Recent developments in osmotic dehydration.
Osmotic dehydration is an operation used for partial removal of water from foods such as fruits and vegetables. In this process, foods are placed in hypertonic (osmotic) solution. Three different mass transfer mechanisms occur in the osmotic dehydration; (i) water migration from food to the solution, (ii) solute migration from solution to food, and (iii) solute concerning product extracted to solution. Osmotically dehydrated fruits and vegetables are generally dried by hot air flow. In recent years, osmotic dehydration has been combined with various methods such as the high hydrostatic pressure, high electrical field pulses, gamma irradiation, ultrasound, vacuum and centrifugal force. These new combination methods increase mass transfer and drying rate and foods by increasing the cell membrane permeability. The operation time in the combined methods is shorter than that in traditional osmotic dehydration, causing further energy saving. In this study, recent advances in osmotic dehydration will be reviewed.
Osmotic dehydration of carrot tissue enhanced by pulsed electric field, salt and centrifugal force
Abstract The osmotic dehydration (OD) kinetics of carrot disc untreated and treated by pulsed electric field (PEF) was studied under centrifugation (2400 × g), stirring (250 rpm) and with a salt addition (NaCl/sucrose solutions 0%/65%, 5%/60% and 15%/50%). The PEF intensity was E = 0.60 kV/cm and the treatment duration was tPEF = 0.05 s (500 rectangular monopolar pulses each of 100 μs). The water loss (WL), solids gain (SG) and water loss/solids gain ratios (WL/SG) were evaluated in the binary (sucrose + water) and ternary (sucrose + salt + water) solutions at the temperature of 20 °C during 4 h. The mass ratio of sample to solution was 1:3. The PEF treatment and salt addition enhanced the OD kinetics. WL and SG were increased under centrifugation (centrifugal OD) and under stirring (static OD). The centrifugal field enhanced the WL, however, decreased the SG comparing to the static OD. Therefore, the static OD has advantages for the higher SG (confectionary adds), while the centrifugal OD is better appropriated if the WL should be increased and the solids (sugar) uptake should be limited (dietetic products). The two-exponential kinetic model fitted well to experimental data for both static and centrifugal OD. The correlation coefficient was R2 = 0.982–0.999 and the standard error was 5–10%.
DESIGN AND SELECTION OF PROCESSING CONDITIONS OF A PILOT SCALE CONTACTOR FOR CONTINUOUS OSMOTIC DEHYDRATION OF CARROTS
A pilot-scale belt type osmotic contactor was designed and installed in a continuous osmotic dehydration process to obtain maximum water removal from fruits and vegetables with a minimum contact time. The carrot was chosen as a representative vegetable and a ternary mixture of sucrose/sodium chloride/water with total solids concentration of about 50 to 55% was chosen for the osmotic solution. The experimental results showed that the optimum condition was found to require a 44% sucrose/7% sodium chloride aqueous solution with 16 min of contact time which gave a water loss of 26% and a highest ratio of water loss over solids gain (about 9) among different treatments. It is concluded that the continuous type of contactor can have potential applications in the food industry.