Available online at http://www.urpjournals.com International Journal of Agricultural and Food Science Universal Research Publications. All rights reserved ISSN 2249-8516 Original Article Effect of varietal differences on the osmotic dehydration of sweet potatoes (Ipomoea batatas Lam) Md. Nahid Hossain Jany, Md. Anisur Rahman Mazumder * and M Burhan Uddin Department of Food Technology and Rural Industries Faculty of Agricultural Engineering and Technology Bangladesh Agricultural University Mymensingh- 2202, Bangladesh * Corresponding author Md. AnisurRahmanMazumder Assistant Professor E-mail: anis_engg@bau.edu.bd Tel: +8801754146914 Fax: +880-91-61510 Received 13 March 2016; accepted 08 April 2016 Abstract This study was concerned with osmotic dehydration rate of sweet potatoes (var. Local White and KomolaSundari) at different sugar concentration (40%, 50% and 60%), different temperature (30 o C, 40 o C, 50 o C and 60 0 C) and two thicknesses (3 and 5 mm). Influence of thicknesses, blanching treatments, sugar concentrations, immersion or contact time and temperatures on the osmotic concentration behavior were studied. The results indicated that the rate of water loss and solid gain increased with increasing solute concentration, temperature and decrease with increasing the thickness. The 60% sugar solution at 60 0 C was found to be the most effective for both varieties of sweat potatoes. Immersion or contact time on osmotic behavior showed that the first two hours was the rapid period for osmotic dehydration for both 3 and 5 mm thickness in both varieties. % water loss (WL) and % solid gain (SG) were found to be much higher for high solute concentration and high temperature. The 3 mm thickness slices showed the highest %WL (for local White variety 48%, 65% for Kamala Sundari) and %SG (for local White variety 13%, 16% for Kamala Sundari) for both varieties. Unbalanced samples had shown the highest %WL (for local White variety 48%, 65% for Kamala Sundari) and %SG (for local White variety 13%, 16% for Kamala Sundari) than others. Kamala Sundari variety showed higher %WL (65%) or %SG (16%) than local variety (48% Water loss or 13% Solid gain, respectively). 2016 Universal Research Publications. All rights reserved Key words: Osmotic dehydration, sugar concentration, blanching, water loss, solid gain. Introduction Sweet potato (Ipomoea batatas Lam) is an important starch rich root crop of Bangladesh. The crop is very popular among the poor people of the country due to its low price. Sweet potato provides more calories than potato (113 vs. 75 per 100 g) and is an exceptionally rich source of vitamin A. Sweet potato processing has a great potential in tropical countries to augment available food resource [1]. Kamala Sundari (BARI-2) variety is one of the high yielding released varieties of sweet potato and contains 5000 µg of beta-carotene per 100 g of sweet potato. It also plays an important role in combating night blindness in children. The farmers cannot preserve sweat potatoes more than 2-3 months due to spoilage resulting in heavy financial loss. Osmotic dehydration is a water removal technique, which is applied to horticultural products, such as fruits and vegetables, to reduce the water content, while increasing soluble solid content [2]. Osmotic dehydration is a simple and inexpensive method of preservation of food materials [3]. Dehydration results primary from osmotic water and solute activity gradients. In osmotic dehydration, the fruits 14
are subjected to osmosis by dipping or spreading them in aqueous sugar syrup under specific conditions, so that the water from the fruits migrates to sugar syrup. Major dehydration of the fruit takes place in this process step. The final drying of the fruits to make it suitable for marketing is carried out by vacuum or air drying depending on the cost considerations. It was reported that an increase in concentration and temperature of the osmotic solution increased the rate of mass transfer up to a certain extent, above which undesirable changes in flavor, color and the texture of the product were observed. In the last few years, numerous studies have been carried out to better understand the internal mass transfer occurring during osmotic dehydration of foods and to model the mechanism of the process. On the basis of the information so far accumulated, the present study has been undertaken with the following objectives: to assess the influence of thickness, solute concentration, blanching, immersion time and temperatures as well as varietal effect on osmotic dehydration behavior of the sample. Materials and Methods 2.1 Materials Local White (moisture content 73%, protein 1.6%) and Kamala Sundari (moisture content 70%, protein 2%) varieties of sweat potato were collected from local market, sucrose (Splenda, Tale and Lyle, UK). 