Mass transfer kinetics during osmotic dehydration of carrot cubes in sugar beet molasses

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1 Romanian Biotechnological Letters Vol. 16, No. 6, 2011 Copyright 2011 University of Bucharest Printed in Romania. All rights reserved ORIGINAL PAPER Mass transfer kinetics during osmotic dehydration of carrot cubes in sugar beet molasses Abstract 6790 Received for publication, April 5, 2011 Accepted, May 10, 2011 MIŠLJENOVIĆ N., M. *, KOPRIVICA G., B., JEVRIĆ L., R., LEVIĆ LJ., B Faculty of Technology, University of Novi Sad, Bulevar Cara Lazara 1, Novi Sad, 21000, Serbia * Corresponding author: Phone: ; Fax: ; nevenam@uns.ac.rs The paper describes the effect of different solution concentrations (80, 60 and 40%) and immersion time on osmotic dehydration/impregnation of carrot cubes in sugar beet molasses as hypertonic solution. Osmotic dehydration process was conducted at the temperature of 45 C and under atmospheric pressure. The highest value of dry matter content, in osmodehydrated carrot (54.67%), was achieved when 80% sugar beet molasses was used as osmotic solution. Mass transfer coefficients (k w and k s ) were calculated using equations proposed by Hawkes and Flink s. Using the LabFit Curve Fitting Software, mathematical equation that fits the calculated mass transfer coefficients was determined. The proposed equation has three parameters and takes into account both variables (time and concentration). That equation satisfactory fits experimental data and high values of correlation coefficients were reached (R = for k w and R = for k s ). In the second experiment, influence of the osmotic solution circulation on the final dry matter content was examined. It was found that the dry matter content, after 5 h of dehydration, was considerably higher if the process took place under conditions of osmotic solution circulation. Key words: Osmotic dehydration, sugar beet molasses, carrot cubes, mass transfer kinetics Introduction Shelf life of fruits and vegetables, as well as seasonal products, is relatively short and taking into account that they are valuable raw materials for food industry, it is very important to increase their sustainability. Conventional preservation methods (convective drying, candying, freezing, etc.), commonly employed to preserve food materials, often cause decreasing in nutritional and sensorial properties of treated fruits and vegetables (loss of vitamins, changes in color, altered taste and texture, bad rehydration). Main disadvantages of the convective drying are high energy consumption and loss of the thermolabile components of food. Osmotic dehydration is a water removal process, which is based on soaking foods (fruit, vegetable, meat and fish) in a hypertonic solution. Water removal in liquid form, usage of mild temperatures and osmotic solution reusing are main advantages of osmotic dehydration process in comparison with other drying treatments [1-3]. The driving force for the osmotic dehydration process is the difference in osmotic pressure between the food material (hypotonic medium) and osmotic solution (hypertonic medium). The diffusion of water is accompanied by the simultaneous counter diffusion of solute from the osmotic solution into the tissue. Taking into account that cell membrane is not perfectly selective, other solutes present in the cells can diffuse into the osmotic solution [4, 5]. For fruits and vegetables dehydration, the most commonly used osmotic agents are sucrose and sodium chloride and their combination. Glucose, fructose, maltodextrin and sorbitol also can be used for osmotic dehydration [6]. Recent research has shown that use of sugar beet molasses as hypertonic solution improves osmotic dehydration processes [3]. Sugar

2 NEVENA M. MIŠLJENOVIĆ *, GORDANA B. KOPRIVICA, LIDIJA R. JEVRIĆ, LJUBINKO B. LEVIĆ beet molasses is an excellent medium for osmotic dehydration, primarily due to the high dry matter (80%) and specific nutrient content: solids (around 80%), 51% saccharose, 1% rafinose, 0.25% glucose and fructose, 5% proteins, 6% betaine, 1.5% nucleosides, purine and pyramidine bases, organic acids and bases, which subsequently results in high osmotic pressure of the solution. From nutrient point of view, an important advantage of sugar beet molasses, as hypertonic solution, is enrichment of the food material in minerals and vitamins, which penetrate from molasses to the plant tissue [7]. In Serbia, sugar beet molasses has not been used as an ingredient in food industry. Hence, extensive research has been conducted with the aim of introducing molasses as a valuable ingredient in bakery, confectionery and meat processing industry [8, 9]. Sugar beet molasses, as a by-product of sugar production, is a cheap source of nutrients, available in large quantities. The rate and dewatering degree of the material and its chemical composition changes depend on the sort of the osmotic solution used, the kind and the size of raw material, as well as the ratio of material to osmotic solution, temperature, dehydration time, agitation of liquid solution and type of apparatus [10, 11]. The rate of osmotic dehydration is the highest at the beginning of the process. It results from the largest difference in osmotic pressure between the osmotic solution and the cell sap of the material and small mass transfer resistance at this stage of the process [12]. The paper describes the effect of different solution concentrations and immersion time on mass transfer kinetics during immersion of carrot cubes in sugar beet molasses as hypertonic solution. Additionally, influence of the osmotic solution circulation on the final dry matter content, was examined. Materials and methods Sample preparation Carrot samples were purchased in a local market in Novi Sad (Serbia) and stored at 4ºC until use. Initial moisture content, X o, was ± 0.66%. Prior to the treatment, the carrots were thoroughly washed and cut into cubes, dimension 1x1x1 cm. Osmotic solutions Different concentrations of sugar beet molasses (40.0%, 60.0% and 80.0% dry matter) were used as osmotic solution. Sugar beet molasses was obtained from the sugar factory Pećinci, Serbia. Initial dry matter content in sugar beet molasses was 83.68%. For the dilution of sugar beet molasses distilled water was used. Experimental procedure In all experiments, a weight ratio of solution to carrot samples of 4:1 was used. The experiments were conducted in apparatus presented in Figure 1, under atmospheric pressure at solution temperature of 45ºC. Romanian Biotechnological Letters, Vol. 16, No. 6,

