Mechanical properties of crisp pear peel and flesh as affect by storage temperature

ABSTRACT It is important to have an understanding of the mechanical properties of crisp pears in order to assess their resistance to damage and investigate their compression patterns. Through tensile and compression tests, this study measured the elastic modulus, tensile strength, compressive strength and other parameters of Pucheng Crisp Pear, explored the effect of storage temperature on the mechanical properties of crisp pears, and developed a storage temperature-mechanical property prediction model. The results of the pear peel tensile test showed that both the tensile strength of the peel decreased with the increase of storage temperature, while the modulus of elasticity was decreased. There were no significant differences in the tensile strength and modulus of elasticity of the peel at the same storage temperature. The results of the flesh compression test also showed a decrease in compressive strength and modulus of elasticity with increasing storage temperature. Under the same storage condition, the compressive strength and modulus of elasticity of the pear flesh differed significantly in different directions. The correlation coefficients for tensile strength and modulus of elasticity in both the axial and radial directions for the peel and flesh of crisp pears were high, and provided accurate predictions for mechanical characteristics. These findings could be used to inform the development of mechanical equipment, post-harvest storage methods, and the reduction of mechanical damage in all working stages of crisp pears.


Introduction
China is the leading global producer of pears, with a high scope of cultivation and production. [1]After apples and citrus, pears hold the third-most important fruit in China.With over 130 varieties of pears, the Pucheng crisp pear is renowned for its large size, small core, bright yellow color, crisp texture, and sweet taste, making it a favored choice among consumers.[4][5] The study of mechanical properties in pear can help to understand the mechanism of compression damage and provide guidance for equipment design, production chain development, and reduction of mechanical damage.
8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] By studying and analyzing the physical and mechanical properties of fruits, researchers determined the volume, density, hardness, elastic modulus, Poisson's ratio and other important parameters of fruits, and established creep models and stress relaxation models for different fruits, which provided data and theoretical basis for exploring the damage mechanism of fruits and the optimization of mechanical equipment.Ebrahiema Arendse investigated the influence of storage temperature and duration on postharvest physicochemical and mechanical properties of pomegranate fruit and arils. [26]He found that weight loss increased with temperature and storage period, and colors of fruit and arils decreased with prolonged storage duration.And when it was stored at 5°C, it could maintain the best quality for a 2 and 3 months.Demet predicted the mechanical properties of two types of pear fruits at different storage times by using artificial neural network. [27]Through a free-dropping experiment and a physical properties experiment, C. Ortiz determined the effect of variety and storage conditions on the resistance of citrus to impact damage. [28]hile previous studies have examined the impact of moisture content, loading rate, loading mode, loading direction, and storage time on the mechanical properties of fruits, few studies have addressed the effect of storage temperature on pears' mechanical properties.As the primary factor affecting fruit freshness, shelf life and quality, storage temperature have different effects on the metabolism and mechanical properties of fruit.Thus, establishing a model based on storage temperature is crucial to evaluate the mechanical properties of Pucheng crisp pear.In addition, this model can also aid in designing optimal post-harvest storage methods and cold chain systems to minimize damage and ensure quality. [29,30]n this paper, we investigate the effect of storage temperature on the mechanical properties of Pucheng crisp pears and establish a prediction model based on storage temperature-mechanical properties.This provides theoretical and data references for the development of mechanical equipment in the various working stages of the crispy pears, the design of optimal post-harvest storage methods and the reduction of mechanical damage.The objectives of this paper were: (i) to analyze the effect of storage temperature on the mechanical properties of Pucheng crisp pear; (ii) to develop a prediction model of the mechanical properties based on storage temperature; and (iii) to validate the prediction model.

Preparation of crisp pear samples
Fresh Pucheng crisp pear samples were obtained from a commercial orchard in Pucheng county, Weinan City, Shaanxi province, China.The fruits were all at the same stage of ripeness (hardness: 5.87 ± 1.05 N), uniform in size and shape, and had no mechanical or insect damage.Within 2 hours of harvesting, the fruits were carefully packed and transported to an experimental laboratory in Xi'an using temperature-controlled vehicles.

