**2. Respiratory pattern and maturity**

The fruits continue metabolic processes such as respiration, transpiration, ethylene biosynthesis, carbohydrate metabolism, among others, even after being harvested and end up rotting naturally. The detachment from the mother plant speeds up all those processes. In the fruit respiration processes, oxygen is absorbed, and carbon dioxide is released using the accumulated carbohydrates (starch and sugars). The ultimate goal of this reaction is energy production. Respiration is a continuous process that takes place in fruits both in the field and after harvesting [1]. When the fruit is separated from the mother plant, it cannot replace carbohydrates and water. Therefore, respirations stops when the reserves of carbohydrates and water are depleted, followed by fruit senescence.

Considering the physiological behaviour of fruits during ripening, and from the practical point of view, fruits have been classified into two groups: climacteric and non-climacteric [2, 3].

Climacteric fruits are characterised by a marked increase in respiration and ethylene production at the beginning of ripening, while non-climacteric fruits do not exhibit such respiratory behaviour [4, 5]. It has been known for a long time that climacteric fruits have in common the presence of ethylene to regulate maturation in [1]. Moreover, the production of ethylene in climacteric fruits is autocatalytic, and the application of exogenous ethylene is able to ripe climacteric fruits [6]. On the contrary, the absence of ethylene can effectively stop their ripening. Already in 1934, Franklin Kidd [7] explained the climacteric ripening process of fruits and the existence of a climacteric peak.

Therefore, the non-climacteric fruits must be ripe at harvest. Non-climacteric fruits have a quite different ripening behaviour, without a peak of respiration or of ethylene production [5]. The behaviour and key regulators for non-climacteric fruits are poorly understood until today. Abscisic acid (ABA) has been suggested as one of the potential key regulators in ripening process of non-climacteric fruits [5]. In short, climacteric fruits can ripen after harvest, where non-climacteric fruits cannot do that.

This classification, although oversimplified, is very useful for practical reasons. Nevertheless, analyses of ripening and related changes in CO2 and ethylene levels, at different fruit growth and maturation stages, have challenged this basic classification [4].

According to Cambridge dictionary, ripeness is the quality of being ready to be collected or eaten [8]. This is the popular concept that does not reflect the physiological knowledge of fruit ripening.

The knowledge of respiration and ripening pattern of each fruit is determinant to choose optimum harvest date, proper management strategies and storage practices, to achieve good nutritional and sensory quality, as well as decrease loss and waste [1].

Storage strategies involve temperature control, to decrease respiration rates and humidity control, to avoid transpiration, and eventually to control gases such as CO2 and O2 and eliminate ethylene in the atmosphere of the storage chambers [9].

Non-climacteric fruits should be kept away from any ethylene source to avoid the possible adverse effects on their ripening and/or quality, until more detailed

Prunus *spp. Fruit Quality and Postharvest: Today's Challenges and Future Perspectives DOI: http://dx.doi.org/10.5772/intechopen.112638*

information on the role of ethylene in these fruits is available. Moreover, the coexistence of ethylene-dependent and ethylene-independent pathways in climacteric and non-climacteric fruits has been described [4].

The mechanical or physiological damage, during harvest or storage, will cause an increase in respiration and consequently shorten shelf-life. Inappropriate temperatures (high temperatures and freezing) and lack of humidity may also cause physiological damages or disorders and an undesirable boosting of ripening [10].

Finally, it has been clearly demonstrated that many regulators of fruit ripening are common to both climacteric and non-climacteric fruits; namely, low O2 and high CO2 in the fruit microenvironment can delay the increase in ethylene and respiration, and consequently fruit ripening [5].

#### **2.1 Ripening of Prunus species**

Considering the wide and diversified range of fruits included in the genus *Prunus*, apricot, peach, plum, nectarine and durian are climacteric, whereas sweet and sour cherry are non-climacteric fruits. However, it is far too simplistic [4].

