Mechanistic Insights into Effect of Sugar Impregnation Pretreatment on Texture and Moisture Stability of Freeze-Dried Pear Slices

Abstract

The product quality of freeze-dried pear slices is limited by moisture absorption, texture softening, and color deterioration. This study evaluated the effects of sugar impregnation using glucose, fructose, and sucrose at 2 M and 3 M concentrations on key quality attributes. Sugar impregnation improved the product’s appearance, texture, and flavor by reducing moisture absorption, reinforcing the cell wall, and forming a surface sugar layer, exceeding the benefits of osmotic dehydration. Among all groups, 3 M sucrose-treated samples exhibited the highest glass transition temperature (Tg), lowest moisture uptake, and most compact structure, indicating enhanced stability and reduced hygroscopicity. Further analyses revealed that sugar impregnation regulated microstructure and water-binding behavior, contributing to better physical properties. These findings suggest that high-concentration sucrose impregnation is an effective strategy to improve structural integrity and extend the shelf life of freeze-dried fruits and vegetables, offering promising applications in food preservation.

1. Introduction

Pear (Pyrus spp.) is a widely consumed fruit known for its high nutritional value, containing vitamin C, polyphenols, dietary fiber, and minerals. Its sweet taste and crisp texture make it popular among consumers. However, due to high moisture and thin pericarp, fresh pears are prone to softening, browning, and decay during postharvest handling, limiting their shelf life and transportability [1]. Therefore, the development of pear-related products using different processing technologies, such as pear juice, pear jam, and freeze-dried products, helps to extend their sales cycle and expand market applications. Among these methods, freeze-drying technology, showing outstanding performance in preserving the original quality of fruits, has gradually become an important approach for the deep processing of pears. Freeze-drying is an effective fruit preservation method that removes moisture through low-temperature sublimation and minimizes thermal damage, retaining the original nutrients, color, and structure of fruits [2,3]. Freeze-dried fruits and vegetables have become a highly favored category of snack foods, among which freeze-dried pear slices stand out as a representative product, renowned for their unique taste and flavor. However, the porous, low-density structure of freeze-dried pear slices makes them susceptible to moisture absorption during storage. Because most freeze-dried products are amorphous, they possess unstable glassy structures prone to glass-to-rubber transitions under high humidity, leading to texture loss, discoloration, and oxidation—factors that compromise shelf stability and market value [4,5].

To address moisture absorption and quality deterioration in freeze-dried fruits and vegetables during storage, researchers have been focusing on various pretreatment methods [6,7,8,9]. For example, ultrasound-assisted vitamin C pretreatment can effectively improve the water activity of jujube slices, preserving their quality and active components during the drying process [10]. The hot air explosion puffing technique improved the texture characteristics of apple slices, enhancing drying efficiency and product quality [11]. These pretreatment methods improve the structural or compositional state of the raw material before drying, thereby enhancing the stability of the final product during storage. Notably, sugar impregnation has attracted considerable attention for its simplicity, efficiency, and safety; therefore, it has gradually been industrialized in food processing plants [12]. This technique involves immersing the products in sugar solutions, where osmotic pressure drives water out and sugar in, altering tissue physicochemical properties. Studies have shown that the type of sugar affects the texture, physicochemical properties, and storage stability of the treated products. Erythritol pretreatment markedly enhanced peach slices’ hardness, whereas trehalose facilitated the development of a homogeneous porous matrix, thereby improving crispness and structural stability [13]. In addition, it has been reported that the type and concentration of sugar can significantly affect the textural properties of dried yellow peach slices by modulating the pectin structure [14]. Therefore, among various pretreatment methods, sugar impregnation has gradually become a research focus due to its potential in regulating the texture of dried products and inhibiting moisture absorption. Although sugar impregnation pretreatment has been applied to various dried fruits, existing research has primarily focused on texture modulation and general moisture control. The underlying mechanisms of its interaction with the cell wall structure and hydrophilic components of high-moisture fruits remain unclear. Pear slices, characterized by their high initial moisture content, cell wall structure rich in pectin and hemicellulose, and susceptibility to browning, serve as an ideal model for investigating such mechanisms.

This study investigated the effects of sugar type (sucrose, glucose, fructose) and concentration (2 M and 3 M) on the quality and hygroscopic behavior of freeze-dried pear slices. This study compared the effects of different sugar treatments on the textural properties, structural characteristics, and moisture distribution of pear slices to elucidate the mechanisms by which they regulate hygroscopic behavior. The results of this study will provide a theoretical basis for improving product quality and offer technical support for the application of sugar impregnation in dried fruit and vegetable processing, highlighting the potential of natural sugars in regulating food stability.

