https://orcid.org/0000-0002-6972-7078
Abstract
The present study aimed to evaluate the combined application of edible coatings (alginate or pectin) and the osmotic dehydration process (glucose solution at 40 °Brix and 60 °Brix, solution temperatures of 20 and 40 °C, and process times of 1, 2, 4, 8, and 16 h) on mass transfer kinetics and physicochemical parameters (weight reduction (WR), water loss (WL), solids gain (SG), and optical and mechanical properties) of pear cubes. The best dehydration performance was observed in the coated samples treated at 60 °Brix-40 °C. At the end of this process, the coated samples showed higher values of WR (77.30% and 75.13%) and WL (77.39%; 74.38%) and lower values of SG (1.69% and 1.24%) than the uncoated samples (WR: 65.97%, WL: 68.38%, and SG: 4.12%). Additionally, optimal retention of optical and mechanical properties was achieved in the coated samples exposed to the 40 °Brix-40 °C condition.
Introduction
The worldwide production and consumption of food result in a significant amount of food waste. Among all stages of the food supply chain, the highest levels of waste are observed in fruit and vegetable production, primarily due to overproduction and low demand in the wholesale market. As these variables are inherently unpredictable, the task of minimising food waste becomes particularly challenging after the production stage.
Several preservation technologies have been extensively investigated to mitigate food waste by developing new functional additives or value-added products. One technology that has garnered significant attention is osmotic dehydration (OD). This process is favoured due to its ability to maintain the natural characteristics of fruits and vegetables, extend their shelf life, and enhance their sensory and nutritional properties (Zhao et al., 2017; Etemadi et al., 2020). Moreover, OD has the potential to reduce energy consumption, making it an environmentally friendly preservation technique.
However, the migration of osmotic solids into the food matrix during OD can adversely affect both the processing efficiency and the nutritional composition of the final products (Jalaee et al., 2011), thus limiting its widespread commercial application. To address this challenge, several researchers have explored the use of different pretreatments to control the absorption of osmotic solids in food (Wang et al., 2013; Akbarian et al., 2015; Rodriguez et al., 2016; Gamboa-Santos & Campañone, 2019; Jansrimanee & Lertworasirikul, 2020; Liu et al., 2020; Sotera et al., 2021; Sui et al., 2022).
Edible coatings, comprising polysaccharides, lipids, proteins, or their combinations, have been studied in different applications, either applied on the surface of food as a pretreatment to improve the efficiency preservation processes (Rodriguez et al., 2016, 2021; Meerasri & Sothornvit, 2023), to extend the shelf-life of fresh produce (Bangar et al., 2023; Ge et al., 2023; Kaur et al., 2023) or as a food packaging material (Saleena et al., 2023).
Polysaccharides such as sodium alginate and low methoxyl pectin have garnered significant interest due to their ability to form robust gels in the presence of divalent cations (e.g., calcium). These polysaccharides are non-toxic, cost-effective, and widely employed in the food industry to enhance the structural properties of various food products (Tharanathan, 2003; Tahir et al., 2019).
Several authors have employed empirical models to characterise the drying process, as they prove valuable for the design and improvement of drying systems. While various approaches based on Fick's second law have been described for modelling mass transfer in osmosis processes (Chausi et al., 2001). More straightforward semi-empirical methods, such as the models introduced by Hawkes and Flink (1978) and Peleg (1988), have found widespread use in characterising mass transfer kinetics in OD. The feasibility of applying Peleg's model to describe the behaviour of fruits and vegetables during OD has been explored in several studies (Khin et al., 2007; Atarés et al., 2008). However, there is limited information available in the literature regarding the OD of coated samples.
Therefore, the objective of this study was to assess the combined application of different edible coatings (sodium alginate/calcium lactate or low methoxyl pectin/calcium lactate) and the OD process under various conditions (temperature: 20 and 40 °C, processing time: 1, 2, 4, 8, and 16 h, glucose syrup solutions: 40 °Brix and 60 °Brix) on the kinetics of mass transfer (weight loss, water loss, and solids gain), as well as on the optical and mechanical properties of pear cubes. The goal was to determine the most favourable operating conditions that would yield optimal dehydration performance, minimise excessive solids gain, and preserve the natural quality of the product. Additionally, the kinetics of mass transfer were modelled using the Peleg model.
