Physiochemical Changes of European Pear cv. Conference and Asian Pear cv. Yali during Cold Storage

Abstract

This study evaluated the physiochemical changes of one commercial European pear variety ‘Conference’ and one Asian pear variety ‘Yali’ during 60 days of storage at 1 °C. Content of organic acids, ethylene and formation of CO₂ were determined using HPLC/UV-VIS and GC/FID and TCD detectors, respectively. This study presents an evaluation of the influence of the above-mentioned factors and long-term storage on quality parameters, including the antioxidant capacity of an important European and Asian pear cultivar. There was a significant effect of temperature on respiration rate in both varieties. Development of the respiration intensity had a similar course in European and Asian pears. The high values of CO₂ production at the beginning of storage for the variety ‘Conference’ (14.08 mg·kg⁻¹·h⁻¹) and ‘Yali’ (23.37 mg·kg⁻¹·h⁻¹) were followed by a sharp decline of 80% and 83% at later stages of storage at 1 °C, respectively. Ethylene formation in fruit increased with storage time and was the highest after 60 days in cold storage, especially in ‘Yali’ (7.14 µL·kg⁻¹·h⁻¹). The results show the relation between ethylene formation and ripening-related fruit parameters. The ‘Yali’ variety showed negligible changes in soluble solids content and flesh firmness during storage ranging from 0.35 to 0.60 MPa. The most represented sugar determined enzymatically was fructose and malic acid was the dominant organic acid in pear fruit. Antioxidant activity determined using the FRAP and DPPH methods did not change significantly during 60 days of storage.

1. Introduction

Pear (Pyrus spp., Rosaceae) is one of the most cultivated worldwide fruit [1] with significant economic and health values. There are two kinds of cultivated pears: Asian pears and European pears (Pyrus communis L.). European pears are mainly cultivated in Europe and America. They require exposure to ethylene [2] and/or a period of chilling for ripening [3]. The main cultivated varieties of Asian pears are derived from the botanical species Pyrus pyrifolia Nakai, Pyrus ussuriensis Maxim., Pyrus bretschneidei Rehd. and Pyrus sinkangensis Yu [2].

Asian pears remain firm but are crispy and juicy at the time of consumer maturity [4], while European varieties are characterized by a buttery and juicy properties with full flavor, yellowing of the skin and an adequate sugar–acid ratio [5].

Pyrus communis L. cv. ‘Conference’ is among the most cultivated and stored pear cultivars in Europe and is highly appreciated by consumers due to its desirable eating quality [6]. If storability is to be extended, it has to be cooled to −1 °C for 21 days immediately after harvest, then a controlled atmosphere is used [7,8,9,10]. The regular atmosphere (RA) storage at −1 to 0 °C allows high quality to be maintained for up to 4 months without significant fruit quality losses [11,12]. ‘Conference’, like most European pear varieties, require a period of exposure to cold temperature after harvest to initiate the autocatalytic ethylene formation in order to ripen [13,14]. Conditioned pears, after rewarming, can soften and reach a satisfactory aroma via ethylene biosynthesis at ambient temperature [15,16,17].

In the ‘Conference’ variety, it is possible to induce the ripening of freshly harvested fruit in air with the addition of exogenous ethylene [18]. Late-harvested pears have the capacity to ripen much earlier than fruit harvested in their immature state, for which a short period of cold or treatment with exogenous ethylene is necessary [3]. Ethylene and temperature conditioning have been successfully used to stimulate fruit ripening compared to unconditioned fruit [19]. The ‘Conference’ pear is a climacteric fruit and therefore perishable. The climacteric phase is characterized by a peak in ethylene production, which is accompanied by a peak in fruit respiration [20]. In the investigations, ‘Conference’ pear was very susceptible to low O₂ and/or high CO₂ partial pressures during a 6-month controlled atmosphere (CA) storage period [6,8].

