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Peach (Prunus persica) is one of the representative climacteric fruits susceptible to environmental stresses, including microbial contamination. This article analyzed major findings from the literature on pre- and post-harvest technologies for maintaining the quality of peach fruit to figure out the strengths and limitations of each treatment strategy. The key implication from studies of pre-harvest agents directly applied to the fruit surface or supplemented as fertilizer was the application of a mixture regarding substances with diverse working mechanisms to prevent excessive use of the agent. The common objectives of previous research on pre-harvest treatments were not only the improvement in the quality of harvested fruit but also the storability during long-term refrigeration due to the short lifespan of peaches. In the case of post-harvest treatments, the efficacy was considerably affected by various determinant factors (e.g., a cultivar of fruit, the sort of technologies, and storage environments), and thus operating conditions optimized for peach fruit were described in this article. Whereas, although the combined treatment of technologies categorized into principles (physical, chemical, and biological approaches) has been adopted to achieve the synergistic effect, undesirable antagonistic effects (i.e., the inhibition of efficacies expectable from singular treatments) were also reported to highlight the importance for exploring adequate treatment conditions.
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- fruit quality
- productivity
- fungal infection
- long-term fruit storage
- fruit ripening
- microbial safety
- climacteric fruit
- stone fruit
- combined treatment
1. Introduction
Peach (Prunus persica) is a globally consumed fruit preferred by consumers due to its exotic taste and rich nutritional substances (e.g., minerals, sugars, and amino acids) [1]. Peaches have several antioxidant compounds, including vitamins, phenolic compounds, volatiles, carotenoids, and organic acids [2]. However, as a climacteric fruit, peaches are susceptible to rot and have a short shelf life due to ethylene emission accompanied by a rising respiration rate during storage [3]. Within the Prunus genus, peach is a stone fruit that has a thin exocarp or skin with the characteristics of a lignified endocarp and a fleshy mesocarp [4]. Although the production of peaches and nectarines is steadily increasing, the problems in storage and transportation due to their rapid rotting and softening at ambient temperature make their exports difficult [5]. Previous studies regarding the quality of peaches have been conducted with similar objectives to those of other stone fruits (e.g., cherry, plum, and apricot) [6,7,8]. However, research focused specifically on peaches is needed because peaches age more rapidly and are more vulnerable to disease caused by pathogens than other stone fruits [9].
To enhance the quality and extend the storage period of fruits, both pre-harvest and post-harvest treatment technologies have been consistently developed and applied from production to storage [10,11]. Pre-harvest treatment is a method performed before harvesting to improve the quality of harvested fruits and/or extend their shelf life during post-harvest storage [12,13]. Post-harvest treatment prevents the deterioration of fruit quality until consumption by consumers [14,15]. Various types of pre- and post-harvest technologies have been reported with differing effects, and even the effects of a particular technology can differ depending on the processing target or environment. Therefore, insights into the design of pre- and post-harvest treatment strategies to secure peach quality and safety can be obtained by the comprehensive analysis of key findings from previous relevant research conducted to determine desirable treatment conditions.
2. Quality Factors of the Pre- and Post-Harvest Treatment of Peach Fruits
The characteristics of fruit include sensory properties (e.g., texture, taste, aroma, and appearance), chemical constituents, functional values, safety factors (e.g., the concentration of toxigenic substances, and population level of contaminated microorganisms), nutritional value, mechanical properties, and defects caused by the growth or metabolism of pathogens [21]. Major quality factors of peaches investigated from previous research regarding pre- and post-harvest treatments can be categorized as the stability of fruit quality, microbial deterioration, and antioxidant capacity. As shown in Table 1, the quality factors can be estimated from the quantitatively measured values by the indicators related to taste, color, and nutrients. Examples of the measurable parameters are as follows: firmness, weight loss, volume, total soluble solid (TSS), titratable acidity (TA), ethylene production, vitamin C content, activation of enzymes related to antioxidant ability, antioxidant content, decay incidence, and antimicrobial function. Taste, one of the sensory characteristics, is mainly divided into a sweet and sour flavor that can be indicated by the values of TSS and TA, respectively.
