Antioxidants and Shelf-Life of ‘Changbang’ Peaches as Affected by Coating after Cooling

Abstract

The combined effects of sucrose monoester coating and precooling on the shelf-life and nutritional value of ‘Changbang’ (Prunus persica L.) peach fruits were investigated. Treatments included control and coated groups with and without precooling and were tested at 3 d intervals over 12 d of storage (12 DAS). Scanning electron microscopy images showed that the control fruits had a porous surface microstructure. Coated pre-cooled fruits showed low respiration rates and high firmness at 6 and 9 DAS, not observed in non-pre-cooled fruits, which retained moisture around the fruit surface after coating. Coating with precooling reduced fruit weight loss (27% at 12 DAS), while pre-cooled control fruit showed a higher weight loss (41%). Coating combined with precooling increased overall fruit taste and sourness, while reducing fruit decay. The concentrations of Ca, total flavonoids, anthocyanin, total polyphenols, and scavenging activity were increased in coated pre-cooled fruits, while coating without precooling had no effect on these levels in year 1 (2021) and only a slight effect in year 2 (2022).

1. Introduction

Peaches (Prunus persica L.), originating from China, are a popular stone fruit that is enjoyed fresh in summer in Korea and other countries around the world [1]. They are rich in antioxidants, minerals, and fiber, which can help lower the risk of various human health problems associated with free radical attacks and oxidative damage [2,3]. During ripening, peaches exhibit high sugar content and low acidity. Their taste texture can be classified into three groups: melting, non-melting, and stony-hard types with melting or no melting [4]. ‘Changbang’ peach fruits are referred to as stony-hard types and are a popular variety that is typically consumed between the end of July and early August. Through post-harvest handling procedures, farmers attempt to improve the storage shelf-life of these climacteric prunus fruits when kept in tightly closed containers under industrial conditions [2,4].

It was found that precooling peach fruit between 3 and 8 degrees Celsius extended storability by two or three days, and preserved fruit taste while preventing discoloration and the occurrence of physiological disorders, which was also commonly used for the climacteric summer fruit species [5,6,7]. Precooling treatment has been found to lower internal CO₂ concentrations in peach fruits by blocking ethylene biosynthesis and oxidative stress, and reducing aging [6,7]. However, ‘Changbang’ peach fruits kept in prolonged cold storage for more than 2–3 d, as is commonly practiced in commercial farmhouses, rapidly developed mealiness while becoming harder and susceptible to chilling injury [8]. This was associated with the internal browning and breakdown of the flesh, reducing the fruit’s marketability [8].

Peaches treated with 1-methylcyclopropene (1-MCP) and an edible coating maintained post-harvest parameters including flesh firmness, soluble solids, titratable acidity, antioxidants, and sensory parameters for marketing values 6 d after storage [9]. The edible coating produced a thin film on the fruit which reduced its permeability to oxygen, CO₂, and moisture, thus preserving fruit quality [9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27]. A sucrose mono-ester coating containing palmitate and stearate, a novel natural coating, created a more durable protective barrier on the surface, considerably extending the shelf life of early apple cultivars and plumcots [14,15,16]. Tiny particles form a uniform coating on the fruit surface which may be combined with precooling to minimize deterioration in fruit quality and improve fruit freshness during the tight storage, although they were not effective for other vegetables treated with the combined treatments reported for the unpublished papers.

In this study, an edible coating was applied, with or without precooling, to ‘Changbang’ peaches under commercial storage conditions. Any changes in the fruit morphological characteristics, fruit quality, mineral nutrients, and antioxidants, as well as any occurrences of physiological disorders, were observed to assess the suitability of the coating for use with precooling under commercial storage conditions.