2. Materials and Methods

2.1. Chemicals and Materials

Mature ‘Huangguan’ pears (Pyrus bretschneideri Rehd. ‘Huangguan’) with uniform color and size, free from diseases, insect pests, and mechanical damage, were selected as raw materials. Before the experiment, the pears were balanced at room temperature for 3 h, then peeled and cored. Sucrose, fructose, and glucose were all of analytical purity (purity > 99%), purchased from Shanghai Yuanye Biotechnology, Shanghai, China. All other reagents used in the experiment were also of analytical purity.

2.2. Preparation of Pear Slices

Pears were sliced into evenly spaced 5 mm thick slices. Pear slices (200 ± 5 g) were separately soaked in glucose, fructose, and sucrose solutions (2 M and 3 M) at a fruit-to-solution mass ratio of 1:5 (w/v) at 25 °C for 1 h. Subsequently, the samples were immersed in distilled water for 15 s, then removed and blotted with absorbent paper to remove excess surface moisture. The pretreated pear slices were evenly spread and pre-frozen at −80 °C for 12 h, followed by freeze-drying processing. CK was the sample without sugar treatment. 2-Glu, 3-Glu, 2-Fru, 3-Fru, 2-Suc, and 3-Suc were samples treated with 2 M and 3 M glucose, fructose, and sucrose solutions, respectively.

2.3. Physicochemical Properties of Pear Slices

2.3.1. Color

The color of pear slices was measured using a colorimeter (CR-10 colorimeter, Konica Minolta, Tokyo, Japan), with the central part of the pear slice selected as the color measurement point [15]. The total color difference (ΔE) was calculated using Equation (1):

$$∆E=\sqrt{{\left(L^{\ast } - L_0\right)}^2+{\left(a^{\ast } - a_0\right)}^2+{\left(b^{\ast } - b_0\right)}^2}$$

(1)

where ΔE represents the total color difference; BI represents the browning index; L*, a*, b* and L₀, a₀, b₀ represent the lightness, red/green value, and yellow/blue value of the pear slices after and before drying, respectively.

2.3.2. Total Sugar Content and Total Phenol Content

The extracts for determining total sugar and total phenolic contents were prepared following previously reported methods [16]. The total sugar content was determined by using the anthrone-sulfuric acid colorimetric method. The extract was mixed with 0.39 mL of distilled water, 0.1 mL of anthrone-ethyl acetate solution (20 g/L), and 1 mL of concentrated sulfuric acid, followed by incubation in boiling water for 1 min. Finally, the absorbance was measured at 620 nm. The total sugar content is expressed in mg/g.

1.0 mL of the extract was mixed with 7.0 mL of distilled water and 0.5 mL of Folin–Ciocalteu reagent, followed by the addition of 1.5 mL of 20% (w/v) Na₂CO₃ solution. The mixture was then incubated at 75 °C for 10 min. After incubation, the absorbance was measured at 765 nm [17]. Total phenolic content is reported as mg GAE/g (gallic acid equivalents).

2.3.3. Antioxidant Activity

The antioxidant activity of pear slices was determined using an ABTS radical scavenging capacity assay kit (Solarbio, Beijing, China), DPPH radical scavenging capacity assay kit (Solarbio, Beijing, China), and ferric ion reducing antioxidant power (FRAP) assay kit (Solarbio, Beijing, China) according to the manufacturer’s instructions [18].

2.3.4. Determination of Hardness and Crispness

The hardness and crispness of pear slices were measured using a texture analyzer. A TA/5 cylindrical probe was used for the compression test, with the test speed set at 0.5 mm/s, post-test return speed at 0.5 mm/s, and test distance at 7.0 mm. The hardness of the samples was represented by the maximum pressure peak in the coordinate diagram, with the unit of g. The crispness was represented by the first obvious pressure peak on the coordinate diagram during the first downward movement of the probe toward the sample, with the unit of g.

2.3.5. Extraction of Cell Wall Components

Pear slice texture is determined by cell wall components. To remove soluble substances that may interfere with the analysis, the cell wall material needs to be extracted first. Pear slices were pulverized and passed through a 100-mesh sieve. A total of 4.0 g of the resulting powder was mixed with 200 mL of 75% ethanol, boiled for 30 min, and centrifuged at 4000 r/min for 10 min. The residue was then washed with boiling ethanol and centrifuged again, followed by sequential washing with a chloroform–methanol solution (1:1, v/v) and with acetone. The final material was filtered through a 100-mesh nylon cloth and dried at 40 °C to obtain the cell wall fraction [19].

