Effect of carrageenan and carboxy methyl cellulose coating during osmotic dehydration on drying time and rehydration of Golden delicious apples

Use of coating treatment prior to osmotic dehydration can improve process performance by reducing solids uptake. In this study, the effect of coating-osmotic treatment on drying of apple cubes was investigated. So, the apple cubes (1 cm3) were coated in two concentrations (1 and 1.5% w/w) of carboxy methyl cellulose (CMC) and carrageenan solutions and osmotically dehydrated in different sucrose solution concentrations of 45 and 60˚ BX, for 150 minute. Then, coated apple cubes were dried in a hot-air drier at 70˚C to final moisture content of 20% (w.b) and the rehydration ratio of dried samples was measured. Our results revealed that the highest performance ratio was related to single-coated apple cubes with 1% solution of carrageenan, osmo-dehydrated in 60˚ BX and single and double-coated samples with 0.5% solution of CMC, osmo-dehydrated in 45˚ BX. Investigation of final drying showed that coated materials created a strong barrier against water removal and decreased the drying rate. Dried coating layers on apple cubes also prevented the samples from water absorption, and the coated apple cubes had a lower rehydration ratio than noncoated ones. We found that coating is effective only during osmotic process and it is not a good and efficient method for the final air drying.