2. Materials and Methods

2.1. Fruit Collection and Treatments

Approximately 300 ‘Changbang’ peaches were sampled on the day of harvest at a private orchard in Yeongcheon, Korea on 31 July 2021 and 2022 (Figure 1A,B). Fruits for the experiment were transported to a laboratory at The Catholic University of Daegu in Gyeongsan, Republic of Korea, near the peach orchard. Half of the fruits were soaked with tap water and left uncoated for use as a control, and the other half underwent a coating treatment of 2.0% edible additives. This was made up of a combination of 5% fatty acid monoesters of sucrose of Naturcover Extra Preservation^® (Decco Co., Valencia, Spain), for 5 min [15,16]. The coating consisted of 15.0% (w/v) ethanol, with 5.0% alpha-D-Glucopyranoside, beta-D-fructofuranans, combined palmitates and stearates, and 2.0% water [15,16].

Half of the control and coated fruits were immediately placed into cold storage at 5.0 ± 0.1 °C/80 ± 2% relative humidity (RH) and cooled for 48 h. The treatments consisted of four groups: a control without precooling, a control with precooling, a coating without precooling, and a coating with precooling. All fruits in each treatment were placed next to each other in a container at a temperature of 25.0 ± 0.1 °C/60 ± 2% of RH for 12 days to simulate fruit storage conditions as are typical in Korea. Also, 12 days was indicated for mimicking the exact time that ‘Changbang’ peaches were stored before reaching the retail market.

2.2. Fruit Quality

A 2 mm thick epicuticular peel 5 × 5 × 2 mm was collected from two of each treated fruit at 6 DAS to examine any substantial difference in the translucent tissue [15,16]. Scanning electron microscopy used an SU-3500 (Hitachi Co., Tokyo, Japan) at 10 kV and 50 and 100× zoom to determine the effects of coating and precooling on the fruit.

The fruit’s physiochemical characteristics, weight, firmness, total soluble solids (TSS), acidity, sensory quality, and levels of decay were measured at 3-day intervals over 12 days of storage (DAS) at room temperature, using 5 individual fruits per treatment.

Three separate fruit samples per treatment, of approximately 100 g to 110 g fresh weight (FW), between 5 and 6 cm in length, were placed into a sealed polyethylene film (3000 mL capacity), and an XE-2000 multi-function (XEAST Co., Shenzhen, China) digital CO₂ sensor employed over 60 min to detect CO₂ concentrations in the fruit samples [15,16].

The fresh weight of three separate fruit samples per treatment, with samples approximately 100 g to 110 g FW and between 5 and 6 cm in length, was determined using an electronic fruit and vegetable weighing balance scale (EB-430HU, Shimadzu Co., Ltd., Tokyo, Japan). Fruit weight loss over the 12-day storage period was estimated by determining the percentage difference in fruit weight at 0 DAS and the weight observed at 12 DAS and dividing the result by the weight at 0 DAS.

The fruit’s flesh firmness was obtained from the averaged values of three different scales reading from the middle point of each fruit after removing a thin layer of peel with a hand-held destructive flesh penetrometer with a diameter of 8.0 mm and an FR-5105 cylindrical tip (Lutron electronic enterprise Co., Taipei, Taiwan). The freshly squeezed juice was obtained from a cotton cloth, and total soluble solids (TSS) were measured with a GMK-706R (G-WON Hitech Co., Seoul, Republic of Korea) digital Brix-acidity meter. The fruit juice was then diluted 100 times to detect acidity and the TSS/acid ratio was obtained by dividing values of TSS and acidity.

A quality rating test of each fruit employed a four-person panel of judges trained on a five-score hedonic scale with values from one (worst) to five (best), that assessed sweet and sour taste and fruit acceptability [24].

Incidences of fruit decay during storage were evaluated by visual observation on a scale ranging from 0 to 5, where 0 indicated no symptoms of decay, with 1 (1% ≤ surface ≤ 20%), 2 (20% ≤ surface ≤ 40%), 3 (40% ≤ surface ≤ 60%), 4 (60% ≤ surface ≤ 80%), and 5 (80% ≤ surface ≤ 100%) [28].