Sample storage conditions
[33] Refrigerators and constant temperature and humidity incubators were used to control the storage temperature.The samples were randomly divided into four groups of 30 each and stored at a humidity uniformly controlled at 80-90% RH for 15 days.

Preparation of peel samples
To investigate the effect of storage temperature on the axial and radial mechanical properties of the peel, Pucheng crisp pear peel was prepared in two directions: pedicel-calyx direction (axial) and the direction perpendicular to it (radial).Peel samples were cut out from the fruit and carefully sliced into equal-sized pieces along the boundary, with the attached pulp removed using a razor blade.The operation process ensured that the peel samples were intact, undamaged, and free of cracks.A total of 60 samples were prepared for each direction, each measuring 40.64 ± 0.86 mm in length, 10.04 ± 0.61 mm in width, and 0.83 ± 0.22 mm in thickness, as measured using a digital caliper (MNT-150, China) with a measurement range of 0-150 mm and a resolution of one hundredth of one millimeter.The sample preparation process of Pucheng crisp pear peel axially is shown in Figures 1a, and 1b depicts the radical preparation process of Pucheng crisp pear peel.The radial peel samples were cut along the latitude, and all other operations were performed in the same way as for axial specimens.Finally, the peel was made into 60 samples, each measuring 40.72 ± 0.82 mm in length, 10.13 ± 0.59 mm in width, and 0.82 ± 0.18 mm in thickness.

Preparation of flesh samples
Cylindrical samples of pear flesh were obtained from the residual flesh using a 10 mm inner diameter punch, drilled in both axial and radial directions, as shown in Figure 1c.The ends were cut parallel with a knife to ensure flatness.The axial sample for compression testing measured 9.95 ± 0.74 mm in height and 10.03 ± 0.55 mm in diameter, while the radial sample measured 9.97 ± 0.80 mm in height and 10.02 ± 0.64 mm in diameter.

Tensile test of crisp pear peel
The ASABEs 368.4 standard was used to determine the mechanical properties of Pucheng crisp pear. [34,35]Electronic universal testing machine (Shanghai Kaiyan Testing Instrument Co., LTD, KY-5KNW, China) was used for the peel tensile and flesh compression tests.The change condition of the pear peel before and after the tensile test is shown in Figure 2. The peel was clamped keeping the upper and lower jigs parallel, and the loading speed was set to 25 mm/min.Breakage in the middle of the peel was considered as the criterion for success.Stress-strain curves and test data were recorded using the machine, and 20 valid data were guaranteed for axial and radial peel samples respectively.

Compression test of crisp pear flesh
The flesh was compressed with a flat indenter placed close to the peel upwards, eliminating the gap by adjusting the height.The maximum loading displacement was set at 5 mm with a loading speed of 25 mm/min.Stress-strain curves and test data were recorded by the electronic universal testing machine. [36,37]To prevent errors due to flesh browning, all testing was completed promptly.The condition of the pear flesh before and after compression is shown in Figure 3.

Parametric solution of the mechanical properties
The specimens of crisp pear peel and flesh were subjected to tensile and compression tests respectively, in order to obtain force-deformation curves.Based on Hooke's elasticity principle and these curves, the modulus of elasticity, the tensile strength and compression strength of crisp pear peel and flesh can be calculated as follows. [38]During testing of the pear specimens, internal forces between the peel and flesh parts are generated.The internal force per unit area of the peel and flesh is referred to as stress, and can be expressed using the formula: where, σ means the stress (N/mm 2 ), F denotes the tensile(compression) force (N), S indicates crosssectional area of the material in the direction of tensile (compression) (mm 2 ).The peel and flesh will be deformed to a certain extent under external force.The degree of deformation is called strain, and the formula is expressed as: where, ε means the tensile (compression) stress (%); ΔL denotes the length after deformation (mm), L indicates the original length before deformation (mm).The modulus of elasticity is one of the main mechanical parameters to measure the elasticity of fruits, and it represents the relationship between stress and strain under load.The formula for calculating the modulus of elasticity of the peel is expressed as: where, E means the elastic modulus of the peel (N/mm 2 or MPa); b denotes the width of the peel (mm); t indicates the thickness of the peel (mm)。The formula for calculating the modulus of elasticity of the flesh is expressed as: where, E means the elastic modulus of the flesh (N/mm 2 or MPa); d indicates the flesh diameter (mm).