Materials and methods
Materials
Pears (Pyrus communis L.) cv. Packham's Triumph were obtained from a local marketplace and stored at 0 °C. Fruits of similar colour, size, and firmness were selected to form homogeneous groups. The initial characteristics were measured in duplicate (Association of Official Analytical Chemist (AOAC), 1990): moisture content 85.8 ± 1.7% (wet basis w.b.) and soluble solids 12.8 ± 1.8 (°Brix).
Sample preparation
The pears were cut into cubes (1 cm3) after being washed and peeled. Then, 50 g of samples were immersed in film-forming solutions (sodium alginate 2% w/w (Sigma-Aldrich, Argentina) or low-methoxyl pectin 3% w/w (CpKelco, Argentina)) for 5 min at 25 °C. Then, they were removed from the solution and dried in tissue paper for 2 min. Subsequently, samples were dipped in calcium lactate solution (5% w/w) for 5 min and dried in tissue paper for 2 min. Experiments were performed in duplicate.
Osmotic dehydration (OD)
Osmotic dehydration was performed in a thermostatic shaker (model TT400, brand Ferca, Buenos Aires, Argentina) with constant agitation (100 cycles/min). The fruit/solution ratio selected was 1/30. Osmotic dehydration was carried out using glucose syrup solutions at two concentrations (40 °Brix and 60 °Brix) and two temperatures (20 and 40 °C). The samples were dehydrated for different OD times (1, 2, 4, 8 and 16 h). Solution concentrations were measured using a digital refractometer (Hanna Instruments model HI96801, USA). All the experiments were carried out in duplicate.
Determination of water content and soluble solids content
Water content (WC %) was determined using an oven (Gallenkamp, UK) at 70 °C until constant weight (~24 h) (Association of Official Analytical Chemist (AOAC), 1990). Soluble solids (SS) were determined using a digital refractometer (Hanna Instruments model HI96801, USA). Results were expressed as °Brix ±0.2 °Brix at 20 °C. Both determinations were carried out in duplicate.
Weight reduction (WR), water loss (WL) and solid gain (SG) were calculated according to Rodriguez et al. (2016). The barrier ability of edible coatings (DP) was calculated as the relation between WL and solig gain of the samples under various operating conditions.
Empirical model
To describe the behaviour of the sample during OD, the Peleg model was applied. According to the model, water content with respect to processing time can be modelled by the following eq. (1):
where t is the OD time, H is the water content at time t, H0 is the initial water content, and k1w, k2w are model parameters. Model parameters have physical meaning (Della Roca & Mascheroni, 2010). From eq. 1, taking limit as t → 0 (short processing times), the following equation is obtained:
From eq. 2, k1 is inversely proportional to the rate initial of mass transfer. On the other hand, if t → ∞, we can find the relation of parameter k2w with the equilibrium moisture He:
These equations are valid to describe the solids content as a function of time substituting H–H0 by solid content (Della Roca & Mascheroni, 2010).
Quality analysis
The optical characteristics of both the untreated and osmodehydrated specimens were evaluated using a Konica Minolta Chromameter (Model CR 400/410, Japan). Calibration of the device was performed with a standard white reflector plate, and the system chosen was CIE L*, a*, b*. The outcomes were presented as the colour variation (ΔC) following the methodology described by Rodriguez et al. (2016). Each treatment involved measurements from twenty-four pear cubes.
The evaluation of the mechanical characteristics of both the uncoated and coated samples was conducted through a puncture test. These tests were performed under controlled conditions at 25 °C utilising a Texture Analyser (model TA-XT2, Stable Microsystems, England) with a consistent displacement rate of 0.5 mm/s. A cylindrical puncture probe SMSP/3 (3 mm in diameter) was employed, allowing for a displacement of up to 50% relative to the initial height of the cube. For each treatment, measurements were obtained from thirty pear cubes to ensure statistical reliability. The parameters under analysis included Firmness (N), defined as the maximum force required to fracture the sample, and Stiffness (N/mm), which represents the slope of the force-distance curve from the beginning of the curve to the point of fracture (Rodriguez et al., 2016). Data were normalised with respect to fresh samples.