2.3.6. Monosaccharide Composition

Monosaccharide composition was analyzed using an ICS-3000 ion chromatograph (Dionex, Sunnyvale, CA, USA) following [20]. Cell wall samples (20 mg) were hydrolyzed in 20 mL of 2 mol/L trifluoroacetic acid at 120 °C for 2 h. The hydrolysate was evaporated under nitrogen and redissolved in 20 mL ultrapure water. After tenfold dilution, the solution was filtered through 0.22 μm membrane and injected. The mobile phases were 15 mmol/L NaOH, 15 mmol/L NaOH with 100 mmol/L sodium acetate, and ultrapure water. The flow rate was 0.3 mL/min. For comparative analysis of relative variations in monosaccharide profiles among samples, individual monosaccharide contents are presented as molar percentages (mol%). The analyzed monosaccharides included arabinose (Ara), rhamnose (Rha), galactose (Gal), galacturonic acid (GalA), xylose (Xyl), glucose (Glc), and mannose (Man). Components present in trace amounts (glucuronic acid, rhamnose, fucose) are collectively denoted as “Others”.

2.3.7. Molecular Weight (Mw)

The M_w of pectin samples was determined using high-performance size exclusion chromatography coupled with multi-angle laser light scattering (HPSEC-MALLS). The system was equipped with a refractive index detector, UV detector, and Shodex 806 column (Tosoh, Tokyo, Japan). 0.1 mol/L NaCl aqueous solution was used as the eluent. After filtration through 0.45 μm membrane, 100 μL of pectin solution (2 mg/mL) was injected for analysis [21]. Mw is expressed as × 10⁴ Da.

2.4. The Structure of Pear Slices

2.4.1. Scanning Electron Microscope

The microstructure of the pear slices was observed using SEM (S-4800, Hitachi, Tokyo, Japan). After gold sputter-coating, the samples were examined at an accelerating voltage of 10 kV and a pressure of 100 Pa, with a magnification of 500×.

2.4.2. Fourier Transform Infrared (FT-IR) Spectroscopy

The cell wall components of the pear slices were thoroughly ground and passed through a 100-mesh sieve. Subsequently, FT-IR analysis was performed using a Fourier transform infrared spectrophotometer (Tensor 27, Bruker, Billerica, MA, USA). FT-IR spectra were recorded over the 4000–400 cm⁻¹ wavenumber range with 64 scans. The intensity of an infrared absorption peak is evaluated by its integrated area. The results are presented as infrared spectra, with the x-axis representing wavenumber (cm⁻¹) and the y-axis representing transmittance (%).

2.4.3. X-Ray Diffraction (XRD)

The physical state of pear slices was analyzed using an X-ray diffractometer (D8 Advance, Bruker, Billerica, MA, USA). The scanning step size was set to 0.02°, and the diffraction angle was recorded over a 2θ range of 10° to 80°. The XRD patterns are plotted with the diffraction angle 2θ (°) as the abscissa and the diffraction intensity (A.U.) as the ordinate. The crystallinity Index (CI) was calculated using Equation (2):

$$CI\left(\%\right)={\displaystyle \frac{A_c}{A_c+A_a}}\times 100\%$$

(2)

where A_c represents the area of the ordered peak, and A_a represents the area of the disordered peak.

2.4.4. Glass Transition Temperature (Tg)

The Tg of pear slices was determined using a differential scanning calorimeter (TG-DSC 3, Mettler, DE, USA). A 10.0 mg sample was placed in a crucible. The sample was initially cooled from 25 °C to −70 °C at a rate of 10 °C/min and held at −70 °C for 5 min. It was then heated to 100 °C at a rate of 10 °C/min. The resulting DSC heat flow curves were analyzed using specialized software (STARe Eval 0.24.2, Columbus, DE, USA). The midpoint of the glass transition process was recorded and taken as the Tg of the sample [22].

2.5. Hygroscopic Stability of Pear Slices

2.5.1. Water Activity (Aw)

The Aw of the sample was measured using an A_w meter (AW1000T, ChangerTek Industrial Co. Ltd., Shanghai China) which was previously calibrated with standard solutions [12].

2.5.2. Low-Field Nuclear Magnetic Resonance (LF-NMR) Analysis

The transverse relaxation time (T₂) was measured using a pulsed nuclear magnetic resonance analyzer (MacroMR12-60, Niumag, Suzhou, China). The instrument parameters were set as follows: proton resonance frequency of 21 MHz, spectral width of 100 kHz, repetition interval of 3000 ms, number of repeated scans of 16, relaxation decay time of 0.16 ms, and echo number of 4000. The T₂ decay curves were inverted using Multi Exp Inv Analysis software embedded in the instrument to obtain the T₂ spectra of samples [23].