Plums are generally classified as climacteric fruits, but some cultivars vary markedly in there ripening behaviour. Some cultivars are known for their rapid softening, while others remain firm enough for commercial purposes, exhibiting a longer shelflife. Actually, there are two distinct patterns of ripening, with some cultivars behave typically as climacteric fruits and others showing a suppressed-climacteric behaviour due to their reduced capacity of converting 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene [11, 12]. El-Sharkawy and colleagues [6, 13–15] studied cultivars of Japanese plums that show different patterns: 'Early Golden' behaves as a climacteric fruit, and 'Shiro' presents a suppressed climacteric pattern. These differences can be caused by a wide range of factors such as different ethylene receptors [15], in the ACC-synthase genes [13], and in the role of auxins in mediating gene expression for ethylene-responsive transcriptional factors (ERFs) [6, 14].

Peaches are clearly classified as climacteric fruits, so the ripening process is controlled by ethylene. Moreover, an increase in auxins production in the fruits leads to the induce expression of the ACC-synthase [16, 17]. Additionally, Trainotti and coworkers [18] used a genomic approach and reported that there is a cross-talk between auxins and ethylene; that is, auxin genes are regulated by ethylene and vice versa, which confirm the role of auxins in regulating the ripening of peaches was confirmed.

#### **2.2 Development, ripening and quality of Prunus spp.**

Since the work of Chalmers et al. [19], it is assumed that a double sigmoid curve characterises the development and growth of some fruits, such as *Prunus* drupes (**Figure 1**).

Stage I is an initial exponential phase, from the fecundation of ovary throughout the morphological changes that occur until the formation of the fruit. During the first half of stage I, an increase in the number of cells is observed, while in the last part of that stage there is an increase in cell size. In cherries, the cuticle reaches its full thickness. Stage II corresponds to a plateau, characterised by no cell division and by a slight tangential elongation of cells, cell wall thickening and endocarp lignification. Stage III corresponds to the second exponential growth phase, when cells start to enlarge to the final fruit size, and it ends with the complete and mature fruit. Finally, stage IV corresponds to the physiological ripening of drupes, so that fruits achieve a

**Figure 1.** *Generalisation of the double-sigmoid growth curve typical of fruits of the genus Prunus.*

high nutritional and organoleptic value, and are appropriate for human consumption, reflected in their economic valorisation.

During ripening, the morphological and physiological changes occur at different levels, the more evident being changes in colour, texture and biochemical composition [20]. Colour changes due to the degradation of chlorophylls, reflected in the loss of green colour, and the increase in anthocyanins and carotenoids, nonphotosynthetic pigments, which confer a reddish and/or orange pigmentation to the fruits [21]. The decrease in fruit firmness that occurs throughout the ripening is a very complex process, which involves the breakdown of complex carbohydrates into sugars, and the activity of cell wall-modifying enzymes, causing reduction in intercellular adhesion, depolymerization and solubilisation of pectins, depolymerization of hemicelluloses, and loss of pectic galactose side chains [20]. All the aforementioned factors are responsible for the loss of firmness and the consequent increase in succulence of ripe fruits [22–24]. Changes in taste are very important for consumers' acceptance, and generally correspond to an increase in perceived sweetness, caused by a decrease in acidity and increase in sugar content. Moreover, in many fruits, the aroma becomes exquisite due to the release of volatile compounds, very appealing to the consumer. Carbohydrates, amino acids and fatty acids are the major fruit flavour precursors. The biosynthesis of volatile compounds is related to metabolic changes that occur during fruit ripening and have different profiles in the different ripening stages, and in different cultivars. Mihaylova and colleagues [25] studied the volatile compounds of eight peach varieties (*Prunus persica* L.) and identified 65 volatile compounds (aldehydes, esters and fatty acids), in different relative quantities depending on the variety.

The adequate ripening refers to fruits that present the best sensory and nutritional characteristics for consumers, and simultaneously allow the proper harvest management, storage and transport minimising loss and waste.

Prunus *spp. Fruit Quality and Postharvest: Today's Challenges and Future Perspectives DOI: http://dx.doi.org/10.5772/intechopen.112638*

This concept of ripening, and consequently quality, although seemingly simple is, in fact, very complex and variable, depending among others, on consumer profile, agronomical practices, and the available facilities, and obviously on the fruit species.