The ‘Yali’ pear (Pyrus bretschneideri Rehd. cv. Yali) is a famous Chinese pear cultivar with a high yield and excellent quality with delicious and juicy flesh. The fruit is very sensitive to a rapid decrease in temperature [21]. Rapid cooling treatment is not recommended for Asian pears, as no advantages have been shown due to the higher loss of firmness and soluble solids, and usually browning of the core and flesh (brown spots have formed in the pulp) has been caused during storage [21,22,23]. The quality of the ‘Yali’ pear can be maintained for 6–8 months if stored under conditions where the CO₂ level does not increase at a temperature of 0 °C with the current very low O₂ content [24,25].

Instead of rapidly cooling the fruit from field temperature to storage temperature (0 °C) in 24 h, the slow cooling treatment is preferred. In current practice, the initial storage temperature is set at 12–15 °C, and then takes 30–45 d to gradually reduce to 0 °C [21,22,26]. Results according to Liang et al. [27] and Li et al. [22] showed that the slow cooling treatment reduces the respiration intensity and ethylene release in pears. Itai et al. [28] classified the Chinese pear cultivar ‘Yali’ in the group “high” based on ethylene formation in ripening fruits.

The storability of pear as a climacteric fruit species [29] depends on fertilization and health, the optimal harvest date, the fruit cooling rate after harvest and storage conditions [30].

Storage temperature substantially affects the rate of respiration-related reactions [31]. Studies have shown that the quality of the new century pear is prolonged by the low-temperature storage [32].

Pears preferentially use organic acids as a substrate for respiration during storage and after their reduction they use sugars. Since respiration is strongly dependent on the composition of the atmosphere and temperature during storage, the respiration rate of fruit can be used as an indicator of storage potential [33].

Pears have moderate antioxidant activity. Despite this, they represent an important source of health-promoting substances due to their high consumption in Europe [1]. Pears have anti-proliferative and anti-inflammatory properties and owe their antioxidant potential to bioactive compounds such as carotenoids, chlorophylls, polyphenols and triterpenoids [34]. Pear, being a climacteric fruit, undergoes changes in antioxidant properties during storage [35] and great variability exists among different pear cultivars [36].

Low temperature storage can significantly inhibit the respiration rate, organic acid metabolism and carbohydrate degradation of fruits after harvest [37]. Konopacka et al. [38] suggested that each pear variety needs an individual strategy during ripening and storage to obtain the best sensory attributes. According to Torregrosa et al. [16], physicochemical parameters of pears such as ethylene formation, flesh firmness, soluble solids content and total titratable acidity are important parameters influencing consumer preferences. Low-temperature storage technology is not only the simplest and most efficient method, but also the most widely used method in fruit commercialization. The analysis according to Łysiak et al. [30], revealed that it is more economically advantageous to sell pears directly after harvest than after six months of storage. The molecular mechanism underlying this process was described in a study by Yi et al. [32]. Currently, the demand for ready-to-eat fruit is increasing. It is therefore important for retailers to understand how pear quality develops during shelf life (SL) [16].

The aim of this study was to compare the postharvest ripening behavior of the Asian variety of pears ‘Yali’ and the European variety ‘Conference’, which were grown in the conditions of the Czech Republic. Emphasis was placed on monitoring the influence of cold storage conditions and shelf life on qualitative parameters in terms of flesh firmness, color, antioxidant capacity, content of saccharides and organic acids, but also, and especially on the physiological processes (ethylene formation and respiration rate) explaining these quality changes.

2. Materials and Methods

2.1. Plant Materials and Experimental Details

The fruits of the Asian pear cultivar ‘Yali’ and the European pear cultivar ‘Conference’ were produced by 6-year-old trees grown in the orchards of Mendel University located in Lednice, Czech Republic (48.79° N/16.80° E). Twenty-five kilograms of fruits were harvested at the same stage of ripeness, which physiologically corresponded to just a few days before the onset of the climacteric development stage. Fruits from ten trees from each variety were harvested all at once, and fruits were randomly selected for the experiment.