2.5.3. Moisture Sorption Curves

The method was performed with slight modifications based on the procedure described in a previous literature [24]. Pear slices (0.5 g) were accurately weighed and placed in desiccators containing saturated sodium chloride solution (relative humidity 75%) at 25 °C. Samples were precisely weighted at specific time intervals. The moisture adsorption percentage was calculated using Equation (3), and the moisture adsorption curve was plotted with adsorption time as the abscissa and adsorption rate as the ordinate.

$$Moisture adsorption percentage \left(\%\right)={\displaystyle \frac{W_n-W_0}{W_0}}\times 100\%$$

(3)

where W_n and W₀ are the weight of sample after moisture adsorption and before moisture adsorption, respectively.

2.6. Statistical Analysis

The data were analyzed by the analysis of variance and Duncan’s multiple range tests. The significant level was p < 0.05 throughout the study. All experiments were conducted in triplicate, and data were reported as mean values ± standard deviation (SD).

3. Results

3.1. Effects of Different Sugar Treatments on Physicochemical Properties of Pear Slices

3.1.1. Color and Total Sugars Content

Color and total sugar content of food products are important quality attributes affecting consumer choices.

Table 1. Color and total sugar content of freeze-dried pear slices.

Sample	L*	a*	b*	ΔE	Total Sugar Content (mg/g)
CK	82.40 ± 3.54 c	1.83 ± 0.74 a	16.40 ± 1.17 a	8.62 ± 2.60 b	273.17 ± 1.68 g
2-Glu	89.64 ± 0.55 ab	0.65 ± 0.39 bc	12.58 ± 1.20 b	12.62 ± 0.62 a	560.62 ± 1.36 c
3-Glu	88.60 ± 1.45 ab	0.26 ± 0.15 c	9.40 ± 1.07 c	11.41 ± 1.44 ab	669.27 ± 1.32 a
2-Fru	86.24 ± 0.51 b	1.33 ± 0.21 ab	15.83 ± 0.20 a	10.61 ± 0.45 ab	301.91 ± 1.10 f
3-Fru	87.65 ± 2.93 ab	1.04 ± 0.07 b	14.76 ± 0.47 a	11.46 ± 2.36 ab	371.47 ± 1.29 e
2-Suc	89.89 ± 1.22 ab	0.22 ± 0.12 c	11.01 ± 1.14 bc	12.66 ± 1.24 a	472.63 ± 1.21 d
3-Suc	90.50 ± 1.04 a	1.27 ± 0.25 ab	9.61 ± 0.55 c	13.23 ± 1.03 a	584.97 ± 1.57 b

CK was the sample without sugar treatment. 2-Glu, 3-Glu, 2-Fru, 3-Fru, 2-Suc, and 3-Suc were samples treated with 2 M and 3 M glucose, fructose, and sucrose solutions, respectively. Different letters within the same column indicate significant differences (p < 0.05).

summarizes the effects of various sugar treatments on the color parameters (L*, a*, b*, ΔE) and total sugar content of freeze-dried pear slices. The CK group had the lowest L* value (82.40 ± 3.54), while all sugar-treated groups showed significantly higher L* values (p < 0.05), with 3-Suc achieving the highest brightness (90.50 ± 1.04). This may be attributed to the fact that the sugar film retards enzymatic browning by reducing the area of pear slices exposed to air. Lower a* and b* values in all treated groups compared to the CK group indicated a reduction in red and yellow hues, which may be attributed to inhibition of phenolic oxidation and non-enzymatic browning. The ΔE values were highest in the 3-Suc group (13.23 ± 1.03), indicating a noticeable color change.

In addition, the total sugar content of the freeze-dried pear slices was increased significantly after sugar treatment, corresponding to the concentration of the sugar solution. At a concentration of 3 M, the total sugar contents of the glucose, fructose, and sucrose groups were 669.27 ± 1.32 mg/g, 371.47 ± 1.29 mg/g, and 584.97 ± 1.57 mg/g, respectively, with significant differences (p < 0.05). Among them, the fructose group showed the smallest increase, which may be due to the high fructose level in pears, which reduced the osmotic pressure difference inside and outside the cell, lowering the osmotic effect [25].

3.1.2. Antioxidant Activity and Total Phenolic Content

As shown in

Table 2. Antioxidant activity (ABTS radical scavenging rate, DPPH radical scavenging rate, and FRAP) and total phenolic content of freeze-dried pear slices.

Sample	ABTS (%)	DPPH (%)	FRAP (μmol/g)	Total Phenolic Content (mg GAE/g)
CK	30.87 ± 1.38 a	63.46 ± 0.40 a	6.98 ± 0.27 a	3.07 ± 0.04 a
2-Glu	24.86 ± 0.25 b	24.49 ± 0.91 b	3.64 ± 0.19 cd	1.34 ± 0.01 b
3-Glu	19.81 ± 0.50 c	17.93 ± 0.90 c	3.34 ± 0.13 de	0.79 ± 0.04 d
2-Fru	24.21 ± 0.11 b	24.79 ± 0.90 b	3.97 ± 0.18 b	1.29 ± 0.03 bc
3-Fru	19.02 ± 1.14 c	18.06 ± 0.04 c	3.23 ± 0.13 e	0.74 ± 0.01 de
2-Suc	24.45 ± 0.16 b	25.04 ± 0.17 b	3.71 ± 0.13 bc	1.27 ± 0.05 c
3-Suc	19.57 ± 0.68 c	17.95 ± 0.04 c	3.16 ± 0.14 e	0.72 ± 0.01 e

Different letters within the same column indicate significant differences (p < 0.05).

