Combined osmotic and microwave drying of strawberries and blueberries.

摘要

This work was aimed at obtaining high quality dried strawberries using microwaves to assist convection air of 2 m/s at 30–45°C. Preliminary trials with whole strawberries were unsuccessful. Fruits would cook rather than dry at low microwave power levels, and burst at higher powers. This was due to the inhibition of moisture movement by the waxy cuticle. Slice and pureed strawberries dried, but were of lower quality than freeze-dried; A treatment consisting of dipping the berries in a solution of ethyl oleate and sodium hydroxide was studied. Such treatments are used in industry to reduce the skin resistance to moisture diffusion. Result showed that the treatment greatly enhanced the drying rates of whole berries in convection and microwave regimes. A 1% concentration of ethyl oleate was sufficient for maximum reduction of drying time, and it is possible that even lower concentrations could be used for strawberries. Rehydration was similar to that of the dipped and freeze-dried samples, but the microwaved samples were a bit softer, and had less aroma, colour and flavour.; Osmotic dehydration was then studied as a technique of binding flavours and aromas and of reducing the time required for finish drying with microwaves. These studies were performed on strawberries and blueberries. Results showed berries that were dipped and then osmotically dehydrated for 24 h in sucrose, yielded a microwave-dried final product that was equal to the freeze-dried one in terms of quality, and this, with a much lower time for finish drying.; The shrinkage ratio of strawberries has a straight line relation to the moisture ratio. The reduction in equivalent diameter is well-described by a reciprocal logarithmic function.; The results of these major aspects of the research suggest that microwave-drying could be a viable and more rapid alternative to freeze-drying when berries are first subjected to a pretreatement of ethyl oleate and partial dehydration by osmosis. It was also found that if microwave energy is applied in continuous mode, the initial applied power should not exceed 0.2 W g–1, otherwise burning may occur. It is also recommended that osmotic dehydration be limited to not much longer than 24 hours, since off-odours, develop. The results apply to convective regimes with inlet air temperatures of 45°C and inlet velocity of 2 m/s.