OSMOTIC DEHYDRATION OF CARROT IN SUGAR BEET MOLASSES: MASS TRANSFER KINETICS

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1 OSMOTIC DEHYDRATION OF CARROT IN SUGAR BEET MOLASSES: MASS TRANSFER KINETICS Gordana B. Koprivica, Nevena M. Mišljenović, Ljubinko B. Lević, Lidija R. Jevrić, Bojana V. Filipčev The osmotic dehydration process of carrot in sugar beet molasses solutions (40, 60 and 80%), at three temperatures (45, 55 and 65 o C) and atmospheric pressure, was studied. The main aim was to investigate the effects of immersion time, working temperature and molasses concentration on mass transfer kinetics during osmotic dehydration. The most important kinetic parameters were determined after 20, 40, 60, 90, 120, 180, 240 and 300 min of dehydration. Diffusion of water and solute was the most intensive during the first hour of the process and the maximal effect was observed during the first 3 hours of immersion. During the next two hours of dehydration, the process stagnated, which implied that the dehydration time can be limited to 3 hours. KEYWORDS: osmotic dehydration, carrot, sugar beet molasses, mass transfer kinetic INTRODUCTION The various methods for extending shelf life of fruits and vegetables are fermenting, pickling, canning or cold storage, freeze drying, etc. Convective hot-air drying is extensively employed as a preservation technique. However, using this method, food materials are exposed to elevated drying temperatures, which leads to an increase in shrinkage and toughness, reduction of both the bulk density and rehydration capacity of the dried product, and also causes serious damage to flavor, color and nutrient content (1). So, there is a need for simple and inexpensive alternative processes that are not only energy intensive and low capital investment but offer a way to make available these low cost, highly perishable and valuable crops available for the regions away from the production zones and also during off - season (1). Osmotic dehydration (OD) is one of these methods (2). The OD is a method for partial removal of water from the plant tissue by direct contact of product with a hypertonic medium. The process is governed by the osmotic pressure difference between the food material (hypotonic medium) and concentrated osmotic solution (hypertonic medium) Dr Ljubinko B. Lević, Prof., bigmum@uns.ac.rs, Gordana B. Koprivica, B.Sc., gordanak@uns.ac.rs, Nevena M. Mišljenović, B.Sc., nevenam@uns.ac.rs, Dr Lidija R. Jevrić, assistant professor, lydija@uns.ac.rs, Faculty of Technology, Bulevar Cara Lazara 1, Novi Sad, Serbia, Dr Bojana V. Filipčev, bojana.filipcev@fins.uns.ac.rs, Institute for Food Technology, University of Novi Sad, Bul. cara Lazara 1, Novi Sad, Serbia 47

2 (3). The choice of optimal hypertonic aqueous solution appears to be the key problem in osmotic dehydration. For fruits and vegetables dehydration, the most commonly used osmotic agents are sucrose and sodium chloride, as well as their combination. Glucose, fructose, maltodextrin and sorbitol also can be used for OD (4). Recent research has shown that use of sugar beet molasses as hypertonic solution improves the OD process (5). Sugar beet molasses is an excellent medium for this purpose, primarily due to the high dry matter (80%) and specific nutrient content: 51% sucrose, 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 a high osmotic pressure of the solution (6, 7). Apart from these ingredients, sugar beet molasses is a significant source of numerous micronutrients (vitamins and minerals), especially of K, Ca, Na and Mg. Of special importance is the fact that all mineral components of molasses are in the dissolved state and that the potassium is in much greater quantity than all other cations with a share of 75%. In Serbia, sugar beet molasses has not yet 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 the meat processing industry (8, 9). Sugar beet molasses as the by-product of sugar production is a cheap source of nutrients, i.e. saccharose, it is available in large quantities, and for osmotic dehydration previous treatment is not needed. During the OD, the tendency is to increase the diffusion of water from the sample into the surrounding solution and to decrease the penetration of solids from the solution into the plant tissue, on the other hand (10). The rate and dewatering degree from the material and changes in its chemical composition 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, and type of apparatus. The rate of OD is the highest at the beginning of the process. It results from the largest difference in the osmotic pressure between the osmotic solution and the cell tissue of the material and small mass transfer resistance at this stage of the process (11). During the OD operation, two main mass fluxes at counter - current take place: water loss, (WL) and solid gain, (SG). So, the determination of the osmotic treatment effectiveness can be evaluated via the WL/SG ratio, taking into account that water removal must be greater than solute acquisition (12). The objective of this work was to study the influence of main process variables such as temperature, concentration of sugar beet molasses and operation time on the OD of carrot in sugar beet molasses. Also, the aim was to investigate the mass transfer during the OD and find out an appropriate mathematical model which would describes the investigated process. Mass transfer model A large number of factors influence the OD process. However, earlier research has shown that the temperature and concentration of the osmotic solution are two most influential factors on the mass transfer rate during the dehydration. In the OD process, three main process variables are usually measured: moisture content, change in the weight and change in the soluble solids. Of these, water loss (WL), weight reduction (WR), solid 48

