3.1. Osmotic Dehydration of Figs
The effect of OD parameters (temperature and time) on the progress of the OD processing was investigated for 1/4 and 1/6 ratios of sample per OD solution (
ws/
wOD). The data obtained for the WL (Equation (1)) and SG (Equation (2)) of the figs during OD at 25, 35 and 45 °C were plotted (
Figure 1).
WL and SG values increased significantly (
p < 0.05) up to the first 100 min of OD for all studied samples, while for prolonged immersion times the process tended to equilibrium. Similar trends in the change of WL and SG have been reported in studies of OD on cherries [
34], apple slices [
35], and figs [
9], where high mass transfer phenomena were observed till the first ~120 min, while for longer durations the system seemed to equilibrate. The highest values for WL and SG, observed at the end of the processing (300 min), were estimated as 1.67, 2.24 and 2.75 g H
2O/g i.d.m. and 0.15, 0.23 and 0.29 g solids/g i.d.m. at 25, 35 and 45 °C, respectively. Higher OD temperature led to increased WL and SG values during OD. It was obvious that the mass transfer phenomena were more pronounced at 35 and 45 °C. A duration of 300 min of OD processing was necessary at 25 °C to reach WL and SG values equal to 1.67 g H
2O/g i.d.m. and 0.15 g solids/g i.d.m., respectively, whereas at 45 °C, only 45 min was sufficient to achieve equal values of WL and SG. Dermesonlouoglou et al. [
12,
13] and Mandala et al. [
14], reported that when increasing the OD temperature and time, the mass transfer phenomena are enhanced for many plant tissues, such as tomato (WL: 0.98–12.10 g H
2O/g i.d.m.; SG: 0.05–4.52 g s/g i i.d.m.), cucumber (WL: 9.02–24.32 g H
2O/g i.d.m.; SG: 0.08–4.95 g s/g i.d.m.), goji berries (WL: 1.12–2.48 g H
2O/g i.d.m.; SG: 0.41–1.96 g s/g i.d.m.), and apple (WL: 0.55–1.65 g H
2O/g i.d.m.; SG: 0.09–0.62 g s/g i.d.m.).
The experimental data obtained were fitted to Equations (4) and (5) and the respective effective diffusion coefficients (namely
Dew,
Des) were estimated (
Table 1).
The obtained values of
Dew and
Des were in the ranges of 0.94–2.11 × 10
−10 and 0.43–1.53 × 10
−10 m
2/s, respectively. No significant differences in
Dew and
Des values were observed between 1/4 and 1/6
ws/
wOD (
p > 0.05;
Table 1). Moreover, the estimated
Dew and
Des values at 45 °C were two and three times higher, respectively, compared to their counterparts at 25 °C.
During OD, a
w significantly decreased as time and temperature increased (
p < 0.05) (
Figure 2), which is mainly attributed to both water removal and solute uptake from the fig. This observation could be related to the initial low a
w value of the osmotic solution (0.5215). Processing at 25 °C led to a decrease in a
w of the figs from an initial value of 0.986 to ~0.946, while at higher temperatures (35 and 45 °C), a
w reached the value of ~0.907, at the end of OD (300 min) (
Figure 2).
Regarding quality changes, a significant decrease in the redness of the fruit was observed, with
a*-values ranging at the end of the OD from 10.41 ± 2.83 to 16.16 ± 2.84 depending on the intensity of the OD conditions, compared to control (23.32 ± 2.83) (
p < 0.05). During OD, a slight decrease in the lightness of OD fig halves was also observed, with
L*-values ranging from 31.51 to 38.76 ± 3.46 at the end of OD, while the corresponding
L*-value of the untreated fig was 43.11 ± 3.46 (up to ~27% decrease). The color of the solution changed during OD, due to the leaching of pigments from the interior of the fig. Pereira, Ferrari, Mastrantonio, Rodrigues, and Hubinger [
36] observed a decrease in the
L*-values of tropical fruits during OD processing, attributing the phenomenon to the sugar gain. In contrast to these results, other researchers have reported that OD treatment increased the luminosity of fruits [
13,
37].
