Ultrasound application as pre-treatment for drying of fruits

Drying is one of the processes that present higher costs in food processing. Pre-treatments are often used to reduce drying time and consequently its cost. In the past years, our group has studied the use of ultrasonic pre-treatments to induce structural changes in the fruit tissue as to improve the effective water diffusivity during air-drying and consequently reduce air-drying time. This study has evaluated the effect of ultrasound pre-treatment on several fruits: banana, papaya, melon, strawberry, sapota, malay apples, star fruit and pineapples. The influence of ultrasound on water loss, sugar gain, effective water diffusivity, drying time and tissue structure modification was evaluated and analyzed. Ultrasound pre-treatment consists in immersing fruit pieces in water or in an osmotic solution and to subject the fruit and solution to ultrasonic waves (at frequencies ranging from 18 to 40 kHz) for a period of time (usually less than 60 min). The ultrasonic waves produce a “sponge” effect which compresses and releases the fruit cells inducing in the appearance of microscopic channels within the tissue structure of the fruit. These microscopic channels are responsible for the increase in the effective water diffusivity in the fruit during air-drying processes. Our studies showed significant differences on the results for different fruits. Ultrasound showed to have higher influence on fruits with high water content (pineapples, melons, malay apples) and high content of fibers and phenolic cells (sapotas). Low influence was observed in very porous fruits (strawberries) and on dense fruits with strong cell attachment (papaya). These observations are in accordance to the modifications observed in the fruit tissue structure by means of photo-micrography, which show different structures and microscopic channels lengths and concentration. Overall, the ultrasound pre-treatment was able to reduce drying time by 20% and to reduce drying costs in up to 30%.