Table 1. Quality factors as the indicator of the effects of pre- and/or post-harvest treatments for peach fruits.
| Category | Examples of the Factors |
|---|---|
| Stability of fruit quality | Weight, volume, length, width or diameter, total soluble solid (TSS), soluble solid content (SSC), titratable acidity (TA), ethylene emission (production), malondialdehyde (MDA) content, firmness, color, vitamin C content, pectin content |
| Microbial deterioration and damage | Infected wounds, decay, disease incidence, lesion diameter |
| Antioxidant capacity | Antioxidants content (phenolics, flavonoids), activity of enzymes (catalase (CAT), superoxide dismutase (SOD), ascorbate peroxidase (APX), peroxidase (POD), pectin methyl esterase (PME), phenylalanine ammonia-lyase (PAL), enzymes of ascorbate-glutathione (AsA-GSH) cycle, polyphenol oxidase (PPO), lipoxygenase (LOX)) |
The stability of fruit quality allows for the maintenance of the product value of peaches from cultivation to harvest followed by storage, and various factors correlated with the changes in physiological characteristics of products have been identified. Quantitative values of morphological characteristics (e.g., volume, length, and width or diameter) can belong to quality factors. One of the major goals of the pre-harvest treatment of fruits is the modulation of metabolisms related to fruit growth and development [22,23]. Whereas, preventing a decrease in weight during fruit storage under refrigerated temperatures due to the loss of water and the response to cold stress is an additional intended function of both pre- and post-harvest treatments [24]. Since an increase in sugar content is closely related to the ripening of peach fruits, TSS can also be an indicator of the product quality of fruit [25]. The measurement of soluble solid content (SSC) is an index of the flavor of peaches to determine the appropriate time for harvest and storage from the perspective of fruit maturation [26]. TA, determined by the titration of internal acid, is a measurement of total acid, which can be used to analyze the growth level and taste of fruits [27]. The ratio of SSC to TA (SSC/TA) is negatively related to the maturity of the fruit and is also used as the representative ripening index [28]. Peach produces ethylene inducing fruit ripening, and the level of ethylene emission or production can be a parameter of maturity [29]. Malondialdehyde (MDA) produced by reactive oxygen species (ROS) is known as an indicator of fruit damage [30]. The color of peach skin and flesh is generally considered a factor in the stability and value of commodities, but is also related to antioxidant capacity because of phytochemicals [31].
The incidence of fungal decay is an important index for shelf life and commercial value because damage to peach fruits is critical from the perspectives of economic loss for producers and retailers. The major fungi reported as the cause of deterioration from peach fruits are brown rot fungi [32], Rhizopus rot fungi [33], blue mold [34], and gray mold [35]. Brown rot is commonly caused by fungal species including Monilinia fructicola, Monilinia laxa, and Monilinia fructigena [36]. Rhizopus rot, blue mold disease, and gray mold disease are generally caused by the fungal species of Rhizopus stolonifer, Penicillium expansum, and Botrytis cinerea, respectively [37]. The level of fungal infection has been generally measured by the visual observation of indicators for the severity of fruit diseases (e.g., infected wounds, decay, disease incidence, and lesion diameter).
Antioxidant capacity is the ability to remove ROS, which is a cause of undesirable quality changes in fruits due to weakened stress tolerance and can be evaluated by factors including the contents of phenolics and/or flavonoid compounds [38]. The activity of major enzymes involved in the production or removal of ROS in peach fruits can also be reported as indicators of the antioxidant capacity as follows: catalase (CAT) [39], superoxide dismutase (SOD) [40], ascorbate peroxidase (APX) [41], peroxidase (POD) [42], pectin methyl esterase (PME) [43], phenylalanine ammonia-lyase (PAL) [44], enzymes that comprise the ascorbate-glutathione (AsA-GSH) cycle [45], polyphenol oxidase (PPO) [46], and lipoxygenase (LOX) [47].
3. Strategic Approach to the Application of Pre-Harvest Treatment Technologies for Peach Fruits
Pre-harvest treatments for general fruits have been used as measures for enhancing the quality and safety of fruits with the treatment of chemical agents (e.g., spraying and spreading, fertilizing) or physical treatments (e.g., bagging fruits, setting a canopy on the fruit tree, and irrigating the field of the orchard) before harvest [16]. Most relevant research using pre-harvest peach fruits reported the application of chemical substances to improve the product quality at harvest and to prevent the deterioration of harvested fruits during storage [48,49,50,51,52,53,54,55,56,57]. This indicates that the effects expected from the post-harvest treatment can also be achieved by pre-harvest technologies. Thus, as shown in Table 2, this review mainly analyzed the previous research regarding the evaluation of the efficacies of pre-harvest treatment technologies with the perspectives of not only harvesting but also storage time [58]. Key chemical agents that can be utilized as pre-harvest treatments for peaches by spreading and/or spraying are the following: calcium salts [48,49,50,51,52], acids [53,54,55], sodium nitroprusside (SNP) [56], and putrescine (PUT) [57]. The use of fertilizer has also been regarded as an effective method to control the quality of peach fruit during cultivation [59,60].
4. Strategic Approach of the Application of Post-Harvest Treatment Technologies for Peach Fruits
4.1. Physical Treatments
Since consumers’ concerns about the hazard of residue when using chemical compounds have increased, physical treatment technology emerges as an alternative to protect the quality and safety of fruits [74]. Major examples of physical treatment technologies are as follows: temperature control [75,76,77,78,79,80,81,82], modified atmosphere [83,84,85,86], and irradiation [87,88,89,90,91,92,93] (Table 3).
