Shrinkage of Mirabelle Plum during Hot Air Drying as Influenced by Ultrasound-Assisted Osmotic Dehydration

Abstract

Convective drying in hot air is still the most popular method applied to reduce the moisture content of fruits and vegetables. Conventional hot-air drying of Mirabelle plum is considered to be a slow and energy intensive process. This is due to the fact that the waxy skin of Mirabelle plum has low permeability to moisture, a fact which results in high shrinkage. The aim of this study was to investigate the effect of ultrasound-assisted osmotic dehydration pretreatment on shrinkage of Mirabelle plum as a function of moisture content with the end goal of optimizing operating conditions that minimize shrinkage of the produce during drying. Results showed that application of ultrasound-assisted osmotic dehydration led to a significant (p < 0.05) decrease in shrinkage (from 76.41 to 64.05%). A linear relation between moisture loss and shrinkage was observed. Results indicated that shrinkage may be easily estimated from changes in moisture content, and independent of the drying rate. Inversely, determination of shrinkage would provide an indirect indication of moisture content.

Keywords:

• Air drying
• Mirabelle plum
• Ultrasound-assisted osmotic dehydration
• Moisture content
• Shrinkage

INTRODUCTION

Convective drying in hot air is still the most popular method applied to reduce the moisture content of fruits and vegetables. However, this method has several disadvantages and limitations; for instance, it requires relatively long times and high temperatures, which cause degradation of important nutritional substances as well as color alteration. Another disadvantage is shrinkage, which results from tissue collapse caused by volume reduction, and is due to the loss of moisture as well as the presence of internal forces.^[1]

Shrinkage is important not only for quantification of the quality of dehydrated foodstuffs but also in the characterization of textural properties of materials.^[2,3] It is known that mass transfer rate is affected by shrinkage of the product and volume changes are dependent of several factors such as geometry, drying method and experimental conditions. Physical properties such as bulk density and porosity change and transport properties like thermal and mass coefficient of diffusion are related to changes in material shrinkage during dehydration.^[4–7] Major shrinkage can indicate structural damage because it implies the collapse of the tissue’s structural organization.^[8] During drying, shrinkage is rarely negligible. Furthermore, it is necessary to take it into account when predicting moisture content profiles in the material undergoing dehydration.^[9,10] Values of the effective diffusivities estimated while taking shrinkage into consideration were smaller than those obtained without considering this phenomenon.^[7] Therefore, any attempt to characterize drying behavior must inevitably address physical parameters—such as shrinkage—of the material.^[11] Attempts have been made to describe shrinkage of different products undergoing different drying processes and conditions.^[12]

Among various fruits and vegetables, conventional hot-air drying of the Mirabelle plum is considered to be a slow and energy intensive process. This is because its waxy skin has low permeability to moisture,^[13] a fact which results in high shrinkage. Skin of this fruit consists of an underlying amorphous wax layer adjacent to the cuticle proper, together with crystalline granules of wax protruding from the surface.^[14] Therefore, any pretreatment for plum drying processes which decreases shrinkage by reducing drying time through reducing the initial moisture content and preserves the prune (dried plum) quality is of considerable interest.^[15] Various pretreatments such as blanching, freezing, piercing, abrasion and chemical additives have been used to increase moisture transport from the plum surface. Methodologies such as ultrasound-assisted osmotic dehydration have also been implemented in a few studies as an alternative pretreatment to increase moisture transport from the plum surface.^[16–18] Reduction of drying time and, consequently, processing costs have been reported at the experimental scale after research was conducted on several fruits and vegetables. Osmotic dehydration pretreatment partially removes water from fruits or vegetables immersed in a hypertonic solution.^[19,20] Regarding low mass transfer rate during osmotic treatment, ultrasound can be used to improve mass transfer rate and dehydration time.^[18] Ultrasonic waves can bring about a very rapid series of alternative compressions and expansions, similar to what a sponge does when it is squeezed and released repeatedly. Forces involved in this mechanical mechanism create microscopic channels that may ease moisture removal. In addition, ultrasound produces cavitation, which can be beneficial for removal of the moisture that is strongly attached to the solid.^[21,22]