Fruits with uniform color and size and free of visible injury were selected visually for investigation. Each variety was harvested at one stage of maturity determined by a starch test and flesh firmness. The pears were harvested manually and transported within two hours to the technological laboratory of the Institute of Postharvest Technology of Mendel University in Brno, Lednice, Czech Republic for immediate analysis. After harvesting and sorting, the fruit was put in the cold chamber for 48 h to stabilize the fruit temperature. The fruit was placed in two experimental gas-tight chambers with a capacity of 1 m³ each. Within 4 h, the fruits were stored at 1 °C and 80–85% relative humidity (RH) at normal atmosphere.

Fruits were evaluated immediately after harvest, after 15, 30, 45 and 60 days of storage at 1 °C, followed by shelf-life storage at 20 °C and normal oxygen atmosphere for 7 days. Ethylene formation and respiration rate was subsequently measured on the fruits, followed by flesh firmness. The juice for subsequent analysis of soluble solids, titratable acidity, antioxidant activity, saccharides and organic acids content was created by homogenizing 5 whole fruits in three repetitions (n = 3), which were always obtained from each batch.

2.2. Measurements of Physiological Indexes

Changes in the color of pear fruits were monitored in 15 fruits always twice on one fruit by measuring transmittance using a Lovibond RT850i. The resulting color of the samples was expressed by a set of L* a* b* parameters (CIELAB). The samples were measured in plastic cuvettes with an optical path length of 10 mm. The OnColor™ Premium software application (Lovibond, Amesbury, UK) was used for the evaluation.

Flesh firmness was determined using a Turoni (Italy) digital firmness tester using 11 mm diameter punches to a depth of 8 mm. Each fruit was analyzed from 2 opposite sides. To eliminate firmness, the skin was always removed before analysis. Results were expressed in MPa. Each sample was represented by 15 fruits in three repetitions.

A homogenate of 15 fruits for each variant was used to determine soluble solids (SSC) and titratable acids (TA). SSC was determined using a digital refractometer PAL-1 (Atago Co., Ltd., Tokyo, Japan), and results were expressed in °Bx.

Titratable acids were determined by potentiometric titration of 10 g of juice with 0.1 mol.L⁻¹ NaOH to pH 8.1. Results are expressed as % malic acid.

2.3. Saccharides Content

The contents of selected sugars (sucrose, glucose, fructose) were determined using the commercial kit K-SUFRG (Megazyme, Wicklow, Ireland) at the beginning and at the end of storage (60 days). The preparation of the reaction solutions was according to the instructions provided by the manufacturer. Absorbances were measured on a SPECORD^® 50 PLUS spectrophotometer (Analytik Jena AG, Jena, Germany) at 340 nm in a 10 mm cuvette. The results of individual sugars are expressed in grams per 100 g of fresh weight.

2.4. Ethylene Formation and Respiration Rate

Measurements of the ethylene formation rate and respiration rate were carried out at harvest and every 15 days of storage at 1 °C followed by shelf life storage at 20 °C. This measurement was repeated three times for each storage interval and treatment.

Ethylene formation and respiratory rate (respiration) were measured in intact fruits using a static system. Both gaseous components were analyzed by a gas chromatography using the HP/GC/FID/TCD working technique from one injection of the gaseous sample into two different capillary columns connected in the thermostat of the gas chromatograph in parallel. On each sampling date, 5 fruits were weighed and placed in a respiratory flask of 1 L volume. The flask was hermetically sealed and stored at 1 °C for 1 h before measurement in the fruits that were in cold storage and at 20 °C for the fruits from shelf life. C₂H₄ and CO₂ were monitored by injecting 1 mL of headspace gas into an Agilent 4890D (Agilent Technologies, Inc., Wilmington, NC, USA). Both gases were determined simultaneously on a dual column. The first column was HP-Plot/Q 30 m, I.D. 0.53 mm; 40 mm film was used for ethylene and this was detected by FID. The second column was an HP-AL/KCL column 30 m, I.C. 0.53 mm, a 15 µm film and CO₂ were detected on the TCD. The rate of ethylene formation was expressed in µL per kilogram per hour; the rate of CO₂ formation was expressed in mg per kilogram per hour.