Experimental design and statistical analysis
The impact of processing conditions, including temperature and osmotic solution concentration, the presence of an edible coating, and processing duration, was assessed through ANOVA analysis. Tukey's test was subsequently employed to discern disparities between the mean values. Statistical significance was determined at the P < 0.05 level. Statistical analysis was carried out using STATGRAPHICS Plus 5.1 software (USA).
Results and discussion
Osmotic dehydration
Weight reduction and WL were statistically affected by all processing conditions (concentration and temperature of solution and processing time) and the application of edible coating (P < 0.05). Figure 1a,b show the kinetics of WR and WL, respectively, in relation to the processing time. Weight reduction and WL in the pear cubes increased significantly (P < 0.05) with higher processing time, concentration, and temperature of the glucose syrup solution. Besides, the application of edible coating improves significantly (P < 0.05) the WR and WL throughout processing time, regardless of the temperature and concentration of the solution. Regarding SG (Fig. 2), coated samples subjected to glucose syrup solution at 40 °Brix presented a greater uptake of solids compared to the uncoated ones (P < 0.05). This trend was observed regardless of the temperature of the solution.
Effect of process conditions and edible coating on the (a) weight reduction (WR %) and (b) water loss (WL %) in pear cubes during osmotic dehydration. Lowercase letters indicate significant differences among alginate-coated (ALG), pectin-coated (PEC), and uncoated samples (S/R) and uppercase letters indicate significant differences in processing time.
Effect of process conditions and edible coating on the gain solids (SG %) in pear cubes during osmotic dehydration. Lowercase letters indicate significant differences among alginate-coated (ALG), pectin-coated (PEC), and uncoated samples (S/R) and uppercase letters indicate significant differences in processing time.
In samples subjected to glucose syrup solution at 60 °Brix-20 °C, alginate presented a barrier effect at the end of the process, while in those treated at 60 °Brix-40 °C, both edible coatings performed as a barrier against osmotic solids uptake (P < 0.05).
The barrier capacity parameter was determined by dehydration performance (WL/SG) for each edible coating (Fig. 3). It could observe that WL/SG values of coated samples were similar to uncoated ones at 40 °Brix, irrespective of the temperature of the solution. Thus, coatings did not show an effect of barrier at this level of solute concentration. Regarding the experiment at 60 °Brix-20 °C, the alginate-coated samples presented better dehydration performance than pectin-coated and uncoated ones (P < 0.05) at the conclusion of the osmotic treatment. As the solution temperature increased, the coated samples demonstrated improved dehydration performance compared to the uncoated samples (P < 0.05). During the osmotic process, under all studied conditions, the dehydration performance increased significantly from 8 h of the process (P < 0.05). Similar results were found by Ebrahimian et al. (2017), who studied the application of carboxymethyl cellulose (0.5%, 1%, and 3%) and pectin (1%, 2%, and 3%) coatings in pumpkin slices subjected to OD process (sucrose 50% and 60% for 3 h). These authors reported that the rate of SG decreased as the concentrations of sucrose, carboxymethyl cellulose, and pectin solutions increased. Jansrimanee & Lertworasirikul (2017) observed that the application of alginate coating (3%) as pretreatment of the OD process in pumpkin (70% w/w sucrose solution with a ratio of solution to samples of 4:1 for 12 h) allowed higher dehydration performance (WL/SG = 5.28) achieving higher WL and lower SG.
Performance ratio of coated and uncoated samples under different conditions of osmotic dehydration. Lowercase letters indicate significant differences among alginate-coated (ALG), pectin-coated (PEC), and uncoated samples (S/R) and uppercase letters indicate significant differences in processing time.
The coated samples subjected to 60 °Brix-40 °C exhibited the best performance as a barrier against solid uptake. This can be attributed to the fact that the application of the coating on the food surface reinforces cellular integrity, enabling resistance against high osmotic pressure. This, in turn, allows for better control of osmotic solids uptake without compromising the rate of WL (Azam et al., 2013). Furthermore, the use of elevated temperatures and solution concentrations can lead to a decreased absorption of osmotic solids as a result of the accelerated WL rate. Previous studies have observed that an increase in temperature or solution concentration accelerates WL without significant changes in SG (El-Aouar et al., 2006; Shi et al., 2009).