Analysis of the relationship among process factors and shrinkage during drying could provide a solid base to optimize drying process.^[16] Analyses of various experimental data have revealed that shrinkage of food materials during drying could be represented only as a function of moisture content without any considerable dependency on inert material, air temperature, and velocity or sample length.^[11] To our knowledge, there has been no study in the literature devoted to investigation of the effect of ultrasound-assisted osmotic dehydration as a pretreatment on shrinkage of the Mirabelle plum during hot-air drying. Therefore, the aim of this study was to investigate the effect of ultrasound-assisted osmotic dehydration pretreatment on shrinkage of the Mirabelle plum as a function of moisture content searching for optimal operating conditions (sonication time, concentration of osmotic solution, and immersion time in the osmotic solution) that help minimize shrinkage of the produce during drying.

MATERIALS AND METHODS

Preparation of the Samples

Mirabelle plums (Prunus domestica subsp. syriaca) were purchased from a local garden. They were sorted visually based on a relative standard of maturity, shape, size, and color. Such a sorting stage was intended to select similar plums to be used in every experiment and to discard ripe and damaged samples. Before experiments, plums were washed with tap water and were dried with a filter paper. Moisture content was gravimetrically measured by drying samples in an oven at 105ºC to reach constant weight.^[16] The average initial moisture content of the plums was 4.54 g water/g dry matter.

Ultrasound-Assisted Osmotic Dehydration Pretreatment

Pretreatments were structured in combinations of two ultrasonication times (both at 40 kHz): 10 and 30 min; two osmotic solution concentrations: 50 and 70% sucrose in water (% w/w) and four immersion times in osmotic solution: 60, 120, 180, and 240 min. No pretreatment (neither ultrasonic nor osmotic treatment) was applied to control samples (Table 1). Results of kinetics studies were obtained before these ultrasonication times were chosen. Results showed that effects of ultrasound pretreatment started to influence the drying process after 10 min. After 30 min, changes inferred in the drying process became insignificant.^[21]

TABLE 1 Abbreviations utilized for different treatments

Abbreviation	U*	B**	T***
Control	0	0	0
U10-B50-T60	10	50	60
U10-B70-T60	10	70	60
U30-B50-T60	30	50	60
U30-B70-T60	30	70	60
U10-B50-T120	10	50	120
U10-B70-T120	10	70	120
U30-B50-T120	30	50	120
U30-B70-T120	30	70	120
U10-B50-T180	10	50	180
U10-B70-T180	10	70	180
U30-B50-T180	30	50	180
U30-B70-T180	30	70	180
U10-B50-T240	10	50	240
U10-B70-T240	10	70	240
U30-B50-T240	30	50	240
U30-B70-T240	30	70	240

*U: Ultrasonication time at 40 kHz (min); **B: Osmotic solution concentration (Brix) [Sucrose in water (% w/w)];

***T: Immersion time (min).

Ultrasonic pretreatments were carried out using an ultrasonic bath (AS ONE Corporation, US-4R, Japan, capacity: 9.5 L, dimensions: 36.5 (height) × 30.5 (width) × 26.2 (depth) cm; oscillating frequency: 28 and 40 kHz, high frequency output: 160 W) without mechanical agitation. The bath was operated at a frequency of 40 kHz. Water temperature inside the ultrasonic bath was maintained constant at 25ºC. Temperature increase during the experiments was not significant (less than 2°C) after 30 min of ultrasonic treatment.