The temperature coefficient (Q_T) was calculated considering the impact of temperature (T_i) on respiration rate (R_i) for the specific range of temperature [39].

$$Q_T={\left(\frac{R_2}{R_1}\right)}^{\left(\frac{10}{{T_2-T}_1}\right)}$$

(1)

2.5. Antioxidant Capacity

Antioxidant capacity was determined by the FRAP and DPPH methods [40,41,42]. The “FRAP” reagent (20 mmol/L FeCl₃.6H₂O, 10 mmol/L 2,4,6-tris(2-pyridyl)-s-triazine in 40 mmol/L HCl and acetate buffer pH 3.6 in a 1:1:10 ratio) was used for the FRAP determination. A total of 25 µL of the diluted sample was added to 2 mL of the “FRAP” reagent and the mixture was stirred intensively for 25 s. The solution obtained was measured in a plastic cuvette (10 mm) in a SPECORD^® 50 PLUS spectrophotometer (Analytik Jena AG) at 593 nm. The solution (0.1 mmol/L) of 2,2-diphenyl-1-picrylhydrazyl in methanol was used for the DPPH method. To 1.9 mL of the “DPPH” reagent, 100 µL of the diluted sample was added and the mixture was intensively stirred for 25 s. After 30 min, the absorbance in a plastic cuvette (10 mm) was determined on a SPECORD^® 50 PLUS spectrophotometer (Analytik Jena AG) at 515 nm. For both methods, the results were expressed in mmol Trolox equivalent (TEA) per kilogram of fruits, converted to fresh weight. Each sample was repeated three times.

2.6. Determination of Organic Acids by HPLC

Organic acids (malic, citric, L-ascorbic) were assessed by HPLC. Assessment conditions: Column: Prevail 5 μm Organic Acid 110 A HPLC Column 250 × 4.6 mm, flow rate of mobile phase 25 mM KH₂PO₄ 1 mL.min⁻¹, temperature +30 °C. A UV-VIS detector was used to detect organic acids and the analysis was performed at a wavelength of 210 nm. Contents of organic acids were expressed in mg·kg⁻¹ of fresh pears.

2.7. Statistical Analysis

The results are presented as the mean of three replications. All statistical analyses were performed using the SAS statistical software package, version 12 (SAS Institute Inc., Cary, NC, USA). All data are represented in terms of mean ± standard error. Means were compared by analysis of variance (ANOVA). When the analysis was statistically significant, the Tukey’s HSD test at p ≤ 0.05 was performed for separation of means.

3. Results and Discussion

3.1. CO₂ and Ethylene Formation at Refrigeration Temperature as a Function of Storage Temperature Change

The release of gaseous CO₂ from the intact fruit is a basic physiological value of a fleshy fruit, expressing the level of ongoing metabolic changes. Respiration rate is an important physiological parameter indicating ripening and senescence of fruit during storage [43]. At the beginning of storage, CO₂ formation was high (14.1 mg·kg⁻¹·h⁻¹ for ‘Conference’ and 23.3 mg kg⁻¹·h⁻¹ for ‘Yali’) (Figure 1). It was only after cooling at 1 °C (day 15 of storage) that the two values aligned (2.7 mg·kg⁻¹·h⁻¹ for ‘Conference’ and 3.0 mg·kg⁻¹·h⁻¹ for ‘Yali’) and their CO₂ formation values did not exceed 5.0 mg·kg⁻¹·h⁻¹ by day 60 of storage, while transferring the fruit to 20 °C meant an increase in CO₂ formation, which indicates a higher metabolic change most often according to the Q₁₀ criterion, expressing this change related to a temperature difference of 10 °C. The results according to Brandes and Zude-Sasse [44] showed a major influence of storage time on the temperature coefficient, which has been proven more relevant for shelf-life quality. The shorter the storage time, the higher the value of the temperature coefficient, usually in the range of 2 to 3 for a temperature increase of 10 °C [44]. Lee et al. [45] found that the respiration rate of Asian pears varies with storage temperature. Respiration rate increased during shelf-life and decreased at the end of storage [44]. The value of CO₂ formation at two temperatures differentiated by 19 °C, then Q₁₀ (the value is dimensionless) was always higher for the ‘Conference’ variety than for the ‘Yali’ variety with the highest value for the 30th day of storage, from which point the Q₁₀ value tended to decrease, but always less for ‘Yali’ than for ‘Conference’. For ‘Conference’, Q₁₀ was as follows on days 15, 30, 45 and 60: 3.33; 3.25; 2.15; and 2.68. For ‘Yali’, Q₁₀ was as follows: 2.17; 2.40; 1.51; and 1.76. According to Łysiak et al. [30], fruit mass loss results mainly from the respiration rate. The rate of respiration in the present study shows a distinctive respiratory climacteric pattern in both varieties of pears, where an initial high CO₂ evolution rate during shelf life was followed by a sharp decline at the later stages of storage at 20 °C. This can be attributed to the full use of the substrate as an energy source during ripening [46].