3 gain (SG), normalized moisture content (NMC) and normalized solid content (NSC) are calculated as follows (13): WR g g W 0 W W 0 g u u SG g W 0 g WL WR SG [3] g X NMC [4] X 0 u NSC [5] u 0 where: W o - initial sample weight (g), W - sample weight after OD (g), u o - initial solid content in the fresh sample (g), u - solid content in the sample after OD (g), X o - initial moisture content of the fresh sample before osmotic treatment (g), X - moisture content in the sample after OD (g). The existence of a simple mathematic model is very important from a practical point of view, because in that case it is possible, for example, to predict the duration of the process for the desired moisture content and vice versa. Modeling of the calculated values of the main kinetic parameters (WL, SG, NMC and NSC) was achieved by employing an empirical model suggested by LabFit Curve Fitting Software and based on our experimental data: P = a t b C c [6] where t is the immersion time, C is the concentration of osmotic solution, P represents WL, SG, NMC or NSC; a, b and c are parameters in Eq. [6]. The purpose of this analysis is to obtain a simple equation that takes into account the both variables (time, concentration) simultaneously. o [1] [2] EXPERIMENTAL Carrot samples were purchased in a local market in Novi Sad, Serbia, and stored at 4ºC until the use. Initial moisture content, X o, was ± 0.99%. Prior to the treatment, the carrots were thoroughly washed and cut into cubes, dimension 1x1 cm. Pure sugar beet molasses (around 80% solid content) and sugar beet molasses solutions (with 40% and 60% solid content) were used as osmotic agents. Solutions were made by mixing pure molasses with distilled water. Sugar beet molasses was obtained from the sugar factory Pećinci, Serbia. Initial dry matter content in sugar beet molasses was 83.68%. In all experiments, a weight ratio of solution to carrot sample of 4:1 was used; it can be considered high enough to neglect the concentration changes during the process. The experiments 49

4 were conducted under atmospheric pressure and static conditions and at temperatures 45ºC, 55ºC and 65ºC. Mass transfer studies lasted 5 hours and the samples were taken out from the osmotic solution at different times (20, 40, 60, 90, 120, 180, 240 and 300 min). After removal, carrot samples were washed with water and gently blotted to remove excessive water. The samples were kept in an oven (Instrumentaria Sutjeska, Serbia) at 105 C for 24 h until a constant weight was attained. Dry matter content was calculated from the samples weights before and after drying. The solid content of osmotic solutions was determined refractometrically. All analytical measurements were carried out in accordance to AOAC (14). All results were treated using Statistica 9 and Origin 6.1 software. RESULTS AND DISCUSSION Water loss One of the most important parameters used to describe the process of osmotic dehydration is WL from sample. Fig. 1 displays the WL as a function of the immersion time in molasses solutions (40, 60 and 80%) at studied temperature 65 o C, because the best OD result was achieved at this temperature. 50 Figure 1. Dependence of WL on dehydration time for the different concentrations of sugar beet molasses solutions (40, 60 and 80% by weight) at 65 ºC As can be seen from Fig. 1, the highest water loss from the samples occurred during the first 20 min of dehydration regardless of the solution concentration or temperature. At this interval, 52.06% (at 65 o C and 80%) of the initial water content was removed from the sample. During the next 100 min of the process, 42.9% of water was additionally removed from the sample (at 65 o C and 80%). After the second hour, WL remained almost

