1. Introduction
Sour (tart) cherry (
Prunus cerasus L.) production has been increasing over the past 20 years. European countries, including Hungary, are the leading producers, accounting for 66.2% (800,116 tons) of the annual production [
1,
2]. In Hungary, sour cherry is one of the most important fruits, ranking second in orchard area and production volume, following apples. The predominant cultivars are the Hungarian-bred ‘Újfehértói fürtös’ (syn.: ‘Ungarische Traubige’, ‘BalatonTM’) and ‘Érdi bőtermő’ (syn.: ‘DanubeTM’) [
1,
3].
Sour cherry is widely used in the food industry for the production of preserved fruits (frozen, dried, or canned), beverages (juice or alcoholic drink), confectionery items (jam, marmalade, and jelly), and as a natural colorant [
4,
5,
6]. The acid/sugar ratio is higher in sour cherry as compared to the sweet cherry (
Prunus avium L.); therefore, many cultivars are considered to have an astringent taste, which limits their fresh consumption. However, Hungarian cultivars (cvs.), such as ‘Újfehértói fürtös’ and ‘Érdi bőtermő’, are also excellent for fresh consumption due to their harmonic sugar/acid ratio [
7,
8]. Moreover, there is an outstanding nutritional content of fresh sour cherry, with high concentrations of vitamins, minerals, and bioactive components (e.g., antioxidants), which can decrease during fruit processing or storage [
9,
10,
11].
Sour cherry is a non-climacteric stone fruit and can only be stored for up to one week in cold conditions [
12]. Sour cherry fruit has low penetration resistance and high rates of respiration [
13]; thus, they quickly soften and lose their moisture content. This results in rapid postharvest quality loss [
14,
15]. The rapid postharvest quality loss exponentially increases with the development of fruit decay during storage and shelf-life [
14]. The biggest obstacle to the worldwide growth in fresh consumption of sour cherries is the perishability of the fruits and the limited options for effective storage technologies.
Many preharvest factors play a critical role in enhancing the storability of fruits, including genetics, environmental conditions, and cultural practices. These factors significantly influence both the development and postharvest quality of fruit crops [
16,
17,
18,
19,
20]. Key preharvest determinants, such as soil composition, environmental variables, nutrition, and varietal selection, affect the fruit quality and storage life significantly [
19,
20,
21,
22]. Soil properties, in particular, have a profound influence on the compositional and physical quality of harvested produce, which, in turn, affects postharvest outcomes [
18]. Nutrient balance is essential for maintaining optimal fruit texture and size; for instance, nitrogen-deficient trees tend to produce smaller, firmer fruits, while excessive nitrogen leads to a rapid decline in firmness and reduced storability. Similarly, potassium deficiency can lead to poor coloration and textural changes in fruits, rendering them small, hard, and inedible due to improper ripening [
19,
20,
23]. Selecting a high-yielding cultivar with desirable qualities and extended shelf-life is a vital preharvest decision that directly impacts the postharvest longevity of fruits [
18,
24]. Moreover, environmental stresses, such as heavy rainfall, excessive moisture, and heat stress, can exacerbate fruit cracking and water loss, particularly in crops like cherries and apples, leading to significant postharvest quality deterioration [
25,
26].
Among postharvest strategies, cold storage is one of the most widespread and oldest methods to preserve the fruits following harvest. Cold storage can suppress decay in many fresh stone fruits [
27,
28,
29,
30,
31,
32] by increasing CO
2 and decreasing O
2 levels in the ambient atmosphere. In the case of sour cherry, Wang and Vestrheim [
28] indicated that fruit decay can be greatly reduced in fruits stored for up to 20 days at 2 °C in a controlled atmosphere (CA) consisting of 25% CO
2 + 10% O
2.