Statistical analysis
A completely randomized group design was adopted for this study.Data on the mechanical properties of Pucheng crisp pear were analyzed using one-way analysis of variance (ANOVA).Differences were considered significant if P < .05,as determined by Duncan's test in SPSS 25.0.Origin 2015 was used for plotting, while all data results were presented as mean±standard deviation.

Mechanical properties of crisp pear peel with storage temperature
The results of the peel tensile test are presented in Figure 4.The mechanical properties of crisp pear peel, including tensile strength and modulus of elasticity, were affected by storage temperature.As the storage temperature increased, both the tensile strength and modulus of elasticity of the peel decreased.However, small temperature changes (less than 10°C) did not have a significant effect on pear peel.It was only when the temperature difference reached 20°C or more that significant variability in the tensile strength and modulus of elasticity of the pear peel was observed.[41] Firstly, as the temperature increased, the activity of enzymes such as cellulase and polygalacturonase (PG) in the fruit increases, leading to enzymatic disintegration of the cell wall and varying degrees of pericarp softening.Secondly, as the temperature difference increased, pectin undergoes denaturation and decomposition, increasing the cell wall porosity, which increased the rate of water dissipation and lead to differences in the tensile strength and modulus of elasticity of the peel.
Figure 4a and experimental data show that the tensile strength of pear peel in the axial direction ranged from 0.99 to 1.24 MPa and from 0.98 to 1.19 MPa in the radial direction.During picking, packing, and transport, the axial tensile strength of crisp pear peel at room temperature (20°C) was a. tensile strength b. modulus of elasticity 1.08 MPa and the radial tensile strength was 1.04 MPa.The differences in tensile strength of crisp pear peel in the axial and radial directions were not significant (p > .05)under the same storage conditions.In J Wu's experiments, he came to a similar conclusion. [42]As the outermost physiological tissue of the fruit, the skin of the pear plays an important protective role in cushioning against mechanical damage.Thus, crisp pears stored at higher temperatures are more susceptible to damage from external forces.The relationship between the modulus of elasticity and storage temperature of crisp pear peel is depicted in Figure 4b.Experimentally, the elastic modulus of crisp pear peel ranged from 11.62 to 14.34 MPa in the axial direction and from 11.18 to 13.88 MPa in the radial direction.During harvesting, packaging, transporting, and marketing, the axial modulus of elasticity of pear peel at room temperature (20°C) was 12.51 MPa, and the radial modulus of elasticity was 12.03 MPa.The axial modulus of elasticity was greater than the radial modulus of elasticity.This is probably due to the alignment of fibers in the fruit along the cell wall parallel to the axial direction, reinforcing the elastic modulus of the peel in that direction. [43]However, the differences between the axial and radial elastic moduli of the crisp pear peel were not significant (p > .05).Consequently, the peel of pears can be considered an isotropic material for future studies related to finite element simulation of pears.
The modulus of elasticity is an essential mechanical parameter that measures the fruit's resistance to deformation.The axial and radial modulus of elasticity of pear peel decreased with increasing storage temperature due to the drying effect and softening of peel tissues, reducing turbidity.This is similar to the results obtained by Singh and Reddy for the mechanical properties of orange pericarp but differs from the trend observed in the modulus of elasticity of kiwifruit and grape pericarp. [8,44,45]his is probably due to the large varietal differences among berry fruits and the variation in the modulus of elasticity depending on the influencing factors.