2.2 Preparation of solution 40%, 50% and 60% sucrose solution were prepared by blending with portable water on a weight to weight (w/w) basis. 2.3 Sample preparation The sweet potatoes were hand peeled and cut into 3 mm and 5 mm thickness and blanching were used as pretreatment. 2.4 Osmotic dehydration At first both 3 and 5 mm thick slices were weighted individually and placed into stainless steel baskets containing 40%, 50% and 60% sugar solution. The basket was placed inside a stainless dehydrating vessel at 30 o C, 40 o C, 50 o C and 60 o C, respectively. Then Sampling was carried out at time intervals of 15, 30, 60, 120, 180, 240, 300 and 360 minutes. Finally the slices were returned quickly to the dehydrating vessel for further osmotic process. The influence of thickness, solute concentration, blanching, immersion time and temperatures as well as varietal effect on osmotic dehydration behavior of the sample was carried out at Department of Food Technology and Rural Industries, Bangladesh Agricultural University, Mymensingh. Results 3.1 Influence of time and solution concentration on osmotic concentration behavior of sweet potato (Ipomoea batatas Lam) For both local White and Kamala Sunduri variety at both thickness (3 and 5 mm) showed increase the %WL and %SG with the immersion of time. The rate is very rapid at the beginning period which falls presumably due to reduction of osmotic potential gradient. In other words, an equilibrium stage is being reached. %WL and %SG increased with increasing the solution concentration and temperature. 60% sugar solution showed the highest %WL and %SG for first three hours and the rates was reduced presumably due to the reduction of osmotic potential gradient for both thickness (3 and 5 mm). The results indicated that solution concentration increased the osmotic potential gradient increasing the diffusivity. 3.2 Influence of thickness on osmotic concentration behavior of sweet potato slices The 3 mm sample thickness showed higher %WL and %SG compare to 5 mm thickness. 5 mm thick samples showed slow rate of water losing capacity than 3 mm thickness. As a result, the solid gain capacity much higher in 3 mm thick samples than 5 mm samples. This result suggested that mass transfer rate is related to the thickness and thicker samples needed a longer path to reach desired destination [3](Islam, 1980). The 3 mm thick, unblanched samples at 60% sugar solution treated at 60 o C showed around 48% water loss (Figure 1, 2) whereas 5 mm thick (Figure 1, 2) samples with same condition showed 35% water loss. Nonetheless, there was no significant difference in case of solid gain (12.87% and 11.19% for 3 mm (Figure 3, 4) and 5 mm thick (Figure 3, 4) samples respectively, with same conditions. 3.3 Influence of blanching on osmotic concentration behavior of sweet potato Unblanched samples showed higher %WL and %SG compare to blanched samples at all the treatment tested. Figure 2 (A), showed 3 mm thick, blanched at 60% sugar solution (local White variety) the percent water loss was about 14.14% whereas at unblanched (Figure 2B) sample with same conditions had water loss about 47.72%. In case of solid gain (Figure 4A), 3 mm thick, blanched at 60% sugar solution (local White variety) the percent of solid gain was about 3.84% whereas unblanched (Figure 4B) sample with same conditions had solid gain about 12.87%. 