4.2. Chemical Treatments
Using chemical agents that can enhance the quality and safety of fruits by controlling fruit maturation (e.g., naturally occurring compounds extracted from growing plants; putrescine, spermidine) and decontamination (e.g., bactericidal and/or fungicidal substances) has been regarded as an efficient post-harvest treatment method. Treatment strategies can be categorized as liquid (solution with dipping and spraying) and gas (vaporized materials) phases of the chemical agents according to the application methods, as shown in Table 4.
4.3. Biological Treatments
Fungal diseases in peach fruits are usually caused by latent infection via wounds during handling in fields, processing, and storage [144,145]. Antagonists are microorganisms that control pathogens by colonization on fruit surfaces or flesh exposed by the wound (i.e., competitive exclusion) and resource competition for nutrients [146]. Moreover, previous research regarding the inoculation of antagonists on peaches also showed an increase in the activities of antioxidant enzymes (e.g., APX, CAT, PAL, POD, and SOD) as an indirect effect of post-harvest treatments [147,148]. Since the antagonists used on fruits have been validated as safe for consumption, there is no concern about the residue, which is not the case for the chemical treatment technologies using toxic fungicides [149]. Major examples of antagonists reported as applicable for peach fruits are as follows: Pichia caribbica [147], B. subtilis [148], Cryptococcus laurentii [150], and Aureobasidium pullulans [151]. The function of antagonism can be diverse according to the determinant factors including region, cultivar, and environment (Table 5).
4.4. Combined Treatments
Many previous studies conducted to enhance the quality of peach fruits combined multiple post-harvest technologies categorized as physical (e.g., heating and irradiation), chemical (e.g., dipping in treatment solution and spraying or fumigating solution), and biological (e.g., co-culture of antagonistic organisms) treatments. The major aim of the combination of technologies is to achieve a greater effect than the sum of individual technologies (i.e., synergistic effect) and/or to complement the limitation of each technology with perspectives on the intervention mechanism of quality changes in peaches. The findings of studies on the development of the combination method of technologies and the establishment of optimal treatment conditions highlight novel effects that were unexpected based on the results of the application of individual technologies for peach fruit post-harvest treatments. Since the combination of technologies results in antagonistic effects (i.e., a lower effect than the sum of individual technologies or the inhibition of the intended effects by the interaction among the combined technologies), the exploration of adequate treatment concentrations is also needed to avoid inefficiency. Table 6 summarizes the findings of the previous research focused on combined treatment methods applied for the post-harvest quality and safety control of peach fruits.
5. Conclusions
This review provides comprehensive information based on the findings from studies regarding pre- and post-harvest treatment strategies optimized for peach fruits to extend the durable intake. Since peaches are vulnerable to environmental stresses under room temperature, most relevant studies aim to ensure fruit quality during long-term cold storage. Recent research has mainly focused on the development of new technologies and the design of novel combined treatment, whereas the in-depth study of pre- and post-harvest processes previously reported as applicable for stone fruits to optimize operational conditions for peaches should also be consistently conducted due to the diversity in the efficacies of treatment methods according to various determinant factors (e.g., a cultivar of fruits, processing environments, storage temperature, and time). Major implications from the analysis of the literature can be summarized as follows: (1) the discovery of side-effects from the overuse of treatment agents (chemical and biological technologies) or severe treatment conditions (physical technology) highlights the importance of the determination of the adequate criteria for the limitation of operational conditions; (2) since the result of the combined treatment is generally unexpectable (e.g., synergistic, additive, and antagonistic effects), the establishment of strategies which can harmonize both the efficacy and efficiency should be followed; and (3) pre-harvest treatment technologies generally aim to achieve sustentative effects allowing the improvement in the stability of fruit quality during the long-term cold storage, and thus the combined (sequential) treatment with subsequent post-harvest treatment is expected to enhance overall efficacies. This focused review suggests practical information for the design of advanced pre- and post-harvest treatments for peach fruits based on insights into advantages and disadvantages of currently reported technologies. As a future perspective on the research area in peaches, the quality control system based on the technologies in the Fourth Industrial Revolution era is expected to be integrated into pre- and post-harvest treatment strategies for peach fruit by sensing the fruit quality, strict pre-harvest quality control in smart farms, and web cloud-based precise quality management during the storage and/or distribution. The sensor-based analysis of the changes in the fruit quality factor can be a promising countermeasure for undesirable antagonistic effects derived from the combined treatment of pre- and post-harvest technologies described in this study.
This entry is adapted from the peer-reviewed paper 10.3390/horticulturae9030315
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