In each ultrasound-assisted osmotic dehydration pretreatment trial, an experimental set of plum samples were immersed in four separate beakers (one for each immersion time in osmotic solution: 60, 120, 180, and 240 min) filled with osmotic pretreatment solution and were then placed in the ultrasonic bath for 10 and 30 min. Experiments were carried out in separate beakers to avoid interference between samples and runs. Osmotic solutions were prepared through mixing food-grade sucrose with distilled water until concentrations (% w/w sucrose in water) of 50 and 70% were obtained. The weight ratio between fruit and the osmotic solution was 1:4. This ratio was used to avoid dilution effects.^[17,21]

After completion of the ultrasound-assisted osmotic dehydration pretreatment for the intended time (10 and 30 min), all the beakers were removed from the ultrasonic bath and the remaining time for osmotic dehydration pretreatment was passed under ambient temperature (25°C) and without mechanical agitation. Total immersion times of the samples in osmotic solutions were 60, 120, 180, and 240 min, considering both the time with and without ultrasound. After reaching the desired time, samples were removed from the beakers, washed with distilled water, and blotted with absorbent paper to remove excess solution on the surface. All experiments were carried out in duplicate.

Hot-Air Drying

After the completion of the osmotic dehydration pretreatment, samples were placed in Petri dishes in a single-layer arrangement and were dried in a pilot plant hot-air drier (UOP 8 Tray dryer, Armfield, UK). Air temperature in the drier was set at 80ºC.^[23] Cross-flow air moved from side to side of the dryer at 1.4 m/s, flowing parallel to the drying surface of the samples. Moisture loss was recorded at 30 min interval by a digital balance of 0.01 g accuracy. Drying process continued until an average moisture content of 0.57 g water/g dry matter was obtained.

Determination of Shrinkage

Shrinkage represents a relative or reduced dimensional change of volume and is represented by:^[24]

where S is the shrinkage (%), V_t is the apparent volume of the sample at a certain degree of dryness after time t and V₀ is the apparent volume of the raw sample. Toluene displacement method was used to measure the volume of the samples gravimetrically.^[25,26] Based on this method, samples were transferred into a flask half filled with toluene after being weighed precisely. The flask was then filled with toluene, the level of solvent being carefully adjusted to ensure consistency, and was weighed. Sample volume (V) was calculated using:^[25]

where V_f is the volume of the flask; M_sf is weight of toluene added to fill the flask; M_t+s is the weight of the flask plus the sample and the solvent; M_f is the weight of the flask; M is the weight of the sample; and ρ_s is the density of toluene (0.87 g/cm³ at 20ºC).

Experimental Design and Statistical Analysis

A 2×2×4 factorial experiment in a randomized complete block design with two replicates was used to study the effects of ultrasonication time, osmotic solution concentration, and immersion time in osmotic solution on shrinkage as a response variable until an average moisture content of 0.57 g water/g dry matter was obtained. Independent variables were ultrasonication time at two levels: 10 and 30 min; osmotic solution concentration at two levels: 50 and 70% (w/w); and immersion time in osmotic solution at four levels: 60, 120, 180, and 240 min. For control samples, no pretreatment [ultrasonication time (0 min), osmotic solution concentration (0%) or immersion time in osmotic solution (0 min)] was utilized. Values in the analysis of variance (ANOVA) table were calculated using the Proc GLM Model procedure of SAS (SAS Software v. 9.1, SAS Institute Inc., Cary, NC, USA). Significant differences within pretreatments were determined at p < 0.05 (95% confidence level). Duncan’s multiple range test was employed to compare means where significant differences occurred within the pretreatment combinations in terms of shrinkage response.

RESULTS AND DISCUSSIONS

Table 2 presents drying time, moisture content and shrinkage during hot-air drying of control and pretreated Mirabelle plum samples as influenced by ultrasonication time, osmotic solution concentration, and immersion time in osmotic solution. Drying continued until reaching an average moisture content of 0.57 g water/g dry matter. Shrinkage rate decreased along with decrease in moisture contents of all the samples (Table 2). This can be deduced from the small gradients of shrinkage between two different moisture contents when reaching the end of the process. This observation can be related to a higher effective moisture diffusivity,^[27] case-hardening of the surface and the fixation of the volume of the sample^[25,28] at the earlier stage of the drying process.^[9] This observation is in agreement with the results of Niamnuy et al.^[29] who noticed that faster drying rate induced extensive cellular shrinkage.