3.4 Influence of variety on osmotic concentration behavior of sweet potato The results showed that %WL or %SL gain was higher in KamalaSundari variety than local one. Kamala Sundari variety of sweat potato at 3 mm thick, under unblanched condition at 60% sugar solution during heating at 60 o C showed the highest percent of water loss (Figure 1A) and solid gain (Figure 3A) (64.65% and 15.97%, respectively) than local White variety at same condition (Figure 2A and 4A) (47.72% and 12.87%, respectively). This might be due to low sugar content and higher amount of carotene content in Kamala Sundari variety. Discussion Osmotic dehydration had been termed as two-way process [3] and in this experiment mass transfer parameter such as %WL and %SG are dependent on the osmotic potential gradient. The behavior of reducing solid gain after first three hours permits assessing that there is an antagonistic effect of the solute on water loss which was agreed with [4-6]. These results suggested the reduction of plasmatic membrane permeability which takes place during the plasmolysis process. Nonetheless, it might be due to the different molecular dimensions and sucrose remains mainly in the extra cellular space thus leading to a reduction of the osmotic pressure gradient [6]. 15
Figure 1.Water loss at different temperature for 3 mm (A, B) and 5 mm (C, D) thick blanched (A, C) and unbalched (B, D) sample of sweet potato (var Kamala Sundari) at 60% sugar solution. Figure 2.Water loss with time at different temperature for 3 mm (A, B) and 5 mm (C, D) thick blanched (A, C) and unbalched (B, D) sample of sweet potato (var Local White) at 60% sugar solution. Raw material in concentrated solutions of soluble solids, water and solute diffusion processes are caused by the water and solute activity gradients across the cell membrane, cell wall and surface of the tissue. The complex cellular structure of food acts as a semipermeable surface. Since these compartments are only partially selective, there is always some solute diffusion into the food. The water transfer is generally accompanied by natural substances (vitamins, flavors, fruit acids, pigments, saccharides, minerals). As a consequence of this exchange, the product loses weight and shrinks. Generally, blanched sample showed lower percent of water loss and solid gain due to partial cooked of samples. Nonetheless, sweet potato content high amount of starch which has been gelatinized 16
Figure 3. Solid gain with time at different temperature for 3 mm (A, B) and 5 mm (C, D) thick blanched (A, C) and unbalched (B, D) sample of sweet potato (var Kamala Sundari) at 60% sugar solution. Figure 4. Solid gain with time at different temperature for 3 mm (A, B) and 5 mm (C, D) thick blanched (A, C) and unbalched (B, D) sample of sweet potato (var Local White) at 60% sugar solution. 17
during blanching and increased the water holding capacity [7]. As a result, blanched sample showed less water loss and solid gain than unblanched sample. Conclusion Increasing the immersion or contact time increased the osmotic potential gradient but highly time dependent. However, blanching treatment, sugar solution concentration and temperature also influence the osmotic potential gradient. Nonetheless, variety differences of sweat potatoes also showed significant effect on osmotic dehydration behavior. References 1. AVRC. Asian Vegetable Research and Development Centre. The world vegetable center.retrieved from December, 20, 2015 from http://avrdc.org/. 2. M. N. Islam, User of solar energy for development of shelf stable potato products.ph.d. Thesis.Royal Veterinary and Agricultural University, Denmark, 1980. 3. M.N. Islam, J.M. Flink, Influence of process parameters on the effectiveness of osmotic dehydration of some fruits and vegetables, Bangladesh J.Agril. Eng. 4(1, 2) (1990) 65-73. 4. F. Kaymak-Ertekin, M. Sultanoglu, Modeling of mass transfer during osmotic dehydration of apples,j. Food Eng. 46 (2000)243 250. 5. A. Lenart, J.M. Flink, Osmotic concentration of potato. I. Criteria for the end-point of the osmosis process,j. Food Tech. 19(1984) 45-63. 6. G. Sacchetti, A. Gianotti, D.M. Rosa,Sucrose-salt combined effects on mass transfer kinetics and product acceptability. Study on apple osmotic treatments, J. Food Eng. 49 (163) (2001) 173. 7. S. Tremblay, The Nutrient Value of White Potatoes vs. Sweet Potatoes, Retrieved from December, 20, 2015 from http://healthyeating.sfgate.com/nutrient-valuewhite-potatoes-vs-sweet-potatoes-4221.html. Source of support: Nil; Conflict of interest: None declared 18