5 constant because of the drop in the osmotic driving force between the carrot tissue and surrounding hypertonic solution. During the following 3 hours, only 5% of water was removed, which leads to a conclusion that the duration of osmotic dehydration can be limited to 3 hours. By applying the empirical model (Eq. 6) and by statistical processing of experimental data, the model parameters were calculated and given in Table 1. High values of the correlation coefficients indicated that the suggested model is appropriate and describes well the influence of concentration on the efficiency of the OD process. Table 1. Correlation data for WL during OD of carrot according to equation [6] Correlation data 45 ºC 55 ºC 65 ºC a ± ± ± b 0.285± ± ±0.022 c 0.546± ± ±0.063 R On the basis of data displayed in Table 1, the value of WL at any time or for any molasses concentration at constant temperature can be determined. Solid gain The SG describes the penetration of the solute from osmotic solution into the sample. The aim of the OD is to remove water from the plant tissue and simultaneously minimize the uptake of solute from osmotic medium into the tissue. Table 3 presents the effects of osmotic medium concentrations (40, 60 and 80%), temperatures (45, 55 and 65 o C) and process duration on the SG of carrot. Table 2. SG values for the OD of carrot using different concentrations of sugar beet molasses solutions at different temperatures Temperature of osmotic solution ( o C) Concentration of osmotic solution (%) The process duration (min) 45 o C 55 o C 65 o C The results showed that rapid increase in solids was registered in the first 60 min of the process at 80% and 65 C. Within this period, 59.5% of solids from the osmotic so- 51

6 lution diffused into the sample as compared to total amount of solids eliminated from the sample during the process. Since the process efficiency is characterized by a minimal SG in the treated sample, the temperature of 65 C cannot be considered as optimal. At 45 C, the diffusion of osmotic active substances from the medium to the carrot was minimal (SG = g/g initial sample) after 5 h of immersion in 40% solutions. On the basis of these data it comes out that the temperature of 45 C can be considered as an optimum for the OD of carrot in molasses as an osmotic medium. Table 3. Correlation data for SG during osmotic dehydration of carrot according to equation [6] Correlation data 45 ºC 55 ºC 65 ºC a ± ± ± b 0.297± ± ±0.048 c 1.023± ± ±0.140 R High coefficient of correlation (Table 3) confirms that the proposed empirical model equation [6] predicted well the solid gain in relation to the solution concentrations. Water loss and solid gain ratio In general, the increased concentration of osmotic medium favors the diffusion of solids into the sample, which leads to decline in the WL/SG ratio. This ratio is considered to predict the best efficiency of the osmotic treatment. High WL/SG ratios point to intensive water removal from the samples accompanied with minimal solid gain. Table 4. Effect of temperature, concentration and process duration on the change of the WL/SG ratio at temperature of 65 o C T ( o C) 65 o C Time (min) C (%)

7 The WL/SG ratio gradually decreased during the osmotic dehydration of carrot, after the initial rise within the first 20 minutes of the process. The values ranged from minimal (registered at 65 C, 80% solution, after 180 min) to a maximum of (at 65 C, 40% solution, after 20 min). Both minimal and maximal ratios were achieved at 65 C, suggesting that the concentration of osmotic solution has a more pronounced effect on this parameter, probably due to its greater effect on solid gain. Normal moisture content and normal solid content Figure 3 presents the effect of concentration and process duration for the molasses solutions on the NMC and the NSC values of carrots during the OD at 65 C. Expectedly, increased molasses concentration and temperature intensified the process and caused an NMC drop and NSC elevation. At all tested temperatures and concentrations, the most drastic NMC and NSC changes occurred within the first 60 min of the process. Minimal NMC was achieved when 80% molasses at 65 C was used. Within the first 60 min, 65% of the initial water content was reduced (from 1 to g/g), whereas during the next 2 h it was reduced by additional 20%. During the last 120 min, the NMC decreased by the negligible 3%. This supports our statement that the dehydration duration can be limited to 3 h. Figure 2. Effect of the process parameters (concentration, temperature) on NMC and NSC during OD of carrot in sugar beet molasses at 65 ºC CONCLUSION It was shown that the process of OD of carrots in sugar beet molasses is the most intensive within the first hour. In the next two hours, the mass transfer weakened due to decreased concentration gradient between the osmotic medium and plant tissue, and consequently lowers osmotic driving force. The water removal within the third and fifth hour of the process duration is negligible therefore it was concluded that the process can be limited to 3 hours. 53