3 Mass transfer kinetics during osmotic dehydration of carrot cubes in sugar beet molasses Figure 1. Apparatus for osmotic dehydration Description of the apparatus for osmotic dehydration Simplified scheme of the apparatus for osmotic dehydration is shown in Figure 2. Apparatus was made of stainless steel and it is equipped with centrifugal pump that enables recirculation of the osmotic solution and with a heater and thermoregulation system, which provides maintenance of constant temperature during the experiment. Main barrel has double wall heat exchanger, through which water circulates, previously heated by a two electric heaters (2x1.5 kw) assembled with apparatus. Along with this apparatus there is a stainless, perforated basket divided into several sections. After a certain time a sample from one section was removed and the rest continues with dehydration. T Figure 2. Simplified scheme of the apparatus for osmotic dehydration (1 Osmotic solution; 2 Basket with sample; 3 Centrifugal pump; 4 Electric heaters; 5 System for thermoregulation; 6 Double wall heat exchanger) Experiment 1 In the first experiment, kinetics of the osmotic dehydration of carrot cubes was examined. Dehydration lasted 5 hours and samples were taken out of the osmotic solution at different times (20, 40, 60, 90, 120, 180, 240 and 300 min). The velocity of solution (circulation) trough drying chamber and carrot samples was constant for all experimental 6792 Romanian Biotechnological Letters, Vol. 16, No. 6,

4 NEVENA M. MIŠLJENOVIĆ *, GORDANA B. KOPRIVICA, LIDIJA R. JEVRIĆ, LJUBINKO B. LEVIĆ units. After removal, carrot samples were washed with water and gently blotted in order to remove the excessive water. The samples were weighed and analyzed for dry matter content. Dry matter content of the samples was determined by drying the material at 105 ºC for 24h in a heat chamber (Instrumentaria Sutjeska, Serbia) and measuring the weight loss of the product, gravimetrically on a scale. The solid content of the osmotic solutions was determined refractometrically by Abbe refractometer, Carl Zeis Jenna [13]. Osmotic dehydration kinetics During the osmotic dehydration process, three main process variables are usually measured: moisture content, change in weight and change in soluble solids. Of these, water loss (WL), weight reduction (WR), solids gain (SG), normalized moisture content (NMC) and normalized solid content (NSC) were calculated as follows: w0 w WR = (1) w 0 u u0 SG = (2) w0 WL = WR+ SG (3) X NMC = (4) X 0 u NSC = (5) where: w o - initial sample weight (g), w - sample weight after osmotic dehydration (g), u o - initial solid content in the fresh sample (g), u - solid content in the sample after osmotic dehydration (g), X o - initial moisture content of the fresh sample before osmotic treatment (g), X - moisture content in the sample after osmotic dehydration (g). WL, SG and WR are expressed in g/g initial sample weight (i.s.w.). A model was proposed by Hawkes and Flink [14] to describe the kinetics of moisture loss and solid gain: 0.5 NMC = 1 k w θ (6) 0.5 NSC = 1 + k s θ (7) Where k w (s -0.5 ) and k s (s -0.5 ) represent the overall mass transfer coefficients for water and solute respectively, and θ (s) is the dehydration time. Based on the calculated data for mass transfer coefficients, equation which describes the effect of time and concentration on mass transfer coefficients was determined using LabFit Curve Fitting Software. b c k = a θ C i (8) The Eq. (8) fits satisfactorily the experimental data and makes possible to estimate mass transfer coefficients for any osmotic solute concentration and contact time. In Eq. (8), symbol θ is immersion time, C is concentration of osmotic solution, k i represents k w or k s, a, b and c are parameters. u 0 Romanian Biotechnological Letters, Vol. 16, No. 6,