Empirical model
Parameters of the empirical model of Peleg were calculated from experimental data. Table 1 revealed that as the temperature and solution concentration increased, the initial rate of moisture transfer (indicated by the inverse of k1w) also increased in both the alginate-coated and uncoated samples. In the case of pectin-coated samples, a similar tendency was observed, except for samples exposed to the highest concentration (60 °Brix), where the impact of temperature on the initial moisture transfer rate was not significant. These findings are consistent with the experimental data obtained for WL. Chuquillanqui Romero (2017) applied the empirical model of Peleg to describe the drying kinetics of kiwi slices subjected to OD (sucrose, honey, and glucose solutions at 40 °Brix, 50 °Brix, and 60 °Brix). The author showed that as the concentration of the osmotic solutions increased, the rate of water transfer toward the solution increased. Della Roca & Mascheroni (2010) modelled the kinetics of mass transfer during OD of potato cubes (30%–10% sucrose-salt solution; 40%–10%, and 50%–10% w/w, 1:4 sample/solution ratio and at 40 °C) and found that WL tended to increase with increasing solution concentration. Ganjloo et al. (2012) employed Peleg's equation to study the impact of process parameters on the mass transfer kinetics, particularly regarding solids gain and WL during OD. They conducted experiments using a sucrose solution at concentrations ranging from 30% to 50% w/w and at temperatures of 30, 40, and 50 °C. In all cases, they observed a noteworthy decrease in the initial mass transfer rate parameter (k1) with a simultaneous increase in the solution concentration and temperature, implying a corresponding rise in the initial mass transfer rate (P < 0.05). Brochier et al. (2015) applied Peleg's model to predict the equilibrium condition and indicated that the model was appropriate for WL and solid uptake during OD (temperature 30 and 50 °C and solutions of glycerol or sorbitol to concentrations of 30%, 50%, and 70%) of Yacon (Smallanthus sonchifolius).
Parameters calculated for Peleg's model applied to water content and solid content values
| °Brix/°C | k1w | k2w | k1w−1 | He | r2 | k1ss | k2ss | k1ss−1 | SSe | r2 | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Uncoated samples | 40/20 | −1422.7 | −0.063 | −0.0007 | 69.846 | 0.943 | 1156.6 | 0.062 | 0.0009 | 28.844 | 0.964 |
| 40/40 | −1052.8 | −0.042 | −0.0009 | 61.778 | 0.908 | 968.2 | 0.043 | 0.0010 | 36.077 | 0.918 | |
| 60/20 | −596.8 | −0.033 | −0.0017 | 55.341 | 0.911 | 490.9 | 0.033 | 0.0020 | 42.617 | 0.940 | |
| 60/40 | −445.9 | −0.021 | −0.0022 | 39.132 | 0.967 | 433.9 | 0.021 | 0.0023 | 61.3104 | 0.936 | |
| Alginate-coated samples | 40/20 | 440.3 | −0.067 | 0.0023 | 73.330 | 0.995 | 392.7 | 0.064 | 0.0025 | 25.683 | 0.996 |
| 40/40 | 424.2 | −0.046 | 0.0024 | 66.544 | 0.976 | 403.1 | 0.044 | 0.0025 | 32.556 | 0.995 | |
| 60/20 | 407.0 | −0.038 | 0.0025 | 62.215 | 0.983 | 372.4 | 0.038 | 0.0027 | 36.280 | 0.988 | |