TABLE 2 Drying time, moisture content and shrinkage of different samples (Table 1) until reaching an average moisture content of 0.57 g water/g dry matter pretreated at four immersion times in osmotic solution: (a) 60 min; (b) 120 min; (c) 180 min; and (d) 240 min

(A)
Treatment	Time (min)	MC (g water/g dry matter)	Shrinkage (%)
Control	0	4.54 ± 0.06	0.00^ν ± 0.00
	120	2.96 ± 0.04	22.18^ζηθι ± 2.22
	240	2.03 ± 0.12	45.71^uvwxy ± 5.96
	360	1.41 ± 0.07	65.52^fghijkl ± 2.44
	480	0.96 ± 0.09	75.49^a ± 5.43
	625	0.57 ± 0.01	76.41^a ± 1.36
U10-B50-T60	0	4.50 ± 0.04	0.00^ν ± 0.00
	130	2.88 ± 0.10	20.89^ηθικ ± 1.71
	230	2.11 ± 0.09	44.02^vwxy ± 2.10
	350	1.42 ± 0.07	63.12^hijklmn ± 2.75
	480	0.90 ± 0.04	73.95^abcd ± 4.18
	565	0.60 ± 0.03	76.05^a ± 1.42
U30-B50-T60	0	3.98 ± 0.03	0.00^ν ± 0.00
	70	2.85 ± 0.06	17.33^θικλμ ± 1.88
	150	2.08 ± 0.06	35.76^zαβγδ ± 4.28
	250	1.40 ± 0.03	52.16^qrstu ± 0.55
	350	0.91 ± 0.03	63.44^hijklmn ± 3.26
	500	0.55 ± 0.00	68.99^abcdefghij ± 3.74
U10-B70-T60	0	4.41 ± 0.06	0.00^ν ± 0.00
	120	2.88 ± 0.02	18.78^θικλ ± 1.57
	210	2.11 ± 0.00	40.55^xyzα ± 0.01
	330	1.42 ± 0.01	58.89^lmnopq ± 0.98
	460	0.89 ± 0.00	70.24^abcdefghi ± 2.47
	590	0.57 ± 0.03	75.21^a ± 3.24
U30-B70-T60	0	3.80 ± 0.14	0.00^ν ± 0.00
	80	2.86 ± 0.04	14.33^κλμ ± 4.25
	170	2.06 ± 0.05	32.50^βγδε ± 3.76
	280	1.39 ± 0.08	48.49^stuvw ± 0.85
	390	0.90 ± 0.05	60.76^klmnop ± 4.75
	525	0.56 ± 0.01	67.55^bcdefghijk ± 0.93
(B)
Treatment	Time (min)	MC (g water/g dry matter)	Shrinkage (%)
U10-B50-T120	0	4.45 ± 0.04	0.00^ν ± 0.00
	110	2.91 ± 0.01	22.12^ζηθι ± 4.37
	210	2.09 ± 0.02	44.30^vwxy ± 2.40
	320	1.43 ± 0.05	62.42^jklmno ± 1.33
	440	0.90 ± 0.08	73.29^abcde ± 4.84
	565	0.57 ± 0.02	74.99^ab ± 4.35
U30-B50-T120	0	3.84 ± 0.06	0.00^ν ± 0.00
	70	2.87 ± 0.05	13.28^λμ ± 1.74
	140	2.11 ± 0.07	33.53^αβγδε ± 3.47
	250	1.39 ± 0.01	50.94^rstuv ± 6.58
	350	0.92 ± 0.06	62.52^jklmno ± 5.18
	490	0.55 ± 0.01	66.69^defghijk ± 2.58
U10-B70-T120	0	4.34 ± 0.04	0.00^ν ± 0.00
	120	2.94 ± 0.02	18.62^θικλ ± 1.76
	220	2.12 ± 0.08	40.57^xyzα ± 2.26
	340	1.42 ± 0.01	58.79^lmnopq ± 6.10
	460	0.91 ± 0.01	69.88^abcdefghij ± 1.51
	580	0.59 ± 0.01	73.27^abcde ± 2.07
