Pretreatment of fruits prior to drying has shown success in reducing drying time and costs. In this work, ultrasound-assisted osmotic dehydration has been implemented as a method to increase water diffusivity and reduce drying time in strawberries. Strawberry halves were immersed in distilled water and in two different concentrations of sucrose solutions while pretreatment time and ultrasonic frequency levels were varied to determine their effect on drying time, water loss, and soluble solids gain. A microscopic analysis was carried out to evaluate the formation of microchannels and other changes to the fruit tissue structure. Greater sucrose concentration used in ultrasound-assisted osmotic dehydration resulted in greater water loss with greatest loss observed for the strawberry halves pretreated for 45 min in a 50% w/w sucrose solution. The pretreatment carried out for 30 min employing an osmotic solution of 50% w/w of sucrose resulted in the highest drying rate among the pretreatments. Osmotic dehydration used alone during pretreatment increased total processing time, whereas osmotic dehydration combined with ultrasonic energy during pretreatment reduced total processing time and increased effective water diffusivity. Cell distortion and breakdown were observed not only in pretreatments employing ultrasound-assisted osmotic dehydration but in conventional osmotic dehydration. Formation of microchannels through ultrasonic application and effects of osmotic pressure differential were considered to be largely responsible for reducing drying time for strawberry halves.

The effect of pre-treatments on the colour of freeze-dried strawberries was studied. Strawberries were subjected to different ultrasound and osmotic dehydration conditions followed by freeze-drying. Two concentration levels of sucrose solution (25 and 50 % w/w) and four levels of processing time (from 10 min to 45 min) were studied. Also, ultrasound application without using an osmotic solution was studied. Colour was quantified with a colorimetric analysis (CIE LCh). Sonicated strawberries presented higher lightness (L) and lower hue (h) than fresh and non-treated strawberries (control samples). The sonicated and osmo-sonicated strawberries have presented a more reddish and vivid colour then the control samples.

Oliveira, Francisca I. P.; Rodrigues, Sueli; and Fernandes, Fabiano A. N., "Dual-stage sugar substitution in strawberries with a Stevia-based sweetener" (2010). Biological Systems Engineering: Papers and Publications. 187.