| 60/40 | 305.2 | −0.020 | 0.0033 | 38.189 | 0.977 | 330.4 | 0.019 | 0.0030 | 62.389 | 0.964 | |
| Pectin-coated samples | 40/20 | −522.7 | −0.060 | −0.0019 | 70.741 | 0.978 | 352.4 | 0.061 | 0.0029 | 27.068 | 0.988 |
| 40/40 | 450.0 | −0.044 | 0.0022 | 64.779 | 0.984 | 390.7 | 0.044 | 0.0026 | 33.398 | 0.985 | |
| 60/20 | −278.3 | −0.043 | −0.0036 | 64.150 | 0.979 | 240.5 | 0.042 | 0.0041 | 34.320 | 0.974 | |
| 60/40 | 373.1 | −0.022 | 0.0027 | 42.709 | 0.974 | 373.9 | 0.021 | 0.0027 | 57.736 | 0.968 |
| °Brix/°C | k1w | k2w | k1w−1 | He | r2 | k1ss | k2ss | k1ss−1 | SSe | r2 | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Uncoated samples | 40/20 | −1422.7 | −0.063 | −0.0007 | 69.846 | 0.943 | 1156.6 | 0.062 | 0.0009 | 28.844 | 0.964 |
| 40/40 | −1052.8 | −0.042 | −0.0009 | 61.778 | 0.908 | 968.2 | 0.043 | 0.0010 | 36.077 | 0.918 | |
| 60/20 | −596.8 | −0.033 | −0.0017 | 55.341 | 0.911 | 490.9 | 0.033 | 0.0020 | 42.617 | 0.940 | |
| 60/40 | −445.9 | −0.021 | −0.0022 | 39.132 | 0.967 | 433.9 | 0.021 | 0.0023 | 61.3104 | 0.936 | |
| Alginate-coated samples | 40/20 | 440.3 | −0.067 | 0.0023 | 73.330 | 0.995 | 392.7 | 0.064 | 0.0025 | 25.683 | 0.996 |
| 40/40 | 424.2 | −0.046 | 0.0024 | 66.544 | 0.976 | 403.1 | 0.044 | 0.0025 | 32.556 | 0.995 | |
| 60/20 | 407.0 | −0.038 | 0.0025 | 62.215 | 0.983 | 372.4 | 0.038 | 0.0027 | 36.280 | 0.988 | |
| 60/40 | 305.2 | −0.020 | 0.0033 | 38.189 | 0.977 | 330.4 | 0.019 | 0.0030 | 62.389 | 0.964 | |
| Pectin-coated samples | 40/20 | −522.7 | −0.060 | −0.0019 | 70.741 | 0.978 | 352.4 | 0.061 | 0.0029 | 27.068 | 0.988 |
| 40/40 | 450.0 | −0.044 | 0.0022 | 64.779 | 0.984 | 390.7 | 0.044 | 0.0026 | 33.398 | 0.985 | |
| 60/20 | −278.3 | −0.043 | −0.0036 | 64.150 | 0.979 | 240.5 | 0.042 | 0.0041 | 34.320 | 0.974 | |
| 60/40 | 373.1 | −0.022 | 0.0027 | 42.709 | 0.974 | 373.9 | 0.021 | 0.0027 | 57.736 | 0.968 |
°Brix, solution concentration; °C, solution temperature.
Parameters calculated for Peleg's model applied to water content and solid content values
| °Brix/°C | k1w | k2w | k1w−1 | He | r2 | k1ss | k2ss | k1ss−1 | SSe | r2 | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Uncoated samples | 40/20 | −1422.7 | −0.063 | −0.0007 | 69.846 | 0.943 | 1156.6 | 0.062 | 0.0009 | 28.844 | 0.964 |
| 40/40 | −1052.8 | −0.042 | −0.0009 | 61.778 | 0.908 | 968.2 | 0.043 | 0.0010 | 36.077 | 0.918 | |
| 60/20 | −596.8 | −0.033 | −0.0017 | 55.341 | 0.911 | 490.9 | 0.033 | 0.0020 | 42.617 | 0.940 | |
| 60/40 | −445.9 | −0.021 | −0.0022 | 39.132 | 0.967 | 433.9 | 0.021 | 0.0023 | 61.3104 | 0.936 | |
| Alginate-coated samples | 40/20 | 440.3 | −0.067 | 0.0023 | 73.330 | 0.995 | 392.7 | 0.064 | 0.0025 | 25.683 | 0.996 |
| 40/40 | 424.2 | −0.046 | 0.0024 | 66.544 | 0.976 | 403.1 | 0.044 | 0.0025 | 32.556 | 0.995 | |
| 60/20 | 407.0 | −0.038 | 0.0025 | 62.215 | 0.983 | 372.4 | 0.038 | 0.0027 | 36.280 | 0.988 | |
| 60/40 | 305.2 | −0.020 | 0.0033 | 38.189 | 0.977 | 330.4 | 0.019 | 0.0030 | 62.389 | 0.964 | |
| Pectin-coated samples | 40/20 | −522.7 | −0.060 | −0.0019 | 70.741 | 0.978 | 352.4 | 0.061 | 0.0029 | 27.068 | 0.988 |