, sugar treatments slightly reduced the antioxidant capacity of freeze-dried pear slices, with no significant difference among all groups. Compared with the CK group, all treated samples showed a significant decrease in ABTS and DPPH radical scavenging activities, FRAP values, and total phenolic content. The extent of decline in antioxidant activity demonstrated a positive correlation with sugar concentration, while no significant differences were observed among the glucose, fructose, and sucrose groups at the same concentration. This indicates that the reduction is attributed primarily to sugar concentration rather than the type of sugar. Drying may induce degradation or structural isomerization of phenolic compounds, resulting in a significant reduction in total phenolic content [26]. Moreover, sugars may occupy active sites on the tissue surface, reducing the extractability or reactivity of phenolic compounds. Even so, the sugar film formed on the surface of freeze-dried pear slices improved visual appearance and taste by delaying browning, contributing to higher product quality and consumer acceptability.

3.1.3. Hardness and Crispness

Figure 1A,B show the effects of sugar impregnation on the hardness and crispness of freeze-dried pear slices, respectively. Sugar treatments significantly improved the hardness of the samples compared to the CK group, in which the hardness increased with sugar concentration. The 3-Suc group showed the highest hardness (18.14 ± 0.21 g), significantly surpassing the 3-Glu (10.02 ± 0.61 g) and 3-Fru (12.47 ± 0.30 g) groups, which indicated that sucrose effectively reinforced the structure of freeze-dried pear slices. Similarly, the CK group showed the lowest crispness value (6.24 ± 0.41 g), and the 3-Suc group had a significantly higher crispness than the other treated groups (8.98 ± 0.45 g), followed by the 3-Glu group (5.76 ± 0.32 g) and 3-Fru group (4.91 ± 0.23 g). Improved crispness is attributed to a more uniform porous structure of freeze-dried pear slices, which is related to the molecular structural characteristics of the three sugars. Sucrose is a non-reducing disaccharide with a relatively large molecular weight and high glass transition temperature, which tends to form a stable and dense glassy matrix during drying [27]. This matrix effectively supports the cell wall structure and helps maintain brittle fracture characteristics. In contrast, glucose and fructose are monosaccharides, with fructose in particular having a five-membered furanose ring structure, an extremely low glass transition temperature, and strong hydrophilicity [28]. As a result, it readily absorbs moisture and softens, making it difficult to form a stable glassy network, which in turn reduces structural compactness and crispness. Therefore, the molecular structure of the sugar plays a critical role in regulating the textural properties of freeze-dried pear slices, with sucrose exhibiting the most favorable effects on enhancing both hardness and crispness. The above results indicated that the high concentration of sugar treatment (especially sucrose) significantly promoted the hardness and crispness of pear slices, thus enhancing the product quality.

3.1.4. Monosaccharide Composition and Mw

The texture of freeze-dried pear slices is jointly influenced by their pectin, cellulose, and hemicellulose contents, with pectin having the most significant impact. In view of this, we measured the monosaccharide composition of all samples. After sugar impregnation, the representative monosaccharides of pectin (GalA), hemicellulose (Xyl), and cellulose (Glu) exhibited different trends. As shown in Figure 2A, the Xyl content significantly decreased, the GalA content increased, while the Glu content remained relatively stable. These results indicated that the plasticizing/sliding effect of hemicellulose weakened, whereas the gelling and cross-linking capacity of pectin enhanced. Since the cellulose framework remained basically unchanged, the overall cell wall network became denser, thus synchronously improving both hardness.

Moreover, compared to the CK group, all sugar-treated groups exhibited higher Mw (Figure 2B), indicating that sugar impregnation may protect cell wall polysaccharides from degradation. The Mw of samples significantly increased with the increase in sugar concentration, with the 3-Suc group exhibiting the highest Mw. Within the same type of sugar, the Mw of the 3 M groups was always higher than that of the 2 M groups, suggesting that higher concentrations have stronger protective and structural stabilizing effects. At the same concentration, the sucrose-treated groups consistently exhibited higher Mw, which may be attributed to its larger Mw and structural filling effect during the drying process. These results are consistent with previous reports [13]. The higher Mw indicates a reduced degradation of cell wall polysaccharides and enhanced molecular entanglement, leading to the formation of a denser and more stable cell wall network. This is consistent with the observed improvement in crisp texture, indicating that sugar impregnation enhances textural properties by increasing the structural density of the slices.