Changes in fruit surface color were determined by sampling three different regions randomly distributed across the equatorial point on the fruit at 0 DAS and the same regions from 3 to 12 DAS with a portable colorimeter with an 8.0 mm aperture (Lutron Electronic Enterprise Co., Ltd., Taipei, Taiwan) after the calibration. The instrumental color parameters L* from the peel of each fruit represented lightness with values ranging from 0 (black) to 100 (white). Positive a* and b* denoted red- and yellowness values, respectively.

2.3. Mineral Nutrients and Antioxidants

The fruits in each treatment were washed with tap water, dried for 7 days at 68 °C, and milled using a WDL-1 (Wonder Blender Co., Tokyo, Japan) to produce macro-nutrient concentrations [29]. An amount of 2 g of dried fruit sample was then liquefied in 2 mL of HCl and extracted for analysis of inorganic phosphorous (P) using a UV-1601 spectrometer (Shimadzu Co., Kyoto, Japan) using the Vanadate method. Extracts from the fruit were analyzed using inductively coupled plasma (ICP Integra XL; GBC Inc., Arlington Heights, Fort Worth, TX, USA) to detect calcium (Ca) content [29].

The final extracts were analyzed using a colorimetric assay for total flavonoid concentration [30]. Next, 5 g of the fresh fruit were suspended in 100 mL of aqueous ethanol (ethanol:distilled water, 80:20 v/v), and immediately centrifuged at 3000 rpm for 20 min. The 1 mL of supernatant was poured into a 10 mL plastic tube containing 0.4 mL of 5% NaNO₂ and 4 mL of distilled water for 5 min, mixed with 2 mL of 4% NaOH and 0.4 mL of 10% AlCl₃ for 6 minutes, and made up to 10 mL with distilled water. The extract absorbance was analyzed colorimetrically using a reagent blank at 510 nm with a UV-1800 spectrophotometer (Shimadzu Scientific Co., Kyoto, Japan) to determine total flavonoid concentration.

Anthocyanin concentration in the fruit was determined using the pH differential method of Lee et al. [31]. An amount of 1.0 g of fruit tissue was digested with 10 mL of 80% methanol and 0.1% HCl at 150 rpm for 2 h at room temperature and was centrifuged at 3000 rpm for 20 min. Then, 3.0 mL of extract from the fruit’s upper layer was diluted with 5 mL of two kinds of buffer solution, one of 0.025 M KCl at pH 1.0 and one of 0.4 M sodium acetate at pH 4.5, for 30 min incubations. Anthocyanin in the final extract was identified at 510 nm with a UV-1800 spectrophotometer (Shimadzu Scientific Co., Kyoto, Japan).

In year 2, fruit vitamin C content was determined using 1.0 g of dried fruit digested in 50.0 mL of 5.0% meta-phosphoric acid, centrifuged at 3000 rpm for 10 min, dissolved in 100.0 mL distilled water, and measured using high-performance liquid chromatography (Waters Alliance 2695, Waters Co., Ltd., Manchester, UK) according to previous studies [15,16]. Next, 2.0 mL of extract from 1.0 g of tissue sample in 99.9% ethanol was combined with 2.0 mL of Folin–Dennis’ reagent for 3 min to determine total polyphenolic content. This was then shaken with 2.0 mL of 10.0% Na₂CO₃ for 1 h and analyzed with an Optizen 3220UV spectrophotometer (Mecasys Co., Daejeon, Republic of Korea) at 700 nm. To simulate scavenging activity (RSA), 2.0 mL of the extract was also fixed with 1.0 mL of 0.2 mM DPPH for 30 min at 37 °C and analyzed with an Optizen 3220 UV spectrophotometer (Mecasys Co., Daejeon, Republic of Korea) at 517 nm.

2.4. Data Analysis

Statistical analyses were performed using a SAS statistical package (SAS, Inc., Cary, NC, USA). The treatment effect means were determined using both one-way and two-way analyses of variance (ANOVA) and Duncan’s multiple-range test at a level of p < 0.05 on all main effect means. The vertical bars over storage time were considered as a standard error for the average of the replicates. There were two main variables, cooling and coating, affecting fruit quality.