U30-B70-T120	0	3.63 ± 0.04	0.00^ν ± 0.00
	50	2.90 ± 0.16	13.14^λμ ± 0.28
	140	2.06 ± 0.12	27.68^εζη ± 0.13
	240	1.42 ± 0.11	42.81^wxyz ± 1.97
	360	0.89 ± 0.04	56.71^nopqr ± 2.41
	495	0.55 ± 0.01	65.54^fghijkl ± 0.20
(C)
Treatment	Time (min)	MC (g water/g dry matter)	Shrinkage (%)
U10-B50-T180	0	4.37 ± 0.07	0.00^ν ± 0.00
	90	2.86 ± 0.01	23.04^ζηθ ± 2.22
	180	2.11 ± 0.01	46.74^tuvwx ± 4.34
	290	1.43 ± 0.01	64.49^hijklm ± 5.16
	410	0.90 ± 0.02	73.18^abcde ± 1.63
	545	0.57 ± 0.01	74.38^abc ± 2.81
U30-B50-T180	0	3.72 ± 0.03	0.00^ν ± 0.00
	50	2.91 ± 0.01	15.16^ικλμ ± 1.62
	130	2.10 ± 0.04	34.25^αβγδε ± 2.45
	230	1.40 ± 0.01	51.23^rstuv ± 2.80
	340	0.90 ± 0.01	62.20^jklmno ± 0.46
	490	0.56 ± 0.00	66.15^efghijkl ± 2.61
U10-B70-T180	0	4.20 ± 0.28	0.00^ν ± 0.00
	120	2.86 ± 0.13	17.92^θικλμ ± 3.00
	210	2.12 ± 0.14	36.91^zαβγ ± 2.79
	330	1.42 ± 0.06	55.26^opqrs ± 2.55
	450	0.91 ± 0.02	67.07^cdefghijk ± 2.91
	565	0.61 ± 0.00	72.18^abcdefg ± 2.46
	0	3.41 ± 0.04	0.00^ν ± 0.00
	40	2.94 ± 0.11	13.90^κλμ ± 1.43
U30-B70-T180	130	2.12 ± 0.07	31.44^γδε ± 6.45
	240	1.38 ± 0.01	47.37^tuvwx ± 3.24
	350	0.91 ± 0.05	59.00^lmnopq ± 2.50
	505	0.56 ± 0.01	64.72^ghijkl ± 3.32
(D)
Treatment	Time (min)	MC (g water/g dry matter)	Shrinkage (%)
U10-B50-T240	0	4.27 ± 0.02	0.00^ν ± 0.00
	90	2.91 ± 0.08	19.40^θικλ ± 5.08
	190	2.14 ± 0.08	39.33^yzαβ ± 8.71
	310	1.41 ± 0.06	57.08^mnopqr ± 2.81
	430	0.90 ± 0.03	69.00^abcdefghij ± 3.69
	560	0.59 ± 0.02	72.42^abcdef ± 0.18
U30-B50-T240	0	3.54 ± 0.07	0.00^ν ± 0.00
	40	2.96 ± 0.07	17.95^θικλμ ± 3.56
	130	2.07 ± 0.13	36.77^zαβγ ± 0.06
	240	1.39 ± 0.11	53.05^qrst ± 0.18
	350	0.92 ± 0.10	62.90^ijklmn ± 1.06
	490	0.60 ± 0.04	65.77^efghijkl ± 7.50
U10-B70-T240	0	4.14 ± 0.03	0.00^ν ± 0.00
	120	2.90 ± 0.06	15.95^θικλμ ± 0.73
	220	2.09 ± 0.00	35.77^zαβγδ ± 1.35
	340	1.41 ± 0.01	53.55^pqrst ± 2.30
	470	0.91 ± 0.06	65.90^efghijkl ± 4.60
	590	0.59 ± 0.03	70.77^abcdefgh ± 5.23
	0	3.37 ± 0.05	0.00^ν ± 0.00
	40	2.88 ± 0.03	11.13^μ ± 2.20
	130	2.06 ± 0.00	28.77^δεζ ± 7.85
U30-B70-T240	240	1.39 ± 0.01	44.52^vwxy ± 2.26
	350	0.92 ± 0.00	57.00^mnopqr ± 5.55
	500	0.54 ± 0.01	64.05^hijklmn ± 1.19