OD caused reduction in the firmness of the figs that was scored positively during sensory evaluation. At the end of OD, the firmness decreased from a value of 6.83 ± 1.32 N (untreated fruit) to 2.98 ± 1.84 N (~up to 56% decrease), independently of the intensity of the processing conditions (
p > 0.05). This phenomenon could be attributed to cellular changes (plasmolysis) that took place, including loss of cell turgor and filling of air spaces with OD solution, resulting in disruption of the cell membranes [
38]. Similar observations were reported by Najafi, Yusof, Rahman, Ganjloo, and Ling [
39], Lewicki and Lukaszuk [
38] and Castelló, Fito, and Chiralt [
40] working on red pitaya, orange and strawberries, respectively. In contrary, there are studies that report increase of firmness of plant tissues after OD mainly attributed to the high solid gain caused by sugar impregnation [
37,
41]. Regarding fig shrinkage, no significant differences (
p > 0.05) between OD pretreated and untreated samples were observed.
According to the sensory evaluation, OD treated samples received high scores on all organoleptic parameters, assessed as positive and desirable, thus confirming that the high glycerol concentration did not cause any negative effect on flavor and texture of the samples.
The selection of the optimal OD process conditions was based on a combination of a high aw reduction, superior quality and as minimum processing duration as possible. A ratio of sample per OD solution 1/4 (ws/wOD), temperature 45 °C and processing time 90 min were selected as the optimal OD processing conditions. Under these conditions, OD samples exhibited adequate mass transfer of WL: 2.17 g w/g i.d.m., SG: 0.19 g s/g i.d.m. and an aw decrease from ~0.9870 to ~0.9292.
Assessment of Reconstitution of OD Solution on the Effectiveness of OD Processing
The main bottleneck for the OD technique industrial scale-up, is the disposal of large quantities of osmotic solution, with potential environmental issues and high costs for the industry. The reconstitution of the diluted OD solution through the appropriate addition of solutes, could provide a partial solution to the problem.
In the present study, in order to evaluate the effectiveness of a reconstituted OD solution in WL, SG and a
w of the figs, four consecutive trials of OD at optimal conditions (45 °C, 90 min) were conducted using a 3-times reconstituted OD solution. At the end of the first trial, WL, SG and a
w values of figs were estimated and found to be equal to 2.170 g H
2O/g i.d.m., 0.190 g s./g i.d.m. and 0.9214, respectively. The corresponding values at the end of the 4th trial were 2.145 g H
2O/g i.d.m., 0.174 g s./g i.d.m. and 0.9253, respectively. No significant differences were detected on the effectiveness of the trials (
p > 0.05) (
Figure 3).
The main osmotic agent (80%) of OD solution was food-grade glycerol. Its low molecular weight resulted in a significant reduction of water activity of the OD solution, approximately equal to 0.48. After 90 min of OD treatment at 45 °C, the water activity of OD solution slightly increased and reached the value of ~0.54. The repetitive reconstitution of OD solution resulted in a decrease of its water activity in initial value (~0.48). The low water activity seems to have inhibited adequately the microbial growth. However, considering that during OD many organic components, released to OD solution, are able to act as substrates for microbial growth, microbiological analysis of the OD solution was conducted after the 4th trial, in order to evaluate the potential of re-using and recycling the OD solution for many cycles. Concerning the microbiological analysis performed, it is well-known that some osmo-tolerant or osmo-philic microorganisms are resistant to high sugar concentrations, causing major issues in industrial recycling of OD solution. Loads of the TVC and yeasts/molds in the OD solution were below the detection limit (<2 logCFU/g),which could be attributed to the low water activity of OD solution that ensures its microbiological stability and to the short processing time (90 min for each batch) that is not sufficient for observable microbial growth.
Thermal treatment of OD solution is recommended after 5 cycles of OD use, ensuring microbiological stability of OD process and not cross-contamination between different processed batches, thus enabling the reuse of OD solution as many times as possible. The reconstitution of OD solution is a need concerning industrial scale-up of OD-technique, since it will address the environmental concerns of discarding the OD solutions, while simultaneously will result in final products cost reduction.
Garcıa-Martınez, Martınez-Monzó, Camacho, and Martınez-Navarrete [
42], studied the effect of the OD on kiwi fruit and reported that the reuse of the OD solution was effective for at least 10-times without issues related to fruit dehydration level or microbiological contamination. Valdez-Fragoso, Mujica-Paz, Giroux, and Welti-Chanes [
43] after reusing the OD solution for up to six times, estimated similar effectiveness to those of the first trial for OD-treated apples.