8 Maximal water loss was achieved at 65 C in 80% molasses (0.835 g/g of initial sample) which is by 0.06 g/g or 0.03 g/g higher as compared to the maximal moisture loss at 45 C or 55 C, respectively. This suggests that the process conduction at higher temperatures is more favorable since the main goal is to maximize the water removal from the sample. However, higher temperatures (65 C) also cause an increased uptake of solids by carrot tissue (SG= g/g initial sample). Higher efficiency of osmotic dehydration is related to a high WL/SG ratio, which can be obtained by the proper choice of process conditions (concentration, temperature). The highest WL/SG ratio (18.636) was achieved at 65 C in 40% molasses solution. 54 Acknowledgement This research is part of the project supported by the Ministry of Science and Technological Development, Republic of Serbia, TR 20112, REFERENCES 1. M. Maskan: Microwave/air and microwave finish drying of banana. J. Food Eng. 44 (2000) J. Shi & M.L. Maguer: Osmotic dehydration of foods: Mass transfer and modeling aspects. Food Rev. Int. 18 (2002), N. K. Rastogi and K.S.M.S. Raghavarao: Mass transfer during osmotic dehydration: Determination of moisture and solute diffusion coefficients from concentration profiles. Food Biprod. Process. 82 (2004) A. Ispir and I. Togrul: Osmotic dehydration of apricot: Kinetics and the effect of process parameters. Chem. Eng. Res. Des. 87 (2009) N. Mišljenović, G. Koprivica, Lj. Lević and T. Kuljanin: Influence of mono- and double- edible coating on improving of osmotic dehydration of apple in saccharose solution and sugar beet molasses. J. process. energy agric. 13 (2009) LJ. Lević, V. Filipović and T. Kuljanin: Osmotski tretman oblikovanog korena mrkve u saharozi i melasi. J. process. energy agric. 11 (2007) G. Koprivica, N. Mišljenović, Lj. Lević, V. Pribiš: Changes in nutritive quality of apple osmodehydrated in sugar beet molasses and saccharose solutions. Acta Periodica Technologica 40 (2009) B. Filipčev, Lj. Lević, V. Pribiš and D. Kabić: Sugar beet molasses as a favorable hypertonic solution for osmotic pretreatment of apple, XIII Conference about Biotechnology. Proceedings, Čačak, Serbia, March (2008) B. Filipčev, Lj Lević, M. Bodroža-Solarov, N. Mišljenović, G. Koprivica: Quality Characteristics and Antioxidant Properties of Breads Supplemented with Sugar Beet Molasses-Based Ingredients. Int. J. Food Prop. 13 (2010) M. Matuska, A. Lenart and H. N. Lazarides: On the use of edible coatings to monitor osmotic dehydration kinetics for minimal solids uptake. J. Food Eng.72 (2006) R. Moreira and A. M. Sereno: Evaluation of mass transfer coefficients and volumetric shrinkage during osmotic dehydration of apple using sucrose solutions in static and non-static conditions. J. Food Eng. 57 (2003)

9 12. F. Chenlo, R. Moreira, C. Fernandez-Herrer and G. Vazquez: Osmotic dehydration of chestnut with sucrose: Mass transfer processes and global kinetics modeling. J. Food Eng. 78 (2007) M. Maguer: Osmotic dehydration: review and future directions, Proceedings of the symposium in food preservation process, Brussels (1988) AOAC (2000). Official Methods of Analysis. Washington, USA. ОСМОТСКА ДЕХИДРАТАЦИЈА МРКВЕ У МЕЛАСИ ШЕЋЕРНЕ РЕПЕ: КИНЕТИКА ПРЕНОСА МАСЕ Гордана Б. Копривица, Невена М. Мишљеновић, Љубинко Б. Левић, Лидија Р. Јеврић, Бојана В. Филипчев У раду је испитиван процес осмотске дехидратације мркве у растворима меласе шећерне репе применом различитих концентрација раствора (40, 60 и 80%) и различитих температура (45, 55 и 65 о C) а извођен је при атмосферском притиску. Главни циљ експеримента је био да се испита утицај времена имерзије, радне температуре и концентрације меласе на кинетику преноса масе током процеса осмотске дехидратације. Најважнији кинетички параметри процеса су одређивани након 20, 40, 60, 90, 120, и 300 мин. Дифузија воде и растворка је била најинтензивнија током првих сат времена имерзије док целокупни учинак достиже максимум у току прва 3h трајања процеса. Током последња два сата дехидратације, процес издвајање воде из третираних узорака је стагнирао, на основу чега се може закључити да целокупни поступак може бити скраћен на 3h. Received 23 September 2010 Accepted 26 October