| 40/40 | 450.0 | −0.044 | 0.0022 | 64.779 | 0.984 | 390.7 | 0.044 | 0.0026 | 33.398 | 0.985 | |
| 60/20 | −278.3 | −0.043 | −0.0036 | 64.150 | 0.979 | 240.5 | 0.042 | 0.0041 | 34.320 | 0.974 | |
| 60/40 | 373.1 | −0.022 | 0.0027 | 42.709 | 0.974 | 373.9 | 0.021 | 0.0027 | 57.736 | 0.968 |
| °Brix/°C | k1w | k2w | k1w−1 | He | r2 | k1ss | k2ss | k1ss−1 | SSe | r2 | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Uncoated samples | 40/20 | −1422.7 | −0.063 | −0.0007 | 69.846 | 0.943 | 1156.6 | 0.062 | 0.0009 | 28.844 | 0.964 |
| 40/40 | −1052.8 | −0.042 | −0.0009 | 61.778 | 0.908 | 968.2 | 0.043 | 0.0010 | 36.077 | 0.918 | |
| 60/20 | −596.8 | −0.033 | −0.0017 | 55.341 | 0.911 | 490.9 | 0.033 | 0.0020 | 42.617 | 0.940 | |
| 60/40 | −445.9 | −0.021 | −0.0022 | 39.132 | 0.967 | 433.9 | 0.021 | 0.0023 | 61.3104 | 0.936 | |
| Alginate-coated samples | 40/20 | 440.3 | −0.067 | 0.0023 | 73.330 | 0.995 | 392.7 | 0.064 | 0.0025 | 25.683 | 0.996 |
| 40/40 | 424.2 | −0.046 | 0.0024 | 66.544 | 0.976 | 403.1 | 0.044 | 0.0025 | 32.556 | 0.995 | |
| 60/20 | 407.0 | −0.038 | 0.0025 | 62.215 | 0.983 | 372.4 | 0.038 | 0.0027 | 36.280 | 0.988 | |
| 60/40 | 305.2 | −0.020 | 0.0033 | 38.189 | 0.977 | 330.4 | 0.019 | 0.0030 | 62.389 | 0.964 | |
| Pectin-coated samples | 40/20 | −522.7 | −0.060 | −0.0019 | 70.741 | 0.978 | 352.4 | 0.061 | 0.0029 | 27.068 | 0.988 |
| 40/40 | 450.0 | −0.044 | 0.0022 | 64.779 | 0.984 | 390.7 | 0.044 | 0.0026 | 33.398 | 0.985 | |
| 60/20 | −278.3 | −0.043 | −0.0036 | 64.150 | 0.979 | 240.5 | 0.042 | 0.0041 | 34.320 | 0.974 | |
| 60/40 | 373.1 | −0.022 | 0.0027 | 42.709 | 0.974 | 373.9 | 0.021 | 0.0027 | 57.736 | 0.968 |
°Brix, solution concentration; °C, solution temperature.
Regarding equilibrium moisture (He), all samples (alginate-coated, pectin-coated, and uncoated) exhibited lower equilibrium moisture as the temperature and concentration of the solutions increased. When comparing the coated and uncoated samples, it became evident that the coating facilitated the initial rate of moisture transfer under all studied conditions. However, contrary to expectations, the coating did not have the desired effect on equilibrium moisture, as it resulted in equal or higher equilibrium moisture values.
In terms of soluble solid content, higher levels were obtained in the uncoated samples as the temperatures and concentrations of the solution increased, as indicated in Table 1. Similar findings were reported by Arias et al. (2017), who investigated the behaviour of SS and moisture during the dehydration of mango (Mangifera indica L.) with sucrose solutions with concentrations of 45% and 60% and temperatures of 20, 35, and 50 °C. Additionally, in the case of the coated samples, this effect was more pronounced with increasing solution concentrations. With longer processing times, higher temperatures, and higher solution concentrations, there was an increase in equilibrium solid values (SSe), with the pectin-coated samples showing the lowest values.
