G. Moraga

Changes in mechanical properties throughout osmotic processes: Cryoprotectant effect

Abstract In osmotic dehydration of fruits, physical and chemical changes occurring throughout the process provoke changes in the product texture and appearance to a different degree depending on the process conditions and product characteristics. Nevertheless, water content reduction and sugar gain have been observed to have some cryoprotectant effects on colour and texture in several fruits. In this work, the specific influence of osmotic treatments with sucrose solutions at 30°C on mechanical properties of mango, kiwi and strawberry are discussed. The combined effect of blanching and vacuum impregnation is analysed in mango. The influence of osmotic solution concentration and vacuum impregnation on kiwi fruit is discussed and the effect of osmotic dehydration, as compared with air drying is analysed in strawberry. In all cases, changes in mechanical properties of fresh and pre-dehydrated fruits due to freezing–thawing are compared in order to quantify the cryoprotectant effect of the osmotic treatment.

Water sorption isotherms and glass transition in strawberries: influence of pretreatment

Abstract Water sorption isotherms and glass transition as a function of moisture content were determined in strawberries as a function of sample pretreatment. Sorption experiments were carried out in chambers at controlled a w , at 30 °C. Samples studied were whole and homogenized tissue, fresh (for the desorption process) and previously freeze-dried (for the adsorption process). Glass transition of samples equilibrated to different moisture contents were analysed by DSC. GAB model was fitted to sorption data and Gordon and Taylor equation was used to model the water plasticization effect. Strawberry pretreatments caused changes in the tissue structure that affected the water binding capacity of the different product phases at equilibrium with a determined a w value. This implied differences in the relationships between the mean equilibrium moisture content of the product and both the a w value (equal for all phases at equilibrium) and glass transition temperature of the amorphous soluble solids.

Combined Drying Technologies for High-Quality Kiwifruit Powder Production

Freeze-drying is a dehydration technique which, from a sensory, nutritional and functional point of view, provides high-quality powder products. Nevertheless, both long processing times and high economic costs are required. In this study, pre-drying the samples using hot air or microwave has been considered in order to reduce the initial product’s water content thereby shortening the freeze-drying time so as to obtain high value products at a reduced cost. The effect of dehydration pre-treatments on the kinetics, antioxidant activity and solubility of freeze-dried kiwifruit products was evaluated. Nine different thin-layer semi-theoretical models were used to fit the drying data. According to the obtained results, the pre-treated samples exhibited higher drying rates than the fresh sample. In turn, the dehydration pre-treatments used did not affect the solubility or the antioxidant activity of the samples.