Ethylene plays a key role in the ripening of climacteric fruits [47]. Ethylene is known as the main factor regulating fruit ripening and is an important physiological indicator during fruit storage. European pears such as the ‘Conference’ variety have a noticeable ethylene formation that is much lower than that of apple varieties [48], but postharvest ripening can be influenced by exogenous ethylene. ‘Yali’ pears have unproven formation values. Current measurements demonstrate the capability of this species, i.e., P. bretschneideri Rehd, to produce significant amounts of ethylene that are comparable to European species. At the beginning of the climacteric phase, the two varieties had comparable threshold concentrations, which were 0.07 μL·kg⁻¹·h⁻¹ for ‘Conference’ and 0.32 μL·kg⁻¹·h⁻¹ for ‘Yali’ (Figure 2). During continued storage at 1 °C, formation increased to values that were no longer very variable over time. For ‘Conference’ the values from day 15 to day 60 were as follows: 0.64; 3.53; 2.66; 3.17 and for ‘Yali’ in the same sequence of time: 3.60; 4.96; 7.84; and 7.15. A statistically significant increase in ethylene formation occurred after storage at 20 °C for both pear varieties. For the Asian variety ‘Yali’, the increase in ethylene formation after seven days of shelf life was twice as high as for the ‘Conference’ variety.

The increase in metabolism through ethylene formation had essentially a range of Q₁₀ values not too dissimilar to CO₂ formation. Increasing the temperature from 1 °C to 20 °C increased Q₁₀, which decreased with storage time, but the values remained in the usual range of Q₁₀, i.e., 2 to 3. For ‘Conference’, Q₁₀ figures were, on day 15, 30, 45 and 60 of storage, in the sequence as follows: 5.52; 2.56; 2.87; 3.17; for ‘Yali’ in the same sequence of time, the figures were 4.15; 3.71; 2.41; and 3.03. Ethylene formation provides the possibility of reducing ethylene in post-harvest technology. Ethylene formation increased in both control and treated groups of “Zaosu” pears [49]. In the study according Yi et al. [32], the ethylene content of pear fruits varied during the low-temperature storage and increased with the prolonged storage time. The results of CO₂ formation provide a very good match with ethylene formation during storage at 1 °C, and also during shelf life at 20 °C. The ethylene formation results are similar to those previously reported for Spanish ‘Conference’ pears [50]. The formation of ethylene affects important fruit quality traits such as fruit softening [51].