5 Mass transfer kinetics during osmotic dehydration of carrot cubes in sugar beet molasses Experiment 2 In the second experiment, the dry matter content of the carrot, after 5 hours of osmotic dehydration process, in condition of circulation and without circulation of the osmotic solution, was compared. Preparation of the material and osmotic solutions was the same as in the first experiment and same apparatus was used, except the basket which was not divided in sections. Data analysis STATICTICA software version 9 (Statsoft, Tulsa, Oklahoma, USA) was used for analyzing variations (ANOVA) and for calculation of parameters in Eq.8. The level of significance was set at p<0.05. For drawing figures 3, 4 and 5 Origin 6.1 was used. Results and discussions The effect of sugar beet molasses concentration on dehydration kinetics is presented in Fig. 3. It was observed that concentration of the osmotic solution and dehydration time have important effect on the carrot dehydration rate, i.e. the rate of dehydration was increased with concentration of solution and processing time. After two hours, due to dilution of the osmotic medium and consequential decreasing in driving force, the rate of mass transfer has tendency of stabilization i.e. reaches the equilibrium state and dehydration has almost a constant rate. Figure 3. Kinetics of the carrot dehydration/impregnation at different solution concentrations of sugar beet molasses and constant temperature (45ºC) 6794 Romanian Biotechnological Letters, Vol. 16, No. 6, 2011

6 NEVENA M. MIŠLJENOVIĆ *, GORDANA B. KOPRIVICA, LIDIJA R. JEVRIĆ, LJUBINKO B. LEVIĆ Figure 4. Water loss (WL) during osmotic dehydration of carrot at different solution concentrations of sugar beet molasses and constant temperature (45ºC) Figure 5. Solid gain (SG) during osmotic dehydration of carrot at different solution concentrations of sugar beet molasses and constant temperature (45ºC) The osmotic dehydration process was studied in terms of water loss and solid gain (Fig. 4. and Fig. 5.). Initial high rate of water removal and solid uptake, followed by slower removal and uptake in the later stages was observed. Rapid loss of water and solid gain in the beginning is apparently due to the large osmotic driving force between the dilute sap of the fresh carrot and the surrounding hypertonic solution. Water loss and solid gain were most intensive in the first two hours of osmotic dehydration process. Higher concentrations of molasses increased the osmotic pressure gradient and, hence, higher water loss. The highest water loss ( g/g of i. s. w.) was observed in the sample which was dehydrated in molasses with 80% solid content for 5 hours. The solid gain value indicates the degree of penetration of solids from the osmotic solution into the samples. The objective of osmotic dehydration is the removal of water from Romanian Biotechnological Letters, Vol. 16, No. 6,

7 Mass transfer kinetics during osmotic dehydration of carrot cubes in sugar beet molasses plant tissue and, at the same time, minimizing the penetration of substances from the osmotic solution into the plant tissue. However, in the case when sugar beet molasses is used as hypertonic solution, the penetration of mineral substances, vitamins, etc. to the tissue can be considered as favorable because the nutritional value of the treated fruits and vegetables becomes higher [7]. Penetration of the solute, primarily sucrose molecules, from the osmotic solution into the sample can be limited by applying edible coatings [15-17]. Solid gain, during the osmotic dehydration of carrot, showed a tendency to increase with increasing the immersion time. Results of this experiment (Fig. 5.) implicate that using sugar beet molasses as hypertonic solution, similarly to other osmotic mediums [18-21], caused increasing of solute penetration in the treated samples at higher solution concentrations. The essence of the osmotic dehydration process is to obtain high dry matter content in the treated samples in order to increase microbiological and enzymatic stability. In Table 1 changes in dry matter content in the carrot samples, after 5 hours of osmotic dehydration at different concentration of sugar beet molasses (80, 60 and 40%), are shown. The increase in concentration resulted in higher content of dry matter in the samples. The highest value of dry matter content (54.67 %) was achieved after 5 hours of osmotic dehydration in sugar beet molasses with 80% solid content. With this relatively simple, low energy required and nutritionally favorable food preservation method, dry matter content was increased almost five times, which implicate that this carrot could be used as a row material in different food processing or could be a suitable pre-treatment for other preservation technique (convective drying, freeze drying ). Significant difference in the dry matter content was observed (Table 1) between the samples dehydrated at higher concentrations of sugar beet molasses (80 and 60%), but significant difference was not confirmed between the samples dehydrated by 60% and 40% of sugar beet molasses. After 5 hours of carrot dehydration in 80% sugar beet molasses, the solid content in the remained solution was 74 %. It still represents high value, therefore the same molasses can be used as a hypertonic solution in another osmotic dehydration process [22]. Table 1. Dry matter content in carrot samples after 5 hours of osmotic dehydration in sugar beet molasses (different letters indicate significant differences (p<0.05)) Concentration, % Dry matter, % 54.67±1.66 a ±0.37 bc 37.27±1.89 c Mass transfer coefficients for water and solute (k w and k s ) were calculated by Eqs. (6) and (7), respectively. In order to correlate the experimental data, equation with three parameters (Eq. 8) suggested by LabFit Curve Fitting Software, was used. The purpose of this analysis was to obtain a simple equation able to take into account both the variables (time, concentration) simultaneously. Parameter values for this equation are shown in Eqs. (9) and (10), for osmotic dehydration of carrot with sugar beet molasses at 45ºC. This equation satisfactorily fits the experimental data, taking into account the obtained values for coefficient of correlation (R = for k w and R = for k s ). Better correlation was achieved for the prediction of mass transfer coefficient for water (k w ). Knowing the parameters in this equation is very important from practical point of view and allows optimization and better control of the process = θ C (9) (10) k w k s = θ C 6796 Romanian Biotechnological Letters, Vol. 16, No. 6, 2011