3.2. Effects of Different Sugar Treatments on the Structure of Pear Slices

3.2.1. Microstructure

Figure 3 shows the effects of different sugar types and concentrations on the appearance and microstructure of freeze-dried pear slices. In the CK group, due to the absence of sugar protection, the cell wall contracted severely during drying, resulting in a sunken appearance and poor surface color. The corresponding SEM images revealed a loose structure with large, uneven pores and evident collapse, indicating weak structural stability. Sugar treatments significantly improved the structural characteristics of freeze-dried pear slices, enhancing brightness, shape integrity, and microstructural uniformity. Treated samples (especially the 3-Suc group) showed smaller, more evenly distributed pores, thicker walls, and reduced collapse. SEM images of 3-Suc samples displayed dense tissue, smooth surfaces, and continuous pore walls, indicating that sucrose formed a robust glassy matrix during drying, stabilizing the cellular framework. The glucose-treated group showed moderate improvements, especially with 3-Glu reducing pore size and increasing wall integrity, though its compactness was still below that of the sucrose group. The fructose-treated group showed the weakest effect, with irregular pores and evident collapse even at higher concentrations. These differences are linked to sugar structure: sucrose, a large non-reducing disaccharide with a high glass transition temperature, supports stable matrix formation. Due to its small molecular size, strong hydrophilicity, and low Tg, fructose tends to absorb moisture and exhibit poor structural stability, which hinders the formation of dense and stable amorphous matrices. Although the Tg of the slices indicates a more stable structure, all measured values were below −10 °C. This suggests that phase transition is unlikely to occur during ambient storage.

3.2.2. FT-IR Spectra

FT-IR analysis is commonly used to identify changes in key functional groups in food and reveal the intermolecular interaction characteristics among different sugar components. Figure 4A displayed the FT-IR spectra of the cell wall components of pear slices. Compared to CK, all sugar-treated samples exhibited a noticeable increase in the absorption peak intensity at 3369 cm⁻¹, which may be attributed to the formation of hydrogen bonds between hydroxyl groups in the sugar molecules and polysaccharides such as pectin [29]. In addition, a stretching vibration absorption peak of C-H bonds was observed at 2939 cm⁻¹ in all samples, and its intensity increased with higher sugar concentrations, indicating sugar retention within the tissue [30]. In the spectral range of 1800–1600 cm⁻¹, FT-IR is often used to assess the degree of methyl esterification of pectin components [2]. Specifically, peaks at 1745 cm⁻¹ and 1620 cm⁻¹ correspond to the stretching vibrations of esterified carboxyl groups (COO-R) and free carboxyl groups (COO-), respectively. The sugar-treated samples showed varying absorption intensities in this region, with particularly noticeable differences at 1745 cm⁻¹ and 1620 cm⁻¹, suggesting that the type and concentration of sugars may influence the methyl esterification degree of pectin. Around 1043 cm⁻¹, all sugar-treated groups exhibited significantly enhanced absorption peaks, primarily attributed to the stretching vibrations of C-O and C-O-C bonds, which are characteristic of sugar molecules and represent their fingerprint region [31]. The intensity of this peak increased with higher sugar concentrations and was most prominent in the 3-Suc group, indicating higher sugar retention and stronger interactions with the matrix in these samples.

3.2.3. XRD

XRD analysis reveals the crystallinity or amorphous characteristics of samples, offering insights into structural order and physical stability. As shown in Figure 4B, all freeze-dried pear samples exhibited broad characteristic diffraction peaks within the range of 18° to 22°, indicating an amorphous state. Compared with the CK group, the crystallinity of the sugar-treated sample decreased from 8.55% to 6.39%. This suggests that sugar impregnation disrupts or replaces parts of the original amorphous matrix (e.g., pectin and cell wall polysaccharides), thereby weakening X-ray scattering. The 3-Suc exhibited the lowest peak intensity, indicating that sucrose, owing to its higher molecular weight and glass transition temperature, likely forms a dense and stable glassy network during drying. This structural reorganization may be attributed to hydrogen bonding between sugar molecules and cell wall polymers, which restricts molecular mobility and enhances matrix rigidity. Furthermore, the embedding of sugar within the tissue likely dilutes or shields the diffraction signal from native components. Such interactions are particularly evident in high-concentration sucrose treatments, where the amorphous structure becomes more stable. Overall, the reduced crystalline peak intensity may result from structural rearrangement or signal masking by amorphously embedded sugars. Sugar in an amorphous form altered the structural profile of freeze-dried pear slices [32].