Firmness is a main quality indicator of the pear flesh [52] and is commercially used to predict the optimal harvest date of pear [53]. Penetrometric firmness of the fruit during storage is a telling value for eating acceptability. During refrigeration storage, two different penetrometric firmness waveforms were evident. For ‘Conference’, the change curves only slightly decreased, with a value of 0.74 MPa at the beginning of storage and 0.66 MPa at day 60, indicating that no significant changes in fruit firmness were expected at the optimum storage temperature, even though there were already stimulating concentrations of ethylene in the fruit to promote fruit softening (Figure 3). It can be assumed that, after an extended storage period at this temperature, the firmness of the fruit would be much higher than 0.22 MPa, a value at which the fruit is soft and suitable for direct consumption. According to Torregrosa et al. [16] consumers most appreciated pears of the ‘Conference’ variety, which showed firmness values in the range of 10–30 N. Softening of the fruit after 7 days at 20 °C is typical of the European pear (‘Conference’). This decrease in firmness in MPa was calculated as the ratio of the firmness achieved after 7 days at shelf-life conditions (20 °C) to the firmness before removal, which in the time sequence represents the firmness coefficient of k_f(k_f = k_1°C/k_20°C): 0.19 (day 15), 0.16 (day 30), 0.39 (day 45) and 0.28 (day 60). At this stage of ripening, the fruit had a firmness of 0.12 to 0.23 MPa, which corresponds to consumption ripeness. A different pattern of firmness was observed for the ‘Yali’ fruit, which did not soften within 7 days when the fruit was warmed to 20 °C, but its firmness was identical to the values before the fruit was exposed to shelf life (Figure 3). For the evaluation dates (day 15, day 30, day 45, day 60), the values of k_f for ‘Yali’ were 1.04; 0.77; 1.38; and 1.09. According to this criterion, ‘Yali’ remained equally firm even when the temperature changed. Ethylene formation in the same time course at the end of the shelf life had a high Q₁₀, so the fruit produced high concentrations of ethylene. The analysis of the relationship between fruit softening and ethylene formation shows that ‘Yali’ was different from the European variety ‘Conference’. The firmness of ‘Yali’ pears decreased very slowly during storage and a similar trend was reported in a previous work by Chen et al. [54]. The flesh firmness of ‘Yali’ pears was maintained to a certain extent during storage, which is not common in other fruit species, and there was no damage during storage, as the flesh of Asian pears is sufficiently firm [55].

3.2. Soluble Solids and Titratable Acidity during Storage at 1 °C and at 20 °C

In refrigeration storage, the soluble solids of the ‘Conference’ variety remained constant and were not affected by higher temperatures during shelf life. At day 15 of storage, there was a decrease in soluble solids at 1 °C that was statistically significant from the subsequent evaluation at 20 °C and was likely due to sampling. ‘Conference’ had higher soluble solids, at 13 °Bx, compared with ‘Yali’ with an average value of soluble solids of 9.5 °Bx, which was the same for the whole storage period. Similar results were obtained in previous studies on European pear cultivars such as ‘Blanquilla’ and ‘Spadona’ [12,56]. Yali is typically harvested at minimum soluble solids concentration of 10–12 °Bx [54,57]. Degradation of polysaccharides to monosaccharides usually increases the SSC value during the initial storage period. The SSC value may decrease during further storage as the fruit uses stored energy for respiration [30]. Our results are similar to those of Arzani et al. [55], where Asian pears maintained flesh quality and soluble solids did not change significantly during storage for five months.

The titratable acidity of both varieties was low, ranging from 0.13% to 0.16% (‘Conference’). For ‘Conference’, the decrease in the warming phase at 20 °C was extremely low and statistically insignificant. ‘Yali’ had a higher organic acid content than ‘Conference’, ranging from 0.18% to 0.24%. The loss of organic acids was statistically significant on days 15 and 30 of storage. From the experiments performed by Łysiak et al. [30], TA decreased during storage and reached about 1/3 of the initial value in RA-stored fruit. The same values of soluble solids and titratable acids were determined for the ‘Conference’ variety in the study according to Kolniak-Ostek et al. [34].

3.3. Comparison of the Physicochemical Parameters of Asian and European Pear Fruit Varieties at the Beginning and End of Storage

Selected physicochemical parameters were compared between Asian ‘Yali’ and European ‘Conference’ pears at the beginning of storage and after 60 days of storage in a normal atmosphere at 1 °C. The measured values of the CIE LAB color parameters and the statistically significant differences found at the 0.05 level of significance are documented in

Table 1. Fruit substance parameters before and after 60 days storage in normal atmosphere at 1 °C.

