Objective  The process to freeze-dry kiwifruits was optimized using an ultrasonic-assisted sugar osmotic pretreatment to shorten the drying time and improve the product quality.

  Method  Single factor tests were conducted to determine the ranges of sugar concentration as well as ultrasonic application time, temperature, and power for the pretreatment prior to process optimization by a Box-Behnken response surface experiment.

  Result   The time required to freeze-dry kiwifruits increased significantly when the treatment temperature was raised, especially, beyond 40℃. On the other hand, the time was shortened as the ultrasonic power and sugar concentration increased. Prolonged ultrasonic application also reduced the drying time to a leveled off minimum. The optimized pretreatment applied 200W ultrasound at 24℃ for 50m on fresh kiwifruits impinged with 45% (w/w) sugar. The freeze dehydration took 43.07h to yield kiwifruit pieces with a hardness measurement of 14,616.01g, a chewiness of 7,795.61mJ, a vitamin C content of 0.19%, and a total sugars content of 62.82%. The sensory evaluation rated the test product much superior to control without pretreatment.

  Conclusion  The freeze-drying process optimized with ultrasonic-assisted sugar osmotic pretreatment on kiwifruits required significantly shortened dehydration time and produced products with superior quality in comparison with the conventional technology.