3.2. Air-Drying Processing of Osmotic Dehydrated and Untreated Figs
The effect of air-drying temperatures equal to 50, 60 and 70 °C on the drying kinetics of the OD-pretreated (at the optimal conditions) and untreated fig halves, was studied (
Figure 4a,b).
Increase of the air-drying temperature significantly enhanced the water removal from the figs (
p < 0.05), as expected [
44].
Based on the Fick’s 2nd law, the apparent diffusion coefficients (
Deff) for the drying temperatures studied, were calculated considering as 0.55 the a
w-value of the final dried figs (
Table 2).
At the drying temperatures studied, OD-pretreated figs showed significantly higher
Deff values compared to the control counterparts (
p < 0.05). Da Costa Ribeiro et al. [
11], observed significantly higher rates in water removal of OD-pretreated pears compared to those of untreated samples. The
Deff values of the OD-pretreated and untreated figs were estimated in the ranges of 0.77–1.21 × 10
−10 and 0.55–0.95 × 10
−10 m
2 s
−1, respectively. Şahin and Öztürk [
24], studied the air-drying process of OD-pretreated and untreated figs and reported
Deff values ranging from 3.57 × 10
−10 to 10.25 × 10
−10 and from 2.75 × 10
−10 to 5.69 × 10
−10 m
2 s
−1, respectively. In literature, there are also other mathematical models that have been used to describe OD and drying kinetics of figs, with a satisfactory fitting on the experimental data, such as
Peleg and
Azuara model [
9] and a proposed thin layer drying model [
2]. The effect of temperature on the
Deff values was expressed through the
Ea (Equation (3)). OD-pretreated samples presented lower
Ea value compared to the respective untreated, equal to 20.80 ± 2.56 and 25.34 ± 1.20 kJ/mol, respectively, expressing less temperature dependence on the OD drying rate of the figs, compared to the use of stand-alone air-drying.
Aiming at assuring microbial safety and reducing the non-enzymatic hydrolysis and Maillard reaction, the final a
w was selected to be equal to 0.55. OD pretreatment led to a significant reduction of the initial a
w (from 0.9870 to 0.9292), resulting in significant decrease of air-drying time (
p < 0.05) in order to achieve the final desired a
w-value (0.55) for “shelf-stable” products. The drying time needed for the OD-treated and untreated figs was 11.2 and 16.1 h, respectively, at 70 °C, showing a significant reduction in the air-drying time by approximately 30.4%. For milder drying temperatures (60 and 50 °C), the air-drying time of the OD-pretreated figs was reduced approximately by half compared to untreated samples (
Table 3).
Dermesonlouoglou et al. [
12] observed that the air-drying times of OD-pretreated tomato and cucumber were significantly reduced up to 28 and 47%, respectively, when compared to control. Moreover, Dermesonlouoglou et al. [
13] reported that the OD-pretreatment of goji berries followed by air-drying, led to a decrease of drying time by 120 min.
3.3. Energy Savings and Yield Increase of the Combined Use of Osmotic Dehydration and Air-Drying on Figs
OD-pretreatment led to a significant reduction of the initial a
w, resulting in a significantly decreased air-drying processing time (
p < 0.05) of approximately 50%, for achieving the final a
w-value equal to 0.55 for “shelf-stable” products. The decrease in the air-drying time, could lead to a final product of increased quality, as well as to increased energy savings. Based on Equation (9), the energy consumption of the untreated and OD-pretreated samples for air-drying at 60 °C was 480 and 331 MJ kg
−1, respectively. When OD pre-treatment was applied, the energy consumption for air-drying at 50, 60, and 70 °C was by 42.5, 31.1, and 18.0% less than the respective energy required for untreated samples. The most pronounced energy savings was observed for drying at 50 °C, estimated as 177 MJ kg
−1. Alibas [
45], estimated a 3-fold decrease in the energy consumption for pumpkin slices using a combined technique of microwave and air-drying, compared to the conventional air-drying.
Based on data received for product weight measurements before and after processing, the yield of the OD-pretreated final product was higher, since for the production of 1 kg of final dried figs, 4 kg of control samples should be dried, in contrary to 2 kg of OD-treated samples (100% yield increase). These results sustain and emphasize the benefits of OD as a pretreatment for drying of fig halves, making it an attractive and feasible approach for implementation in an industrial drying process line.