3. Results and Discussion

3.1. Fruit SEM Analysis and CO₂

The SEM images showed a porous microstructure on the control fruit surface at 6 d after harvest (DAS) (Figure 2A,C). The surface color of the control fruit was darker and less glossy than those of coated peach fruits without precooling (Figure 2B,D), as has been previously observed under SEM in banana, apple, pear, and plumcot fruits [11,16,22]. The coating substrate evenly adhered to fruit surfaces and filled the pores and cracks on the external tissues with colloidal suspensions of oils spread across the water [16,21,23]. On the other hand, a previous study on plumcot fruits found that the coating of the small fragments of sucrose fatty acid esters did not completely adhere to the plumcot’s fruit surface. This was attributed to uneven coating due to factors such as the hairy skin of the fruit and substantial physical changes in the fruit surface from the approximately 20% weight loss that occurred during the 10 d storage period [16].

Maximum fruit respiration occurred at 9 DAS and declined quickly at 12 DAS, showing distinctive climacteric behavior typical of the species prunus after harvest (Figure 3A,B). This climacteric stage is referred to as the ripening process, with the promotion of internal ethylene and increased respiration both stimulated by the increase in air temperature similar to that described in various studies with pre-cooled prunus fruits [5,6,7,32,33]. Fruit respiration was elevated by coating without cooling at 6 DAS but did not significantly differ between the treated fruits for the rest of the time in storage. Unfavorable storage conditions caused by keeping the hairy fruits together may have accelerated a humid microclimate and adversely affected the gas barrier of the coating on the climacteric fruits [21]. In contrast to the samples that did not undergo precooling, the coating treatment was effective at suppressing respiration in the pre-cooled fruits during storage at 0, DAS, 6 DAS, 9 DAS, and 12 DAS (p < 0.05). This was attributed to reduced humidity around the fruits in cold storage after harvest. However, the treatment had no effect on fruit respiration during the whole storage period (p = 0.998).

3.2. Fruit Quality

Flesh softening is the critical variable amongst many that impacts consumers’ evaluation of the freshness of stone fruits [10,25,34]. Coating treatment decreased fruit firmness without precooling at 6 DAS but retained strong fruit firmness at 6–9 DAS with precooling (Figure 4A,B), likely corresponding to respiration levels between the treatment fruits. The sucrose ester coating suppressed oxygen concentrations that produced cell wall degrading enzymes in prunus [16,25,34], apples, bananas, and pear fruits [11,12,13,14,23]. The treatment appeared to have no effect on the fruit’s firmness during the whole storage period based on the two-way analyses. Fruit weight loss during the storage was reduced by combining coating with precooling and was recorded as 27% at 12 DAS, with 41% weight loss observed for the control fruit without precooling (Figure 4C,D). This was mostly attributed to reduced water evaporation and internal respiration from the degradation of carbohydrates in the cell walls [18]. However, no effects were observed on the fruit weight loss during the whole storage.

Flesh TSS did not significantly differ between treatments without precooling but was higher in the control fruits at 3 DAS and 9 DAS compared to those of the coated fruits (Figure 5A,B). Acidity was higher at 9 DAS in control fruits but higher at 3 DAS and 6 DAS in the coated fruits (Figure 5C,D), with a high ratio of TSS/acidity observed in the pre-cooled fruits (Figure 5E,F). Coating combined with precooling would have retarded the consumption of organic acids, substrates, respiration, and ripening, maintaining the high acidity observed in the fruits, which was in agreement with results for other coated stone fruits [10,16,25,34]. Based on the two-way analyses, the main effect of coating was on the TSS and TSS/acidity over the storage period (p = 0.690).

