally occurring substance that affects the development of flowering, ripening, and ageing of fruits. By affecting even at low temperatures, it can cause many biochemical changes, accelerating the metabolism of fruits and vegetables [45]. Therefore, this hydrocarbon significantly extends the shelf life of fruits and vegetables after harvest. Increasing the amount of ethylene causes fruit and vegetables to soften, as well as the degradation of chlorophyll, resulting in progressive spoilage. Apples, sensitive to ethylene content, can be protected by coating to delay the ripening process [46]. The use of edible coatings delaysethylene production during apple storage compared to detecting the amount of ethylene produced during the storage of uncoated apples. The use of edible coatings also delays microbiological degradation [47]. The degradation of chlorophyll under the influence of ethylene is commercially used to remove the green colour of, e.g., lemon peel [48]. Ethylene removal systems contained in packaging can effectively slow down the undesirable effects of ethylene during storage. The research shows that the ethylene volume in the packaging of fresh products can vary from 0.017-0.035 \(\upmu\)L in shopping centres to 0.06 \(\upmu\)L in distributor centres. In addition, using potassium permanganate (KMnO4) removes ethylene--causing the breakdown of the double bond, while zeolite and activated carbon can adsorb it. Potassium permanganate is used for its cost-effectiveness, but some studies indicate its toxic effects on the human body [49,50]. Due to the role of ethylene during fruit ripening, they can be divided into climacteric and non-climacteric. The first group concerns fruits that are capable of secreting a large amount of ethylene, which is conducive to the ripening process, e.g., peaches, apples, bananas, or kiwi.