Cold storage can also be combined with other preservation techniques, like modified atmosphere packaging (MAP). MAP is used to supplement low-temperature management to delay senescence, reduce physiological disorders, and suppress decay in many fresh fruit and vegetable products [
33]. Although the positive effect of MAP storage on the weight loss and decay of sweet cherries has already been demonstrated in several studies [
28,
34,
35,
36,
37,
38,
39,
40], few MAP studies are available for sour cherry. For example, Lurie and Weksler [
14] showed that sour cherry fruit storage is better in modified atmosphere packaging than in regular air storage at 0 °C. Moreover, the weight loss and fruit decay of the sour cherry cultivars, ‘Érdi jubileum’ and ‘Érdi bőtermő’, were significantly reduced by MAP storage compared to regular cold storage conditions [
41]. However, fruit decay as well as the subsequent shelf-life conditions of various sour cherry cultivars were not compared between regular cold storage and cold storage with MAP. Moreover, previous studies focused on the postharvest features of cherry fruits under conventional or integrated production systems; however, there is a significant gap in the research regarding the fruit decay of sour cherries under cold storage with MAP and subsequent shelf-life conditions, particularly for fruits originating from various production systems.
The objectives of our study were twofold: First, to evaluate the incidence of fruit decay during shelf-life for both freshly harvested and cold-stored sour cherry fruits under normal atmospheric conditions and MAP for three cvs.: ‘Érdi bőtermő’, ‘Újfehértói fürtös’, and ‘Petri’. Second, to assess the fruit decay incidence for harvested, cold-stored fruits in combination with MAP and the subsequent shelf-life of cold-stored fruits in combination with MAP across three production systems (conventional, integrated pest management [IPM], and reduced IPM) for the cv. ‘Érdi bőtermő’. Additionally, we measured fruit firmness and weight loss for all three cultivars to provide a comprehensive analysis of their general fruit quality characteristics prior to shelf-life evaluation.
4. Discussion
Our research showed that sour cherry fruit decay incidence was variously dependent on storage temperature conditions (shelf-life vs. cold storage), storage atmosphere conditions (normal atmospheric (NA) vs. modified atmosphere packaging (MAP)), and cultivars’ susceptibility (‘Érdi bőtermő’, ‘Újfehértói fürtös’, and ‘Petri’). In addition, we showed that cold-stored fruits in combination with MAP showed significantly lower fruit decay incidence in the conventional production system as compared to the reduced IPM production system for the cv. ‘Érdi bőtermő’.
The fruit firmness was determined for sour cherry in several previous studies, and a significant difference was detected among the various genotypes [
6,
15,
28,
41,
42,
43,
44,
45,
46,
47,
48]. In this study, sour cherry fruit firmness (Durofel Index) ranged between 23.6% and 41.9%, 23.9% and 46.0%, and 26.7% to 43.5% for cvs. ‘Érdi bőtermő’, ‘Újfehértói fürtös’, and ‘Petri’, respectively, at harvest and under cold storage NA and MAP treatments (
Figure 3). These results are similar to those of Desiderio et al. [
48], who showed that the fruit firmness of cv. ‘Újfehértói fürtös’ ranged from 28.4% to 44.5%. There was also a significant difference in fruit firmness among the three studied Hungarian cultivars. The fruits of cv. ‘Érdi bőtermő’ were the softest, cv. ‘Újfehértói fürtös’ had higher firmness, and cv. ‘Petri’ had the highest firmness at harvest. Upon comparisons of Hungarian and other sour cherry cultivars (including cvs. ‘Érdi bőtermő’, ‘Petri’, and ‘Újfehértói fürtös’), cv. ‘Újfehértói fürtös’ exhibited high firmness values [
41], which were higher compared to the most well-known sour cherry cultivars, including ‘Schattenmorelle’ and ‘Montmorency’ [
6,
44,
45]. However, higher firmness was also shown by some Iranian cultivars [
15], providing a good option for postharvest storage.