Fruit sweetness as assessed by sensory evaluation was not consistently affected by coating with or without precooling (Figure 6A,B). Flesh sourness was higher at 9 DAS in the control fruits without precooling and at 6 DAS and 9 DAS in the coated fruits with precooling (Figure 6C,D), as was similarly observed for fruit acidity (Figure 5C,D). The coating did not affect overall fruit quality in samples without precooling but improved quality in samples with precooling later in the storage period (Figure 6E,F). Precooling significantly maintained lower sweetness, higher sourness, and overall fruit quality over the majority of the storage time (p < 0.05). The application of an edible coating to peaches contributed to modified atmospheric conditions between the stone fruits in tight storage typical of a commercial packhouse. However, the coating treatment did not prevent post-harvest deterioration of fruit flesh in the absence of precooling and rapidly increased fruit decay at 6 DAS precooling (Figure 6G). This was not consistent with other reports that the Chitosan coating had induced the defense enzyme, chitinase, and slowed decay in fruits by suppressing the germination and growth of fungal spore cells [10,35], as reported for other edible coatings of stone fruits [25]. The high respiration and adverse storage conditions could have increased post-harvest deterioration and slowed the oxidation process of the fruits [10,35]. Precooling significantly reduced fruit decay at 6–12 DAS (p < 0.001), in particular for samples with the coating treatment (Figure 6H), as the process can rapidly reduce the temperature of the fresh produce [33]. Based on two-way analyses, coating was observed to affect sourness and rate of fruit decay, over the storage period.

Levels of L* and a* in the fruit surface declined in most fruits over the storage time, with little change observed on b* (Figure 7A–F). There was little color change reported for coated stone fruits, attributed to reduced oxygen concentrations in the fruits consequently decreasing enzyme formation for pigmentation, such as anthocyanin and carotenoids [19,25,26,27]. The coating treatment mostly did not affect fruit color during ripening, with or without precooling, due to dilution effects from the adverse storage conditions coupled with the short storage period conducted in this study. However, coating was only observed to have an effect on L* over the storage period based on the two-way analyses.

3.3. Mineral Nutrients and Antioxidants

The total concentration of p, a prime component of phospholipids in the cell membrane, did not significantly differ between the treatment samples with or without precooling (Figure 8A,B). The permeability of cell membranes in the peel increased in the coated fruits due to the interruption of fatty acids and leakage of electrolytes such as phosphate, calcium, and boron [17,36,37,38]. Concentrations of Ca, responsible for the strength of cell membranes and stability of cell walls, were found to be similar in fruits that did not undergo precooling (Figure 8C) but higher in the coated fruits that had undergone precooling (Figure 8D).

Levels of flavonoid and anthocyanin, important antioxidants in peach fruits [33,39], were lower in the coated fruits that had undergone precooling than in the control fruits (Figure 9A,C), presumably due to the increased oxidation associated with elevated respiration [10,28,30]. Coating coupled with precooling was more effective at inducing a higher concentration of total flavonoids and anthocyanin (Figure 9B,D). This has been attributed to reduced oxygen in the fruits being available for enzyme activity, preventing oxidation in the total flavonoids and anthocyanin [10,28,30]. Increased total polyphenols and RSA but vitamin C at 7 DAS was not observed in the coated fruits without precooling but was shown in the coated fruits with precooling at 7 DAS in year 2 (Figure 10A–F). Fruit coating treatment was likely to not only facilitate movements of nutraceuticals, such as calcium, antioxidants, and other functional substances but also maintain levels of nutraceuticals in the fruit although the effects were different for the types of coating substrates and the concentrations of the components [40,41]. In particular, the edible coatings, adding Ca components, and chitosan-based substrates, were mostly effective in increasing storability and Ca concentrations in fresh and frozen berry fruits. Main effects and interaction effects were not mostly observed to have an effect on the concentrations of antioxidants for the storage period based on the two-way analyses.

4. Conclusions

The combination of precooling and coating had a synergistic effect on fruit quality, enhancing antioxidant defense systems and cell integrity. This resulted in reduced heat stress from room temperature storage, improving the commercial marketability of the fruits. Further research is needed into the effect of combining coating with precooling, and into the physiological underpinnings of oxidative stress that occurs during storage at various stages of fruit maturation. This research could help to develop new strategies for improving the post-harvest quality of stone fruits.