FOOD. Science. Technology. Quality

Food. SCIENCE. Technology. Quality

Food. Science. TECHNOLOGY. Quality

Food. Science. Technology. QUALITY




Colour and physical proprieties of osmotically dehydrated and freeze-dried strawberries


The objective of this research was to examine the effect of osmotic dehydration on the colour and selected physical proprieties of freeze-dried strawberries. The process of osmotic dehydration of strawberries was carried out in a 67.5 % starch syrup with the dextrose equivalent of DE 30-35 at a temperature of approx. 20 and 50 °C under the dynamic conditions. In addition, prior to its initial pre-treatment, one part of strawberries was covered with coatings made of low methyled pectin solutions (2 %) or with alginate sodium solution (0.75 %). Regardless of the initial water content in raw material, 24 h freeze-drying resulted in a similar level of final water content (approx. 0.0432 g water/g db). The lyophilised strawberries, previously just frozen, obtained the lowest water activity (aw = 0.163) compared to dried fruit after their osmotic dehydration in a syrup starch (aw from 0.195 to 0.250). The coatings formed on the fruit surface limited the penetration of osmotically active substance into the tissue. The process of osmotic dehydration, regardless of the applied parameters, caused, in the majority of the samples, the product colour to lighten, and the lowest surface lightness was obtained for fruit dehydrated for 3 h at the temperature of 50 °C. The increase in absolute colour difference, ∆E, of the freeze-dried strawberries occurred along with the lengthening of the osmotic dehydration duration period. The initial pre-treatment duration period increased during the first phase of deformations registered and resulted in the clear rise in the compressive strength of freeze-dried fruit. The most fragile fruit were strawberries dehydrated for 20 h and freeze-dried.


freeze-drying, sugar content, water activity, colour, mechanical proprieties, strawberries