Quality analysis
Optical properties
Figure 4 shows the colour difference in relation to the processing time of the coated and uncoated samples subjected to OD. Colour differences were statistically affected by the temperatures and concentrations of the solution (P < 0.05). The results obtained for OD at 40 °Brix-20 °C showed that processing time had no significant effect (P > 0.05) on all samples and that alginate-coated samples showed the smallest colour difference as the process progressed. By increasing solution temperature, results showed a decreasing trend of colour difference with the processing time, being significant after 8 h (P < 0.05). Under these conditions, pectin-coated samples presented the smallest colour difference (<0.05). In the case of samples subjected to glucose syrup solution at 60 °Brix, an increasing trend of the colour difference on the sample surface was observed, being significant (P < 0.05) after 4 h in the process at 20 °C and after 8 h in the process at 40 °C. On the other hand, in the process at 20 °C, we could observe that the presence of the edible coating had a significant effect (P < 0.05) on the colour surface since the coated samples presented lower colour differences than the uncoated ones. In turn, the alginate-coated samples showed better preservation of the surface colour than the pectin-coated ones (P < 0.05), regardless of the solution concentration. However, in the process at 40 °C, the pectin coating allowed good preservation of the colour surface in pear cubes. These results agreed with those found by Ferrari et al. (2013) who worked on cut melon coated with pectin and osmodehydrated in sucrose solution (40 °Brix and 15 °C) and Silva et al. (2015) who studied pineapple coated with pectin and osmodehydrated in sucrose solution (50% of sucrose, 4% of calcium lactate, and 2% of ascorbic acid). These authors reported that pectin coating allowed maintaining the colour surface of fruit during OD.
Effect of process conditions and edible coatings on the total colour difference (ΔC) in pear cubes during osmotic dehydration. Lowercase letters indicate significant differences among alginate-coated (ALG), pectin-coated (PEC), and uncoated samples (S/R) and uppercase letters indicate significant differences in processing time.
In terms of process conditions, Dermesonlouoglou & Giannakourou (2018) studied the OD of mango and attributed the total colour change to the liquid phase composition and structural changes induced by the osmotic process. Furthermore, Seraji et al. (2012) investigated the application of a carboxymethylcellulose coating with ascorbic acid on the pumpkin and reported that the coated and osmotically dehydrated samples exhibited better colour quality compared to the uncoated samples subjected only to OD. Jalaee et al. (2011) analysed the effects of edible coatings (low methoxyl pectin, carboxymethyl cellulose, and corn starch) combined with OD (using sucrose solutions at 50% and 60% at 30 °C) on Golden Delicious apples. These authors reported that, after the air-drying process, the coated samples exhibited smaller colour differences compared to the uncoated samples following OD. They attributed this result to the barrier capacity of the coating, which allowed better control of gas exchange between the samples and the environment, limiting solid uptake. In the OD of mango, Sakooei-Vayghan et al. (2020) observed that the use of a pectin coating (2%) in combination with OD (using a sorbitol solution at 35 °Brix for 30 and 45 min) as pretreatments resulted in improved optical properties. The authors identified a synergistic effect between the pectin coating and the OD process, leading to the inactivation of the polyphenol oxidase enzyme and preventing the oxidation of phenolic compounds.
Mechanical properties
Figure 5a,b show the behaviour of normalised firmness and stiffness of coated and uncoated samples subjected to different conditions of the OD process. Firmness (NF) and stiffness (NS) were statistically affected by the processing conditions (temperatures and concentrations of solution and time) (P < 0.05). The results obtained during OD at 40 °Brix-20 °C demonstrated a significant increase (P < 0.05) in NF and NS from 1 to 4 h of processing time. However, beyond this point, both parameters gradually decreased until the end of the process. When the solution temperature was increased, the firmness of the samples was not affected by the processing time (P > 0.05). However, the stiffness of the samples decreased significantly (P < 0.05) during the OD process.
Effect of process conditions and edible coatings on (a) normalised firmness (NF) and (b) normalised stiffness (NS) in pear cubes during osmotic dehydration. Lowercase letters indicate significant differences among alginate-coated (ALG), pectin-coated (PEC), and uncoated samples (S/R) and uppercase letters indicate significant differences in processing time.
In the case of samples exposed to the highest solution concentration (60 °Brix), a significant decrease (P < 0.05) in firmness and stiffness was observed at the end of the 16-h OD process when the samples were subjected to a temperature of 20 °C. However, with an increase in the solution temperature, a significant increase in firmness of the samples was observed as the processing time progressed. Additionally, stiffness showed an increase starting from 2 h (P < 0.05), but beyond this time point, it exhibited a decreasing trend until the end of the processing.