Variable	‘Conference’	‘Yali’	‘Conference’	‘Yali’
	Before Storage		After Storage
Chromaticity L*	55.08 ± 1.63 a	61.47 ± 1.38 bc	56.59 ± 2.04 ab	62.82 ± 0.53 c
Chromaticity a*	−0.31 ± 2.16 bc	−7.71 ± 0.24 a	2.43 ± 2.65 c	−6.75 ± 0.15 ab
Chromaticity b*	31.54 ± 1.91 a	41.37 ± 1.46 b	34.00 ± 2.43 a	42.96 ± 0.14 b
DPPH (mmol Trolox/kg FW)	0.416 ± 0.007 ab	0.443 ± 0.012 b	0.377 ± 0.010 a	0.429 ± 0.007 b
FRAP (mmol Trolox/kg FW)	0.439 ± 0.015 a	0.648 ± 0.023 c	0.386 ± 0.012 a	0.550 ± 0.014 b
Soluble solids (°Bx)	11.9 ± 0.2 b	9.4 ± 0.2 a	13.1 ± 0.3 b	9.3 ± 0.2 a
Saccharose (g 100 g−1)	3.00 ± 0.06 c	0.13 ± 0.05 a	1.55 ± 0.38 b	0.06 ± 0.03 a
Fructose (g 100 g−1)	5.49 ± 0.07 b	3.88 ± 0.09 a	6.65 ± 0.14 c	3.56 ± 0.21 a
Glucose (g 100 g−1)	0.79 ± 0.04 a	1.71 ± 0.04 c	1.47 ± 0.02 bc	1.29 ± 0.12 b
Titratable acidity (%)	0.132 ± 0.011 a	0.183 ± 0.006 b	0.149 ± 0.006 a	0.194 ± 0.002 b
Malic acid (mg·kg−1)	2900 ± 54 c	1700 ± 6 a	2700 ± 43 b	1800 ± 12 a
L-ascorbic acid (mg·kg−1)	200 ± 17 a	700 ± 12 b	280 ± 24 a	680 ± 23 b
Citric acid (mg·kg−1)	140 ± 8 a	1300 ± 46 b	130 ± 14 a	1300 ± 46 b

* Values are means and standard errors, calculated from three pears subjected to treatments. Tukey’s HSD test, p < 0.05; significant differences between variants are indicated by letters.

. Both varieties had a skin color with a higher percentage of yellow tint (b* = 31.54–42.96). The highest green tint value was observed for ‘Yali’ (a* = −7.71) at the time of harvest/early storage. There were no significant changes in the skin color for any of the varieties studied during storage, especially in the expression of the yellow tint (b*) and the lightness parameter (L*). However, all the fruits increased the red tint during storage at the expense of the green tint (higher a* value), most notably for ‘Conference’. In Asian pears, there is generally an increase in the a* value during maturation of the fruit, which reflects an increase in the red shade of the skin colour [45]. A similar trend was observed for parameter b*, an increase which indicated an increase in the yellow shade of the skin. In the Asian pears observed in this study, there were no demonstrable changes in parameter b during storage. Similar conclusions were also reached by Arzani et al. [55] in their study.

The most represented acid in the ‘Yali’ pear is malic acid, followed by citric acid. Chen et al. [58] reported in his study that the same acid composition is also found in other pear varieties [54]. Malic acid was the most abundant. ‘Yali’ contained statistically significantly higher amounts of citric acid than ‘Conference’ (Table 1). Different organic acid ratios depending on variety were observed previously [59]. Gao et al. [60] reported that pears from different places, such as the ‘Yali’, contained mainly malic acid. After storage, there is an increase in respiratory processes in which malic acid serves as a metabolic substrate together with sugars [55].

Gao and Wang [61] reported that there is a close relationship between the respiration rate of pears and the sugar change during storage. The fructose, glucose and sucrose are the major sugars in pears [13,62]. The most common acids in the fruit of most pear varieties are malic and citric acids. The ratio between sugars and acids is important for a positive sensory perception of flavor [59].