The highest shrinkage was observed in the control sample (Table 2a). Generally, the shrinkage of the pretreated samples was significantly (p < 0.05) decreased by increasing ultrasonication time from 10 to 30 min at different immersion times (60, 120, 180, and 240 min) in osmotic solutions (Table 2). Shrinkage of the pretreated samples was also decreased by increasing osmotic solution concentration from 50 to 70% at different immersion times in osmotic solutions; however, this decrease was not statistically significant (p > 0.05). This could be due to the stronger influence of ultrasonication time compared to the concentration of the osmotic solution. In accordance with the results obtained in this study, Koc et al.^[9] also reported that the extent of shrinkage is generally higher for air drying than for osmotic dehydration. With respect to the solution concentration, a smaller moisture content, and a consequently higher shrinkage were observed for samples with lower osmotic solution concentrations. This is due to the formation of a dense layer of solutes in the surface of the fruit when concentrated solutions are used. This layer makes transfers between the fruit and the solution more difficult.^[30] Fante et al.^[30] also observed lower shrinkage values by increasing sucrose solution concentration during plum drying. However, Nowacka et al.^[24] and Schössler et al.^[26] observed that ultrasound treatment had no significant effect (p > 0.05) on product shrinkage.

At constant ultrasonication time and osmotic solution concentration, increasing immersion time from 60 to 240 min decreased the shrinkage. Many aspects of cell structure are affected during osmotic dehydration of fruits, such as alteration (deformation) of cell walls, splitting of the middle lamella, lysis of membranes (plasmalemma and tonoplast), and tissue shrinkage.^[31] During osmotic dehydration, plasmolysis is also accompanied by a loss in the turgor pressure, pectin solublization, and solute uptake in the cells.^[32] These tissue changes, which strongly alter the cellular compartmentalization, wall matrix, and membrane permeability, could greatly influence the transport properties of the product during processing.^[31] Because of the complex situation in the microstructure of plant tissue, the phenomena observed during osmotic dehydration cannot always be explained just in terms of osmotic processes in which cell membranes act as a semipermeable barrier and allow the passage of water. Disruption of cell membranes during osmotic dehydration puts an end to the osmotic mechanism and from then on, diffusion, capillarity or free convection become the mechanisms that control the mass transfer as the process advances.^[4] This, in turn, could lead to a higher moisture diffusivity, lower drying time and lower shrinkage.

On the other hand, Rodriguez et al.^[33] studied the effect of ultrasound on the microstructure of apple tissue during drying by means of scanning electron microscopy (SEM). Microphotographs of fresh and dried apples showed that during drying, one of the most important phenomena is cell shrinkage, which leads to a major modification in the structure of the product and allows the release of water. Through microstructural analysis, it was observed that ultrasound application disrupted the cellular structure and resulted in pores which were larger than those in fresh samples. This fact could improve the drying rate by making an easier water pathway,^[33] which, in turn, could lead to higher moisture diffusivity, lower drying time, and lower shrinkage.