Based on the data received during air-drying and taking into account the no-significant differences between the estimated required time for air-drying at 60 and 70 °C (
p > 0.05) (
Table 3) and the reduced energy consumption at lower temperatures, the optimal condition selected was 60 °C.
3.4. Cost-Effectiveness Analysis of OD-Assisted Air Drying of Fig Halves
A cost analysis approach was performed, estimating the total cost of OD process at the selected optimal conditions (45 °C, 90 min, 1/4 (ws/wOD) (and formulation of OD solution: 80% glycerol, 1% salt, 1% vinegar, 0.5% ascorbic acid and 17.5% water). The costs for the OD ingredients, the energy consumption during OD and unit operation costs were provided by collaborating suppliers and industries, respectively. It was estimated that the OD solution cost would be approximately to 0.63 € kg−1 fresh fig, assuming that the reconstitution of OD solution would be at least 4 times (as also suggested in our study). The energy consumption during OD (90 min, 45 °C) was estimated as 0.021€ kg−1 fresh fig when industrial scale operation units were used. The total cost of the OD process was estimated as approximately 0.65 € kg−1 fresh fig.
The cost for the conventional and OD-assisted air drying at 60 °C was calculated based on the energy consumption (kWh) of industrial equipment (considering a cost of 0.06 € kWh
−1). According to the obtained results (
Section 3.2), the drying time was approximately 16 and 11 h for the conventional and OD-assisted air-drying process, that correspond to 1.18 and 0.67 € kg
−1 fresh fig, respectively. Considering also that in OD-assisted drying the yield of the product was increased (for the production of 1 kg of dried figs, 2 kg of OD pre-treated figs are needed or 4 kg of conventionally air-dried figs) the total cost of conventional air drying was estimated as more than 3-fold the cost per kg for OD pre-treated figs.
3.5. Quality Evaluation of Osmotic Pretreated and Untreated Air-Dried Figs
Quality characteristics of the OD-pretreated (45 °C, 90 min, 1/4 (
ws/
wOD)) air-dried figs, processed at the optimal drying conditions (60 °C), were evaluated, and compared to those of conventionally air-dried figs (
Table 4).
OD-pretreatment led to the production of dried figs of improved quality characteristics such as brighter color and softer texture due to the significant reduction of the required air-drying time, as well as to structural changes that occur in the food surface during OD. The firmness of the OD-pretreated dried figs was decreased by up to 40% (18.84 N) compared to the non-pretreated ones (31.84 N). According to Yadav and Singh [
46], OD-pretreatment helps the fruit structure to be unaffected during the subsequent drying, mainly due to structural changes caused in the waxy layer of the fruit surface during OD. Mandala et al. [
14] have reported that OD-pretreatment of apple slices using a glucose and sucrose (30%) solution, resulted in softer surface during drying. Shamaei, Emam-Djomeh, and Moini [
47] also showed that OD and air-dried cranberries had a softer texture compared to untreated samples, due to the use of lower drying temperature and reduced drying time.
Regarding the luminosity (
L*-value) of the flesh, for the OD-pretreated figs a slight increase was observed increased compared to the non-pretreated ones, however this was not perceived during the sensory evaluation. Based on the estimated
L*,
a* and
b*-parameters, OD-pretreatment did not cause any significant color changes to the fig halves (
p > 0.05) (
Table 4).
Mandala et al. [
14] reported that OD pretreatment (45% sugar solution) led to color retention of apple slices during drying. Da Costa Ribeiro et al. [
11] proved that the combination of OD and conventional drying resulted in a 17% higher color acceptability than the one obtained by the conventionally dried pears. Dermesonlouoglou et al. [
12] observed that OD-treated, air-dried goji berry retained their color characteristics when drying time was decreased, compared to untreated samples.
Sensory properties, such as color, odor, flavor, texture and taste of both dried figs, were also examined. The odor and flavor of the untreated dried fig halves were scored lower than OD-pretreated samples, due to the loss of volatile organic compounds during the time-consuming air-drying [
48]. El-Gendy [
21] reported that the OD-pretreatment of figs with 70% syrup, resulted in increased scores of sensory characteristics, compared to the untreated counterparts.
The nutritional and bioactive compounds of both OD-pretreated and control dried fig halves, were also measured (
Table 5), in order to assess the impact of the OD process on their nutritional profile. OD-pretreated samples had significantly increased content of intracellular compounds, compared to the control samples (
p < 0.05).