In the ‘Yali’, fructose was the most abundant sugar, followed by glucose and sucrose (Table 1). For both varieties, fructose was the predominant carbohydrate, ranging from 3.88 ± 0.09 g.100 g⁻¹ for ‘Yali’ to 5.49 ± 0.07 g.100 g⁻¹ for ‘Conference’. Fructose content in ‘Yali’ pears was approximately 2 times higher than glucose content and 7 times higher in ‘Conference’ pears. Sucrose levels decreased with increasing storage time, but fructose and glucose levels did not change significantly [54]. This behavior was also reported by Itai et al. [63] in ‘Gold Nijisseiki’ pear. Measured fructose values were similar to the ones reported by Torregrosa et al. [20] for ‘Conference’ pears. Glucose and fructose contents in this study agree with the previously reported ranges for ‘Yali’ pear [64].

Pears have attracted the attention of the researchers and food industry for their content of active compounds. Therefore, pears seem to have a strong potential in the production of functional food [65]. In general, the antioxidant capacity of both varieties decreased statistically insignificantly during 60 days of refrigeration storage. This fact may improve the promotion of the consumption of ‘Conference’ and ‘Yali’ pears long after harvest. The antioxidant capacity in the study conducted by Łysiak et al. [30] significantly decreased after storage in normal atmosphere. Silva et al. [66] found that the antioxidant properties of pears were preserved for up to 8 months under good storage conditions. The FRAP values measured for ‘Yali’ were identical to those measured for Asian pears by Guan et al. [67]. This result was similar to that of another study [68], whereby the antioxidant activity of Oriental pears (P. bretschneideri or P. ussuriensis) was higher than those of Occidental pear (P. communis). The oxidation of phenolic and AsA compounds is also related to the decrease in antioxidant activities in fruit during storage [69].

Fresh fruit undergoes chemical and physical changes during the postharvest period that result in a change in nutritional and sensory quality [70]. ‘Conference’ and ‘Yali’ pears are typical climacteric fruits in which ethylene induces fruit ripening and causes a number of physiological changes.

4. Conclusions

Varieties of European pear (Pyrus communis L) represented by ‘Conference’ and Asian ‘Yali’ had the same respiration rate at 1 °C. When evaluating shelf life, ‘Yali’ had a lower respiration rate than ‘Conference’. The temperature quotients of Q₁₀ for respiration were higher for ‘Conference’ while Q₁₀ values for ethylene formation were higher for ‘Yali’. For ‘Yali’, ethylene formation in refrigeration storage ranged from 3.55 to 7.14 µL·kg⁻¹·h⁻¹. The softening of ‘Conference’ fruit was significant, reaching a consumption firmness of 0.12 to 0.23 MPa after seven days of storage at 20° C. ‘Yali’ showed minimal softening during 67 days of storage (0.40 to 0.59 MPa), while ‘Conference’ softened by an average of 74% at shelf life. Although the concentration of ethylene in the indoor atmosphere was as high for ‘Yali’ as for ‘Conference’, these fruits did not soften, were still juicy but did not acquire the buttery consistency typical of ‘Conference’. Antioxidant activity determined by FRAP and DPPH methods was higher for ‘Yali’ and did not change statistically significantly during 60 days of storage. For both varieties, the titratable acidity was low, ranging from 0.14% to 0.16%, and was influenced by the selection of fruit with unequal organic acid content. For organic acids, the decrease in the warming phase at 20 °C was extremely low and statistically insignificant. ‘Conference’ had a higher soluble solids content (an average of 13 °Bx) compared with ‘Yali’ (an average of 9.5 °Bx), which had the same soluble solids content throughout the storage period.

Asian pear trees are of increasing economic importance in temperate climates. They have great storage potential, as their quality parameters change less during storage than standard European varieties. ‘Yali’ was less susceptible to the presence of ethylene, although it produced significantly more than the European variety and its response to changes in respiration rate and firmness was lower.

It is therefore not suitable for storage with European pears, which would react strongly to their ethylene production. Research on the use of 1-MCP during postharvest ripening and its effect on ethylene production in Asian pears may be of interest.