Fernandes et al.^[21] demonstrated that osmotic and ultrasound pretreatments increased moisture diffusion of melons through different effects. Ultrasonic waves created microscopic channels in the fruit; water could use these microscopic channels as an easier pathway to diffuse toward the surface of the fruit.^[34] Fernandes et al.^[21] verified in microscopic images that micro-channels were formed by the elongation and flattering of cells in some regions of the melons submitted to ultrasound. Besides, authors argued that no cell breakdown was observed in the samples. On the other hand, osmotic dehydration increased moisture diffusion by breaking down parts of the cell walls and, therefore, reducing the resistance for water to diffuse through the cells. In a similar study, Garcia-Noguera et al.,^[17] in experiments with strawberries, showed that increasing the time of ultrasound pretreatment reduced moisture content of the samples and consequently resulted in reduction of air-drying time. This result may be due to higher creation of microscopic channels in higher ultrasonication time (30 min).

Table 2 shows that different samples needed various drying times to reach an average moisture content of 0.57 g water/g dry matter. Mirabelle plums treated with ultrasound for 30 min and dehydrated at osmotic solution concentration of 70% for 240 min (U30-B70-T240), prior to drying, were found to have the lowest shrinkage (64.1%) compared to control (76.4%). Thus, processing conditions in terms of ultrasonication time, osmotic solution concentration and immersion time in osmotic solution can be optimized to reduce shrinkage to a minimum, if it is desired from an industrial point of view.

Figure 1 shows the relationship between shrinkage and moisture content of all treated samples shown in Table 2 (a, b, c, and d). As can be seen from Fig. 1, a uniform behavior was observed between shrinkage and moisture content; such behavior is essentially independent of each set of experimental conditions and suggests a linear relation between moisture loss by the samples and shrinkage. The fundamental equation of shrinkage during drying is normally developed on the basis of the hypothesis that variation of the volume of the product corresponds to the volume of the evaporated water.^[35] A linear relationship between shrinkage and moisture content during drying of various fruits and vegetables, during the whole process or at least in part of it is reported in several works using different drying procedures. Results indicate that shrinkage may be easily estimated from changes in moisture content of the sample, and independent of the drying rate. Inversely, determination of shrinkage would give an indirect indication of moisture content of the product.^[4] It has also been noted that if development of pores during drying is not negligible, a linear model may not be adequate to model the shrinkage behavior. This is the case for drying at higher temperatures or lower moisture contents.^[36] Dissa et al.^[5] stated that although experimental shrinkage curves were not strictly linear, they could be fitted by the fundamental linear model. In addition, analysis of the experimental data by Souraki et al.^[7] revealed that the shrinkage of apple could be represented only as a linear function of water loss without any considerable dependency on the osmotic solution temperature and concentration. Similar results were also obtained by Koc et al.^[9] and Schössler et al.^[26] for different fruits and vegetables. However, Panyawong and Devahastin^[12] and Yan et al.^[25] found the relationship between the degree of shrinkage and the moisture content to be more or less of a second-order in nature at every tested condition. Shrinkage modelling by Aversa et al.^[37] also revealed a non-linear dependence of eggplant sample volume on the food’s moisture content.

FIGURE 1 Relationship between shrinkage and moisture content of all treated samples shown in Table 2 (a, b, c and d).

CONCLUSION

Shrinkage of pretreated plum samples was decreased by increasing ultrasonication time from 10 to 30 min and osmotic solution concentration from 50 to 70% at different immersion times (60, 120, 180, and 240 min) in osmotic solutions. At constant ultrasonication time and osmotic solution concentration, increasing immersion time from 60 to 240 min decreased the shrinkage. Ultrasonication time, osmotic solution concentration and immersion time in osmotic solution all had a significant effect (p < 0.05) on shrinkage of the samples. Mirabelle plums treated with ultrasound for 30 min and dehydrated at osmotic solution concentration of 70% for 240 min (U30-B70-T240), prior to drying, were found to have the lowest shrinkage (64.1%) compared to control (76.4%). Thus, processing conditions in terms of ultrasonication time, osmotic solution concentration and immersion time in osmotic solution can be optimized to reduce shrinkage to a minimum, in case it is desired from an industrial point-of-view.