Our study showed that MAP effectively preserved (or even increased, to some extent) the firmness of the samples compared to control fruits for the three tested Hungarian cultivars (
Figure 3). In one case, this result corresponds to a previous study by Szabo et al. [
47], where authors found no significant differences in firmness values during the storage of control and MAP-treated cv. ‘Újfehértói fürtös’ (days 0–14). In another case, a similar increase in fruit firmness was reported previously for sour cherry cultivars ‘Érdi jubileum’ and ‘Érdi bőtermő’, following six weeks of MAP storage at 0 °C [
41,
43], and for sour cherry cv. ‘English Morello’ with 20 days of controlled air storage at 2 °C [
28]. The possible reasons for this can be explained by the biochemical processes in fruits, i.e., fruit softening, a biochemical process typically attributed to changes in cell wall composition due to enzymes, such as polygalacturonase, that modify cell walls [
49]. Low oxygen and high CO
2 levels inhibit the activity of these enzymes, thus helping to maintain fruit firmness during storage. Moreover, reduced transpiration and water retention provide flexibility to fruit cells [
50]. This also means that fruit firmness is highly correlated with the water content of fruits and the corresponding measure of fruit weight loss [
41,
51].
Our results showed that, as expected, the fruit weight loss of the three sour cherry cultivars was reduced significantly under MAP storage compared to NA storage (
Figure 4). In agreement with our study, many sweet cherry studies have shown a positive effect of MAP storage on fruit weight loss [
28,
34,
35,
36,
37,
38,
39,
40]. However, only a few sour cherry studies [
41,
47] demonstrated that the weight loss of the sour cherry was significantly reduced by MAP storage. Aryanpooya and Davarynejad [
41] showed that the weight loss of the sour cherry cvs. ‘Érdi jubileum’ and ‘Érdi bőtermő’ was significantly reduced by MAP storage compared to ambient air packaging. The study by Szabo et al. [
47] measured 3.9 and 6.4% fruit weight losses in the control stored samples of cvs. ‘Kántorjánosi’ and ‘Újfehértói fürtös’, respectively, while MAP-stored fruits showed 1% weight loss during the 14-day storage. Our results showed larger fruit weight loss (ranging between 1.18 and 26.1%) in the MAP treatment after 6 weeks of storage compared to the previous two sour cherry studies, which was probably due to a longer storage duration in our study as well as differences in cultivars and experimental setup.
Our results showed that in all years, the yield loss of harvested fruits continuously increased over the observed 14 days, causing considerable loss from day 6 in all cultivars (
Table 3). This corroborates previous findings that sour cherry fruits quickly soften and lose moisture and can only be stored for up to one week [
12], due to their low penetration resistance and high respiration rates [
13]. This leads to rapid postharvest quality loss [
14], which exponentially increases with the development of fruit decay during storage [
14,
15,
47,
52]. In our study, consistent with these findings, the incidence of fruit decay in harvested fruits significantly increased during shelf-life in all the cultivars (
Table 3). However, decay incidence was lowest across all shelf-life days for the harvested cv. ‘Újfehértói fürtös’ compared to the other two cultivars, ‘Érdi bőtermő’ and ‘Petri’. Soltész et al. [
52] also demonstrated that the storage capability of cv. ‘Újfehértói fürtös’ was similar to that of ‘Petri’ but better than cv. ‘Érdi bőtermő’. The study by Szabo et al. [
47] reported a decay incidence of over 50% for cv. ‘Újfehértói fürtös’ after 12 days of shelf-life, which was considerably higher than in this study. The differences between these studies could be due to different seasons, different plant protection technologies used during growth, and different shelf-life conditions applied to the harvested fruits.
It is important to note that this study aimed to show the incidence of overall disorders in sour cherry fruits, expressed as fruit decay incidence, at harvest and under postharvest treatments. Therefore, the specific types of postharvest fruit decay were not identified in this study. However, it is generally known that more than 80% of fruit decay is caused by fungal pathogens belonging to the genera
Alternaria,
Penicillium,
Monilinia,
Botrytis, and
Rhizopus, which is consistent with previous studies on sour cherries under postharvest treatments [
14,
53,
54].
Fruit decay incidence values after 6 weeks of normal atmosphere (NA) cold storage were low (below 20%) and similar for all cultivars (
Table 5). In line with our results, Wang and Vestrheim [
14] indicated that the decay of sour cherries can be greatly reduced when fruits are stored for up to 20 days at 2 °C in controlled atmosphere storage. However, Soltész et al. [
52] showed that postharvest fruit decay incidence was significantly higher for cv. ‘Érdi bőtermő’ (above 50%) than for cv. ‘Újfehértói fürtös’ (below 40%) after 6 weeks of cold storage, while cv. ‘Újfehértói fürtös’ and cv. ‘Petri’ showed similar postharvest decay incidence after the 6-week cold storage. The possible differences among the studies may be due to the different seasons of the tested cultivars and the fact that CO
2 concentration was higher in our cold storage technology. This aligns with the previous study by Lurie and Weksler [
14], who showed that increasing CO
2 concentrations (higher than 5%) had a better fungistatic effect against the development of decay.