Mechanical properties of crisp pear flesh with storage temperature
The results of the flesh compression test are presented in Figure 5.The mechanical properties of crisp pear flesh are affected by storage temperature.Increasing storage temperature led to a gradual decrease in the compression strength and modulus of elasticity of crisp pear flesh in both axial and radial directions.These findings are consistent with Panmanas' study on mechanical properties of pear fruit. [46]Differences in compression strength and modulus of elasticity of pear flesh were significant (p < .05) at different storage temperatures, indicating that pear flesh is more sensitive to temperature changes than the peel.This difference in sensitivity may be due to the higher moisture content of the flesh, which changes more drastically in respiration rate when the temperature rises, leading to greater changes in mechanical properties. [26,40,47]he compression strength of crisp pear flesh ranged from 0.19 to 0.29 MPa in the axial direction and from 0.18 to 0.28 MPa in the radial direction, as shown in Figure 5a.At room temperature (20°C), the axial and radial compression strengths were 0.24 MPa and 0.23 MPa, respectively.To minimize mechanical damage during production, externally generated pressure should be kept below 0.23 MPa throughout the process.Moreover, significant differences were observed between the axial and radial directions in compression strength at the same storage temperature (p < .05).It was consistent with J. Wang's finding that fruit tissue orientation had a significant effect on the mechanical properties of the fruit. [48]igure 5b shows the relationship between storage temperature and modulus of elasticity of crisp pear flesh.The axial and radial elastic moduli ranged from 0.85 to 1.28 MPa and 0.75 to 1.20 MPa, respectively.At room temperature, the axial and radial moduli were 0.24 MPa and 0.23 MPa, respectively.At the same storage temperature, there were significant differences in the modulus of elasticity of the flesh in different tissue orientations (P < .05).Thus, for later experiments, we can consider the flesh of the crisp pear as an anisotropic material. [17]he experimental data showed that the modulus of elasticity of crisp pear flesh differed significantly at a 5% level of confidence in four storage temperatures.Pears stored at 0°C had the highest modulus of elasticity, while those stored at 30°C had the lowest.This is because increased storage temperature leads to accelerated pectin dissolution and decreases intercellular adhesion, affecting the elastic modulus of the flesh. [49,50]In addition, the cells of fruits are made up of cell walls, cytoplasm, vesicles and soft cell membranes, whose vesicles contain a large amount of water.The cell turgor is the hydrostatic pressure exerted by the intercellular fluid on the cell membrane, which enables the entire cell tissue to maintain a certain degree of elasticity.However, high storage temperatures cause fruit tissue to lose water and result in weakened ability to resist deformation due to decreased cell expansion pressure and plasmolysis. [16,51]In addition, the interaction between proteins and phenolic compounds, which are co-existing components in the fruit, changes with increasing temperature.The complexes formed as a result of the interaction also affect the mechanical properties of the fruit. [52]odulus of elasticity is a key parameter for measuring fruit hardness. [8]Hardness, as one of the most important indicators of fruit evaluation, can directly influence fruit quality.Therefore, storing crisp pears at 0°C can help maintain their hardness and nutritional and economic value.

