Osmotic Dehydration of Selected Commercial Crops

Osmotic dehydration can partially reduce the moisture content of most fruits and vegetables. This method was applied to New Zealand chestnuts and Philippine cassava. The effect of osmotic dehydration on mass transfer kinetics (water loss, solids gain and normalized moisture content) and quality of osmosed products were investigated. Shelled (i.e. shell removed) and unshelled (i.e. shell intact) chestnuts in bulk conditions (1kg samples) were immersed in sodium chloride, sucrose, glucose and calcium chloride solutions of 15, 22; 50, 60; 56.5 and 50 % (w/w), respectively for 2, 4, 6, 8, 10 up to 24 hours. Cassava slices with 15 mm thickness in bulk condition (1kg samples) were immersed in sodium chloride and sucrose solutions of 22 and 25; 60% (w/w), respectively for 30, 60, 90, 120, 150, 180 and 210 minutes. The solution to product ratio was kept above 10 to minimise the concentration change of the osmotic solutions during the dehydration experiments. Experiments were conducted at ambient conditions. For chestnuts, using glucose (56.5% w/w) and calcium chloride (50% w/w) solutions produced unfavourable results. The normalised moisture content for both shelled and unshelled chestnuts using 22% (w/w) sodium chloride and 60% (w/w) sucrose solutions was reduced to about 75 and 80%, respectively after 8 hours at 20.5°C. Thus, the chestnut shell was not a significant barrier during osmotic dehydration with these solutions. Water loss rates were higher compared to solids’ gain values both for shelled and unshelled chestnuts. The optimum osmotic dehydration time for shelled and unshelled chestnuts was 6-8 hours. Furthermore, using sodium chloride (22% w/w), the recovery of wholenuts after mechanical shelling (i.e. shell removal) showed a statistically significant improvement from 28.5±0.04% (unosmosed) to around 45.3±0.04% (osmosed). In addition, with a different batch of chestnuts, using sucrose (60% w/w) solution, an improvement of almost 16.5±.08% (osmosed) from the control (unosmosed) on wholenuts shelling recovery achieved. Chestnuts osmosed with 22% (w/w) sodium chloride were darker compared to the control samples while chestnuts osmosed with 60% (w/w) sucrose was no colour difference to unosmosed chestnuts. However, chestnuts osmosed with sucrose were more brittle than unosmosed chestnuts. For chipped cassava the optimum osmotic dehydration time was found to be 210 minutes for 25% (w/w) sodium chloride and 180 minutes for 60% (w/w) sucrose. The sodium chloride (25% w/w) solution achieved a slightly higher moisture loss compared to that of sucrose (60% w/w) solution at 24.5°C. After osmotic dehydration, air drying was done for final drying (to a moisture content of 0.13, wet basis) of the chipped cassava. The optimum drying temperature was 70°C at 1.8 m/s air velocity. Drying rate was not affected significantly by osmotic solutions and osmosed cassava was not affected by a 10°C increase in the drying temperature. Osmosed products with 25% (w/w) sodium chloride or 60% (w/w) sucrose showed no gelatinization after air drying to 70°C. Cassava osmosed with 60% (w/w) sucrose was darker compared to samples osmosed with 25% (w/w) sodium chloride and to unosmosed, air dried chipped cassava. If a chestnut processor’s aim is to get higher wholenuts recovery after mechanical shelling, a 6 hour osmotic dehydration period using 22% (w/w) NaCl is recommended. Otherwise, if the processor prefers good shelling recovery and considers the broken nuts to be used for second stage by-products processing (e.g. crumb, chestnut flour), 8-hour osmotic dehydration time before mechanical shelling is more advantageous. Osmotic dehydration for chipped cassava using 25% (w/w) sodium chloride and 60% (w/w) sucrose is only effective up to 210 and 180 minutes, respectively. The optimum air drying temperature for osmosed cassava chips with no gelatinization effect was 70°