Our results for both NA and MAP indicated that six weeks of cold storage was effective for maintaining the quality of sour cherries; however, the shelf-life of the fruits was significantly reduced at room temperature, regardless of the storage method (
Table 5 and
Table 6). Lurie and Weksler [
14] also showed that the main fruit loss of sour cherries occurred after cold storage during the shelf-life period. MAP has been previously reported to reduce the decay of cherry fruits during storage, but this effect was maintained only until day 5 of shelf-life at 20 °C [
55]. The proportion of decayed fruits was the lowest on the 6th day for cv. ‘Érdi bőtermő’ following MAP storage. However, the other two cultivars had a significantly higher proportion of decayed fruits, following both NA and MAP storage. This may be the result of inappropriate gas concentration achieved by the applied packaging, leading to anaerobic fermentation during MAP storage. MAP storage can induce anaerobic fermentation if the O
2 concentration drops to a level where aerobic respiration can no longer be maintained or if the CO
2 concentration exceeds an acceptable level [
56]. Anaerobic fermentation (an excessive decrease in O
2 levels and an excessive increase in CO
2 levels) can occur when the gas permeability of a MAP sachet does not match the respiration dynamics of the fruits inside it. It has been observed that there is a difference between the respiration intensity of different sweet cherry [
57] and sour cherry [
47] cultivars. Kappel et al. [
58] found that the respiratory intensity of cultivars harvested later was lower compared to earlier ones. Crisosto et al. [
59] found in their experiment that sweet cherries with higher respiration intensity had softer fruits. In the case of raspberry cultivars, it was concluded that fruits with higher respiration intensity are more prone to decay [
60]. Based on this information, we can assume that the earlier harvested cv. ‘Érdi bőtermő’ has a higher respiration intensity than the other two cultivars, as a shorter shelf-life, lower fruit firmness, and a higher degree of decay were observed. Cultivar ‘Érdi bőtermő’ may have achieved the proper gas composition earlier with the applied MAP bag, which ensured that quality was maintained longer following storage. In the other two cultivars, the perforation of the MAP sachet was presumably inadequate.
Our results showed that fruit decay incidence at harvest was highest in the reduced IPM system and significantly higher than in the conventional and IPM systems (
Table 8). This finding aligns with the study by Holb et al. [
42], which showed that the incidence of disease in apples at harvest was higher in the reduced IPM system compared to the IPM or conventional systems. The efficacy of disease management in the reduced IPM system is closer to that of organic production systems than to classical IPM systems, where disease management efficacy is lower than in IPM and conventional systems [
61,
62,
63,
64,
65]. Due to less effective disease and pest management during the season, higher disease occurrence was observed at harvest in the reduced IPM system in our sour cherry study (
Table 8). Additionally, some fungal diseases (e.g.,
Monilinia laxa) can cause latent infections by penetrating immature fruits and remaining dormant until conditions become suitable, often manifesting after harvest as postharvest decay [
66,
67,
68]. Moreover, latent infections in fruits at harvest can also be higher in these systems compared to conventional or classical integrated systems. These latent infections express symptoms during storage, which likely occurred in our experiment with the reduced IPM system.
Fruit decay incidence values after MAP cold storage increased in the order of conventional, IPM, and reduced IPM systems, but significant differences were detected only between the conventional and reduced IPM systems (
Table 8). Although previous studies have not compared various production systems for stone fruits under MAP cold storage followed by shelf-life conditions, our results confirm that the health status of sour cherry fruits at harvest influences the longevity of postharvest quality. Additionally, fungicides applied during the growing season may also influence the early stages of storage by preventing postharvest diseases and latent infections [
67,
69] in sour cherry fruits compared to reduced IPM fruits in our study.