3.2.4. Tg

Tg represents the transition point at which amorphous substances in food shift from a ’glassy’ state to a ‘rubbery’ state, which significantly affects various physical, chemical, and storage-related properties of food products [33]. Different sugar types and concentrations had distinct effects on the Tg of freeze-dried pear slices. As shown in

Table 3. Glass transition temperature (T_g) and water activity (A_w) of freeze-dried pear slices.

Sample	Tg (°C)	Aw (10−2)
CK	−22.33 ± 1.58 f	11.05 ± 0.13 a
2-Glu	−16.14 ± 0.58 d	10.33 ± 0.38 b
3-Glu	−14.07 ± 1.15 bc	8.68 ± 0.34 cd
2-Fru	−18.26 ± 0.86 e	9.20 ± 0.54 c
3-Fru	−15.66 ± 0.48 cd	8.65 ± 0.34 cd
2-Suc	−13.27 ± 1.30 b	8.45 ± 0.79 d
3-Suc	−10.98 ± 0.74 a	7.75 ± 0.13 e

Different letters within the same column indicate significant differences (p < 0.05).

, the Tg values of all sugar-treated groups (from −16.14 °C to −10.98 °C) were higher than that of the CK group (−22.33 ± 1.58 °C), indicating that sugar impregnation helped enhance the thermal stability of the products. Among the treatments, the 3-Suc group showed the best effect, with its Tg increasing to −10.98 ± 0.74 °C, suggesting that a more stable glassy structure formed during drying. This result can be attributed to the high intrinsic glass transition temperature of sucrose (60 °C–70 °C) and its stable molecular structure, which facilitated the formation of a dense glassy matrix on the surface of the pear cell walls during drying [27,34]. This glassy matrix effectively restricted the mobility of water and cellular contents, and reduced the free volume, thereby significantly improving the thermal stability of the freeze-dried pear slices. In contrast, the glucose group showed a moderate increase in Tg. Although glucose itself has a relatively high Tg, its smaller molecular size and more flexible structure result in a less compact and less stable glassy network. The fructose group exhibited the smallest Tg increase, with the Tg of the 3-Fru group reaching only −15.66 ± 0.48 °C, which is close to the CK level. This is due to the extremely low Tg of fructose (5–10 °C) and its strong hydrophilicity [35], which makes it prone to moisture absorption and softening during drying, making it difficult to form a stable glassy state and leading to lower thermal stability compared with other sugars. High- Tg sugars like sucrose possess excellent anti-plasticizing properties in dried foods, which can significantly inhibit moisture uptake and prevent stickiness, thereby improving storage stability [36]. The type and concentration of sugar had a significant impact on the formation and maintenance of the glassy structure in freeze-dried pear slices, with sucrose providing the best thermal protective effect. As shown in Table 3, all measured Tg values were below −10 °C, which is lower than the ambient storage temperature. Under such conditions, a phase transition is unlikely to occur in the pear slices. Therefore, the Tg values here primarily serve as a comparative indicator of the relative structural stability imparted by different sugar treatments, with a higher Tg representing greater matrix rigidity.

3.3. Effects of Different Sugar Treatments on Hygroscopicity of Pear Slices

3.3.1. Aw

Aw is a key indicator of the physical stability and hygroscopic sensitivity of freeze-dried products. As shown in Table 3, all sugar impregnation treatments significantly increased the Tg while notably reducing Aw compared to the CK group, indicating enhanced structural stability and reduced hygroscopicity after sugar pretreatment. Among the treatments, the 3-Suc group exhibited the most favorable results, with the highest Tg and the lowest Aw, clearly outperforming other groups. In contrast, the CK group had the lowest Tg and the highest Aw. This inverse relationship suggests that higher Tg corresponds to a more stable glassy state, which limits molecular mobility and moisture migration, thereby lowering water activity [37].

Furthermore, increasing sugar concentration enhanced this trend, particularly in the sucrose groups. This is likely due to sucrose being a non-reducing disaccharide with a higher molecular weight and a higher Tg. During freeze-drying, sucrose readily forms a dense network structure that encapsulates cell wall components and water, thereby effectively stabilizing the system. Glucose and fructose are monosaccharides with smaller molecular sizes. Especially for fructose, its five-membered furanose ring structure, low Tg, and high hygroscopicity make it prone to moisture absorption and softening, hindering the formation of a stable glassy matrix. As a result, these treatments yield less compact structures and reduced crispness.