The concentration in total phenolics and flavonoids of the untreated air-dried figs was estimated as 22.31 mg CAE/100 g d.w. and 4.47 mg catechin/100 g d.w, respectively. The use of OD increased significantly (p < 0.05) the concentration of both total phenolics and flavonoids bioactive compounds by 18% (26.34 mg CAE/100 g d.w.) and 15.4% (5.16 mg catechin/100 g d.w.), respectively. The higher retention of the intracellular bioactive compounds for the OD-pretreated figs could be attributed to the reduced oxidation reactions due to the shorter duration of air-drying (9.2 h less compared to control samples) or/and due to the use of OD solution that has a protective effect on exposure of fig in oxygen. OD-pretreated dried figs presented a significantly (p < 0.05) higher antioxidant activity (14.18 mg Trolox/100 g d.w.) compared to the control ones (13.14 mg Trolox/100 g d.w.), due to their increased bioactive compounds content.
No significant differences were observed in total fibers content for the OD-pretreated and untreated dried fig halves, estimated as 10.5 and 8.5 g/100 g d.w., respectively. Similarly, to this result, El-Gendy [
21] reported slight increase of fibers in OD-treated dried fig halves by ~4.5%, compared to control.
No significant differences were observed for the proteins and the main sugars (glucose and fructose) of both conventionally air-dried and OD pretreated-air-dried figs (
p > 0.05). OD-pretreated dried figs contained an amount of 3.0% glycerol, mainly attributed to the solid uptake of the fig halves during OD. At the optimal OD conditions, it was estimated that the SG was equal to 0.09 g/g d.w. after 90 min of OD. During OD, the diffusion phenomenon of solids took place with two countercurrent flows: a major solute flow from the OD solution to the fig halves, enriching its nutritional value, and a minor simultaneous flow of solute from the fig halves to OD solution, decreasing the concentration of some soluble solids that leached into the OD solution. The polyvalent organic acids measured were found to be significantly decreased for the OD-pretreated dried figs (
p < 0.05) compared to the control dried samples, due to solute transfer from the fig into the OD solution. Phisut, Rattanawedee, and Aekkasak [
49] concluded that during OD treatment, natural solutes such as acids, vitamins, and small molecules were extracted from the fruit into the OD solution. The content of the OD-pretreated dried samples in ascorbic acid was estimated as 15.61 mg/100 g d.w., ~50% decreased compared to the untreated samples. The main mechanisms of the loss in vitamin C, appears to be due to its water solubility, mass transferability (leaching out of the vegetative cell), and heat sensitivity.
3.6. Shelf-Life Determination
In order to estimate the shelf-life of both pre-treated and untreated air-dried figs, an accelerated experiment was conducted, including the monitoring of quality parameters and sensory evaluation of samples during storage at 25, 35, and 45 °C for ~2 months. The hardness of all fig halves increased during storage, deviating significantly from their initial value for storage times longer than 30 and 51 days at 45 °C, for OD-pretreated and untreated samples, respectively (
Figure 5).
OD-pretreatment led to a better retention of the hardness of the already improved fig texture (softer), during storage, leading to a value of approximately 26.17 N after 51-days of storage at 45 °C, instead of 74.89 N for the control. Based on measurements of the hardness during the sensory evaluation, the OD-pretreated figs received higher scores compared to the untreated ones throughout the whole storage period.
Despite the increase of fig hardness, their color was considered by the organoleptic panel to be the main parameter for the sensory rejection of the product. In all cases, increase of storage temperature led to increased color change (Δ
Ε), mainly attributed to the non-enzymatic browning that took place (
Maillard reaction) (
Figure 6).
Increase in storage temperature led to increased rate constants of the color change for the untreated dried figs (
Figure 7) showing significantly higher values compared to the OD-pretreated ones (
p < 0.05) (
Table 6).
The effect of storage temperature on the rate constants of the fig color change was expressed through the activation energy (Ea), calculated for both OD-pretreated and untreated dried figs as 93.6 and 79.9 kJ mol−1, respectively (Equation (8)).
The acceptability of the dried figs was expressed through an average sensory score of 5 which was found to be well-correlated to a color change value (ΔΕ) of 20. The shelf-life of the control and the OD-pretreated figs for storage at 25 °C, was estimated through extrapolation of the linear regression of Equation (11) as 9.9 and 13.7 months, respectively.