8 NEVENA M. MIŠLJENOVIĆ *, GORDANA B. KOPRIVICA, LIDIJA R. JEVRIĆ, LJUBINKO B. LEVIĆ Comparison of the predicted and experimental results for both mass transfer coefficients (k w and k s ) are shown in Fig. 6 and Fig. 7. The validation of results showed a very good agreement to each other, which indicates the good performance of the equation Predicted value of k w Experimental value of k w Figure 6. Validation of the model for mass transfer coefficient for water (k w ) Predicted value of k s Experimental value of k s Figure 7. Validation of the model for mass transfer coefficient for solute (k s ) Information about the influence of agitation of osmotic solution on the mass transfer rate are contradictory. Many authors have indicated that agitation or circulation of osmotic solution can enhance osmotic dehydration process [23]. Generally, that improvement is so small that in some cases it might be more economical not to use agitation when considering equipment needs [24]. Experiment 2 was performed in order to check the influence of osmotic solution circulation on the final dry matter content of carrot (after 5 hours of the process, at solution temperature of 45 ºC). Changes in the dry matter content of carrot, dehydrated with and Romanian Biotechnological Letters, Vol. 16, No. 6,

9 Mass transfer kinetics during osmotic dehydration of carrot cubes in sugar beet molasses without the osmotic soluton circulation, are compared and presented on Fig. 8. Significantly higher dry matter content (p<0.05) was achieved with circulation of osmotic solution, due to recovering of thick diffusion layer, in all three examined concentration of the osmotic solution. However, in the case of industrial application of this process, economic justification of the osmotic solution circulation should be considered (costs VS increasing of dry matter). Figure 8. Comparison of the dry matter content in carrot, osmodehydrated with and without circulation of the osmotic solution (different letters, at the same solution concentration, indicate significant differences (p<0.05)) Conclusions The results of this work allow to determine the effect of concentration and immersion time on osmotic dehydration kinetics of carrot using sugar beet molasses as hypertonic solution. Higher solution concentration and longer immersion time give higher water loss and solid gain. The highest content of dry matter (54.67 %) was achieved during osmotic dehydration with 80% sugar beet molasses. Suggested equation, for the prediction values of mass transfer coefficients, satisfactorily fits the experimental data. In order to attain the specific mass transfer coefficients (k w or k s ), concentration of osmotic solution or process time can be predicted by using the proposed equation when the parameters in equation are known. Circulation of the osmotic solution had significant influence on the final dry matter content in the osmodehydrated carrot samples. Acknowledgements This research is a part of the project supported by the Ministry of Science and Technological Development of the Republic of Serbia, TR 31055, References 1. M. DELLA ROSA, F. GIROUX, Osmotic treatments (OT) and problems related to the solution management. J. Food Eng., 49, (2001). 2. D. TORREGGIANI, Osmotic dehydration in fruit and vegetable processing. Food Res. Int., 26, (1993). 3. G. KOPRIVICA, N. MIŠLJENOVIĆ, LJ. LEVIĆ, T. KULJANIN, Influence of nutrients present in sugar beet molasses and saccharose solution on the quality of osmodehydrated carrot. J. Process. Energy Agric., 13, (2009) Romanian Biotechnological Letters, Vol. 16, No. 6, 2011

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