The health-promoting properties of stone fruits also contribute to their quality and are due to the presence of vitamins, namely A, C, E, and folates, dietary fibres, and phenolic compounds, mostly flavonoids [26]. Phenolics display antimicrobial properties that are important in the preservation of fresh fruits [26]. Moreover, flavonoids may protect against chronic diseases and play a preventive role in neurological disorders [27, 28].

All fleshy fruits of the genus *Prunus* share their short shelf-life. The use of cold storage is undoubtedly mandatory. There are many variations of cold storage and many methods that can be applied to maintain quality and increase shelf-life in a sustainable way. It should be remembered that fruits are complex and dynamic biological systems, whose thermophysical properties vary with numerous factors, such as temperature, moisture content, species and even cultivar. These data are essential for the study and optimization of postharvest handling processes, such as pre-cooling and cold storage, predicting practical situations, namely cooling rate, cooling time, cooling uniformity and cooling energy utilisation, and allowing the monitoring of temperature-induced changes in fruit quality [28].

#### **3. Some facts about the international trade of stone fruits**

According to a recent report published by the United States Department of Agriculture (USDA), the production of peach and nectarine in 2022/23 will increase by 1 million tonnes, reaching 23.7 million tonnes [29]. This forecast is justified by the expected increase in production in China, the European Union and Turkey, which are the largest producers in the world.

China's peach production is expected to rise to 16.8 million due to higher yields despite area declines, because growers are switching to currently more profitable crops, such as cherries.

Russia lifted restrictions in February 2022, so exports are expected to increase. Remarkable is the investment of the China government in new smart farm tools, namely online selling. However, the political instability that is being witnessed at the moment, due to the invasion of Ukraine, leads to fears of some unexpected embargoes with unpredictable consequences on the international market. EU production is expected to improve to 3.1 million tonnes, with a significant recovery in French and Greek production. However, Spain has suffered a cold spring this year, reducing production, and this will have a negative effect. As most EU exports come from Spain, EU shipments are also expected to decrease.

Turkey is the world's third largest producer of peaches and nectarines. The investment of government programs in these commodities is remarkable. Turkey's production is expected to reach a record 940,000 tonnes, with nectarine supply increasing steadily. Exports decreased slightly due to the reduction of shipments to Russia.

According to the 2021 statistical data of the Food and Agriculture Organisation of the United Nations (FAO) [30], the world production of peaches and nectarines was of 1504682.00 ha of harvested area, with a yield of 166110.00 hg ha−1, corresponding to 24994352.05 tonnes, whereas the harvested area for the European Union is 194050.00 ha, with a yield of 157908.00 hg ha−1, corresponding to 3064200.00 tonnes.

Plums are one of the most important commodities in *Prunus* genus, occupying the second place and evidencing rapid worldwide growth in popularity [31]. According to FAO, the world's plum production reached 12 million tons in 2021, and the leading producer being China (5 million tons per year), followed by Romania and USA [30].

Sweet cherry (*Prunus avium* L.) is a highly valued fruit, with a large international market. In 2021, the top exporters of fresh cherries were Chile, Hong Kong, USA, Turkey and Spain [30].

Almonds are in high demand around the world and their long shelf-life makes them easy to store and transport. Increasingly health-conscious consumers are driving the rise in demand for almonds. According to the FAO 2021 statistical data, there is an harvested area of 2,283,414 ha for almonds that correspond to a yield of 17,491 hg ha−1 [30]. The top three producer are the United States, Spain and Australia [30]. The United States is undoubtedly largest producer and marketer of almonds, with a current production figure of 2 M million tonnes that remain stable since 2019 [32]. For the first time in more than a quarter century in California almond area has declined in 2022, due to a faster rhythm of orchard removals than new planting grow, with a drop of 1.2% relative to 2021. There appears to be a declining trend in the area under almond cultivation in California [32]. A 10% decrease is expected for the 2022/2023 season, around 1.5 million tonnes, without the shell. Even so, the USA is by far the largest producer with 79% of the world production, followed far behind by Australia, with 7% and Spain with 4% of the world production. Iran, Morocco, Syria and Turkey are traditional producers of almond. The large increase of areas cultivated with almonds in Turkey and Portugal is remarkable. The consumption is increasing all over the world, with a special reference to the new Asian markets.