Regarding the edible coating, coated samples treated at 20 °C showed better preservation of the mechanical properties than the uncoated samples (P < 0.05), regardless of the concentration of the solution. It could be observed that the best preservation of the mechanical properties was obtained with pectin-coated samples treated at 40 °Brix (P < 0.05) and alginate-coated samples treated at 60 °Brix (P < 0.05). By increasing the solution temperature, alginate-coated samples subjected to 40 °Brix showed better preservation of the mechanical properties than pectin-coated and uncoated samples (P < 0.05). However, with increasing solution concentration (60 °Brix) the protective effect of coatings on mechanical properties was not observed.
In a previous study (Rodriguez et al., 2016) we observed a similar behaviour in alginate-coated pumpkin cubes osmotically dehydrated with glucose solution at 40 °Brix-20 °C. This effect was attributed, on the one hand, to the uptake of calcium into the sample surface during the application of the coating, which reinforced the integrity of the samples. On the other hand, the barrier properties of the coating against the absorption of osmotic solids prevented damage to the cellular tissues of the fruit. Jalaee et al. (2011) reported that the OD process improves firmness in both coated and uncoated samples due to the osmotic solids absorption that occurs simultaneously with the outflow of water. On the other hand, they indicated that the presence of calcium in the coating causes some ions to enter the sample reinforcing the integrity of the tissues, which prevents tissue shrinkage during the osmotic process. Ebrahimian et al. (2017) observed that the firmness of pumpkin cubes coated with pectin (1%, 2% and 3%) was higher than those coated with carboxymethylcellulose (0.5%, 1% and 2%) and those uncoated ones. In addition, they found lower resistance to osmotic solids uptake in the uncoated samples, which led to a reduction in the elasticity and firmness of the samples.
Conclusion
The findings from this study demonstrate the pivotal role of operating conditions and the choice of edible coating in influencing mass transfer kinetics, optical characteristics, and mechanical properties of osmotically dehydrated pear cubes. Notably, the application of edible coatings proved to be a critical factor, substantially enhancing WR and WL throughout the dehydration process. The coated samples exposed to a glucose syrup solution at 60 °Brix-40 °C exhibited a remarkable decrease in solid uptake compared to their uncoated counterparts. This translated into a notable improvement in the dehydration performance of the pear cubes.
Furthermore, the results pertaining to optical and mechanical properties revealed that the OD condition for preserving the quality of pear cubes involved a solution concentration of 40 °Brix and a temperature of 40 °C. Remarkably, distinctive behaviours were noted with different coatings under 40 °Brix-40 °C conditions – pectin coating excelled in preserving optical properties, while alginate coating demonstrated superior preservation of mechanical properties.
These results not only show the strong interplay between operating parameters and coating types but also underscore the necessity for further investigations. Future studies should delve deeper into defining the optimal conditions for the synergistic application of edible coatings and OD, aiming to achieve a delicate balance between high dehydration performance and the preservation of both optical and mechanical properties of pear cubes. This work sets the stage for advancing in the understanding and optimising these processes for enhanced fruit quality.
Acknowledgments
We thank the Physical Metallurgy Research Laboratory (Department of Mechanics of the Faculty of Engineering – UNLP).
Author contributions
Laura Campañone: Conceptualization; formal analysis; writing – original draft; supervision; project administration; writing – review and editing. Edgar Soteras: Conceptualization; formal analysis; investigation; methodology. Anabel Rodriguez: Writing – review and editing; supervision; methodology; writing – original draft; formal analysis.
Conflict of interest
The authors have declared no conflicts of interest for this article.
Ethical approval
Ethics approval was not required for this research.
Funding information
This study was funding by the Agencia Nacional de Promoción de la Investigación, el Desarrollo Tecnológico y la Innovación with the project PICT-2017-0923 “Aplicación de técnicas de conservación y deshidratación de alimentos para la obtención de productos de excelente calidad final”.
Peer review
The peer review history for this article is available at https://www.webofscience.com/api/gateway/wos/peer-review/10.1111/ijfs.16961.
Data availability statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.
References
This research work provides the tools to apply Peleg's model to osmotic dehydration experiments.
This research work applied Peleg's model to the experimental data of seedless guava during osmotic dehydration process. Their results allow us to discuss our findings.
This work is pioneering in the field of osmotic dehydration with coated samples, introducing all of us to this subject.
This work was the starting point of our studies in this area.