Mechanical properties prediction model based on storage temperature of crisp pear
The mechanical properties of fruits and vegetables are crucial indicators of their eating quality, as well as playing a vital role in predicting and assessing morphological changes under loading.Crisp pears, being a fruit that is predominantly consumed fresh, undergo rapid physiological metabolism after picking and are susceptible to mechanical damage from external loading forces.Storage temperature is the most critical factor affecting the quality and shelf life of fruits, with suitable conditions required to preserve optimal quality.To ensure the economic value and market competitiveness of crisp pears, it is necessary to develop a predictive model that considers the mechanical properties of crisp pear storage temperature.
In conducting tensile and compression tests, mechanical properties such as peel tensile strength, flesh compressive strength, peel modulus of elasticity, and flesh modulus of elasticity were studied.Figure 6 illustrates the correlations between these mechanical properties and storage temperature.The correlation coefficients ranged from 0.94 to 0.99, indicating high linear correlations between the mechanical properties of different parts of the crisp pear and storage temperature.Hence, a predictive model based on storage temperature and mechanical properties can accurately and reliably predict the quality of crisp pear samples.
The correlation coefficients for the crisp pear flesh's mechanical properties were lower than those of the peel, possibly because the flesh is more sensitive to changes in environmental temperature.Thus, the sampling and measuring process needs to be conducted as quickly as a. compression strength b. modulus of elasticity possible to minimize errors due to external factors.Additionally, the cutting process of the samples, which involves some external force on the flesh, may lead to an increase in relative error.

Validation of the prediction model
To further validate the performance of the prediction model, four sets of data not utilized in modeling were selected as validation samples to test prediction results.The mechanical properties of crisp pears were verified using the linear regression equation in Figure 6.Table 1 shows the experimental and predicted values of the mechanical characteristics of crisp pears.The results indicated that the model mechanical properties had high agreement with predicted values, with average relative errors between predicted and experimental values of 6.90%, 4.50%, 4.81%, and 4.47%.Thus, the pear mechanical properties-storage temperature model can accurately predict the mechanical properties of crisp pears.Moreover, as shown in Table 1, the average relative errors for peel in both axial and radial directions were higher than those of fruit flesh.This may be due to water dissipation from the crisp pear peel. [40,53]Additionally, external pressure from peeling the peel from the intact fruit resulted in additional water loss, and the same loading effect occurred when the knife was used to separate the flesh attached to the peel sample.Therefore, fast sampling and appropriate handling of test samples are necessary to reduce systematic and accidental errors. [54]

Conclusion
This study investigated the effect of storage on the mechanical properties of crisp pear peel and flesh in various directions through tensile and compression tests.For the first time, this study obtained the tensile strength, compression strength, and elastic modulus of crisp pear peel and flesh under different storage conditions.The main conclusions are as follows: (i) Tensile and compression tests showed that under the same storage temperature, the tensile strength and modulus of elasticity of the crisp pear peel in both axial and radial directions were isotropic, while the compression strength and modulus of elasticity of the pulp in different directions were anisotropic due to its nature.(ii) Under the same storage condition, the axial modulus of elasticity was higher than the radial modulus of elasticity, indicating that placing crisp pear axially can reduce mechanical damage.(iii) As the storage temperature increased, the tensile (compression) strength and modulus of elasticity of crisp pear peel and pulp in different directions (axial and radial) decreased due to changes in moisture content, pectin denaturation and dissolution rate, cell expansion pressure, hardness, and viscoelasticity.(iv) The mechanical properties of the pulp were more sensitive to changes in storage temperature than those of the peel due to its higher moisture content.(v) Keeping crisp pear at 0°C preserves its nutritional and economic value better.Future research can focus on storage time, humidity, and choice of edible coatings.(vi) The high linear correlation coefficients between the mechanical properties and storage temperature suggest that the models based on storage temperature and mechanical properties can accurately predict the quality of samples.

Figure 1 .
Figure 1.Sample preparation of crisp pear peel and flesh in the axial and radial direction.

Figure 2 .
Figure 2. The status of crisp pear peel before and after tensile test.

Figure 3 .
Figure 3.The status of crisp pear flesh before and after compression.

Figure 4 .
Figure 4. Changes in mechanical properties of crisp pear peel with storage temperature.

Figure 5 .
Figure 5. Changes in mechanical properties of crisp pear flesh with storage temperature.

Figure 6 .
Figure 6.Correlation between mechanical properties and storage temperature values of (a) tensile strength of peel (b) modulus of elasticity of peel (c) compression strength of flesh (d) modulus of elasticity of flesh.

Table 1 .
Experimental and predicted values of mechanical properties of crisp pear.