3.3.2. LF-NMR

T₂ relaxation time is widely used to evaluate the state and distribution of water in food, as well as its interactions with other components. It plays an important role in quality control, storage stability, and the assessment of processing degree [37]. Figure 5A presents the T₂ relaxation time distribution of freeze-dried pear slices under different sugar treatments, highlighting differences in water-binding states. The CK group showed a distinct free water peak, indicating a loose structure with weak water retention and high water mobility. In contrast, sugar-treated samples exhibited a broader T₂₁ peak and a weakened or absent free water signal, suggesting enhanced water-binding capacity and restricted water migration. The 3-Suc group had the lowest T₂₁ intensity and nearly no free water peak, indicating that water existed mainly in a bound state. This was likely due to sucrose forming a dense glassy matrix during drying, which blocked water diffusion. The trend aligns with the Tg results: sucrose-treated samples were more hydrophobic, leading to improved moisture absorption. In addition, the sucrose-treated group exhibited the most uniform microstructure with minimal structural collapse, further demonstrating its strong ability to prevent moisture penetration. The findings indicated that sugar type and concentration significantly influenced water-binding behavior in freeze-dried pear slices.

3.3.3. Moisture Absorption Rate

Figure 5B shows the effects of different sugar types and concentrations on the moisture absorption behavior of freeze-dried pear slices over time. All groups followed a typical three-phase pattern: rapid absorption (0–24 h), a slowing phase (24–48 h), and a plateau (48–96 h). The CK group exhibited the highest moisture uptake, reaching over 28.44 ± 0.33% at 24 h and 33.90 ± 2.09% at 96 h, reflecting its loose structure, strong surface hydrophilicity, and poor water-binding capacity. In contrast, sucrose-treated samples, especially the 3-Suc group, showed the lowest final absorption rate (24.24 ± 1.07%) and stabilized after 48 h, indicating superior moisture resistance and structural stability. This may be due to a sugar layer forming on the surface during drying, limiting water penetration. The glucose group showed moderate absorption, while the 3-Fru group had relatively high moisture absorption, close to CK, suggesting that fructose’s strong hydrophilicity and low glass stability led to a less effective moisture barrier. These results confirm that sucrose treatment enhances the structural compactness, restricts water mobility, and maintains surface hydrophobicity of freeze-dried pear slices, which may contribute to an extended shelf life.

3.4. Mechanism Analysis

Figure 6 illustrates the proposed mechanism by which sugar impregnation enhances the structural stability and moisture resistance of freeze-dried pear slices. Untreated samples are prone to cell wall shrinkage and structural collapse during drying, leading to the formation of large pores, discontinuous walls, and a loose matrix, which in turn compromises texture and storage stability. Sugar impregnation (e.g., sucrose) introduces sugar molecules into the tissue through osmotic pressure, where they interact with cell wall components via hydrogen bonding during drying. This provides structural support and filling, thereby improving the microstructure by reducing pore size and enhancing wall integrity. Notably, under 3-Suc treatment, the glass transition temperature increases significantly, resulting in a more stable system and denser structure that effectively inhibits external moisture penetration. In summary, sugar impregnation, particularly with sucrose, can markedly improve the structural properties and moisture resistance of freeze-dried pear slices, offering an effective strategy to enhance product quality and extend shelf life.

The mechanism proposed in this study constitutes a conceptual model derived from correlative analysis of structural, thermal, and physicochemical data. While it offers a plausible explanation for the observed enhancements in texture and moisture resistance, it remains a hypothesis that requires further direct experimental validation, such as through molecular interaction analysis. Additionally, this work has two main limitations. First, the evaluation of moisture stability and structural changes was conducted under controlled short-term hygroscopic conditions rather than real time long-term storage; thus, the actual shelf-life extension under practical storage requires further confirmation. Second, as the experiments were performed only on ‘Huangguan’ pears, the applicability of the sugar impregnation strategy to other fruit varieties with distinct cell wall compositions and moisture characteristics remains to be investigated. Future studies should incorporate long-term storage trials and extend this approach to a wider range of fruits to validate and generalize the present findings.

4. Conclusions

This study investigated the effects of sugar impregnation with different sugar types (sucrose, glucose, fructose) and concentrations (2 M and 3 M) on the appearance, structural stability, and moisture behavior of freeze-dried pear slices. Sugar treatments significantly enhanced brightness, reduced non-enzymatic browning, and improved the structural integrity of samples. Among all samples, the freeze-dried pear slices treated with a high concentration of sucrose (i.e., 3-Suc group) had a desirable visual appearance and the highest hardness and crispness with dense and uniform organization. This was attributed to the fact that sucrose promoted the formation of a stable amorphous structure and had a strong interaction with the cell wall matrix. In addition, the sucrose group had lower water content, excellent glassy-state stability, and hydrophobicity. The above results indicated that sucrose treatment was beneficial to improve the storage stability of freeze-dried pear slices. In contrast, fructose-treated and glucose-treated samples showed weak structural and moisture control due to low Tg and strong hydrophilicity. Overall, sugar type and concentration significantly influenced glassy structure formation and moisture stability, with sucrose delivering optimal results for enhancing the storage and quality of dried fruit products.